IMPACT OF THE SOIL INSECTS AFRICAN BLACK BEETLE, HETERONYCHUS-ARATOR (COLEOPTERA, SCARABAEIDAE) AND WHITEFRINGED WEEVIL, GRAPHOGNATHUS-LEUCOLOMA (COLEOPTERA, CURCULIONIDAE), ON POTATOES AND EFFECTS OF SOIL INSECTICIDE TREATMENTS IN SOUTH-WESTERN AUSTRALIA

Populations of the soil insects African black beetle, Heteronychus ara tor (Fabricius), and whitefringed weevil, Graphognathus leucoloma (Boh eman), and the damage caused to potato crops in south-western Australi a were measured with and without the insecticide chlorpyrifos incorpor ated into the soil prior to planting. Low populations of both insects were highly damaging. Destruction of stems by H. arator reduced tuber yield and both species damaged tubers from the time of their formation . Tuber damage increased with time because both insect species on aver age damaged multiple tubers and caused multiple attacks on tubers. Gro wth of G. leucoloma larvae caused increased abundance of the more dama ging later instars in spring. The insecticide reduced resident H. arat or abundance and hence damage to newly-emerging potato stems in summer , but had less effect on reducing attacks on tubers in summer crops be cause adult beetles flew into some crops during growth. Exceptionally high tuber damage per H. arator adult in winter crops was ascribed to enhanced activity during its spring breeding season. The insecticide w as inconsistent in reducing the abundance of G. leucoloma larvae. Inse cticidal effects were greatest near the soil surface, resulting in an increase in the relative proportion of both insects deeper in the soil . Since a greater proportion of the tubers occurred there, the resulti ng greater potential for the insects to cause tuber damage tended to o utweigh reduction in their abundance.