Susceptibility of grape berry moth, Paralobesia viteana (Clemens) (Lepidoptera: Tortricidae) to insecticides in Michigan vineyards

Grape berry moth is a major pest of vineyards across eastern North America. Increasing infestations in Michigan commercial vineyards under active pest management have raised concerns about insecticide resistance in this species. We tested the susceptibility of male moths of this pest from 5 vineyards in southwestern Michigan over 2 years using a range of concentrations, followed by probit analysis of the resulting mortality data. In 2021 and 2022, topical applications of phosmet and zeta‐cypermethrin were applied to moths collected from each vineyard, and the LC50, LC90 and LC99 values were calculated for each population. The LC50 values were similar among sites, whereas the LC99 values for phosmet were highly variable across years, with sensitivity 2.78 times higher in 2022 than 2021 at Farm 2 and 61.66‐fold higher in 2021 at Farm 3. Sensitivity to zeta‐cypermethrin was generally similar between years at the LC50 level, but it was variable at the LC99, with two sites showing lower sensitivity in 2022 compared to 2021 and three farms with an increased sensitivity in 2022. The response to carbaryl was similar for Farm 3 across both years. However, in 2022 the LC50, LC90 and LC99 values in Farm 3 were 2.34, 6.33 and 14.29‐fold, respectively, higher compared to those in Farm 5. In conclusion, our results show that sensitivity to insecticides in grape berry moth populations in southwest Michigan vineyards is highly variable between different farms and across years. These results highlight the need for continued monitoring of sensitivity to insecticides in this species, and development and implementation of alternative control strategies in vineyards to maintain pest susceptibility to insecticides.

there is concern regarding potential resistance to one or more insecticide classes including organophosphate, pyrethroid and carbamate insecticides, which are used in Michigan vineyards. To date, few studies on insecticide resistance in P. viteana have been conducted. Nagarkatti et al. (2002) found that 6 out of 7 GBM populations collected from Pennsylvania and New York vineyards were resistant to carbaryl, which was applied widely due to its relatively low cost and historical effectiveness. The ratios of LC 50 between the resistant populations and the lab susceptible strain ranged from 7.1 to 71.4-fold indicating there were GBM populations with resistance to this carbamate insecticide. However, the population with 71.4-fold higher resistance was controlled effectively by the recommended rate of carbaryl (i.e. 2245 g [AI]/ha). The combination of recent outbreaks of P. viteana in southwestern Michigan vineyards and the history of resistant P. viteana populations in other regions led us to conduct this study to determine the relative sensitivity of this pest to key insecticides used for its control in Michigan vineyards. Once male moth flight started, lures and sticky inserts were placed into each trap across sites on days preceding evening conditions ideal for moth capture. Each site was visited on the following morning and the inserts with moths were placed into holders in coolers with ice packs, then brought back to our laboratory. To keep survival high during the bioassays, dead moths and moths stuck on the inserts by their heads or antennae were removed before the treatment. For each replicate, 20-40 alive individual moths were treated topically onto the dorsal surface of the thorax using the test insecticide dissolved in acetone at various concentrations. Acetoneonly treated groups were used as controls. Each moth was treated with 0.5 μL of material, and within each year and for each insecticide at different concentrations, and there were four replicates for each population. Moths were held in an environmental chamber at 22°C, 65 ± 5% RH, and a photoperiod of 16:8 (L:D) for 24 h after treatment without feeding and then their mortality was assessed. Each moth in the experiment was gently touched with a fine paintbrush to determine whether they moved (alive) or did not (dead).
All the insecticides were diluted in 99.9% acetone (Sigma-Aldrich, St. Louis, MO) to achieve a range of concentrations that provided from <5% to 100% mortality of male P. viteana moths.

| Data analysis
Probit analysis (SAS Institute Inc, 2013) was conducted to estimate the LC 50 , LC 90 and LC 99 values (concentrations killing 50%, 90% and 99%) and to determine the slope and intercept of the dose-response curves (Robertson et al., 2017). Abbott correction was conducted if there was any mortality in a control group before probit analysis (Abbott, 1925); however, our methods using cooling and the cleanbrake trap inserts with a thin glue layer resulted in no control mortality for most experiments.  (Table 3).

| DISCUSS ION
Our results highlight significant variation among different vineyards in the susceptibility of P. viteana to insecticides. We found populations from the same vineyard such as Farm 3 had significantly lower susceptibility than other populations to multiple chemical classes, which may indicate cross resistance. When we compared LC 99 values of phosmet among the populations in 2021 and 2022, we found that values in 2022 increased from 2.78-fold in Farm 2 to 61.66-fold in Farm 3 highlighting the significant inter-annual variation. We were not able to establish a laboratory susceptible strain to compare with the populations from commercial sites due to a lack of untreated sites with large P. viteana populations where we could capture moths. However, the comparisons between years indicate that susceptibility to phosmet decreased over time at all five sites. We conclude that selection by frequent applications of this insecticide is increasing the ability of this pest to survive and cause fruit damage. Over-reliance on the same class of insecticides is a key driver of resistance development (Casida & Quistad, 1998;Roush & Croft, 1986), so it will be important to rotate to other classes such as diamides or insect growth regulators. TA B L E 1 Probit analyses for mortality of five vineyard field-collected populations of grape berry moth, Paralobesia viteana (Clemens) in response to varying concentrations of phosmet. LC values are presented as μL L −1 , with associated 95% fiducial limits. LC 99 values of zeta-cypermethrin were lower or similar in four farms in 2022, except for Farm 3 where we found a 2-fold increase.
We are not sure why the susceptibility to zeta-cypermethrin declined in the Farm 3 population, but repeated pesticide exposure and isolation from wild populations are likely to be drivers of susceptibility (Onstad & Carrière, 2014). Longer term monitoring is needed to understand the larger patterns of susceptibility of P. viteana to insecticides and the dynamics of resistance in these vineyards.
Although our sample size in this study is relatively small, it provides a basis for future comparison to determine the degree of resistance and cross resistance in P. viteana populations across eastern North America. In vineyards with high pest pressure, maintaining protection against P. viteana requires effective residues for the long period of egg laying during the summer generations (Isaacs et al., 2012). Although registered rates of the tested insecticides may still control moths when recently applied, the results presented here suggest that residual control may decline as the insecticide residues degrade, helping to explain the greater levels of infestation observed recently. These results highlight the importance of implementing a comprehensive Integrated Pest Management (IPM) strategy within vineyards that includes a resistance management component to maintain insecticide susceptibility in pest populations. In addition, our current study has opened an opportunity for the investigation of the molecular mechanisms of cross resistance as described in other insect pests (Fournier et al., 1993;Hubhachen et al., 2020;Russell et al., 2004).

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors have no conflict of interest to declare.