The biology of Agriotes sordidus Illiger (Col., Elateridae)

This paper describes the life cycle, including adult behaviour, oviposition, larval and pupal development rate of Agriotes sordidus Illiger. Each larva passed through up to eight to 13 instars. The larval size range of each instar was defined. Larvae need live vegetable tissues to survive and grow, otherwise most die within 40 days. Resistance to starvation increases with the age of the larvae (last instars can survive up to 1 year without food at 20degreesC). Each instar passes through three phases: mandible hardening and darkening, feeding, pre-moulting. The intense feeding (damaging plants) phase lasted <25% of the whole development time. They are poliphagous and the rate of larval development does not vary with host-plant type (maize, alfalfa). Provided sufficient soil moisture and food are present, larval development rate strongly depends on soil temperature. The duration of each instar increased with the age of the larvae. No larval growth was observed below 9degreesC. Under laboratory conditions the average heat sum (above a base of 9degreesC) required for development from egg to adult was about 3900 DD. Similar results were found in the rearing cages and in the open field. At the latitudes of the regions where this study was carried out (northern Italy, Veneto between 45degrees34'00N and 45degrees42'00N and central-south Italy, Molise, between 41degrees49'720N and 41degrees56'501N) the 6th instar (which normally is the first one passing 10 mm in length) is attained by September of the same oviposition year. Pupae can be found between the end of May and September mostly in the upper soil layer. Their transformation into adults took about 16 days at 25degreesC. Larvae of different stages overwintered by burrowing deep into the soil. Vertical migrations during the year are described: they depend mostly on soil temperatures from October to early spring. The adults overwintered and laid eggs in the subsequent spring. At lower latitudes or in warm seasons most of the population completed its life cycle (from egg to egg) in 24 months over three calendar years. At more northern latitudes, part (sometimes most) of the population completed the whole life cycle in about 36 months over four calendar years.