Monitoring vine mealybug resistance to imidacloprid

Authors: Sisterson, Mark; Burbank, Lindsey; Naegele, Rachel; Wallis, Christopher

Submitted to: CDFA Pierce's Disease Control Program Research Symposium
Publication Type: Proceedings
Publication Acceptance Date: 11/8/2021
Publication Date: 12/14/2021
Citation: Sisterson, M.S., Burbank, L.P., Naegele, R.P., Wallis, C.M. 2021. Monitoring vine mealybug resistance to imidaclopridIn, Symposium Proceedings: Pierce’s Disease and Other Designated Pests and Diseases of Winegrape, pp. 366.

Interpretive Summary:

Technical Abstract: The vine mealybug (Planoccocus ficus) represents a worldwide threat to grapevine production. Contact insecticides have limited efficacy because vine mealybugs are thigmotactic and often found under bark or in grape clusters. Therefore, systemic insecticides are popular for mealybug control. Imidacloprid, a neonicotinoid, was registered for use in the United States in 1994 and is commonly applied in vineyards to suppress a wide range of insect pests including mealybugs. Baseline testing for vine mealybug susceptibility to imidacloprid by Prabhaker et al. (2012) between 2006 and 2008 provided no evidence of resistance. As it has been a decade since mealybug populations were screened for resistance to imidacloprid, testing is underway to determine if susceptibility has changed. For screening, a modified version of the assay used by Prabhaker et al. (2012) was developed. The modified assay consists of infesting a 10 cm tall grape seedling planted in a 20 ml pot with 5 2nd instar vine mealybug nymphs per seedling. The plant was then treated with an 8 ml drench of a solution containing a known quantity of imidacloprid. After 6 days, the number of nymphs surviving was determined. For each set of assays, 6 doses of imidacloprid were evaluated, with 10-20 replicates completed for each dose. Three colonies initiated from collections made in the San Joaquin Valley (1 from Stanislaus County, 1 from Tulare County, and 1 from Kern County) during 2019 and 2020 and a reference laboratory colony have been evaluated. An additional 6 collections were made during fall of 2021 (2 from Stanislaus County, 1 from Fresno County, and 3 from Tulare/Kern County) and will be subjected to testing once colonies have reached a sufficient population size. For all colonies tested, application of 8 ml of imidacloprid at a dose > 1 ug [AI]/ml was required to observe mortality, with occasional survivors at doses of 100 and 1,000 ug [AI]/ml. Survivors at high doses (>1 ug[AI]/ml) displayed delayed development and were less vigorous than survivors at lower doses. To determine if mortality was dependent on assay length, a subset of tests compared mortality of mealybugs in tests conducted over 12 versus 6 days, with similar results. While survivors at high doses were observed 12 days post-treatment, given their lack of vigor, additional testing is required to determine if such survivors can complete development and reproduce. To determine how doses applied to seedlings relate to field doses, all plants from assays were frozen and the quantity of imidacloprid per gram of leaf tissue will be determined using high performance liquid chromatography (HPLC) based methods. For reference, leaves from vineyards treated with imidacloprid were collected throughout the summer of 2020 and 2021 and will be assayed to determine peak quantities of imidacloprid in field treated vines. Processing of leaf samples for HPLC testing will be conducted during fall and winter of 2021.