Dimensional range in Traumatocampa pityocampa (Denis & Schiffermüller) eggs

Traumatocampa pityocampa (Denis & Schiffermüller) (Pine Processionary Moth, PPM) is one of the most threatening defoliator in Mediterranean pinewoods, because of its high biotic potential. PPM is becoming an increasing threat even where its presence has never been reported. PPM lays scale-covered egg-batches around pine needles. In Northeastern Italy, a longitudinal gradient in egg size has been evidenced, which can be explained as an adaptation to local host plant features; e.g. in the Eastern regions pine needles are harder, and PPM eggs and larvae are bigger, to attack them successfully. However, this adaptation implies a greater egg parasitization. The present study investigates the morphological structure of eggs in two PPM populations, situated at the poles of this longitudinal gradient. The ultrastructure of egg-batches has been observed, and divided in two zones (external and internal) to determine a dimensional ratio. . Results showed different sizes in the two populations: eggs are bigger in the Eastern part (1,240X0,994 mm) than in the Western (1,140X0,863 mm). Chorion thickness has been observed by dividing in four zones the transversal section perimeter: A – anterior pole external margin; B – anterior pole; C - side; D – posterior pole. Bigger eggs showed a thinner chorion in correspondence of sections A (45,9 contro 67,6m) and B (22.8 contro 25.5m), explaining their susceptibility to ooparasitoids. Minimal differences have been evidenced among external and internal eggs inside the same egg-batch (A: 45,9 vs 46,1m; B: 21,6 vs 23,3m).