LIFE CYCLE AND HABITS OF WIREWORMS

Wireworms such as Ctenicera species and Limonus species. Adult wireworms, which are commonly called click beetles, do not generally cause economic damage on established mint in the Pacific Northwest, but larvae can be found feeding on mint roots, rhizomes, and stems at the soil surface. Serious infestations occasionally occur on first year mint following potatoes, onions, sugarbeets, grass seed, or mint planted in newly recovered land after fallow. The most common species encountered in mint include the Pacific Coast wireworm, Limonus canus, the sugarbeet wireworm, L. californicus, and the Great Basin wireworm, Ctenicera pruinina.

The adult beetles are slender, tan to nearly black, and range from 1/2 to 3/4 inch long. Larvae are hard, segmented, measure 1/2 to 1 inch long, and vary from yellow to brown. Larvae have three pairs of legs and the last abdominal body segment is elongated and may end in a keyhole-shaped structure.

Wireworms overwinter as larvae or as recently developed adults which do not emerge from the soil until the following spring, usually from early May to June. Adults migrate by flying within fields or to new fields. The females mate and burrow into the soil to deposit eggs. Eggs are laid singly 2 to 6 inches deep in the soil and hatch in 3 to 4 weeks. The larvae move easily through the soil in search of food. They can feed in the soil for 2 to 5 years before pupating in July or August. In the Northwest, most wireworms take about 3 years to complete their life cycles.

MANAGEMENT AND CONTROL OF WIREWORMS

Wireworm populations can be estimated by taking square foot soil samples to a depth of about 2 to 3 inches around mint plants from several different locations in the field. Usually, wireworm larvae, if present, also will be collected at the same time samples are taken for redbacked cutworm, strawberry root weevil, mint root borer, and symphylans. Soil should be screened in the field or placed in Berlese funnels to extract these pests.

No treatment threshold has been developed for wireworms on mint. Wireworms seldom cause serious damage on established mint, but they may be devastating to new plantings. Damage may occur if mint is to be planted in fields with a previous history of wireworm injury or in noncrop or fallow land. Fields that have been fallowed for several years or planted in potatoes, cereals, or sugarbeets should be sampled before planting mint. Using mint roots free of insects to establish new fields is of utmost importance. If mint is to be planted in fields with a previous history of wireworm injury, growers should consider treating the field with Dyfonate or fumigating the soil prior to planting mint. Use Telone II or C-17 at the rates recommended on the label. Fumigation is expensive and therefore usually used to control multiple pests. If Dyfonate is used, do not apply Sinbar until at least 3 weeks after the Dyfonate application. Fields with a previous history of wireworms may be fallowed and frequently cultivated to kill weed hosts for 1 year prior to planting mint, but, since wireworms may take up to 3 years to complete development, some wireworm larvae may remain in the field even after 1 year of fallow.