The larvae of click beetles (commonly known as 'wireworms') are important, polyphagous pests. They attack the subterranean parts of plants, and are usually most numerous in permanent grassland, where populations may reach or exceed several million per hectare. After destruction of infested grassland (e.g. by ploughing), surviving wireworms readily transfer their attention to following crops. Damage is most severe in spring and early summer, especially on potato tubers and on vegetable crops such as celery, leek, lettuce and onion; infestations may also be of significance on beet, hops, mangold, oats, spinach, strawberry, wheat (especially in the second year following grass), and on many other crops. Some plants, including barley, beans, clover, fodder brassicas, linseed, lucerne, mustard, oilseed rape, peas and rye, are resistant to attack. Wireworms browse on the roots or bite through the stems at about soil level; they also produce ragged holes in the basal parts of stems or bore into planted seeds, swollen tap roots or tubers, producing distinctive, small, rounded entry holes.
Adult click beetles are active from April to June. Eggs are then laid in the soil in batches of up to 100, usually amongst grass or other vegetation. The eggs hatch 5-6 weeks later. Wireworms (larvae) develop slowly, especially in their later instars, development usually extending over 4 or 5 years. They cause most damage in early spring, especially from March to May, with a second period of activity in the late summer or early autumn. Pupation takes place in August of the final year of larval development, each larva first forming an earthen cell 10 cm or more below the surface. The adult stage is reached 3-4 weeks later; however, the beetle does not emerge but remains within the pupal cell until the following spring.
Adult 7-10 mm long and mainly dark yellowish-brown, with a darker head; elytra with alternating dark and pale longitudinal striations; pronotum relatively long, with a moderately distinct median longitudinal cleft (Fig. 204). Egg 0.6 mm long, more or less oval, whitish. Larva up to 25 mm long, shiny yellowish-brown, with a darker head, powerful jaws and small thoracic
legs; ninth abdominal segment pointed and with a pair of distinctive dorsolateral pits (Fig. 205a).
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