Nontarget effects of insecticides on Ganaspis brasiliensis in Michigan tart cherries, 2023

This trial was conducted to evaluate the nontarget effects of select insecticides against the new classical biological control agent for spotted wing drosophila (SWD) (Drosophila suzukii), the Samba wasp (Ganaspis brasiliensis, G1 strain) in tart cherry. Plots each containing 9 trees across 3 rows were set up in a 2001 planting of ‘Montmorency’ tart cherries planted at 20’ x 18’ spacing, located at the Northwest Michigan Horticultural Research Center in Traverse City, MI, in 2023. Treatments were replicated four times in a randomized complete block design. Treatments were applied at first detection of larval SWD in cherries on 13 July 2023 using a Pul-Blast air-blast sprayer set to deliver 60 gal/A at 2.5 mph. Treatments were selected for their frequent use by cherry growers for control of SWD and were applied at their highest maximum labeled rate (Table 1). One and 6 days after the application, two 1–3 inch shoots each containing 2 leaves and 2 fruits were clipped from each plot and brought back to the lab. The individual shoots were then placed in a 12 oz. plastic deli cup container, secured from the top with a paperclip. Ten lab-reared G. brasiliensis were introduced to the container, and mortality of the wasps was counted 24 and 48 h after exposure to the fruits. The mean percent mortality of G. brasilienis for each DAT and exposure time was analyzed with a negative binomial distribution using a generalized additive model with a negative binomial function (glmer.nb) in R. Models were checked for overdistribution and appropriateness of fit prior to any post hoc comparisons. Post hoc comparisons were conducted using Tukey’s HSD. Chi-square, degrees of freedom, and the p-values are given in Table 1.

Mortality was lower with the shorter exposure time compared to longer exposure time for both 1 and 6 DAT (Table 1). Control mortality was negligible for all assessments except after 48-h exposure on 1 DAT (13.5%). Imidan caused the highest mortality throughout the trial and was significantly higher than the control for the duration of the experiment (Table 1). Mustang Maxx also had significantly higher mortality compared to the untreated control for all assessments except 6 DAT after 24-h exposure. Mortality from Delegate and Exirel was not zero but was lower than the other products evaluated.¹

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