Pot trial screening of chemical, biological and natural insecticides for the management of white grubs (Coleoptera: Scarabaeidae) during eucalypt and wattle establishment

Abstract

In South African forest plantations, white grubs (Scarabaeidae larvae) can contribute to high transplant mortality following re-establishment. Currently registered insecticides are considered highly hazardous and their use is restricted by the Forest Stewardship Council (FSC). Therefore, there is an immediate need to find effective alternative products for the control of white grubs. To circumvent sporadic spatial variations associated with field trials, a pot trial bioassay method was developed to screen a broad range of alternative chemical, biological and natural products for the control of Schizonycha affinis on eucalypt (Eucalyptus grandis W.Hill) and black wattle (Acacia mearnsii De Wild.) seedlings. The first pot trial screened 14 insecticide options against two white grub instar stages. Treatments included two registered chemical insecticides (deltamethrin, imidacloprid), one alternative chemical insecticide (clothianidin), two natural insecticides (azadirachtin, diatomaceous earth), and four biological insecticides (two Beauveria spp., a Bacillus and a Trichoderma sp.). In addition, potassium silicate was tested in combination with natural and biological insecticides. The most promising insecticides were re-tested against a single larval stage. Results from both trials showed that treatment with imidacloprid tablets and liquid (registered products), clothianidin and B. brongniartii were significantly better than all other products in reducing the impact of white grub damage on E. grandis and A. mearnsii seedlings. The use of pot trials with planned inoculations of the viable target pest proved useful in the initial screening of control products. The information generated in this study will contribute to the development of an alternative FSC-compliant strategy for the control of white grubs in plantation forest regeneration.

Keywords: Biological control, chemical control, Forest Stewardship Council, Schizonycha affinis, soil-borne pests