Abstract

In South Australia the underground grass caterpillar, Oncopera fasciculata (Walker), is found only in the higher rainfall area in the south-eastern portion of the State. It appears that the distribution and numbers of this pest of improved pastures are mainly restricted by unfavourable weather (dry and hot) during the egg and early larval stages (late spring-early summer). These stages, as well as older larvae established in subterranean burrows, stand a better chance to survive the dryness and heat where the surface of the ground is covered with dense herbage. Unfed, first-instar larvae are much more susceptible to loss of water than are the eggs. Larvae remain virtually dormant during the summer, but from about April onwards they grow rapidly. The resumption of active feeding and growth is associated with the first substantial autumn rains. During autumn, when the rainfall may be intermittent, the larvae feed more actively during wet periods and become relatively inactive again during dry periods. The relative humidity of the air at the base of a subterranean burrow where the larvae lives was usually above 95 per cent., even when the relative humidity of the air just above the mouth of the burrow was as low as 65 per cent. No measurements were taken during the height of the summer, when the burrows may have been drier than this.