Effect of a Bacillus thuringiensis subsp. tenebrionis insecticidal spray on the mortality, feeding, and development rates of larval Tasmanian eucalyptus leaf beetles (Coleoptera: Chrysomelidae)

Abstract

The leaf beetle Chrysophtharta bimaculata (Olivier) is an endemic insect pest that can severely defoliate eucalypt forests and plantations in Tasmania, Australia. Damaging levels of insect defoliation on plantation eucalypts are minimized currently using an integrated pest management (IPM) system. The effects of the biological insecticide, Novodor FC (active ingredient: Bacillus thuringiensis variety tenebrionis) on the mortality, feeding, and development rates of C. bimaculata larvae are being evaluated for its suitability for inclusion in this IPM program. In a dose-response trial, Novodor was very effective at killing 1st instars. However, older larvae showed an increasing tolerance to the toxin that was not proportional to their body weight. At 20°C, maximum mortality occurred at 4 d after treatment, although further natural mortality occurred until adult emergence. Novodor also prolonged the larval development time but deterred feeding by all stages of the surviving larvae. A Bt variety tenebrionis-based insecticide shows potential for inclusion in the IPM program with appropriate monitoring to identify when the majority of the population is at 1st-instar stage. Its feasibility would be enhanced if the toxicity to older larvae could be improved.