Lydella thompsoni Hertin, Tachinidae, is an endoparasitoid frequently associated in southern France with larvae of Ostrinia nubilalis, Pyralidae, Sesamia nonagrioides, Noctuidae, Archanara geminipuncta and A. dissoluta, Noctuidae. The tachinid was reared successfully at 21°C in the laboratory by providing the adult flies with a high humidity, a light intensity of 8,000-10,000 lux for mating and a mixture of casein proteolysate and honey as food. Under such conditions, the flies lived for about 30 days and about half of them mated successfully. Hosts were infested by dissecting mature tachinid females and placing 1-2 of the extracted planidia onto each moth larva. Half of the planidia successfully entered their larval hosts. The biology of the tachinid larvae was studied on the 2 main hosts, O. nubilalis and S. nonagrioides. At 21°C, 25°C and 28°C, larval development took less time on O. nubilalis than on S. nonagrioides. At 25°C, female larvae on O. nubilalis required 9.0±0.5 days and on S. nonagrioides 10.5±0.3 days, male larvae on O. nubilalis required 8.3±0.5 days and on S. nonagrioides 10.6±0.3 days. Pupal duration was also influenced by the larval host. In winter, 2nd instar larvae of L. thompsoni enter a resting or quiescent condition. This condition is terminated sooner (December-January) in larvae developing on S. nonagrioides than in those developing on O. nubilalis (February-March). The life cycle of the tachinid in the field was studied by trapping flies in water dishes and by collecting parasitized host larvae from various plants. Flies were caught from April to October, mainly in September. Larvae of the spring generation of the parasitoid developed on larvae of species of Archanara that fed on the reed, Phragmites communis. From 1976 to 1982, parasitism averaged about 16%. Parasitism by summer generation on S. nonagrioides was highest (4-5%) in June. Parasitism of O. nubilalis did not change by more than 2 fold in either of the 2 summers studied (10-17 % in 1981, 6-10 % in 1982). By September the numbers of host larvae had increased to 10-20 times the number available earlier in the season. Larval populations of L. thompsoni similarly increased from 3-400 larvae per ha in June to 4-5,000 per ha in September. The stability of the relationship between this parasitoid and its hosts in southeastern France is discussed and compared to relationship described elsewhere. © 1985 Lavoisier Abonnements.
