Predaceous Gall Midges
Predators of Grape Mealybug
by Devin Carroll - 2005
Many years of observations have convinced me that the predaceous gall midge, Dicrodiplosis californica, is a key to biological control of the grape mealybug. Encyrtidae parasites and the mealybug destroyer, Cryptolaemus, are also important, and many other insects eat some mealybugs. But I have observed the “midge maggot” clean up serious mealybug infestations far more often than other natural enemies.
Midge maggots eat all stages of the mealybugs, except perhaps the eggs. The size of the midge doesn’t matter- the tiny white first stage larvae will feed on the largest mealybugs, and the full-grown red midge will feed on the smallest mealybug crawlers.
With practice, you can recognize the shriveled carcasses of mealybugs that have been sucked dry by midge maggots. Mealybugs killed by Lannate tend to cup. Small mealybugs eaten by midges are twisted and crinkled. Full-grown mealybug victims may appear at first glance to be parasitized, but they are flat and “bready”, without parasite remains and holes.
The midge maggots like heavy infestations of mealybugs. Oftentimes, a berry touching bark will have 50 or more mealybug crawlers, all or most dead. They have been sucked dry by midge maggots. The midge maggots will also move around under the bark or on a bunch, searching for prey. But they are probably not as good at searching as the parasites. It’s best to have both.
Midge maggots appear to be less bothered than parasites by ants. We have often seen infestations cleaned up in the presence of ants. But controlling the ants will help.
We have seen at least two and maybe three generations of midge maggots. The first young larvae appear near the end of May and are full grown by mid-June. The second generation appears near the end of June and matures around the middle of July to early August. There may be a third generation starting in August. The midges overwinter as larvae in cocoons.
If midge maggots can be found in June, you can count on good biological control. If you can’t find them until mid-July, you may still get good control.
We have seen good activity following Lannate and Applaud sprays. Many of the midge maggots are protected under the bark or under berries. The sprays get a lot of credit for cleaning up mealybug that really belongs to the midge maggot. These are the farmer’s hidden friends.
So far, we have not seen midge maggots attacking vine mealybugs. This may be an important reason why vine mealybugs are more damaging than grape mealybugs.