The University of Maryland Extension Agriculture and Food Systems and Environment and Natural Resources Focus Teams proudly present this publication for commercial vegetable and fruit industries. Special Alert Edition #1 May 16, 2018 Striped Cucumber Beetle Biology and Management: A Resource for Cucurbit Farmers Cerruti RR Hooks $ and Alan W. Leslie * $ Associate Professor and Extension Specialist, * Research Associate College of CMNS, University of Maryland Department of Entomology The striped cucumber beetle is the most important insect pest of crops in the family Cucurbitaceae, which are widely grown as vegetable crops in Maryland. In this special edition, have compiled information about this beetle s biology, life history, and different management strategies from a wide range of sources and summarized them here in a single volume as a convenient resource for cucurbit farmers. By presenting a broad overview of information in a single volume, we hope to create a handy reference for information related to this pest, and potentially help farmers to create new or modify existing management plans that result in better control of striped cucumber beetles and increased yields and profits. Many of the control measures outlined in this article offer alternatives to control through pesticide applications. Many of these methods will not provide adequate control on their own, but can be incorporated into a well-designed integrated pest management program to suppress striped cucumber beetles using multiple tactics at once. Introduction. The striped cucumber beetle (SCB), Acalymma vittatum, is a serious insect pest of cucurbit plants throughout most of North America, from southern Canada to Mexico east of the Rocky Mountains. Adults can often be found among flowers of their host plants (Fig. 1A). Their damage can cause substantial yield reductions in cucurbit crops such as cucumbers, melons, muskmelons, squash and pumpkins. Adult beetles feed on the foliage, which reduces leaf area (Fig. 1B), as well as stems, flowers, pollen and fruits of their host Local Governments U.S. Department of Agriculture It is the policy of the University of Maryland, College of Agriculture and Natural Resources, Maryland Agricultural Experiment Station, and University of Maryland Extension that all persons have equal opportunity and access to programs and facilities without regard to race, color, gender, religion, national origin, sexual orientation, age, marital or parental status, or disability.
plants. Adult feeding during early plant growth, especially on cotyledons, can cause substantial stand reductions. Their feeding may also cause stunting of older plants, fruit deformation and scarring, which reduces market quality, or serve as entry points for pathogens. Research has shown that cucumber beetle adults can vector several plant diseases, such as squash mosaic virus, as well as contribute to increased incidence of powdery mildew and black rot, and SCB feeding can cause a predisposition to develop Fusarium wilt. The most damaging pathogen vectored by SCB is, Erwinia tracheiphila, which causes bacterial wilt of cucurbits. Erwinia tracheiphila lives in the gut of the SCB, and relies on this beetle as a host for overwintering and as a vector for transmission to new plants. Young cucurbit plants are primarily vulnerable to bacterial wilt disease and stunting, whereas damage to older plants comes primarily from fruit scarring. In truth, older plants can tolerate as much as 25% defoliation due to beetle feeding with no significant reduction in yield. Larval stages of the SCB feed on roots and stems but this damage is thought to be minimal. Howbeit, their feeding has been reported to lower root density and to be correlated with Fusarium wilt and viral transmission. Thus, significant economic losses caused by SCB feeding occur when adult beetles cause early-stage foliar damage or when beetles vector plant diseases. Most evidence suggests that the magnitude of cucurbit crop damage, either by feeding or disease, is strongly linked to the arrival of a large gathering or group of beetles at a single site within a habitat, also known as beetle aggregation. As such, disaggregating (separating) SCBs would likely be an effective strategy for reducing their impact below levels that require chemical intervention. Adult cucumber beetles damage cucurbit crops primarily by feeding directly on cotyledons, foliage and stems of newly emerged seedlings, thereby reducing plant stands, and also by transmitting the pathogen E. tracheiphila, which causes bacterial wilt disease. It has been suggested that seedlings need to be protected until plants reach the 3-true-leaf stage or beyond. Protecting older plants generally does not reduce plant death due to bacterial wilt or direct feeding. However, excessive defoliation and damage to blossoms or fruit by SCB may delay growth, reduce yields or render fruit unmarketable in mature plants. Identification. The adult SCB is roughly 1/5 inch long, 1/10 inch wide, and has yellow wings with three distinct longitudinal black stripes as well as a black underside. It also has a black head and antennae and a prothorax (the first area behind the head) that tends to be more orange than the yellow wings. Proper identification of SCB is important because they are easily confused with western corn rootworm (WCR) adults. The WCR are not economic pests in cucurbits but may sometime be found feeding on cucurbit flowers. An easy way to distinguish between the two species is to check their legs. The SCB adults have faint yellowish bands on their legs (Fig. 2) whereas WCR adults have solid black legs (Fig. 3). Additionally, stripes on the SCB are straighter than those on the WCR. The WCR has black and yellow lines on its back, but edges of the lines are curved and less distinct (blurred), and the underside of the beetle is yellow. Another beetle that may be mistaken for the SCB is the spotted cucumber beetle. This beetle is often present on 2
cucurbit crops but at lower numbers than SCB. It is of similar size as the SCB but has a greenish-yellow body and instead of stripes, it has 12 distinct black spots on its back (Fig. 4). Lifecycle. Striped cucumber beetles overwinter as adults in protected sites near buildings, fence lines, bordering vegetation, and forests adjacent to cucurbit fields or plant and old crop debris. They begin to emerge from their overwintering sites in the spring when temperatures exceed 55ºF, and afterwards feed on pollen from wild flowering plants. These beetles are relatively specific to cucurbits when available. However, when cucurbit plants are not available in early spring and late fall, SCB adults often feed in wooded areas on pollen from various plants (e.g., asters, goldenrods, willow, and several species of Rosaceae). However, SCB larvae can only complete their development on cucurbit roots. As such, adults quickly locate host plants in the spring. Beetles use cucurbitacin, a plant kairomone (chemical substance emitted by an organism), to locate cucurbit fields. Once cucurbit crops begin to emerge from the soil, or are transplanted into a field, beetles migrate to cucurbit plants to feed and mate. Adult SCB aggregate in cucurbit fields in response to a male-produced aggregation pheromone that functions best when they are actively feeding. The aggregation pheromone signals the presence of mates and host plants and serves as prerequisite for mating and reproduction. Colonizing beetle populations are initially highly concentrated along the field margin, but become less aggregated as the season progresses. Feeding, mating and egg laying begin shortly after colonization of a cucurbit field. From mid to late mornings, SCBs aggregate in flowers of cucurbit plants to feed and mate. Female SCB are long-lived and lay ~125 eggs/individual over their lifetime under optimal conditions. Eggs are laid just below the soil surface or at the base of cucurbit plants 10 to 20 days after mating occurs, and hatch approximately one week later. Larvae take two to four weeks to mature, and then pupate over the next seven days before emerging as next generation adults in mid-summer. Larvae of SCB are sensitive to high soil temperatures, with temperatures of 36 C (97 F) or greater being lethal to SCB larvae. Pupation occurs in the soil and adults emerge 40 to 60 days after eggs are initially laid. Summer adults emerge and feed in flowers, on foliage or on the surface of fruits. In Maryland, SCB may complete one to three generations per year, depending mostly on local soil temperatures. The role of cucurbitacins. Cucumber beetles are attracted to cucurbitacins in the foliage of cucurbit plants. Cucurbitacin is biochemical compounds that some plants produce, which functions as a defense against herbivores. When cucurbit plants are flowering, volatile chemicals emitted from flowers can attract cucumber beetles over relatively large distances. This phenomenon was demonstrated in a study that showed SCBs were attracted to a floral odor from male blossoms of winter squash in the absence of contact with and visual detection of squash plants. The study showed that significant numbers of adult SCB came to sticky traps containing concealed squash seedlings or flowers. Adult beetles may vary in their attractiveness to different cucurbit cultivars. Variation in their attractiveness to different cucurbit species is generally believed to be due to different concentrations and ratios of the bitter cucurbitacin compounds within plants. 3
For example, a study showed that SCBs were highly attracted to Blue Hubbard and buttercup squash, which are relatively high in cucurbitacin B compared to butternut squash which is relatively low in cucurbitacins. Feeding area and damage. Foliar feeding by SCB results in a characteristic pattern of small holes in portions of the leaves serviced by the smallest veins (Fig. 5). If populations are high, adult beetles will feed on stems of host plants as well. Adults may conceal themselves in the soil around plants, under clods of soil, or in cracks within the soil to as a refuge from predators or as shelter from mid-day heat. Therefore, feeding on the stem will often occur at the plant s base where the stem meets the soil. Adult feeding is of particular concern when plants are in the cotyledon and 1-3 true-leaf stages. At this size, plants are small enough that high adult populations can completely defoliate plants or girdle their stems. As plants grow beyond the 3 true-leaf stage, several cucurbits (cucumbers and pumpkins) can tolerate high levels of defoliation. Once flowering occurs, SCB will usually move off the foliage and begin feeding on blossoms and pollen. Fruit or flower feeding does not typically have a significant impact on yield. However, fruit feeding may cause some cosmetic damage when SCB populations are high. Bacterial wilt transmission. Cucumber beetles are of great concern because they can infect cucurbits with bacterial wilt. The bacterium that causes bacterial wilt can survive and overwinter in the gut of striped and spotted cucumber beetles. When beetles become active in the spring and begin feeding, it is believed that they spread bacteria to healthy plants through their feces or from contaminated mouthparts. Chewing damage on young leaves or cotyledons open entry points for the pathogen. Once inside the plant, the bacteria multiply quickly, forming colonies that effectively block the plant xylem and subsequently cut off the plant s water supply, resulting in wilting. Beetles are attracted to infected plants and can pick up bacteria and move it to healthy plants. Strong circumstantial evidence supports the presumption that transmission of E. tracheiphila by SCB occurs through foliar feeding when fecal pellets (frass) containing E. tracheiphila fall onto open leaf wounds at sites of feeding damage. However, recent research from Penn State University suggests that the bacterium may also survive in symptomless weed hosts where SCB can acquire the bacterium after emerging from overwintering sites, before they move into cucurbit fields. Bacteria wilt symptoms. Once a plant is infected with bacterial wilt it cannot be resuscitated. Wilt symptoms typically develop 7-15 days after infection, and the disease is nearly always fatal once symptoms appear. Death occurs 1-3 weeks after symptoms appear. The first symptom of bacterial wilt on cucumber and muskmelon is a distinct flagging individual leaves. Beetle feeding is not always obvious on wilted leaves. Soon, adjacent leaves and finally the entire vine will wilt. The wilting spreads as the multiplying bacteria move within the plant. Eventually, the entire infected plant wilts and leaves may discolor before eventually dying. The only way to avoid bacterial wilt is to prevent beetles from feeding on susceptible plants. Fruits produced on wilting plants usually are not marketable. 4
Wilt diagnosis. One method used for field diagnosis of bacterial wilt is to take a stem that exhibits symptoms from a plant and cut it in half. Hold the two cut ends together for 10-15 seconds and slowly pull them apart. If bacteria are present, a sticky sap should ooze from the two cut ends (Fig. 6). A plant can also be tested for bacterial wilt by cutting it with a knife and then squeezing each end. Infected plants should ooze a milky white bacterial substance. However, these techniques are sometimes difficult to use for diagnosis and should not be relied on as the sole indicator that a bacterial infection is present or absent. Caution should be taken so as not to confuse bacterial wilt with other cucurbit problems. For example, two cucurbit insect pests, squash bugs (Fig. 7) and squash vine borers (Fig. 8) can inflict damage that causes leaves to wilt, producing a similar symptomology to bacterial wilt. Watch for high numbers of squash bugs or orange colored frass exuding from stems (a symptom of squash borer damage). Drought is another factor that will cause cucurbit leaves to wilt. Aggregation and wilt disease. Early in the growing season when SCB populations are small, males release aggregation pheromones when feeding on the leaves of cucurbit seedlings that attract male and female cucumber beetles. This leads to concentrated feeding on some seedlings. It has been observed that wilt disease is more likely to occur on seedlings where concentrated feeding occurs. This finding suggests that the dosage of E. tracheiphila is important for disease transmission under field conditions. As cucurbit plants grow, foliar feeding becomes less concentrated and the leaves possibly become more resistant to E. tracheiphila invasion. However, even in large cucurbit plants, adult SCBs are attracted to flowers, and may concentrate feeding in those areas. Under heavy infestation, a large proportion of flowers may contain frass contaminated with E. tracheiphila. Thus, it is believed that E. tracheiphila can be transmitted through infected frass that falls on or near floral nectaries while SCBs are feeding in the flower. Plant susceptibility. A recent study showed that the proportion of beetles harboring E. tracheiphila is typically five times greater than the proportion of beetles that actually transmit the disease. Other studies have shown that the efficiency of transmission depends on the size of the wound, the inoculum dose, and the amount of time infected beetles 5
feed on a plant. Although all cucurbits can become infected, a key point to remember about bacterial wilt is that not all cucurbits are equally susceptible. Cucumbers and muskmelon are most susceptible and likely to die from bacterial wilt. On the other hand, some squashes and pumpkins are generally more tolerant or resistant to bacterial wilt. Management. Plants are most susceptible to damage by SCB during their first to third true-leaf stages (i.e., leaves that emerge after the first seed leaves). Plants should be scouted regularly for SCBs during these stages. If at least 25% defoliation is found during this time, it is recommended that plants be treated to protect them from SCB feeding damage. It isn't as important to treat SCBs that are found later in the summer as they typically do little or no damage to plants. Controlling adult beetle populations will also limit the risk of SCB transmitting bacterial wilt to cucurbit plants. Management tactics that result in fast knockdown are the most effective for preventing bacterial wilt. If plants display signs of wilt, an option for very small acreage is to remove infested plants before more SCB can feed on them and spread the bacteria. However, this is typically not feasible for larger plantings. The majority of the control should target the plant s cotyledon and 1-3 true-leaf stages because of its susceptibility to feeding damage and disease (Fig. 9). Cucurbits were reported to suffer the greatest yield loss from bacterial wilt caused by E. tracheiphila when they are infected as young plants, making early season protection from beetles critical. However, bacterial wilt can occur in any sized plant. Bacterial wilt is controlled via management of its vectors (beetles) through the use of systemic and foliar insecticides. Insecticides would have to be applied repeatedly to entirely block wilt transmission, as new beetles constantly colonize the crop. However, this could become costly, especially because control may be required until fruiting as larger plants of some cucurbits are also susceptible to bacterial wilt. However, treatment of plants just before they are transplanted into the field could help get some cucurbits past the vulnerable early stages. Chemical control. Once SCB infestations have been detected and action threshold levels have been reached, there are several different insecticides that provide effective SCB control. When applying any pesticide, always read and follow all directions on the pesticide label. Cucurbit crops require insect pollination to produce maximum yields, therefore excessive use of some materials that are toxic to pollinators may cause yield reductions. When flowers are present, every effort should be made to apply insecticides in the early morning or late evening to reduce contact with pollinators. Currently, chemical treatment thresholds vary depending on the crop, and can be as low as one beetle per cotyledonstage plant. Treatment thresholds increase as plants grow larger. Once threshold has been reached, insecticides may need to be applied to the foliage in a timely fashion (5-10 d schedule) to prevent excessive crop damage particularly if wilt is present. Certain cucurbits such as muskmelon or cucumber may require a more extended treatment period because of their high susceptibility to wilt. Other cucurbit crops that are not very susceptible to wilt disease, such as watermelon, only require protection when plants are small and beetle populations are high. In the absence of concern over bacterial wilt, more mature cucurbit plants can withstand defoliation from beetle feeding without impacting plant productivity. For example, it has been proposed that fully developed, healthy cucurbit plants can withstand 25-50% defoliation before yields are dramatically affected. In contrast to conventional cucurbit systems where beetles are 6
effectively controlled with foliar insecticides or systemic seed treatments such as neonicotinoids, beetles can be especially serious on organic farms where there are few effective insecticidal options. Natural materials such as kaolin clay, pyrethrum, and spinosad have been reported to have some effectiveness in managing cucumber beetles. Kaolin clay is an insect repellent that supposedly suppresses light populations of cucumber beetles. Not all spinosad formulations are approved for organic use, so care must be taken in selecting any chemical used for organic plantings. Additionally, the efficacy of organically approved products may be low compared to synthetic pesticides. Efficacy may be further reduced because many organic products lack residual activity, and new SCB will often colonize a field over a lengthy period of time. As a consequence, organic-approved insecticides have not always been found to provide adequate control of SCB. Cultural. In some cropping areas there may be perennial SCB problems. Cultural control, which mainly consists of preventive measures, should be a part of any SCB management program under these conditions. Cultural controls include crop rotation, the use of transplants rather than direct seeding, row covers, trap cropping, mulching for predator conservation, the use of reflective or black plastic mulches, choosing resistant cultivars, and intercropping. However, many cultural management tactics may not be fully effective as standalone remedies and may require integration with other control measures. Delayed Planting. Delaying cucurbit plantings until overwintering beetles have emerged and using high seeding rates can help ensure adequate plant stands. Growers can avoid the most significant damage by simply delaying the planting of summer cucurbits by a few weeks. This tactic avoids damage from the peak population of dispersing adult beetles in the spring. As a result, seedlings will have reduced pressure during the most vulnerable stages, and can grow into larger plants capable of enduring beetle pressure. Some drawbacks of these approaches include the additional cost associated with a heavier seeding rate. Furthermore, if there is a lengthy time period in which overwintering beetles enter the field, this tactic would be less likely to succeed. Finally, this delayed planting tactic may not be possible for those who benefit from better returns on an early harvest. Rotate. Crop rotation is the cornerstone of any pest management program, with no exception for SCB management. Rotating cucurbit crops to a field site that is distant from a prior cucurbit field can help reduce beetle infestation. Simply rotating to new ground within a field, or to a neighboring field will not be very effective due to beetles mobility, and the fact that they often overwinter near the previous years cucurbit crop. As such, it is best to plant cucurbits as far as possible from the last year s crop. Still, crop rotation may only contribute moderately to SCB management, and other tactics will likely still be necessary. Transplant. Seedlings and small plants are most susceptible to cucumber beetle feeding damage and to bacterial wilt. Using transplants can avoid exposing cucurbits to SCB feeding during the most susceptible plant stages. Transplants will mature faster than direct-seeded cucurbits, which means they are more likely to withstand SCB infestations without yield reductions. Transplanting also reduces the total time that cucurbit plants are in the field each season. This provides less time for SCB densities to build up and for disease symptoms to develop. Another advantage of transplanting is that the crop canopy should close more quickly. This should reduce the critical weed free period. The critical weed free period involves the amount of time that weeds have to be managed to prevent yield reductions due to competition. 7
Row Cover. Floating row covers can be used to exclude SCB from plants during the seedling stage. They provide arguably the most reliable defense against cucumber beetles while in place. This protection allows plants to mature and develop extensive leaf mass and a strong root system, enabling the plant to withstand moderate pest pressure. However, row covers must eventually be removed by the time bloom begins to allow bees and other pollinators access to cucurbit flowers for adequate pollination. Challenges of row covers include their high cost and the fact that they block access to the crop for weeding. Because row covers foster weed growth, many producers use weed-suppressing mulches in combination with floating row covers. Plastic or organic mulches may be combined with floating row covers to reduce weed problems. Black plastic mulch. Growing cucurbit crops on black plastic mulch can reduce damage by SCB through several mechanisms, including improving crop growth, physically blocking beetles from the soil or repelling them from plants, and modifying soil temperature. Plastic mulches can increase soil temperature, and soil moisture and reduce fertilizer runoff, which can all lead to faster growth of cucurbit crops. This increased growth rate may reduce the amount of time that early, vulnerable stages are exposed to SCB feeding, and produce healthier, more vigorous plants. Results of a study in Virginia suggest that metallic-colored plastic mulches repel cucumber beetles, reducing beetle feeding damage and the transmission of bacterial wilt. A study conducted to examine effects of black plastic mulches on the distribution of immature and adult cucumber beetles on plants showed that the presence of plastic mulch appears to limit adult beetles' access to plant roots for egg oviposition. Using black plastic mulch significantly reduced the number of beetle eggs and larvae in the soil around muskmelon plants. Research has shown that temperature strongly influences developmental rate and survivorship of SCB. As soil temperature increases, developmental rate of SCB also increases but survivorship declines rapidly at higher soil temperatures. Using black plastic mulch will increase soil temperatures, with the rate and depth of increase influenced by soil and moisture conditions. It was reported that planting on black plastic mulch can reduce SCB larval survival by up to 50%. However, more research is needed to help explain the exact mechanisms by which black plastic mulches increase SCB larval mortality. Disadvantages of black plastic mulches include the added expense of purchasing materials and removal and disposal after use can incur additional challenges, with few options available for recycling. Further, studies in Maryland indicate that greenhouse gas emissions are higher from soils that are covered by black plastic mulches. Organic mulching. Using straw, hay, plastic, or fabric as mulch can deter cucumber beetles from laying eggs in the ground near the plants. While mulching will not completely halt egg-laying or feeding, it will limit direct access to the stem, as well as significantly slow down larval migration through the soil. Straw mulch is thought to help reduce cucumber beetle problems in at least three different methods. First, mulch might directly slow beetle movement from one plant to another. Second, the mulch provides refuge for wolf spiders and other predators from hot and dry conditions, helping preserve them so that they can then feed on SCB prey. Finally, straw mulch serves as food for springtails and other insects that eat decaying plant material. These decomposers may serve as alternative food for spiders, helping to further build up their numbers. A study showed that cucumber plants grown in richly mulched soils harbored fewer cucumber beetles than those in soils with less organic content. It was suggested that the organic matter promoted diverse populations of beneficial soil microorganisms that triggered the plants internal defenses. 8
Tillage. Cucumber beetles can overwinter in crop residues above and below ground. As such, practicing cultivation after fall harvests has been suggested to be an important practice. Deep tillage, compost application, and cover-cropping in the fall have been recommended to encourage decomposition of residue which may harbor beetles through the winter months. It has also been suggested that diseased plant matter be burned or otherwise discarded rather than composted for future use. However, deep tillage and cultivation are not compatible with conservation tillage and the many benefits associated with reduced tillage practices. Further, removing the residue may prove to be more beneficial to SCB if it also negatively impacts their natural enemies. For example, some predators need a protected place to overwinter, mate and lay eggs. These protected habitats should also provide alternate food, a favorable microclimate, and shelter from other predators. Thus, fields with high residue can provide a suitable environment and protected winter cover for beneficial arthropods such ground beetles. However, tilling a field can make habitats unsuitable for natural enemy survival and conservation by destroying their refuge. Trap cropping. A trap crop is a plant that is used to attract or lure agricultural pests, usually insects, away from the main or protected crop area. By luring insect pests into a concentrated area, control measures can then target that area only; localizing the damage and limiting the spread of disease or damage. Striped cucumber beetles are vulnerable to this management practice because they show a distinct preference for certain types, varieties, and plant stages within the cucurbit family. Beetles prefer to feed on cotyledons of seedlings, wilting plants and certain cucurbit types or varieties due to higher concentrations of cucurbitacins, which serve as a feeding stimulants for SCB. Once feeding begins, beetles use an aggregation pheromone to call others to the acceptable food source. There is a long list of cucurbit varieties that are favored by cucumber beetles, and are thus potential trap crops. Preferred cucurbit crops include some gourds, certain winter squash varieties like Turk's Turban and Blue Hubbard, zucchini, cucumbers, yellow summer squash and acorn squash. Cucurbit crops that have been reported to be slightly less preferred by SCBs include pumpkins, muskmelons, butternut squashes and watermelons. Trap cropping and bacterial wilt. The trap cropping tactic could affect the incidence of bacterial wilt in cucurbits. Research suggests that the incidence of bacterial wilt is positively correlated with the level of beetle aggregation. If that is indeed the case, then one could reasonably expect that the incidence of wilt in the protected crop acreage would be reduced as beetles are disaggregated and drawn to the trap crop. Conversely, the aggregation of beetles would likely increase the incidence of wilt in the trap crop. If the trap crop were sacrificial, that would not present a problem, but if there is a desire for it to be marketable, losses should be acceptable. Additionally, there is risk that the trap crop could serve to increase the incidence of bacterial wilt in the protected crop area. Thus, it is generally not recommended to use a trap crop that will act as a reservoir for the bacterial wilt pathogen as disease incidence may increase and yields may decline. This suggests the best tactic involves using a trap crop that lures SCB from the main crop field area but is resistant to and will not harbor the bacterial wilt pathogen. Perimeter trap cropping. The SCB is known to overwinter in woods surrounding cucurbit fields and move into a crop from the border, and with good crop rotation practices, adult cucumber beetles will always be moving into a crop from somewhere else. Regardless of where they come from, cucumber beetles generally aggregate at field edges, and attractive trap crops may further enhance this behavior. Together, this makes SCB a good candidate to manage using perimeter trap cropping (PTC; Fig. 10). Perimeter 9
trap cropping is a technique in which the trap crop forms a complete barrier around or encloses the main/protected crop field area. As such, the principle of PTC is that beetles will be concentrated at the field borders before dispersing into the field s interior, allowing insecticides to be applied to a much smaller area. If the trap border is treated with a systemic insecticide at planting, the crop may be protected throughout the critical early growth period. Additionally, this targeted application can significantly reduce the amount of insecticide needed. It is recommended that multiple rows of trap crops be planted if beetle pressure is expected to be severe. Just as important, it is suggested that the PTC be planted a week or two earlier than the protected cucurbit planting to proactively direct migration to the trap crop. Research indicates that the Blue Hubbard and buttercup varieties of Cucurbita maxima, and zucchini (C. pepo) are particularly attractive to cucumber beetles. A study showed that using Blue Hubbard squash as a PTC reduced the need for insecticides to control SCB by as much as 90% in butternut and other squash. It was also found that buttercup squash performed equally as well as Blue Hubbard as a trap crop, with 97% reduction in total insecticide use compared with control fields without a trap crop. The study reaffirmed the effectiveness of the PTC system and subsequently offered growers a more marketable trap crop (buttercup squash) than Blue Hubbard for managing SCB damage. Does a trap crop have to be more appealing? Though it is recommended that a trap crop be more appealing to SCB than the primary or protected cucurbit acreage, a trap crop can be any cucurbit variety that attracts SCB. The critical factor is planting the trap early, roughly two weeks prior to planting the primary cucurbit acreage and the trap crop should encircle the primary field area. This method should be successful because overwintering SCB will usually migrate to the earliest emerging, or most mature cucurbits, on a given farm. As such, if the cucurbit is planted around edges of fields it should attract migrating beetles. However, once SCB starts to build-up in these traps, they should be treated with an effective insecticide to minimize further movement into the primary acreage. Even with light-to-moderate beetle populations, it may be necessary to spray the trap crop to control beetles so as to prevent the population from redistributing throughout the field over time. This approach should be effective in reducing early-season SCB in a given area, and the need for treatment in later-emerging cucurbits in the field s interior. Scouting and monitoring for SCB should be conducted in the trap and primary field area to ensure populations are being kept in check. Trap cropping challenges. Though trap cropping has proven to be a viable option, there are some anticipated challenges to using this tactic. First and foremost, SCB exhibits a range of preferences among various cucurbit species and cultivars, and findings from choice trials are highly dependent on choices being offered. Given the many types of species and cultivars of cucurbits available, conducting research to compare all the different types available would be impractical. Second, the success or failure of trap cropping seems highly dependent on growth stages of the main (protected) crop and trap crop. This presents logistical difficulties to growers, who would have to stagger the seeding and transplanting of trap and protected crops to get the correct stages in the field at the appropriate times. Previous research has failed to distinguish whether the trap crop being tested was attractive to beetles because of a preference for that specific cultivar, or whether the aggregation was due to a preference for the developmental stage of the trap crop. For example, cucurbit flowers that are known to be attractive to beetles, and therefore a flowering trap crop may be more attractive than the main crop no matter what species or cultivars are paired together. Further, PTC may not work in fields that were not rotated, especially under severe SCB pressure, as adult beetles may have overwintered in the field as opposed to migrating in from outside overwintering quarters. Still, under the appropriate conditions, PTC is an effective management practice. 10
Intercropping. Intercropping or inter-planting, which involves growing two or more different plant species within the same field is another habitat manipulation technique that may be used to reduce SCB numbers. A field experiment found that intercropping cucumbers with corn and broccoli reduced SCB numbers significantly compared to plots planted in a monoculture of cucumber. As a result of reduced SCB numbers, the researchers also measured a reduced incidence of bacterial wilt disease. A study conducted in Maryland showed that zucchini plants interplanted with a sunn hemp (Crotalaria juncea) living mulch (Fig. 11) had significantly fewer spotted and striped cucumber beetles than monoculture plots. Similarly, fewer SCB were found in cucumber intercropped with red clover (Trifolium pratense) compared with cucumber grown as a monoculture. Strips of buckwheat (Fagopyrum esculentum) grown as a living mulch reduced SCB populations by 60% in cucumber and pumpkin compared to monoculture plots. Numbers of striped and total (striped + spotted) cucumber beetles were reduced significantly by the combined use of three companion plants: radish (Raphanus sativus), tansy (Tanacetum vulgare), and nasturtium (Tropaeolum spp.) or buckwheat, cowpeas (Vigna unguiculata), and sweet clover (Melilotus officinalis). Findings from these studies suggest that many different types of intercrops can help reduce SCB numbers on cucurbits. An advantage of intercropping is that it is compatible with several other management tactics. However, intercropping may impose greater production challenges because of added complexities associated with managing more than one plant species within the same field. Further, intercropping or inter-planting tactics may not be a suitable strategy if SCB populations are extremely high. As such, these tactics may require an integrated approach consisting of additional management options especially under severe pest pressure. Resistance Plant resistance. Variation in susceptibility to SCB damage exists among cucurbit cultivars because of beetle behavior and host plant factors. Cucumber beetles are attracted to host plants by a chemical called cucurbitacin, which gives cucurbits their bitterness and likely is used as a defense against less-specialized herbivores. As a cucurbit specialist, the SCB absorbs cucurbitacin into its body and uses it to defend itself against predators and pathogens. Thus, cucurbit varieties or species with lower cucurbitacin levels may be less attractive to cucumber beetles. This suggests that low cucurbitacin levels may confer some resistance to beetle feeding but only when crops are grown in proximity to a high cucurbitacin crop, as is the case with PTC systems. Some varieties have however, been reported to be resistant to the bacterial wilt disease transmitted by beetles. For example, County Fair, Chinese Long and Tokyo Green cucumbers have been reported to be resistant to wilt, as are buttercup squash and Black Beauty zucchini. Some varieties of watermelon tested demonstrated resistance once they had grown enough to contain 10 leaves. Still, plant resistance levels have not been high enough to manage the disease in cucurbits. Thus, this tactic has not received much attention in SCB management programs. 11
Biological Control Insect pathogens and entomopathogenic nematodes. An insect pathogen is a disease-causing organism that can sicken or kill an insect. Most insect pathogens live in the soil, and would consequently be deployed to target the SCB larval stage. Fungal pathogens and insect-attacking (entomopathogenic) nematodes are commercially produced and available as bio-pesticides. Entomopathogenic nematodes (EPN) are soft bodied, non-segmented roundworms that are parasites of insects. They occur naturally in soil environments and locate their host by tracking carbon dioxide, vibrations and other chemical cues. Soil drenches of bio-insecticides have shown some activity against SCB larvae. Research has shown that the entomopathogenic nematode, Steinernema riobravis, was capable of causing a 50% decrease in the survival of SCB larval under conventional and organic soil management conditions. These nematodes are readily introduced through drip irrigation, and the addition of black plastic mulch provides a moist soil environment that is conducive to their survival. On the contrary, black plastic mulches are not favorable to SCB survival. Thus, the potential exists to use black plastic to enhance the survival of entomopathogenic nematodes while making environmental conditions less conducive to the survival of SCB larvae. Predators. An adult SCB is a relatively large insect, and as it is a beetle, it is protected by a relatively hard shell, or exoskeleton. As such, they will be attacked mostly by relatively large predators capable of subduing a large, protected beetle. Predacious ground beetles in the family Carabidae and especially large wolf spiders (Figs. 12A, B) appear to be major predators of SCB adults and have been shown to feed heavily on cucumber beetles in cucurbit crops. Studies have shown that cucumber beetles avoid wolf spiders, and feed less when spiders are present even when spiders do not actually attack or kill the cucumber beetles. Laboratory experiments showed that in the presence of spiders, SCB fed less frequently. Field observations conducted at night in cucurbit gardens showed that a spider within 15 cm (~ 6 in) of a group of SCBs increased the rate at which they left the plant by 1.6-fold. Further experiments revealed that the SCB consistently relies on tactile (touch) cues and sometimes on visual cues to detect the wolf spider s presence. The most consistent SCB behavioral response to the presence of wolf spiders was to leave the plant at a higher rate. A field cage study conducted in Maryland showed that cucumber beetles more readily left plants in which wolf spiders foraged. This occurred even when spiders were removed before introducing cucumber beetles to test plants. This suggests that cucumber beetles are capable of detecting spider cues when they are no longer on plants. 12
Parasitoids. The tachinid fly (Celatoria setosa, Fig. 13) and a braconid wasp (Centistes diabroticae) parasitoids are known to parasitize SCB adults. Celatoria setosa females lay eggs that contain fully-developed first instar larvae directly into their host, a process termed larviposition. Centistes diabroticae deposits its egg within the SCB body. A study conducted in central New York showed that both can significantly impact SCB survival. During the study, parasitism by the tachinid and braconid reached maximum rates of 43% and 54%, respectively. Parasitism of SCB was also monitored at two field sites in central PA between June and October 2014. Parasitism rates reached levels up to 56% for Cel. setosa and 17% for Cen. diabroticae. These studies showed that the tachinid and wasp parasitoids are active in cucurbit fields, suggesting that more research attention should be given to their potential impact on SCB mortality and determining methods to increase their efficacy. There is anecdotal evidence that parasitoid populations may build up over several years in organic fields, such that parasitoid impacts in organic fields may be far greater than in conventional fields. Once the tachinid fly and wasp parasitoids deposit their offspring inside the SCB, immature stages of these parasitoids develop inside the SCB body. As such, it is not easy to assess whether adults have been parasitized. The current protocol includes rearing SCBs in cages until parasitoids emerge. Another tachinid fly, Celatoria diabroticae, is known to parasitize spotted and striped cucumber beetles in New England. Collections of adult beetles from cucumber plots in southern and western MD in 2016 indicated some adult striped and spotted cucumber beetles were parasitized by Cel. setosa (Fig. 13) and Cel. diabroticae, respectively. Future surveys are planned in Maryland to assess rates of SCB parasitism by tachinid and wasp parasitoids. Conclusion Cucurbit growers face an assortment of economically important pests. Spotted cucumber beetle, squash bug, squash vine borer, aphids, SCB and plant pathogens that they vector are of concern. Arguably, the SCB is the most serious cucurbit pest globally. They feed on all plant parts; and their feeding can cause significant yield reductions in melons, cucumbers, squash and other cucurbits. As such, they are the primary target of insecticide spray programs for cucurbits grown in the Northeastern US. However, chemicals used in their management can prove fatal to pollinators and other beneficial organisms, disrupting services they provide. This suggests that alternative SCB management strategies should be considered. There are a number of non-chemical management (e.g., trap cropping, plastic mulch, biological control) tactics that may be considered in addition to insecticide applications. In most instances, these non-chemical practices may not be used as a standalone practice, especially if SCB populations are expected to be severe. Additionally, many nonchemical practices are preventive and cannot be used in response to a heavy beetle infestation. However, they may be deployed as preemptive tactics, and under low to moderate SCB pressure they may prevent SCB from reaching levels requiring chemical intervention. Further, preventive tactics may be combined or used in concert with chemicals, consequently reducing the number of sprays or dosage required. Any SCB management program that reduces the dosage or number of chemical applications may help conserve beneficial organisms. Subsequently, this will help in the natural management of other economically important cucurbit pests. Though there has not been a lot of research conducted to show the benefits of SCB natural enemies, studies have shown that entomopathogenic nematodes, large 13
wolf spiders and certain parasitic wasps and flies may use SCB as prey or host. This research area requires greater attention so that natural enemies can be better incorporated into SCB management programs. To this end, research is currently underway in Maryland to determine the potential for spiders and parasitic flies to suppress SCB populations as well as develop production practices to enhance their numbers and effectiveness in cucurbit plantings. Acknowledgements Mention of chemical products in this publication is not an endorsement of their use. Some of the research reported and composing of this article were supported by USDA NIFA EIPM grant 2017-70006-27171 and a University of Maryland Sustainability Grant. Vegetable & Fruit News Published by the University of Maryland Extension Focus Teams 1) Agriculture and Food Systems; and 2) Environment and Natural Resources. Submit Articles to: Editor, R. David Myers, Extension Educator Agriculture and Natural Resources 97 Dairy Lane Gambrills, MD 21054 410 222-3906 myersrd@umd.edu Article submission deadlines for 2018 at 4:30 p.m. on: April 25, May 23, June 18, July 25, August 22, September 19 and October 24 (Special Research Edition). Note: Registered Trade Mark Products, Manufacturers, or Companies mentioned within this newsletter are not to be considered as sole endorsements. The information has been provided for educational purposes only. The University of Maryland Extension programs are open to any person and will not discriminate against anyone because of race, age, sex, color, sexual orientation, physical or mental disability, religion, ancestry, national origin, marital status, genetic information, political affiliation, and gender identity or expression. 14