Improvement of the in vitro culture of Exorista larvarum (L.), a tachinid parasitoid of Lepidoptera

The tachinid larval parasitoid Exorista larvarum (L.) is well-known as an antagonist of lepidopterous forest and agricultural pests in several regions of the palearctic area. This beneficial insect was successfully reared from egg to adult on artificial media composed of crude components and containing agar as a physical support. For E. larvarum the consistency of the food is important to allow gaseous exchange, as its metapneustic larvae breathe atmospheric oxygen and reject CO2 from the beginnning of their development. In the host they form primary integumental respiratory funnels, while in the gelled media they penetrate the substrate and maintain the spiracles in contact with air. The in vitro culture of E. larvarum can be made cheaper by using absorbent cotton in replacement of more expensive agar as a support of the liquid medium, at the rate of 15 ml medium/15 mg cotton. Parasitoid yields did not differ significantly between agar-containing and cotton-absorbed media. The artificial rearing procedure of E. larvarum is usually performed under aseptic conditions in order to control contamination by fungi. When it was conducted out of a laminar flow hood, with a medium devoid of antifungal agents, no significant increase of mould contamination or decrease of adult yield however occurred. The possibility to carry out the in vitro culture out of a laminar flow hood could lead to significant savings in costs in case of large-scale production of E. larvarum on artificial media. E. larvarum perform parasitization by laying macrotype eggs on the host body. Both in nature and captivity conditions eggs are however deposited also in the environment if host larvae are not sufficient, a behaviour which is common amongst tachinids. In the absence of host, these eggs are obviously wasted. At moment, for the in vitro rearing of E. larvarum, the eggs are collected from superparasitised Galleria mellonella L. larvae and subsequently transferred onto the medium. We have found that the eggs laid on a plastic sheet, that had been placed on the bottom of the cage, were more easily removed to be transferred onto the medium than those deposited on the host body. On the artificial substrate, these eggs hatched and the parasitoids developed up to the adult stage at rates equivalent to those obtained from host-collected eggs. This technique may thus be recommended to simplify the in vitro culture of E. larvarum at least until the goal of obtaining the direct oviposition on the artificial substrate is achieved. Studies aimed at stimulating oviposition on the plastic sheet and finding the better conditions to store the eggs are now in progress.