Cranberry Fruitworm

Cranberry Fruitworm ,Acrobasis vaccinii (Riley), affects cultivated blueberries that are poorly maintained. This pest overwinters as a fully grown larva in the litter near the soil surface under the bushes. The small, brown, adult moths emerge when the berries of early varieties begin to form, and they begin inserting eggs along the rim of the calyx cup. After hatching from the egg, the larva enters the berry. It eventually webs several berries together with silk, feeding inside as many as four. One generation hatches each year. The cranberry fruitworm infestation is characterized by masses of brown frass and silk.

Adults of both species are small, dark gray moths with a wingspan of about 1/3 inch (9 to 10 mm). Cranberry fruitworm moths have white patches on their wings. Larvae are smooth caterpillars that will grow to 3/8-inch (9 mm) in length. Cherry fruitworm larvae have pink-red bodies with brown or black heads. Cranberry fruitworm have green bodies with dark heads.

Since the cranberry fruitworm feeds exclusively on the fruit, it is one of the most damaging insects on cranberry. Chemical and non-chemical techniques help suppress the population but the methods must be well timed with the newly emerged larvae. If left untreated, bogs in Massachusetts have reported more than a 50% crop loss directly due to this insect.

Life Cycle: Fruitworms overwinter as large larvae in cavities usually made in the dead wood on the bush. The adult moths emerge in the late spring. Green-white flattened eggs are laid on the underside of leaves as well as on developing small green fruit. After hatching, larvae enter the berries. They usually feed on one berry and then penetrate and feed on another. Cherry fruitworms seal entrance holes with silk so that frass is not visible outside the berries. Both species have one generation per year.

This Lepidopterous pest is found on a number of host trees including cherry, apple, rose and hawthorne, all of which are commonly found near blueberry fields.

Damage: Soon after feeding begins, the injured berries become reddish prematurely. The fruit shrivels due to the loss of pulp and dries out. However, this occurs after the larvae have already entered the berry and began feeding on the seeds. Once all the seeds and pulp is consumed, the larva exits, leaving behind a frass-filled, hollowed out fruit. Such dried berries are sometimes called “raisins”. Each larva can consume three to six berries during its life.

Natural Control: Some natural egg and larval parasitism occurs, but often not enough to provide adequate control.

Cultural Control:In Massachusetts, late water (a 30 day reflood before the end of dormancy) has been shown to greatly reduce fruitworm activity. This is not practiced in Wisconsin.

Biological Control: None currently available. Some Bacillus thuringiensis products are registered, but efficacy is minimal and multiple applications are needed.

Chemical Control:Several broad spectrum insecticides are registered against cranberry fruitworm and are effective. The Insect Growth Regulator tebufenozide is also registered for cranberry fruitworm control; refer to the label for specific usage information. Applications should be timed for the egg stage. Applications time for control of larvae within the fruit provide minimal benefit.

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