integrated pest management

Strawberry Crown Borer
Tyloderma fragariae

Adult crown borers are short-snouted, reddish-brown, flightless weevils about 1/5-inch long. Their wing covers are marked with three pairs of darker spots. Adults, which overwinter in plant debris in strawberry fields or in surrounding areas, become active in the spring at about the same time that strawberries begin to bloom. They feed in crowns to open holes into which they lay eggs that hatch in about one week. Egg-laying continues through mid June. Yellowish, legless grubs feed for several weeks in strawberry crowns before pupating in late summer. Adults emerge in the fall, feed on strawberry foliage, and then seek shelter in plant debris to pass the winter.

Strawberry corn borer adult and larva

Strawberry corn borer adult and larva.
Adult is about 1/3 inch (8 mm) long;
larva is about 1/4 inch (6 mm) long.

Although adults eat small holes in leaves in the fall, this defoliation rarely is economically damaging. Larval feeding is far more harmful. As one or more larvae bore downward through strawberry crowns, plants are weakened, stunted, or killed. Field borders or the portions of fields nearest older, infested plantings are often most heavily damaged.

Because adult strawberry crown borers cannot fly, isolating new fields from existing infestations greatly reduces the likelihood that this insect will cause significant losses. Commercial growers should use new plants that are free of crown borer and establish new fields at least 300 yards from existing fields. To prevent crown borer survival and migration, infested fields should be destroyed and tilled soon after the final picking. Although chemical control is rarely advised, some insecticides applied to control other insects may kill crown borers as well. Prebloom sprays intended to limit damage by tarnished plant bug kill some crown borer adults, but peak adult activity occurs slightly later. Egg-laying adults are especially active during bloom, a time period when insecticides should not be applied. Insecticides applied between bloom and harvest for the control of leafroller or sap beetle will also kill crown borer adults if they are still active on foliage, but killing adults at this time is unlikely to significantly reduce crown damage. Postharvest sprays may be used to reduce the population of newly emerged adults in late summer before they overwinter, but this practice is seldom warranted. Check the latest edition of the Illinois Commercial Small Fruit and Grape Spray Guide for a list of registered insecticides.