IPMP3.0, Oregon State University, Copyright 2000





Sampling and Action Threshold

Development Model

Registered Insecticides


[Insect Management]


Strawberry Root Weevil Larva

The major root weevil species attacking mint is the strawberry root weevil, Otiorhynchus ovatus. However, the black vine, O. sulcatus, rough strawberry, O. rugosostriatus, and obscure root weevil, Sciopithes obscurus, also may be present in some mint fields.

Most strawberry root weevils overwinter as larvae in the soil, but a few adult weevils also overwinter in protected areas. These adults are the first to deposit eggs the following spring. Overwintering larvae mature during the end of April and early May and form earthen cells in the soil where they pupate. Adults begin emerging during mid-May and early June and are usually present in mint fields through late September (Cacka, 1982; Emenegger and Berry, 1978; Emenegger, 1976). All adults are females and, in the absence of fertilization by males, begin depositing eggs around the bases of plants about 2 weeks after emergence. Most of the eggs are deposited during late June and July. There is one generation each year. Select Development Model from the contents menu to calculate a degree-day model for strawberry root weevil.

The accumulation of day-degrees (DD) may be used to predict the occurrence of the different life stages of strawberry root weevil using a base temperature threshold of 8.8oC (48oF) and accumulating DD above the threshold beginning January 1. For instance, in central Oregon, overwintering larvae are found in soil samples until late May or early June (Cacka, 1982). Pupae are present in samples from late April to early June (350 DD). Peak pupation occurs from mid-May to early June. Teneral adults are present in late May to early June (500 DD) and peak adult emergence occurs in mid-June and early July (700 DD). Development of ovaries and subsequent egg laying usually begins about 2 weeks after adult emergence or after about 950 DD have been accumulated. Control of adults with insecticides in central Oregon should be timed to coincide with the accumulation of about 700 to 800 DD in early July. Development of all stages of strawberry root weevil occurs earlier in western Oregon; pupae and teneral adults are present in soil samples in early May and most adults have emerged by early June. Therefore, applications of insecticides for adult control in western Oregon should be made in mid-June.

Larvae begin feeding on small mint roots very soon after emerging from the eggs (the majority of larvae emerge during late July and early August, 1300 to 1400 DD). Larvae feed through the summer and early fall, at which time they reduce activity until the following spring when feeding is resumed. In western Oregon and Washington, where winters are mild, the larvae may continue feeding during the winter months.

Adult damage, which may be evident during late May, June and July, consists of small notches on the leaf margins and stems, particularly near the soil line. Adult feeding on mint is usually inconspicuous and is not economically important. Adults are active on mint foliage at night after sunset and by dawn have usually descended to the soil surface and become inactive during the daytime in cracks or under sheltering debris.

Natural spread of root weevil infestations in and among fields is gradual because adults do not fly. Serious infestations can originate when roots and rhizomes are dug from infested fields and transplanted in uninfested areas. Obviously, growers buying mint rhizomes for new plantings should purchase "weevil free" rootstock, or at least remove as much of the soil as possible from rhizomes to be transplanted.