Use of Neonicotinoid Insecticides to Manage Cucumber Beetles on Seedling Zucchini

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1 2006 Plant Management Network. Accepted for publication 3 August Published. Use of Neonicotinoid Insecticides to Manage Cucumber Beetles on Seedling Zucchini Paul McLeod, Professor, Department of Entomology, Agri 321, University of Arkansas, Fayetteville Corresponding author: Paul McLeod. pjmcleod@uark.edu McLeod, P Use of neonicotinoid insecticides to manage cucumber beetles on seedling zucchini. Online. Plant Health Progress doi: /php rs. Abstract The use of neonicotinoid insecticides applied into soil at planting and when zucchini [Cucurbita pepo var melopepo (L.) Alef.] plants had two true leaves was effective against both the spotted (Diabrotica undecimpunctata howardi Berber) and the striped [Acalymma vittata (Fabricius)] cucumber beetles. At 17 days after planting, significant increases in mean numbers of dead cucumber beetles were detected in all plots receiving insecticides at planting when compared to nontreated plots. All insecticides applied at planting resulted in significant reductions in feeding damage ratings. The duration of residual toxicity was long with the insecticide applied at planting. Dead beetles were detected in treated plots on each sample date throughout the 56-day test. When seeds were dipped in either a or 0.086% a.i. imidacloprid mixture, emerging zucchini seedlings were effectively protected from cucumber beetle feeding. Introduction Commercial production of zucchini squash, Cucurbita pepo var melopepo (L.) Alef., and several other cucurbits has increased in the south-central US in recent years. Reasons for this increase include a desire to diversify crops from historically common agronomic crops like cotton and soybean, high profit potential, and increased demand by the food processing industry and fresh market consumers (William Russell, Allen Canning Co., Siloam Springs, AR, personal communication). With these high-value crops, any threat to production is often regarded as serious and management is generally extensive. Among the threats to cucurbit production, insects are often considered a major pest. Generally, the most common and destructive insect pests of cucurbits in the south-central US is the cucumber beetle complex. Soon after seeds germinate and seedlings emerge, the new plants are attacked by cucumber beetles including both the spotted (Diabrotica undecimpunctata howardi Berber) and the striped [Acalymma vittata (Fabricius)] cucumber beetles. Adult beetles pass the winter and early spring months in weeds and brush near production fields (1,6). Although beetles can be found feeding on several noncucurbit hosts, they prefer cucurbits and are capable of inflicting severe damage to emerging seedlings. When numbers are high, as often occurs in the Arkansas River Valley and southwestern Missouri, their feeding may result in death of cucurbit seedlings. Current management is centered on early detection of adults and damage and use of insecticides applied to foliage. Although several insecticides are effective against cucumber beetles, migration of additional beetles from surrounding areas together with rapid growth of new plant tissue with no effective insecticide residue, may require multiple applications. Depending on location and cucumber beetle population level, the number of foliar sprays can vary from 1 to more than 5 (2,7). Once plants develop several true leaves, the beetle impact greatly decreases and further management of cucumber beetles is generally not justified. Most of the newer insecticides possess improved safety characteristics to applicators and to the environment yet retain high toxicity to pest insects. Among these is a group of insecticides called neonicotinoids, which are synthesized with the active component of nicotine embedded within the insecticide molecule. Properties possessed by neonicotinoids include low

2 mammalian toxicity, reduced effects on non-target insects and low potential for environmental hazards (8). Neonicotinoid insecticides may be applied to soil (drench), seeds, and foliage (3,4,5). Another characteristic of neonicotinoids is their systemic activity, i.e., rapid uptake, especially through the roots, and translocation throughout the plant (9). Neonicotinoids have been available in agronomic crops for almost 10 years. Labeling constraints and limited potential for profit, however, have delayed their development in horticultural crops. Recently the neonicotinoid imidacloprid has been registered for use on many vegetables. Although it was originally targeted against piercing and sucking insects such as aphids, it has been shown to have activity against a wide range of insect pests including Coleoptera (1). The objective of this study was to evaluate at-planting soil or seed applications of neonicotinoid insecticides as an alternative to multiple foliar insecticide applications for cucumber beetle management in zucchini. Soil Application Trial 2002 Field tests were initiated during the 2002 production season to explore the potential for replacing multiple foliar insecticide applications with the soil applied neonicotinoid insecticides imidacloprid (Admire 2SC, Bayer CropScience, Research Triangle Park, NC) and thiamethoxam (Platinum 2SC, Syngenta Crop Protection Inc., Greensboro, NC). Plots were located on the University of Arkansas Vegetable Station, Kibler, AR, and consisted of a single six-foot-wide bed 35 ft long. Experimental design was a randomized complete block with four replications. Black Beauty zucchini squash was planted with a single-cone Hege planter on 29 April A single seed was placed every 1 ft within each plot. Insecticide treatments at planting were applied within the planting furrow with a single TeeJet 8002E nozzle positioned just behind the seed tube and in front of the press wheel. The sprayer was propelled with CO 2 and calibrated to deliver 2864 ml/1000 row ft at 25 psi (126 ml/15 sec). Planter speed was 2 mph or 341 sec/1000 row ft. Additional imidacloprid and thiamethoxam insecticide treatments were applied on 16 May when adult cucumber beetles reached the threshold of 2 per seedling (two unfolded true leaves). Insecticides were applied as described for the at-planting applications with the exception of treatment location within the bed which follows. A 1.5- inch-wide plow with open bottom was fitted with the single nozzle of the sprayer and driven at 2 mph with a Kubota tractor approximately 3 inches from one side of the drill line. Depth was approximately 4 inches. Sampling was initiated on 16 May (17 days after planting) when plants had two true leaves. Five sample areas, each 18 inches 18 inches, were randomly selected along the drill line of each plot. Plants and ground within each of the five areas were searched for dead (inability to walk) striped and spotted cucumber beetles. Also, within each sample area the two cotyledons of each of two plants were rated for cucumber beetle feeding. Ratings were 0 (no damage), 1 (1 to 20% foliar loss), 2 (21 to 40% foliar loss), 3 (41 to 60% foliar loss), 4 (61 to 80% foliar loss), or 5 (>80% foliar loss). All plots also were sampled on 22 May (23 days after planting). To estimate the duration of residual toxicity within the plants, counts of dead beetles were taken 39 and 53 days after planting. Measured variables included damage ratings and numbers of dead beetles. Data were analyzed with ANOVA and when F values were significant means were separated with LSD (SAS Institute Inc., Cary, NC). Soil Application Trial 2003 Plots were again located on the University of Arkansas Vegetable Station, Kibler, AR. Black Beauty zucchini seeds were planted on 30 April as described above. Plot and experimental design and insecticide application were similar to those for Imidacloprid and thiamethoxam were again tested along with dinotefuran (Dinotefuran 20% SC, Valent Corp., Walnut Creek, CA).

3 Seed Trial 2004 In order to further reduce the amount of applied insecticide a study was initiated 2004 to explore the use of imidacloprid treated seeds for cucumber beetle management. Imidacloprid was diluted in water to produce the following concentrations: 0, 0.002, 0.009, 0.017, 0.043, and 0.086% a.i. Approximately 120 Black Beauty zucchini seeds were placed into 150 mm diameter Petri dishes containing one of the concentrations. Seeds were removed after 10 min and placed on paper toweling. The following day, 28 June, dry seeds were transferred to the field for planting. The plot was located along the edge of a commercial zucchini field 12 miles east of Neosho, MO. Soil type was sandy loam. Seeds were hand planted and spaced two feet apart in a single row that ran the length of the field. Established zucchini plants were located east of the plot and field corn was located west of the field. Both areas provided large populations of cucumber beetles. Experimental design was a randomized complete block with 4 replications. Sampling included dead beetle counts and damage ratings as described above. Data were analyzed with ANOVA and LSD. Because of the lack of information on use of imidacloprid as a cucurbit seed treatment, a test was initiated to determine the phytotoxic effects. Seven commonly-grown cucurbit cultivars were selected for testing. Seeds were obtained from Green Seed Co. Inc., Springfield, MO and included Black Diamond watermelon [Citrullus lanatus (Thunb.) Matsum.] and Nakai; Straight 8' cucumber (Cucumis sativus L.); Acorn Squash winter squash (Cucurbita maxima Duchesne); Connecticut Field pumpkin (Cucurbita pepo var. pepo L.); Black Beauty and Yellow Crookneck summer squash [Cucurbita pepo Var. melopepo (L.)]; and Hale s Best cantaloupe (Cucumis melo L.). Approximately 120 seeds of each were placed into a petri dish containing 100% Admire 2E. After 10 min, seeds were removed and dried overnight on paper toweling. The following day, 21 June 2004, seeds were hand planted at the University of Arkansas Main Experimental Station, Fayetteville, AR. Seeds were spaced two feet apart in a single row. Plots were 60 ft long and row spacing was 3 ft. Experimental design was randomized complete block with 4 replications. Phytotoxicity was assessed at weekly intervals for 6 weeks by visually inspecting each of 25 plants in each plot. Soil Application Results 2002 Five insecticide treatments were applied at planting and each resulted in a significant increase in the mean number of dead striped cucumber beetles and total dead beetles when compared to non-treated plots at 17 days after planting (Table 1). Although the number of dead spotted cucumber beetles was only significantly greater in plots receiving thiamethoxam, numbers were low. Also, all treatments applied at planting significantly reduced the level of foliar damage. At 23 days after planting, the mean number of dead striped cucumber beetles ranged from 0 in the check to 2.4 for the split application of thiamethoxam, i.e., 0.06 lb a.i./acre at both planting and at the two true leaf stage. Although numbers of each beetle species varied somewhat, all treatments except the low thiamethoxam rate at planting, significantly increased the number of total dead beetles. The addition of the second application at the two true leaf stage was of little benefit with imidacloprid. When the two imidacloprid treatments, i.e., 0.25 lb a.i./acre at planting versus the two lb a.i./acre applications, were compared, no significant differences in the number of dead beetles were detected. However, with thiamethoxam the mean number of dead striped, spotted, and total beetles were significantly greater for the double treatments.

4 Table 1. Effects of soil applied insecticides on striped (STCB) and spotted (SPCB) cucumber beetles on zucchini, days post-plant Rate (lb a.i. /acre) schedule No. dead beetles/sample planting threshold STCB SPCB Total Foliar damage rating Thiamethoxam 0.06 X 1.4 ab 1.1 a 2.5 bc 0.3 b Thiamethoxam 0.06 X 0.1 c 0.0 b 0.2 d 3.0 a Thiamethoxam 0.06 X X 2.5 a 1.4 a 3.9 a 0.0 b Thiamethoxam X 2.0 ab 1.2 a 3.2 ab 0.2 b Imidacloprid 0.25 X 1.4 b 0.5 b 1.8 c 0.2 b Imidacloprid X X 1.4 b 0.4 b 1.8 c 0.0 b Check c 0.2 b 0.5 d 2.6 a 23 days post-plant Thiamethoxam 0.06 X 0.2 c 0.6 cd 0.9 cd nd Thiamethoxam 0.06 X 0.6 bc 2.1 bc 2.8 bc nd Thiamethoxam 0.06 X X 2.4 a 3.9 a 6.2 a nd Thiamethoxam X 0.1 c 2.0 bc 2.1 bc nd Imidacloprid 0.25 X 1.2 abc 0.9 cd 2.1 bc nd Imidacloprid X X 2.2 a 2.2 abc 4.5 ab nd Check c 0.0 d 0.0 d nd 39 days post-plant Thiamethoxam 0.06 X 4.0 abc 9.1 bc 13.1 bc nd Thiamethoxam 0.06 X 7.6 a 10.0 b 17.6 b nd Thiamethoxam 0.06 X X 5.5 ab 19.1 a 24.6 a nd Thiamethoxam X 3.5 bc 7.0 bcd 10.5 c nd Imidacloprid 0.25 X 3.8 bc 3.4 de 7.5 c nd Imidacloprid X X 5.9 ab 4.8 cd 10.6 c nd Check c 0.0 e 0.4 d nd 56 days post-plant Thiamethoxam 0.06 X 1.4 bc 2.6 bc 4.0 cd nd Thiamethoxam 0.06 X 2.8 bc 5.2 b 8.0 bc nd Thiamethoxam 0.06 X X 7.0 a 9.9 a 16.9 a nd Thiamethoxam X 3.4 b 10.1 a 13.5 ab nd Imidacloprid 0.25 X 4.1 b 3.6 bc 7.8 bc nd Imidacloprid X X 2.4 bc 2.8 bc 5.1 cd nd Check c 0.5 c 1.0 d nd Within each sample date, within column means followed by same letter do not differ significantly (LSD; P = 0.05). Ratings were 0 (no damage), 1 (1 to 20% foliar loss), 2 (21 to 40% foliar loss), 3 (41 to 60% foliar loss), 4 (61 to 80% foliar loss), or 5 (>80% foliar loss). Although the principal objective of the 2002 study was to determine the effectiveness of the soil applications for protecting squash seedlings, weekly samples were taken from plots receiving the high rates of imidacloprid and thiamethoxam and from non-treated plots through 56 days after planting to

5 determine the length of toxicity to cucumber beetles (Table 1). In plots receiving imidacloprid (0.25 lb a.i./acre) at planting, the mean number of dead cucumber beetles was significantly greater than in non-treated plots at both the 39- and 56-day sample periods. The likely reason for the continued increase in the number of dead beetles is related to the increase in plant size. At 56 days after planting, plants were larger and had more time to attract beetles. The number of dead cucumber beetles in plots receiving thiamethoxam was more variable. However, thiamethoxam toxicity was still evident 56 days after planting when the mean number of dead cucumber beetles in plots receiving the lb a.i./acre rate was Soil Application Results 2003 Similar results were obtained during the 2003 soil treatment study (Table 2). At 14 days after planting, significantly more dead striped, spotted, and total cucumber beetles were counted in plots receiving imidacloprid when compared to non-treated plots. Significantly more dead striped cucumber beetles also were observed in plots receiving thiamethoxam or dinotefuran. Foliar damage ratings were significantly lower for all insecticides when compared to that in nontreated plots. At 24 days after planting, each of the insecticide treatments significantly increased the number of dead striped cucumber beetles and total beetles when compared to those in non-treated plots. Application of each insecticide also resulted in significantly lower levels of foliar damage from cucumber beetle feeding. Table 2. Effects of soil applied insecticides on striped (STCB) and spotted (SPCB) cucumber beetles on zucchini, days post-plant Rate (lb a.i./acre) No. dead beetles/sample STCB SPCB Total Foliar damage rating Thiamethoxam b 0.1 ab 2.4 b 1.4 b Imidacloprid a 0.7 a 7.5 a 0.9 ab Dinotefuran b 0.2 ab 2.2 b 0.5 a Check c 0.0 b 0.2 c 2.4 c 24 days post-plant Thiamethoxam a 0.3 ab 5.6 a 0.6 a Imidacloprid a 0.6 a 8.2 a 0.5 a Dinotefuran a 0.4 a 6.5 a 0.3 a Check b 0.0 b 0.2 b 1.5 b Within each sample date, within column means followed by same letter do not differ significantly (LSD; P = 0.05). Ratings were 0 (no damage), 1 (1 to 20% foliar loss), 2 (21 to 40% foliar loss), 3 (41 to 60% foliar loss), 4 (61 to 80% foliar loss), or 5 (>80% foliar loss). Seed Results 2004 Dead beetle counts were not taken during the 11-day-after-planting sample because of heavy rain that occurred the previous night. Foliar damage ratings ranged from 2.4 for non-treated seeds to 0.1 for seeds dipped in 0.086% imidacloprid (Table 3). Significant reductions in foliar damage ratings were detected for all seed treatments. At 14 days after planting, significantly greater numbers of dead spotted cucumber beetles were observed in plots with seeds treated with the two highest levels of imidacloprid. Although foliar damage ratings were again significantly lower for all seed treatments when compared to non-treated seed, the two highest levels of imidacloprid (0.043 and 0.086% a.i.) were significantly lower than other seed treatments. At 23 days after planting, only the two highest levels of imidacloprid treated seeds produced significant

6 increases in numbers of dead beetles over those for non-treated seeds. These two treatments also had the lowest foliar damage ratings. No symptoms of phytotoxicity were observed on any of the seven tested cucurbit cultivars from plant emergence until the end of the test six weeks after planting. Table 3. Effects of imidacloprid seed treatments on striped (STCB) and spotted (SPCB) cucumber beetles on zucchini, days post-plant Seed dip concentration No. dead beetles/sample STCB SPCB Total Foliar damage rating Imidacloprid nd nd nd 1.5 c Imidacloprid nd nd nd 1.0 bc Imidacloprid nd nd nd 1.0 bc Imidacloprid nd nd nd 0.7 b Imidacloprid nd nd nd 0.1 a Check --- nd nd nd 2.4 d 14 days post-plant Imidacloprid a 0.0 b 0.0 b 3.1 c Imidacloprid a 0.1 ab 0.1 b 3.0 c Imidacloprid a 0.1 ab 0.5 a 1.6 b Imidacloprid a 0.3 a 0.5 a 0.3 a Imidacloprid a 0.4 a 0.5 a 0.2 a Check a 0.0 b 0.0 b 4.4 d 23 days post-plant Imidacloprid b 0.0 b 0.1 b 2.5 de Imidacloprid b 0.1 b 0.2 b 2.0 bc Imidacloprid b 0.2 ab 0.3 b 2.3 cd Imidacloprid a 0.4 a 1.0 a 1.4 a Imidacloprid a 0.4 a 0.9 a 1.7 ab Check b 0.1 b 0.1 b 2.9 e Within each sample date, within column means followed by same letter do not differ significantly (LSD; P = 0.05). Ratings were 0 (no damage), 1 (1 to 20% foliar loss), 2 (21 to 40% foliar loss), 3 (41 to 60% foliar loss), 4 (61 to 80% foliar loss), or 5 (>80% foliar loss). Conclusions and Recommendations Each of the three tested neonicotinoid insecticides was effective against cucumber beetles when applied to the soil at planting. This was evident in increases of dead beetles in treated plots and in reductions of foliar damage. Iimidacloprid and thiamethoxam also were effective when applied into soil after plants had emerged and had developed two true leaves. However, substantial foliar damage was not prevented with the later applications. Had cucumber beetle populations been greater, it is unlikely that the delayed application could have prevented substantial plant mortality. The use of imidacloprid as a seed treatment was also effective against the cucumber beetle complex and resulted in increased levels of beetle mortality and reductions in foliar feeding. Seed treatments also reduced the amount of insecticide per acre and reduced the costs of chemical and application. Although concern with unwarranted insecticide use always exists with preventative applications at planting,

7 historically, cucumber beetle populations are highly predictable in certain locations like the Arkansas River Valley in Arkansas and southwestern Missouri. In these areas, it is likely that the preventative insecticide applications for cucumber beetle management on cucurbits are justified and offer the cucurbit producer a viable alternative to multiple foliar sprays. Literature Cited 1. McLeod, P Identification, biology and management of insects attacking vegetables in Arkansas. Sirena Press, Santa Cruz, Bolivia. 2. McLeod, P., Rashid, T., Eaton, S., and Martin, L Evaluation of soil applied insecticides for control of cucumber beetles on summer squash, Arthrop. Manag. Tests 29:E Nauen, R., Tietjen, K., Wagner, K., and Elbert, A Efficacy of plant metabolites of imidacloprid against Myzus persicae and Aphis gossypii (Homoptera: Aphididae). Pestic. Sci. 52: Palumbo, J. C., and Kerns, D. L Effects of imidacloprid as a soil treatment on colonization of green peach aphid and marketability of lettuce. S. W. Entomol. 19: Pike, K. S., Reed, G. L., Graf, G. T., and Allison, D Compatibility of imidacloprid with fungicides as a seed-treatment control of Russian wheat aphid and effect on germination, growth and yield of wheat and barley. J. Econ. Entomol. 86: Sorensen, K. A., and Baker, J. R Insect and related pests of vegetables. Bulletin AG-295, North Carolina Coop. Ext. Serv., Raleigh, NC. 7. Sorensen, K. A., and Cooke, D. G Pickleworm and spotted cucumber beetle control with insecticides, Arthrop. Manag. Tests 29:E Thomson, W. T Agricultural Chemicals - Book I: Insecticides, Acaracides and Ovicides. Thomson Publications, Fresno, CA. 9. Westwood, F., Bean, K. M., Dewar, A. M., Bromilow, R. H., and Chamberlain, K Movement and persistence of [14C] imidacloprid in sugar-beet plants following application to pelleted sugar-beet seed. Pestic. Sci. 52: