Studies on the ecology of Oncopera intricata Walker (Lepidoptera: Hepialidae). 4. Effects of density of oviposition, pasture cover, and disease on survival from the egg to the final larval instar.

Abstract

The survival rate in populations of Oncopera intricata Walker in plots 1 sq ft in area was measured in two seasons. The plots were seeded with 500 and 2000 eggs in 1958 and with 100, 500, and 1500 eggs in 1959. In both years three levels of pasture cover were provided: short and kept short; initially short; and long. In 1958, two levels of moisture were provided: natural rainfall; and natural rainfall supplemented by 0.5 in. of moisture on two occasions. The survival rates were measured in June and September 1958, and May, July, and September 1959. In both years the initial density of eggs in the plots significantly influenced survival rate. In June 1958 there were more larvae in the plots seeded with 500 eggs and a proportional decrease in survivors between June and September. In 1959 there was a proportional decrease in survivors until May but a relatively greater rate of decline in the denser population from May to September. The effect of density in 1958 is explained in terms of the greater number of larvae that established in unfavourable sites at the higher density and were subject to high mortality when heavy rain fell in May. In 1959 the effect is attributed to crowding of the larvae in the presence of the pathogenic fungus Cephalosporium sp. Long cover favoured survival in 1958 but in 1959 the survival rate was highest when the pasture cover was kept short. The causes of this reversal are discussed in relation to soil moisture conditions. Additional moisture in 1958 did not significantly alter the survival rate as natural rainfall in late March and early April provided reasonable conditions for the survival of the eggs and young larvae. The significant density x cover and density x time interactions in 1959 are discussed. Further information is required before the results of these experiments can be fully applied to natural populations.