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Mealybug Biocontrol

GrapeGrower Magazine (April 2003)

by Devin Carroll

     In a previous column, I covered the biology of mealybugs and timing of treatments.  Now I will discuss biological control.

     A mealybug biocontrol program requires a grower with the courage to delay spraying even when mealybugs are found, and a pest control advisor who understands the biology of the mealybug and its natural enemies and takes the time to read the signs.

     Signs of mealybug natural enemies can be found during winter under the bark.  Look for dead mealybugs that have turned a yellowish color.  These “mummies” have been killed by larvae of tiny parasitic wasps, the most common being Acerophagus notativentris.  Most mummies will have exit holes, indicating that adult wasps have emerged.  These tell us that parasites have been active in the vineyard.  Some mummies will still have the white larvae inside, overwintering.  If you find some of these live parasite larvae, you can predict successful biological control.

     If you find mealybug egg sacs with masses of dead, shriveled crawlers, you are probably looking at the work of the predaceous gall midge, Dicrodiplosis californica.  Larger, flattened mealybug carcasses have also been sucked dry by this “midge maggot”.  If you are lucky, you may see the red larvae or pupae overwintering.  The adult, a small, fragile midge with a red abdomen, is rarely seen.

     These parasites and midge maggots are the most important natural enemies of grape mealybugs in the San Joaquin Valley.  Often, in years when growers see damage at harvest, these two will clean up many of the mealybugs before winter, and be ready to attack the survivors in the spring.  If signs are good, spraying is not necessary.

     In late March or early April, examine the mealybug crawlers on the spurs and canes.  Look for some that have died and turned yellow.  These first mummies of the season have a single parasite larvae inside.  Larger mealybug instars will host multiple parasites.  New parasites appear at roughly two week intervals, with around five parasite generations during each mealybug generation, so even five percent parasitism at the start can turn into good control.  The last generation attacks mealybugs that have already begun to lay eggs, so the mummy will be found with mealybug eggs attached.

     Parasitism will increase much faster if ants are controlled.  Both grey field ants and western fire ants protect the mealybugs like “cows”, and they will kill parasites or scare them away.  Commercial ant baits, such as Clinch and Esteem Ant Bait, are not registered in grapes, but farm-mixed baits can be used.  Cultural practices, such as flood irrigation, french plowing, or planting common vetch between rows, can also help keep ants off of vines.

     When the larger mealybugs return to the trunks around May and June, look for midge maggots.  The tiniest maggots are difficult to see, but dried mealybug carcasses tell of their presence.  Larger maggots are bright red and easy to see if you look.  Sometimes two maggots will be feeding on one large mealybug.  Ants seem to be less harmful to midges than to parasites.  If midge maggots are found during the spring mealybug generation, good control is very likely.

When looking under the bark, you may also find larvae of the “mealybug destroyer” lady beetle, Cryptolaemus montrouzeri, or the cast skins of pupae after adult emergence.  The larvae have wax filaments, making them resemble mealybugs.  These predators sometimes control heavy mealybug infestations, but they are less common than midge maggots.  They can be purchased, but they do not always establish well. 

     In the second mealybug generation, parasite mummies first appear among the crawlers during the last week of June or early July.  Five percent early parasitism is fair, 10-20 percent is good.  Parasitism should increase every two weeks, unless ants interfere.  If a Lannate spray is necessary, try to time it when the parasites are protected inside their mummies.