Biologie d'Agriotypus armatus curtis (Hymenoptera: Agriotypidae), parasite de nymphes de Trichoptères

Abstract

This investigation deals with Agriotypus armatus, an ichneumonid parasite of pupae of Trichoptera (families Goeridae and Odontoceridae). The life cycles of the parasite and its host have been studied by regular sampling in the field and by special techniques in the laboratory. 1. - The family Agriotypidae contains a single genus with two species, the European A. armatus and a Japanese A. gracilis. The European species is polymorphic. According to the host, two “forms” are recognizable: an “armatus” form parasitic on Goeridae (Silo), and a “major” form parasitic on Odontoceridae (Odontocerum). The results do not allow us to state whether it is a question of two species, of two sub-species, or simply polymorphism. 2. - The host in this study is Silo piceus (Goeridae). This species is univoltine. Larval development includes 5 instars, and takes place in 11 types of cases. The larvulae hatch in July-August. The wintering takes place in instar 5. Entry into the pre-pupal stage occurs in March-April. It is at this point that Agriotypus females start to oviposit. 3. - Larval development of Agriotypus armatus includes 5 external instars. Larvae of instar I are agriotypiform. Larvae of instars II, III and IV are vermiform, and are separated chiefly by size. At instar I, the larva places itself under the case wings of the young pupae and starts to suck there. The larva remains there until instar IV. Then it passes to the rear of its victim and rapidly completes the consumption. The last instar larva occupies the whole pupal case of the host and weaves in it a cocoon of silk, which extends outside the case as a ribbon. At the same time, the larva fills the cocoon with gas. 4. - Development from egg to instar IV takes about 5 weeks. Larvae remain in instar V for up to 2 months. Pupation and the imaginal moult take place in September-October. The imago then undergoes a long winter diapause which enables it to wait for the spring. 5. - During this long stay under water, the ribbon with its vacuolated structure allows a gaseous exchange between the air in the cocoon and the surrounding water. 6. - The emergence of the adults is perhaps induced, under certain conditions, by a rise in temperature. 7. - Oviposition compels the female to stay several hours under water. The search for a host seems to be chiefly at random. The female may be capable of recognising certain type of case, but the characteristics of the host itself appear to have little influence on the oviposition behaviour of the parasite. Although a number of eggs can be laid on the same host, only a single Agriotype attains complete development. Therefore Agriotypus is a solitary external parasite. © 1970 Masson.