Investigation of the grape mealybug complex and its natural enemies to improve biological control

The grape, obscure, and longtailed mealybugs belong to the Pseudococcus maritimus-malacearum complex, a group of closely related and biologically similar species that overlap in their host ranges and natural enemies. Economic losses from these pests have increased in West Coast vineyards and some pear orchards over the past decade. Our previous research showed that natural enemies can control obscure and grape mealybug populations in the absence of tending ants (particularly Argentine ants) and insecticide treatments. Unfortunately, most of the insecticides that effectively control ants are more toxic than those used to control mealybugs. We have therefore investigated the use of less toxic ant controls suitable for IPM systems.

In field studies conducted in commercial winegrape vineyards in the Central and North Coast regions, baits consisting of 25%sugar-water and small amounts of pesticides (boric acid at 0.5%; thiamethoxam or imidacloprid at 0.0001%) were dispensed from 250-ml plastic tubes, at a rate of 50 tubes per acre. Plots treated with either boric acid or thiamethoxam showed significant, season-long reductions in ant activity. The reduction in ant activity was associated with significant reductions in mealybug populations and decreased crop damage. One vineyard proved to be more heavily infested with European fruit lecanium scale than mealybugs; we found that ant control allowed natural enemies to lower the densities of the scales as well. In a step toward extending this information, and developing a commercially available baiting system, we established larger plots in the Central Coast region. In these plots, the chosen toxicant was imidacloprid, because it is already labeled for use in grapes. Unfortunately, results at these sites were not conclusive due to the high ant densities at these sites, the late start to the season, and possibly to the instability of imidacloprid when exposed to UV light. In similar experiments employing a sugar-protein bait, we tested the effects of an entomopathogenic fungus, Beauvaria bassiana, on ant activity. To date no significant reduction of ant densities has been detected in this treatment.

Finally, we released a grape mealybug parasitoid (Pseudaphycus angelicus) in combination with ant controls in one of the North Coast vineyards. Parasite recovery rates were very low, however, and the results showed no difference in the level of parasitism between areas with and without ant controls.