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. Determine how increasing concentrations of imidacloprid in leaf tissues impacts Tetranychus urticae and Eotetranychus willamettei behaviors (lifespan and fecundity).

We had a tough time in spring 2019 keeping our spider mite colony free of predatory mites and predatory insects and our T. urticae colony collapsed. We were able to get it re-established by mid-July. All the work completed in 2019 was on T. urticae. No work was completed on E. willamettii in 2019.

On August 21, 2019 a vineyard block was treated with imidacloprid by hanging a plastic cup under a drip emitter putting a dose of insecticide in the cup and then irrigating the vineyard to chemigate the insecticide into the root system of the grape vines. Replicate size was 5 vines in row. Leaves were only collected from the middle 3 vines for use in bioassay. Rates applied were 0, 50, 100, and 200% of the maximum field rate of 0.5 pounds active ingredient of imidacloprid. Leaves were collected on August 23, August 29, September 5, and September 13, 2019. Bioassay arenas were created by punching10 cm leaf discs out from leaves and placing these leaves in Petri dishes filled with water soaked cotton. Ten adult female and 3 adult male T. urticae were then transferred to each leaf disc. Each treatment was replicated 16 times. The number of mites out of the original cohort of 13 mites was counted after a week from each bioassay period and percent mortality was calculated (Table 1). Concurrently, abundance of juvenile mites present (hatched larva or protonymphs) (Table 2) and the number of eggs laid and unhatched (Table 3) were counted.

The results of these bioassays were inconsistent among weeks of study. Mortality was low throughout the weeks of study but in weeks 1 and 2 (Aug 23-Aug 30 & Aug 29-Sep 5) mortality was greater in the 0% (control) and 50% imidacloprid treatments. However in week 3 (Sep 5-Sep 12) mortality was greatest at nearly 13% in the 200% imidacloprid treatement. Mortality in week 4 (Sep13-Sep 19) was a wash among imidacloprid treatments and ranged from between about 10% and 15%. Abundance of hatched juvenile mites and abundance of unhatched mites varied among weeks as well. This data is inconclusive at this point.

Objective 2. Determine how increasing concentrations of imidacloprid in leaf tissues impacts the biology and survivorship of predatory mites. : No studies on predatory mites were completed in 2019 or 2020.

Objective 3. Manipulate temperature in combination with variable concentrations of imidacloprid to determine how these factors impact mite fecundity and lifespan.

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Pest & Disease //