Chemical Control of Grape Mealybug, 2016*

The purpose of this test was to determine the effects of various pesticides on GMB, a vector of little cherry virus 2, a causal pathogen of little cherry disease in sweet cherry. This test was conducted on 1-yr-old potted (1.5 gal pots), ‘Bing’ cherry trees on Mazzard root stock, housed in the Washington State University, Tree Fruit Research Extension Center greenhouse. Replicates consisted of individual trees. A randomized complete block design was used to assign four replications of each of the four treatments and an untreated check. Treatments were blocked based on mealybug pretreatment count numbers. The GMB colonies were started from field collections and were reared in vented plastic containers on sprouted potatoes in a growth room (83 °F, 14:10 (L:D), and 34% RH). Mealybugs were removed from potatoes by using a razor blade to cut out a GMB-infested eye and then transferred to greenhouse trees. The removed potato eye was attached to the bark of the tree using a push pin. Mealybugs were given 7 days to become established on trees before a pretreatment count, followed by treatment. A plastic tube, large enough to contain the entire plant, with a mesh lid over the top, was place on each potted tree to keep mealybugs from crawling away. Pesticides were applied on 6 May. It was determined that 480 ml of water was required to reach soil saturation in the 1.5-gal pots. Admire Pro and Aza-Direct were applied as soil drenches in 480 ml of water/tree; other treatments received 480 ml of water. A foliar-systemic compound, Ultor, was applied at a concentration of 14 fl oz/100 gal, and an insect growth regular, Centaur, was applied at a rate of 46 oz/100 gal. Leaves were sprayed to drip using a 5-gal backpack sprayer. A paper plate, with a hole cut out for the tree trunk, was placed over the soil to avoid soil contact of foliar-applied materials. Counts of GMB crawlers (0.1–0.5 mm in length), nymphs (0.6–2 mm in length), and adults (>2 mm in length) were completed 1 day before (pretreatment count); and 19, 27, 35, and 56 days after treatments were applied. The average number of crawlers, nymphs, adults, and total GMB was calculated for each treatment for each sampling date. Statistical analysis was done by using the difference between the pre- and the posttreatment means for a given date. Data were analyzed using the Statistical Analysis System (SAS 2016). PROC MIXED was used to conduct an ANOVA, and treatment means were separated using the lsmeans/pdiff option.

Average crawler and total mealybug differences were similar for all treatments and all dates (data not shown). Nymph numbers were reduced to zero on Centaur-treated plants at 19 days post treatment (Table 1) and nearly to zero on trees treated with Admire Pro and Aza-Direct at 27 days. Admire Pro, Aza-Direct, and Centaur reduced adult numbers to zero 19 days after application. Ultor did not significantly reduce GMB numbers for any life stage; however, the addition of an adjuvant (suggested on the label) may have increased the effectiveness of this product.

Author notes