The biology of Agriotes ustulatus Schaller (Col., Elateridae). II. Larval development, pupation, whole cycle description and practical implications

Following the first part (FURLAN, 1996) dealing with adult behaviour and oviposition, this paper describes the larval and pupal development of Agriotes ustulatus Schaller. Larval size (head width and length) and weight of the 11-13 instars is defined. Larvae need live vegetable tissues to survive and grow. Most of the young larvae die within 30 days without live vegetable tissues while resistance to starvation increases with the age of the larvae. Each instar passes through three phases: mandible hardening and darkening, feeding, premoulting. The intense feeding and then the damaging phase lasts less than 20% of the total time. They are polyphagous and the rate of larval development does not vary according to the different crops supplied, nor did the kind of soil influence the number of instars and the rate of development. Provided with enough soil moisture, the rate of development strictly depends on soil temperature: the duration of each instar and pupae was studied at different temperatures. In laboratory conditions the centigrade degree day accumulation (CDDA) (above 10 degrees C) required to complete the whole cycle ranged between 3700 and 4500 (average 4156). In the rearing cages and in the open field this data was confirmed and the whole cycle was completed in about 24 months. At the latitude of the region where this study was carried out (45 degrees 34'00 N-45 degrees 42'00 N) the seventh instar (which normally is the first instar passing 10 mm) is attained by June of the year subsequent to oviposition, while most of the larvae reached the last instars by November and pupated the following year. Pupae can be found between the end of May and September mostly in the upper soil layer. The duration of the pupal stage lasts 13 days at 20 degrees C. Larvae of different stages overwinter burrowing deep into the soil. Vertical migrations during the year are described. Finally the practical implications of the reported results are discussed.