In vitro rearing of Exorista larvarum (L.) from eggs laid out of host and effect of short-term storage at different temperatures on egg viability

Exorista larvarum (L.) (Diptera: Tachinidae) is a polyphagous gregarious larval parasitoid of Lepidoptera, the females of which lay macrotype, unincubated eggs on the integument of the host larvae. This tachinid has repeatedly proved to be suitable for in vitro rearing on artificial media devoid of insect components. This procedure is usually performed starting from eggs removed from previously superparasitized larvae of the factitious host Galleria mellonella (L.). Direct oviposition of the parasitoid eggs onto an artificial medium has not until now been achieved, either for this species or for any tachinid that oviposits on or in the host body. In our laboratory colony of E. larvarum, we have observed that although host larvae are available, many parasitoid eggs are laid around the cage. We thus evaluated the possibility of using the out-of-host-eggs to rear E. larvarum on artificial media with the aim of (1) disengaging the parasitoid in vitro culture from the steady availability of a living host and (2) retrieving the eggs laid out of host (that are normally lost) for parasitoid production. For the experiment, E. larvarum eggs laid on a plastic sheet or on G. mellonella larvae were removed from the oviposition substrate and placed on an artificial medium based on skimmed milk (Mellini & Campadelli, Boll. Ist. Ent. “G. Grandi” Univ. Bologna, 50: 95-106, 1995). The out-of-host eggs hatched at the same rate as those removed from the host integument. Also for the percentages of puparia and adults and the times from egg to adult, no significant difference was found between the two treatments. These results showed that E. larvarum may be reared in vitro with a total exclusion of the host insect at least for one generation and that the out-of-host-eggs may be successfully retrieved for parasitoid production by placing them on an artificial medium. With the aim of making the in vitro rearing of this tachinid more flexible and easy to manage, an experiment was carried out to study the effect of short-term storage at different temperatures on viability of E. larvarum eggs. This experiment represents a first step toward assessing the optimal storage time and temperature conditions for E. larvarum eggs. Unincubated (0-day-old) eggs were exposed to 5, 10, 15°C for 1, 2, 3, 4, 5 days and then restored at the standard rearing temperature of 26°C. The eggs stored for 5 days at 15°C hatched at the same rate as the control eggs, which were always kept at 26°C. For the eggs stored at 5 and 10°C for 2-5 days, the hatchability was dramatically lower compared to controls. These temperatures therefore proved to be unsuitable for the storage of E. larvarum eggs.