Entomologia Experimentalis et Applicata 87: 217–220, 1998. 217 c 1998 Kluwer Academic Publishers. Printed in Belgium. Short communication Response of first-instar Aleochara bilineata larvae to the puparium morphology of its dipteran host Lucie Royer 1 , Joseph Le Lannic 2 , Jean-Pierre N´ enon 3 & Guy Boivin 4 1 Department of Natural Resource Sciences (Entomology), Macdonald Campus of McGill University, 21 111 Lakeshore Road, Ste-Anne-de-Bellevue, PQ H9X 3V9 Canada; 2 Centre de Microscopie Electronique ` a Balayage, Universit´ e de Rennes 1, Campus de Beaulieu, 35042 Rennes C´ edex, France; 3 Laboratoire d’Ecobiologie des Insectes Parasito¨ ides,Universit´ e de Rennes 1, Campus de Beaulieu, 35042 Rennes C´ edex, France; 4 Horticultural Research and Development Centre, Agriculture and Agri-Food Canada, 430 Boulevard Gouin, St-Jean-sur-Richelieu, PQ J3B 3E6 Canada; Author for correspondence Accepted: November 25, 1997 Key words: cabbage maggot, Delia radicum, Aleochara bilineata, entrance hole distribution, preference, external puparium relief, incidental protection, host vulnerability, Coleoptera, Staphylinidae Introduction Herbivores have various defense mechanisms such as temporal and spatial refuge (concealment), evasive and aggressive behaviours, defensive (repellent, sticky and noxious) secretions, ant or parental protection, and morphological protective traits (Gross, 1993). Those mechanisms may have evolved in response to selective pressure from parasitoids or they may be exaptations (Gould & Vrba, 1982), characters that appear for other biological functions but incidentally provide some pro- tection. Some morphological adaptations could reduce parasitoid pressure by either reducing the accessibil- ity of the host or increasing the handling time of the parasitoid (Cole, 1959; Pak et al., 1990; Rotheray & Barbosa, 1984; Salt, 1938). The cabbage maggot, Delia radicum (L.), devel- ops in cruciferous roots. The late 3rd-instar larvae leave the roots to pupate at depths of 4 to 8 cm in the surrounding soil. As in other cyclorrhaphous flies (Fraenkel & Bhaskaran, 1973), the tanned cuticle of the last-instar larva, the puparium, provides a protect- ive covering for the cabbage maggot pupa. Aleochara bilineata (Gyll.) is a predator and a parasitoid of vari- ous species of Diptera, including the cabbage maggot. Females of A. bilineata oviposit in the soil near the plant roots (Fuldner, 1960) and the campodeid 1st- instar larva forages to find a puparium through which it gnaws an entrance hole. Fuldner (1960) indicated that entrance holes of A. bilineata were located prin- cipally on the caudal end of the dorsal surface of the puparium. He associated this non-uniform distribution at some locations to the presence of interstices between the puparium and the pupa. However, no mechanism was suggested to explain how the larvae locate such spaces through the opaque, sclerotized puparium. We determined the distribution of A. bilineata entrance holes on cabbage maggot puparia and observed with scanning electron microscopy a lon- gitudinal dorsal section of the puparium in order to verify if the entrance holes were made in relation to the external morphology of the puparium. Materials and methods Insect rearing. Colonies of D. radicum and A. bilin- eata were established using cabbage maggot pupae col- lected from the field at Ste-Clotilde, Qu´ ebec (Canada), and maintained at 20 0 05 C, 70 5% r.h. under a L18:D6 photoperiod. Cabbage maggot adults were provided with a source of water, a 10% honey solution, and mixture of 50:50% soybean flour and brewers’ yeast, as well as with an oviposition site [half rutabaga (Brassica napus) deposited on 2 cm of moist sand]. The oviposition sites were changed twice each week and the cabbage maggots developed into the rutaba- gas until pupation. A. bilineata adults had access to a