Final Report: 2020-2021 Funding Cycle

Principal Investigator: Doug Walsh, Professor of Entomology
Washington State University Irrigated Agriculture Research and Extension Center
24106 N. Bunn Rd, Prosser, WA 99350
Tel. 509.786.9287

Summary: Objective 1. Collect different populations of grape mealybugs. Two distinct populations of grape mealybugs were collected in 2019. The first was from Ste Michelle Wine Estates Vineyard 10 location near Paterson, WA. The second population was collected from a pear orchard in a WSDA certified organic production system. It appears that the grape population was developing resistance to imidacloprid while the pear population appears to still be susceptible to imidacloprid. We made considerable attempts to find populations in Concord vineyards and failed. We tried to work with crop consultants from the cherry industry in finding sites. We were for the most part ignored by these orchardists.

Objective 2: Develop and validate a whole leaf bioassay technique utilizing test tubes as arenas. We did validate that we could observe treatment effects with our whole-leaf bioassay method. With this technique we can chemigate a vineyard with a systemic insecticide like imidaclorprid by hanging a plastic cup under a drip emitter putting a dose of insecticide in the cup and then irrigating the vineyard to deliver the insecticide. Chemigation was completed on June 10, 2019. Rates of imidaclopid were at the equivalent of 0 (control), 50, 100, and 200% of the maximum labeled field rate. Leaves were collected for bioassay arena construction on 13 June, three days after chemigation. Arenas consisted of individual excised leaves with their petioles placed in large test-tubes full of water. These arenas sat for one week to determine viability of leaves. On leaves that were successful in establishing arenas small rootlets developed on the petioles.

On June 20, 2019, ten mealybug crawlers were placed on ten replicate leaves from each of the treatments detailed above. Our initial study was with the mealybug population collected from the vineyard near Paterson, WA. The mealybugs were permitted to settle and feed for one week. On June 27, 2019 these leaves were scanned and the number of surviving mealybugs quantified.

Mealybug crawlers were collected from the organic pear block near Wenatchee, WA on July 10, 2019. On July 11, 2019 arenas were constructed as detailed above. This was 31 days post chemigation of the vineyard. Crawlers were placed on the leaves on July 18, 2019. These were scanned after one week.

Results: There were substantial differences in the mortality observed between the two mealybug populations (Table 1.). Although it is statistically impossible to compare these two populations against one another the differences in mortality between these populations is of great concern. Additionally, the grape population was bioassayed on leaves between 10 and 17 days following chemigation. The population from the organic pear orchard was completed on leaves between 38 to 45 days following chemigation. The simple lack of mortality in the vineyard collected mealybug population leads us to believe that grape mealybugs are developing tolerance to imidacloprid in Washington State
vineyards. We are still working on our bioassay method. These studies will initiate after we develop a bioassay method that provides us with consistent results.

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Pest & Disease //