Abstract

Eggs of the underground grass caterpillar, Oncopera fasciculata (Walker), are laid on the surface of the ground during the spring and may be exposed to desiccation. A field experiment showed that there was a higher survival rate among the eggs and early-stage larvae when eggs were placed under tall, dense herbage. The most likely explanation for this result is that the dense cover protected these stages from dryness and heat. Tall, dense cover is found where improved pastures (herbage consisting of clovers and introduced grasses) have not been heavily grazed or cut for hay. Laboratory experiments showed that the eggs are easily killed by dryness because they lose water rapidly when exposed to evaporation. The median lethal dose for eggs that had completed 15 per cent. of their development was a 43 per cent. loss of water. As the embryo develops, the eggs become more susceptible to loss of water, and water that is lost cannot be regained when free water becomes available.