STRAWBERRY ROOT WEEVIL LIFE CYCLE AND HABITS

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. In IPMP there is also a strawberry root weevil development model:

Run development model

See also management and control of strawberry root weevil.

Click here for Picture

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 Orthene 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 Orthene 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.

MANAGEMENT AND CONTROL OF STRAWBERRY ROOT WEEVIL

The immature or larval stage of the strawberry root weevil causes damage to mint by feeding on roots. However, it has become obvious that the best way to control this pest is in mid-June or early July with Orthene 75S applied in the evening (after sunset for best success) as a foliage spray to kill the adults.

SRW larvae are best estimated by taking 1000 cm[[twosuperior]] soil samples after harvest in late August and September or the following spring in March, April or early May. In western Oregon, samples for root weevils may be combined with samples for mint root borer. In central Oregon, they may be combined with samples for redbacked cutworm in April and May. Take at least 25 soil samples from different areas of the field (one site per 2 1/2 acres), screen the soil, count the number of larvae and calculate the average number per sample. If the average number of root weevil larvae exceeds two per sample, treatment of adults with Orthene applied at 1.0 lb a.i./acre is recommended (or see below for the treatment threshold which includes the total larvae, pupae, and adults/1000 cm[[twosuperior]]). Malathion applied at 1.0 lb a.i./acre also is registered for adult control, but it is not as effective as Orthene.

A sequential sampling program has been developed for SRW in central Oregon (Cacka, 1982). This sampling plan is based on taking a minumum of 25 1000 cm[[twosuperior]] soil samples and accumulating the number of adults, pupae, and larvae found in each sample. Using this method, treatment is recommended if the total accumulated number of adults, pupae, and larvae exceeds 5.5 weevils/1000 cm[[twosuperior]]. Treatment is not recommended if the total weevils/sample is less than 2.7/1000 cm[[twosuperior]]. These values vary with the cost of treatment and the price of mint oil. To generate new thresholds and sequential sampling plans based on current values, run the

Strawberry Root Weevil Sequential Sampling Calculator

All SRW are females capable of laying eggs without fertilization. In general there is a 2- week period before the females begin laying eggs after emergence from the soil. This biological fact helps time an insecticide application for optimum control when most of the adults have emerged from the soil, but prior to any egg laying. In practice, mint fields suspected of being infested with SRW should be sampled with a sweep net in the evening a couple of hours after sunset. Still, warm and dry evenings are best. Windy, cool and/or rainy periods produce fewer weevils per sweep, giving the impression of a smaller field population than is actually present. Take 10 straight line sweep samples in at least five different sites in fields up to 30 acres. Add one additional sample site for each additional 10 acres.

Even though no treatment threshold has been developed for adult SRW, we have seen that, once an infestation becomes established in a field, it can be difficult to control. An evening application of Orthene against adults may be justified even if the population is at a relatively low level. If treatable populations occur, apply 1.0 lb a.i. Orthene/acre in the evening after 90 to 100 percent of the adults have emerged, and prior to egg laying in June or early July. Weevils will not have mature eggs in their bodies until about 2 weeks after emergence (Cacka, 1982). Therefore, monitor for the presence of eggs in the weevils by collecting five or ten adults and, after squashing their bodies, inspecting for small round yellow eggs through a 15x hand lens. If you spray too soon, there will be a residual population of weevils emerging postspray to lay eggs. If you spray too late, egg laying will have already occurred. Field observations indicate that two applications of Orthene may be required in central Oregon, but only one west of the Cascade Mountains.

[$] # Using parasitic nematodes to control strawberry root weevil

The parasitic nematode, Steinernema carpocapsae, marketed under the tradename Biovector-Mint by Biosys Company, provides control of strawberry root weevil larvae in mint. For best results, applications should be injected through sprinkler irrigation after harvest in the evening or at night at a rate of 3.0 billion juvenile nematodes per acre. Applications of Biovector-Mint applied post-harvest will control both strawberry root weevil larvae and mint root borer larvae. Applications to control strawberry root weevil in the spring have been less successful largely because of low soil temperatures (< 60oF). Therefore, additional research is required to establish proper timing of applications for SRW control in the spring. See also the information about using parasitic nematodes to control mint flea beetle and mint root borer.

A carabid beetle, Pterostichus vulgaris, found in western and central Oregon mint fields, feeds on strawberry root weevil adults, pupae, and larvae and may substantially reduce the population (Cacka, 1982). However, populations of this important predator may be significantly reduced by summer applications of Orthene used for adult strawberry root weevil control