Strawberry Insect Scouting

Transcription

1 Strawberry Insect Scouting James F. Dill and David T. Handley University of Maine Cooperative Extension Highmoor Farm, P.O. Box 179, Monmouth, ME Strawberries are hosts for many insect species, some of which can become significant pest problems under certain conditions. Managing insect pests efficiently and effectively requires an understanding of each species biology and behavior, so that control strategies can be used to their best advantage. Preventing infestations of insects should be a primary part of any pest management program. Removing alternate hosts of strawberry pests, including weeds in and around the planting is a critical step in preventing both insect and disease problems. These plants act as hosts and harbors for pest species. Proper row spacing and keeping plant rows no wider than 18 inches allows good air movement through the plants to reduce diseases and plant stress, which can also encourage insect infestations. Finally, encourage natural enemies of pests in your planting, such as predatory mites, lacewings and lady beetles, by using pesticides only when absolutely necessary, and avoiding broad-spectrum compounds that are very toxic to natural enemies whenever possible. A good integrated pest management (IPM) program depends on monitoring insect populations and/or damage in a field and using action thresholds to determine when insect control is necessary. Monitoring insect populations and/or injury is generally carried out by field scouting. This is a regular examination of plants within a field for the presence of insects or insect damage. A relatively small sample of plants or insects in a field can provide good estimates of the total number of insects or the amount of damage present. In order to be accurate, a sample must be representative of an entire field. Typically, when a field is sampled or scouted, the scout stops at ten locations in a field following an X or Z pattern, and will examine a certain number of plants or area at each location, looking for either the presence of pests or the damage they cause. Once pests or damage is found, the amount is compared to an action threshold to determine if any control measures are necessary. An action threshold is, simply put, the level at which the cost of the damage being caused by the pest will soon exceed the cost of controlling it. If only a few pests are present in a field, controlling them may not be justified by the cost. Using an action threshold is a more efficient way of controlling pests than other methods, such as calendar schedule (e.g. spray every 5 to 7 days), or the plant growth stage (pre-bloom, bloom, petal fall, etc.), because these methods do not take into account whether or not a pest is actually present. Listed below are some of the major insect pest of strawberries in New England along with some information about their biology and, when available, monitoring and management techniques. Tarnished Plant Bug, Lygus lineolaris The tarnished plant bug is a small (1/4") bronze-colored insect with a triangular marking on its back. The immature stages, or nymphs, are smaller and bright green. Tarnished plant bug overwinters in the adult stage. In the spring these adults move in to strawberry fields to feed on growing points and lay eggs in plant stems. There are typically two to three generations of

2 tarnished plant bug per year, but the overwintering adults and the first spring generation of nymphs and adults are the most threatening to June-bearing strawberries. When tarnished plant bugs feed during the bloom period, severe deformities of the fruit result, described as buttoning or "cat-facing". The ends of the berries are seedy and the fruit may be dimpled and small. Both adults and nymphs feed on the developing flowers and fruit, sucking out plant juices with strawlike mouthparts. With day-neutral cultivars, control of tarnished plant bug can be especially difficult because the insects will remain on the plants throughout the extended bloom period. Monitoring for tarnished plant bug in strawberry fields should begin just prior to bloom. Nymphs can be sampled by shaking flower clusters over a white surface (e.g. a paper or plastic plate). The shaking causes nymphs to fall out of the flower cluster for counting them. At least 30 flower clusters should be sampled evenly across the field. For example, if you stop at 10 locations, sample 3 clusters per location. If the average number of nymphs per cluster exceeds 0.25 before 10% bloom, or more than 4 clusters are infested with nymphs (regardless of how many), then an insecticide application should be made. Monitoring this way should be carried out at least twice a week during the bloom period in a V or X -shaped sampling pattern across each field. If tarnished plant bug nymphs are not found until mid to late bloom the control threshold can be raised to 0.50 nymph per flower cluster, and sprays can be delayed until just post-bloom. A sequential sampling method for monitoring tarnished plant bug also has been developed and can save scouting time in large fields if nymph populations are especially low or high. Using Table 1, scout as many blossoms as are necessary to determine if control is required. For example, after scouting 15 blossoms, check the table to see if a decision can be made. If no nymphs have been found no control is needed. Under the low threshold (2% injury allowed), if 3 or more nymphs have been found, a control should be applied. If you choose to use the high threshold (4% injury allowed), and tolerate more injury, than a control should not be applied until 5 or more nymphs are found. If one or two nymphs are found, scouting should continue, i.e. examine five more clusters and move down the chart to 20 clusters examined. Continue scouting until the table provides a control or no control decision for the number of infested flowers.

3 Table 1. Sequential sampling chart for monitoring tarnished plant bug in strawberries (N.J. Bostanian, Agriculture and Agri-Food Canada, St. Jean-sur-Richelieu, P.Q.). Number of clusters examined Control not required Number of flowers infested Control required Low threshold (2%) or more 5 or more or more 5 or more 25 1 or less 4 or more 6 or more 30 2 or less 4 or more 7 or more 35 3 or less 5 or more 7 or more 40 3 or less 5 or more 8 or more 45 4 or less 6 or more 9 or more 50 5 or less 6 or more 9 or more High threshold (4%) Strawberry Bud Weevil or Clipper, Anthonomus signatus The strawberry bud weevil or "clipper" is a small beetle (2-4 mm) with a copper-colored body and a black head with a long snout. It over winters in the adult stage, either in strawberry fields or in nearby woods or brush. The weevils become active in the spring when air temperatures start to reach about 60 F, just as strawberry flower buds begin to emerge. After locating a suitable host plant the weevils chew single, small holes in unopened flower buds. They feed on the developing pollen and lay eggs in the flower buds. After an egg is laid the weevil girdles the pedicle just below the bud. The flower bud dries up and dangles from the stem, eventually falling to the ground. The immature stage is a legless grub that develops inside the girdled bud, and later pupates in the soil. New adults emerge in the early summer, feeding briefly on other pollen sources, such as brambles and potentilla, then seek out protected sites, usually in the duff under low growing plants, and go into diapause until the following spring. Because the buds that are clipped will not develop into fruit, the potential yield losses to strawberry bud weevil have been considered quite high. Action thresholds have been developed based on potential yield loss from girdled buds. Scouting is carried out by regularly examining plants for clipped buds as soon as the buds emerge in the spring. The sample size is two feet of row length at five to ten locations in a field. Use the larger sample size for larger fields. Count all clipped buds found within the sample areas. If the average number of clipped buds for all the two-foot samples exceeds 1.2 (or 0.6 clipped buds per foot), a control measure is recommended. Recent studies of clipper injury in New York have noted that some fields with high levels of bud injury appeared to have little yield loss. This is thought to be the result of compensation in fruit size from the remaining buds. The compensation effect varies among cultivars; some have strong compensation ability while others appear to have little or no compensation ability. For example, Seneca and Mohawk had strong yield compensation, while Honeoye and Northeaster had very little, making the latter more susceptible to significant yield loss. This

4 work has led to a proposed modified action threshold that allows a higher level of injury before a control is justified. The new monitoring procedure (Table 2) suggests examining flower clusters rather than just flower buds, evaluating the level of damage in relation to bud position on the cluster. A cluster is considered highly damaged if it has one clipped primary bud, or two clipped secondary buds, or three clipped tertiary buds. The action threshold for highly damaged flower clusters is three per meter. If scouting buds is preferred, the proposed action threshold is three or more clipped primary buds per meter or 30 or more clipped secondary or tertiary buds per meter. Table 2. Proposed Monitoring Procedure for Strawberry Bud Weevil (M. Pritts, M.J. Kelly, G. English-Loeb, HortScience 34 (1): ). Unit examined Assessment Threshold Old method New method New method Flower buds Flower clusters Flower buds Clipped Buds or Not Clipped 2 clipped buds/m Cluster highly damaged or Cluster with low damage 3 highly damaged Clusters/m Clipped buds or Not Clipped 3 clipped 1 buds/m or 30 clipped 2 or 3 buds/m Two-Spotted Spider Mites, Tetranychus urticae Two-spotted spider mites are very small (1/50 inch) insect-like organisms that feed on the foliage of many horticultural crops, including strawberries. They over winter as adults, and begin laying eggs in the late spring. Mites are usually found on the underside of leaves where they feed on the plant juices, causing the leaves to have a yellow-flecked appearance. Under severe infestations the leaves develop a bronze color, and webbing may be noticeable under the leaves. Spider mites are most active under warm, dry conditions. They generally form colonies that may be localized in hot spots around a field, so it is important to monitor entire fields thoroughly and regularly. Sampling for two-spotted spider mites should begin in the spring after the new leaves become fully expanded. Sample the field weekly, using an X or V pattern. Gather 60 leaves from throughout the field and carefully examine the underside for the presence of mites. This may require a magnifying glass. The mites do not have to be counted, only the presence or absence of any mites on each leaf needs to be noted. If 15 or more leaves out of the 60 leaves sampled (i.e. 25%) have mites, the action threshold has been exceeded and control measures should be taken. Excellent spray coverage of the leaf undersides is critical in order for chemical control applications to be effective. High spray pressure, high spray volume, and good nozzles will provide the best coverage. The use of predatory mites to control two-spotted spider mites has become a relatively common management technique. There are several species of predatory mite that feed upon two-spotted spider mites, including Amblyseius fallacis and Phtytosiulis persimilus. Amblyseius fallacis is a

5 native predator and can survive the winters in the northeastern United States, but it must be released into a field in high numbers in order to achieve good control. One strategy for using predators is to spray a chemical miticide on the plants once the action threshold has been exceeded to reduce the immediate threat, wait seven to ten days, then apply predatory mites at the rate of 10,000 to 20,000 per acre to provide continued control. The success rate of using predatory mites alone has not been very high in most commercial plantings, and growers should approach this technique only on a trial basis. However, it is important to encourage the natural population of spider mite predators in a field by reducing the use of broad-spectrum pesticides (especially pyrethroid and carbamate type insecticides) that are highly toxic to them. A technique that appears promising for managing two-spotted spider mites on strawberries is the application of 1% oil with a mist blower early in the season. The oil is very effective against two-spotted spider mites, but does little harm to their natural enemies, which can then keep the spider mites under the action threshold. Please note that oil-compatible pesticides should not be applied prior to the oil spray. Strawberry Sap Beetle, Stelidota geminata Sap beetle adults are small (2-3mm) and dark brown. They chew small holes in ripe strawberries, similar to slug feeding, and often introduce fruit rot organisms as they feed. Although the damage may be obvious, sap beetles can be difficult to find because of their habit of dropping to the ground when disturbed. Adults emerge from protected overwintering sites in the spring to mate. Females lay eggs near fermenting and decomposing vegetation. The larvae feed on this vegetation on or near the soil surface and pupate in two to three weeks. Adults emerge from the pupae in June and July and look for suitable hosts, especially over-ripe strawberries, on which to feed. Sanitation is an important part of managing sap beetles. These insects are highly attracted to fermenting and decaying fruit, so keeping strawberry fields free of over-ripe fruit through regular and timely harvests can prevent them from becoming a significant problem. There is some evidence that suggests early renovation of strawberry beds after harvest may reduce sap beetle populations the following year. Trapping beetles using baskets of over-ripe fruit (strawberries or pineapple chunks) placed between the edges of the field and wooded areas may reduce infestations. Insecticide sprays may be applied to control sap beetles, but because this would have to occur during the harvest period, the potential benefits must be weighed against the likely customer resistance to spraying. Flower Thrips, Franliniella tritici Flower thrips are a relatively new pest of strawberries. Although they were reported to cause damage to fruit as early as the late 19 th century, only in the last ten years have recognized as an economically significant pest. Thrips, are tiny, slender, cigar-shaped insects. Nymphs and adults have the same general shape. Nymphs are wingless, whitish yellow when small, and yellow when fully grown. Adults are yellowish brown, 1/16 inch long, and have narrow wings that are fringed with hairs. While resting, the wings are folded lengthwise over the back. The life cycle can be completed in several weeks; there can be many generations per year. Eastern flower thrips do not overwinter in the Northeastern United States. The thrips arrive each year as a result of long-distance migrations from southern states on south to north moving weather fronts. Therefore, past infestations are not an indicator of future infestations. Thrips have a wide host

6 range, feeding on the flowers and developing fruit of many different plants. On strawberry flowers and fruit, they appear to feed on the tissue between achenes as the receptacle expands. These insects are often overlooked because they are small and often hide in protected sites, such as under the calyx. Damage from thrips infestation is most often seen as dullness or bronzing of the fruit surface as a result of feeding. Fruit cracking has also been reported. Light infestations may go unnoticed, but yield reductions of over 80% have been reported in the Midwest as the result of heavy thrips damage. To determine if thrips control is needed, strawberry growers should sample for thrips at the same time as they sample for tarnished plant bug nymphs. Tap flower clusters over a plate or saucer, and look for the slender yellow thrips. Although the correlation between thrips counts and strawberry damage is not well understood, control is probably warranted only if populations exceed 10 thrips per blossom. Once berries are 1/4 inch in diameter, 50 randomly selected fruit can be picked and examined, control is suggested if an average of 0.5 or more thrips per fruit are detected. Insecticides used for controlling tarnished plant bug will likely also control thrips. Applications should be timed to avoid killing pollinators. Potato Leafhopper, Empoasca fabae The potato leafhopper does not over winter in the northeastern United States but migrates into the region from southern states to feed on strawberry plants and many other host plants late in the spring. The adult leafhopper is bright green with whitish wings, bullet-shaped and about 1/8 inch long. It flies quickly when disturbed, so sweep nets should be used to capture and identify them. The nymphs are smaller and lighter green. They cannot fly and are easily identified by their habit of crawling sideways when disturbed. Leafhoppers feed with long, straw-like mouthparts, sucking plant juices from the veins along the undersides of leaves. During feeding they inject the plant tissues with saliva containing toxins that causes new growth to be stunted and yellow. On strawberries, feeding injury is often most notable in new plantings in early to mid-summer. Leaves on affected plants are stunted and have yellow streaking visible on the upper side, especially along the leaf margins. The symptoms can easily be mistaken for herbicide injury. Brush the leaf canopy with your hands and watch for adult leafhoppers flying off the plants. Examine the undersides of leaves to find the nymphs. Leafhoppers can usually be managed with several broad-spectrum insecticides, but it is important to apply controls before the injury is widespread, as it can reduce growth and lower of next season s yield. Spittlebug, Philaenus spumarius Spittlebug can be a pest of strawberries throughout North America, but tends to be most serious in regions with high relative humidity, including the Northeast. Adult spittlebugs resemble leafhoppers. They are about ¼ inch long and vary in color from green to mottled brown Spittlebug nymphs feed on leaf and flower stems of strawberry plants and excrete a white, frothy mass over their bodies to protect them from predators. Damage to the plant tissues can result in distorted leaves and reduced fruit size. Generally, damage to plants is limited, however high numbers of spittle masses in a field may create a great annoyance to pickers. Scouting for spittle masses should begin in the spring at 10% bloom. Inspect ten one square foot areas per acre. Spittle masses are usually more obvious early in the morning and under conditions of high humidity. Spittlebug nymphs under the frothy masses will be orange to light green in color. An

7 action threshold of two spittle masses per square foot is recommended. Spittlebugs are more often a problem in weedy fields. Strawberry Rootworm, Paria fragaria The adult stage of the strawberry rootworm is a small (1/8"), copper-colored beetle with dark markings on its back. The beetles feed on strawberry leaves during the late spring, causing numerous small holes in the leaves and, in severe infestations, skeletonizing much of the foliage. Another generation of adults appears in the late summer, after renovation. The immature stage is a small (1/8 inch), creamy white, root-feeding grub, with 3 pairs of legs. The grubs feed on strawberry roots in the spring and early summer causing them to be stunted and weak. This insect only rarely to builds up to damaging numbers. However, damage by the grubs can lead to infection of the roots by diseases, such as those causing black root rot. Control of the grub stage is very difficult, so actions are directed at the adults. If feeding injury is observed on the leaves in May or June, an insecticide application will reduce the number of egg laying females and therefore, the number of grubs feeding during the summer. When the next generation of adults emerges in July or August, control measures may be needed again. Strawberry rootworm should not to be confused with root weevil, which is a larger insect and causes more serious damage when present in a field. Strawberry Root Weevil, Otiorhynchus ovatus Black Vine Weevil, Otiorhynchus sulcatus Strawberry root weevil and black vine weevil are part of a complex of weevils that attacks strawberries. The strawberry root weevil adult is brown to black, about ¼ inch long with an elongated snout. The black vine weevil adult is larger (1/2 inch) and black with small yellow flecks. Both species emerge from pupae in the soil in the late spring and early summer. They cannot fly so they must move through a field by walking, or on farm equipment. The weevils feed on strawberry foliage at night and spend the day protected in the duff under the plants. Adult weevil feeding is distinctive, appearing as hook-shaped notches along the leaf margins, but it is not usually significant. After about two weeks the weevils begin to lay eggs in the soil at the base of the plants. Hatching begins in about ten days. Most of the eggs hatch by early fall and the larvae feed on the plant roots, then migrate deeper into the soil to overwinter. The strawberry root weevil larvae are ¼ inch long crescent-shaped legless grubs. Black vine weevil larvae are quite similar, but larger (1/2 inch). They vary in color from creamy white to light pink. The heaviest feeding damage occurs in the spring when the larvae again feed on the strawberry roots and tunnel into the plant crowns prior to pupating in mid-spring. Infested plants wilt, turn dull and reddish, and become stunted, often collapsing when stressed by a fruit load or high temperatures. Damaged areas in a field can be large and often have a circular pattern. By the second year of an infestation, damage can become so severe that early termination of a field becomes necessary. To prevent the spread of weevils to other fields, plow under infested fields as early as possible. Repeated disking of the field through the remainder of the season will repeatedly expose grubs to the elements and predators. Fall plowing will also help eliminate the weevils. Rotate the field into non-susceptible crops such as corn or pumpkins for at least two years. Plant new beds as far as possible from infested fields and clean all farm equipment before moving from one field to another.

8 While economic thresholds for root weevils have not been determined, it is known that more than two weevil larvae per plant can cause economic damage. Insecticides presently registered for root weevil are designed to control the adult stage prior to egg laying in the summer, but effectiveness of this strategy has been limited. Soil fumigation, which requires first plowing under the bed, is presently the only effective chemical means to control the larval stage. Recent research suggests that applying insect parasitic nematodes to the soil can provide good control of root weevil larvae. A spring (May) application of the species Steinernema feltiae at a rate of one billion per acre and/or Heterorhabditis bacteriophora at a rate of ½ billion per acre are recommended for trial. Applications at the end of August may also be effective. Nematodes are most effective when populations of root weevil larvae are high, and when used in combination with insecticides applied for the adults. Soil conditions must be moist for the nematodes to survive and become established. These nematodes may not be able to survive the winters in the coldest locations of the northeast, but they may provide control for more than one season in milder climates. White Grubs, Scarabaeidae: Various Species (Japanese Beetle, Rose Chafer, June Beetle, Asiatic Garden Beetle) White grubs are the larvae of various beetle species in the family Scarabaeidae. These grubs feed on the roots of a number of host plants, including strawberry. The feeding can weaken and kill the plants and may also encourage infection by root rot fungi through the feeding wounds. The adult stage of these insects may feed on strawberry foliage, occasionally causing damage levels that require management. The grubs of these different species all look quite similar. They are white, C-shaped, up to ¾ inch long, have three sets of legs and a brown head capsule. They differ from root weevil larvae, which are smaller and have no legs. The grubs overwinter in the soil, pupate and emerge as adults during the summer. The beetles mate and lay eggs in the soil during the summer months. Once white grubs have become established in the soil they can be difficult to manage. Chemicals are currently registered for control of the grubs and adults. The application of a commercially available bacteria, Bacillus popilliae or milky spore disease, to the soil may reduce white grub populations, but results in the colder soils of the northeast have been poor. Certain pre-planting practices can provide effective prevention of white grub infestations. Do not plant strawberries into recently plowed sod land. Sods (grass species) are a preferred host of white grubs. Fields presently in sod should be plowed and left fallow, with regular cultivation for one season, or planted to non-susceptible cover crops, such as buckwheat, pumpkins or squash for at least one full season, preferably two or three, before planting to strawberries. Avoid planting strawberries adjacent to large, grassy fields that act as hosts for these pests.