Pygostolus falcatus (Nees) (Hymenoptera, Braconidae), a parasite of Sitona species (Coleoptera, Curculionidae).

Abstract

Pygostolus falcatus (Nees), an endoparasitic Braconid of the subfamily Blacinae, was reared from Sitona lineatus (L.), S. hispidulus (F.) and S. humeralis Steph. of Swedish origin. The life-history and immature stages of P. falcatus were studied in the laboratory at 74°F. and in field cages preparatory to its liberation in Canada as a control agent of the sweetclover weevil, S. cylindricollis Fhs.The immature stages develop in the haemocoele of the weevil. After deposition, the egg increases in volume to a maximum of 335 times. At eclosion, the cells of the trophamnion dissociate and increase in volume in relation to the growth of the larva. Supernumerary larvae are eliminated soon after eclosion, and one larva only develops in a host weevil. At 74°F., the final-instar larva emerges from the host weevil 15–16 days after deposition of the egg. Development of many first-instar larvae in a wide range of Sitona species was arrested by diapause. Five larval instars are described and illustrated. Oviposition is prevented in weevils in the summer of their development and eliminated in overwintered weevils by the parasite egg and first-instar larva. The weevil dies within hours of emergence of the parasite larva.Reproduction is parthenogenetic and thely tokous. The maximum number of eggs laid by a female was 46; the number of eggs laid per day per female varied from 0 to 11. Oviposition takes place both in light and in complete darkness and a preference among species of Sitona was not evident. The maximum number of weevils parasitised by a female in a field cage was 39; the number parasitised per day varied from 0 to 8.The species overwinters as a first-instar larva in weevils which became adult in the previous summer. There were two discrete parasite generations in 1958 in Swedish weevils, both of which developed in early and mid-summer within overwintered weevils. The maximum rate of parasitism of S. lineatus by larvae of the mid-summer parasite generation in 1958 was 73·3 per cent.The value of P. falcatus as a control agent of Sitona populations is limited by low parasitism of the new weevil generation of the current year and diapause of the first-instar larva.