Root Beetles

Root beetles are occasional problems on California strawberries primarily in nonfumigated and/or second-year plantings in the San Joaquin Valley. They feed at night and hide around the crowns of plants during the day and, with the exception of adult hoplia beetles, they cannot fly. The adults, nearly all females, emerge in late spring or summer, feed on strawberry foliage, and lay their eggs around the crowns about 1 month after emergence. After hatching, the larvae work their way into the soil and feed on strawberry roots and crowns through the fall.

Root weevil larvae have curved, white or pink bodies that are about 0.38 inch (9.7 mm) long when fully grown. They have distinct brown heads and no legs. In spring, they resume feeding and can cause extensive damage before they pupate. Root weevils have a single generation each year.

The Fuller rose beetle, also called Fuller rose weevil, can be distinguished from the other weevils by an oblique, white band on the side of each wing cover. In addition, their larvae have pale, almost white heads. The black vine weevil is the largest and has a distinct black color. The woods weevil is the smallest of the group.

Hoplia are scarab beetles that are brown and 0.40 inches (10 mm) long. They are primarily a problem in San Joaquin Valley plantations that have not been fumigated. In the San Joaquin Valley, adults emerge in mid-April and are active for about 1 month. They are attracted to strawberry flowers and fruits, where they feed on petals and young, green fruit. Eggs are laid on the soil or on strawberry crowns; the resulting larvae enter the soil to feed on roots and are found associated with the roots from fall through spring. The larvae are 0.45 inches (11 mm) long and characteristically C-shaped. They feed for 2 years before pupating.

Larvae of all of these beetles feed on the roots of strawberry plants and can completely devour small rootlets and destroy the bark and cortex of larger roots. Soon after feeding begins, plants wilt because the roots can no longer provide moisture for leaves. Hoplia larvae will severely stunt and eventually kill infested plants. It is not uncommon to find beetle larvae that have penetrated into the lower portion of the plant's crown.

Adult weevils feed on foliage and remove large scallops from the leaves. Such leaf damage is a good indication that weevils are present but is not economically damaging to the plants. Adult hoplia beetles feed on flower petals, but it is not known if their feeding injures young fruit.

The rapid removal of plantings following harvest and preplant fumigation destroy beetle larvae and pupae in the soil. Soil solarization may be effective for hoplia beetles in the Central Valley. The use of sudangrass as a cover crop may serve to increase beetle numbers in the field.

Parasitic nematodes that target immature insects in the soil are available commercially. However, preliminary research did not show success using them for control of beetle larvae infesting strawberry roots.

Annual plantings reduce the likelihood of high numbers of beetles building up in fields. Rotating to a nonhost crop (such as lettuce or cole crops) will further reduce beetle numbers in the soil.

Use cultural controls, especially annual plantings, soil solarization for hoplia beetles, and crop rotation on organically certified strawberries.

Even one hoplia grub in the crown or roots will cause significant damage. If plants wilt or appear stunted or reddish, larvae may be present. Examine roots to determine if root weevil larvae are present, because cold temperatures can also induce reddening. Dig several plants and look for C-shaped grubs in the crown or roots.

Soil fumigation for weed and disease control will destroy larvae and pupae in the soil, and root weevils and hoplia beetle do not appear to become problems in fumigated fields. In nonfumigated fields, chemigation can be effective.