Lepidopteran Pest Control in Sweet Corn With Insecticides Allowed for Organic Production, 2018

The objective of the study was to evaluate the efficacy of insecticides allowed for organic production to control Lepidopteran pests, especially corn earworm (CEW), on sweet corn. Sweet corn ‘Obsession’ was direct seeded on 6 July 2018 at Cornell AgriTech’s Fruit and Vegetable Research farm in Geneva, NY (GPS: 42°52′35.3″N 77°01′27.4″W). Seeds were planted 8 inches apart within rows, and rows were spaced 30 inches apart. Each plot consisted of two 25-ft long rows and was flanked by two unplanted rows. Plots were separated from each other within rows by 10 ft of bare ground. Five replications of each treatment were arranged in an RCB design. All insecticides evaluated were listed by the Organic Materials Review Institute (OMRI) (Table 1). Insecticides were applied using a CO₂–pressurized backpack sprayer and boom equipped with two flat-fan nozzles (XR-Teejet 8002) calibrated to deliver 25 gallons per acre at 40 psi. Nozzles were separated by 20 inches and directed horizontally toward the primary ear and applied to only one row at a time. All treatments were co-applied with the OMRI-listed surfactant, Nu Film P, at a rate of 8 fl oz/acre. Insecticide applications were initiated on 31 August, shortly after most ears had produced green silks, and additional applications were made on 3, 6, 9, and 12 September (five sprays total). There was an average of 57 CEW moths captured per trap per night from first silk until several days before harvest. Silks were brown and dry shortly after 12 September.

On 24 September, up to 30 primary, market-sized ears were harvested within each plot. Efficacy of treatments was evaluated by recording the number of CEW, European corn borer (ECB) and Fall armyworm (FAW) larvae within each ear as well as the location of their collective damage, which was binned into three categories: 1) no feeding damage (= fresh market); 2) feeding damage to the ear tip only (the top inch or where developed kernels had made a complete ring) (= processing); or 3) damage from one inch below the ear tip to the ear base (= unmarketable). Data were analyzed using a generalized linear mixed model in SAS (ver. 9.4; PROC GLIMMIX) with treatment considered as a fixed effect and replication as a random factor in the model. Treatment means were compared using Tukey’s Studentized Range (HSD) Test at P < 0.05.

A majority of larvae infesting ears in this trial was CEW (96%), followed by ECB (2%) and FAW (2%). The CEW population was so high that there was an average of nearly two larvae per ear in the untreated check and many of the other treatments (Table 1). Although the number of CEW in treated plots did not differ significantly from those in the untreated check, there were significantly fewer CEW in plots treated with both rates of Entrust and Gemstar compared with those treated with Javelin (Table 1). The average sum of ECB and FAW per ear was very low in all plots and there were no significant differences in larval densities between treated plots and the untreated check (Table 1).

Feeding damage was so extensive that no ears were considered acceptable for fresh market and only 49% were considered acceptable for processing in the untreated control (Table 1). Sweet corn treated with the 6 fl oz/acre rate of Entrust had a significantly higher percentage of fresh-market quality ears than those in the untreated control and all other treatments, except the 4 fl oz/acre rate of Entrust (Table 1). The percentages of processing quality ears in all insecticide treatments did not differ from those in the untreated control (Table 1). Overall, none of the insecticide treatments provided adequate ear protection under such enormous CEW pressure. Future research is needed to examine other options for managing such high CEW infestations.¹

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