Osservazioni sullo sviluppo e il comportamento delle larve di Malacosoma neustria (L.)(Lep. Lasiocampidae)

In a northern Sardinia cork oak stand the development of Malacosoma neustrium larvae, starting from egg hatch, was studied in order to verify the number of larval instars, characterise the nests built during the different instars and show a possible periodicity of their daily activity. Tue data obtained by direct open field observations (n. 16 cork oak trees infested with 22 egg masses) and laboratory rearing (25°C; RH=60%) using cork oak leaves as food source for larvae, were analysed. In the selected area, temperature, relative humidity, global radiation (MJ/m2) and rainfall (hourly and daily average data) were recorded. The insect completed 5 instars both in the field and in the laboratory; each instar was properly defined by using head capsule (cps) and frass (frs) measurements (Hcps=559.05; Hfrs=541.43; d.f.=4, P<0,05; Kruskal-Wallis's test). Up to 3rd instar, the larvae showed aggregation behaviour both in searching for food and in building the characteristic silk shelters. First instar larvae, hatched from the same egg masses, built various refuges around the young buds they eat but they completed the first moult in only one of them. In contrast, both 2°d and 3rd instar larvae generally built only one nest, of a more complicated structure, offering also an inside refuge to the colony. Fourth instar larvae maintained aggregation behaviour for a short while and showed a premature inclination to scatter in the canopy. In all observed cases, fourth instar larvae did not build nests but spun only some tents on primary branches and at their junctions, which were used as a stand to rest and moult. Fifth instar larvae did not show any aggregation behaviour and in some cases, they left the host tree to search for food. Direct observations of larval activity revealed that the larvae, up to 41h instar, are not active between midnight and 3.00 a.m. whereas they eat intensively at dawn (between 6.00 and 8.00 a.m.) and at sunset (between 7.00 and 9.00 p.m.). During the last instar, the larvae seemed continuously engaged in foraging activity except for a short interval in the course of the day.