Vine mealybug is one of the most prolific pests of table grapes. Its exponential growth rate and affinity for feeding within clusters close to harvest make its management the highest priority for grape growers dealing with infested vineyards.
Traditionally, vine mealybug is managed with a combination of post-harvest and/or delayed-dormant Lorsban treatments in combination with in-season treatments of the growth regulator buprofezin (Applaud), and neonicotinoids such as Admire or Venom.
However, in many cases these treatment programs do not suffice and additional contact insecticides are needed close to harvest to keep the clusters free of mealybugs.
Over the past few years, methomyl (Lannate) has been the product of choice for late-season insecticide treatments aimed at keeping clusters free of mealybugs. This has been due to its contact mode of action and ability to use the product close to harvest when PHIs for other products preclude their use.
In late May of 2008 we conducted an insecticide trial to evaluate the effectiveness of 1 pound of Lannate, as well as an alternative program of a tank mix of 2.5 ounces of Assail 30SG and 12 ounces of Applaud 70DF, on vine mealybug.
The trial was located in a bearing, two-year old vineyard that was grafted over to ‘Summer Royal’ in 2006. A total of 0.4 acres was divided into 12 plots that were each 2 rows by 10 vines long, and that were each assigned in a randomized complete block design to one of the two treatments or an untreated check.
Treatments were applied at 200 GPA on May 20 using an air-blast sprayer. Plots were evaluated prior to treatment, 3 days after treatment (DAT), 8 DAT and 17 DAT by doing timed searches on six or eight vines in the center of each plot. Searches were performed by stripping bark and counting all motile forms of vine mealybug that could be found within three minutes.
Results showed that Lannate and the Assail/Applaud tank mix both caused significant reductions in the number of vine mealybugs per three-minute search on all post-treatment evaluation dates. Comparisons of the two treatments to each other revealed no significant differences, with both knock-down and residual activity very similar.
There are, however, some additional considerations that growers and PCAs should make when interpreting this data for their own use.
On one hand, efficacy of these treatments is likely overestimated because 1) bark stripping the trunk during our pre-counts caused increased exposure of the mealybugs to the pesticides than would occur normally, and 2) vines were young and therefore had relatively small amounts of bark compared to mature vines.
On the other hand, efficacy of these treatment programs is likely underestimated because 1) at our treatment timing most mealybugs were still under the bark and not as exposed as they would be when mealybugs get up on the leaves and enter the clusters, and 2) timed searches underestimate high mealybug populations.
This means that vines with low populations might have every mealybug counted during the three minutes, whereas only a portion of the mealybugs can be counted in three minutes on a heavily infested vine.
In conclusion, both the Lannate and the Assail/Applaud treatments provided good knock-down of vine mealybug close to harvest. However, these treatments were far from perfect, and should only be utilized as a last resort. This is especially true due to the cost of the Assail/Applaud tank mix, as well as the ongoing regulatory scrutiny of Lannate residues that has the potential to cause table grapes to be removed from the label.