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Strawberry Root Weevil

August 29, 2001

Strawberry root weevil, Otiorhynchus ovatus, adults are numerous in landscapes and houses in many areas of the state. The adult beetles are about 3/16 to 1/4 inch long, with a somewhat bulbous abdomen and a head that narrows into a broad muzzle. Most are black, but brown individuals are common.

Adults start emerging from the soil in June but become noticeable again in late July and August. They feed primarily at night for 10 to 14 days before laying eggs. Adult feeding damage is very noticeable on various flowers, such as rose, black-eyed Susan, coralbells, azalea, rhododendron, and trumpet creeper. They chew even notches about 1/8 inch across in the margins of flower petals and leaves. Where adults are common, additional notches may be chewed at the edge of previous damage, resulting in more severe injury. Rose leaflets may be eaten to the midvein; rose petal damage commonly extends 1/4 inch or more into the petal. In addition, the beetles tend to hide among the rose petals, further damaging the bloom's appearance with their presence and black feces.

They also feed on arborvitae, other conifers, grasses, brambles, cucurbit and crucifer vegetables, and many deciduous trees and shrubs. On arborvitae, the beetles girdle the twigs of terminals and eat the younger foliage. The adults are long-lived, with some individuals living through the winter. These beetles also enter buildings, where they may be found wandering about indoors throughout the winter.

Flightless, the adults lay eggs near the vicinity of summer feeding damage. Larvae feed on roots of many of the hosts listed earlier. Root feeding by the grubs kills hemlock seedlings. Fully grown larvae or grubs are about 3/8 inch long. They are white, legless, and thick-bodied, with a brown head.

This insect is difficult to control in the larval stage, partly because it is difficult for an insecticide to be effective deeper in the soil, where many of the larvae feed. Insecticides tend to move slowly through the soil, binding to organic matter and losing effectiveness. The wide host range of the grubs also makes control difficult. One would end up treating almost the whole landscape, with a relatively low level of control. Hb nematodes, Heterorhabditis bacteriophora, have been shown to be effective. These searching nematodes burrow through moist soil, seek out, and kill the root weevil larvae. Their cost reduces their usefulness except in small areas or valuable crops, such as seedling beds or nurseries.

Feeding adults can be controlled with acephate (Orthene) or bifenthrin (Talstar) by spraying the foliage. Avoid spraying blossoms because these insecticides are toxic to bees and other pollinators. Typically, there is enough feeding on the foliage of attacked plants to obtain control. If you still have bendiocarb (Turcam), it controls adults on the foliage, and what drips to the ground controls the larvae.

Nonfeeding adults, particularly those entering houses and other buildings, are not easily controlled with insecticides. Caulking cracks and crevices along the foundation and making sure that door thresholds fit tightly are the most effective measures. Removing bark mulch and fallen leaves along the foundation reduces beetle hiding places and reduces the number next to the house. Indoors, vacuum or hand-remove individuals as they are found.

Author: Phil Nixon


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