Laboratory evaluation of a scarabid-specific isolate of Bacillus thuringiensis against white grub Holotrichia serrata

Abstract

A scarabid-specific isolate (Bt 62) of Bacillus thuringiensis Berliner was bioassayed against different instars of the white grub Holotrichia serrata F. (Coleoptera: Scarabaeidae). Carrot disc contamination method was followed since preliminary tests established carrot as a suitable food material vis-à-vis potato, sugarcane root and farmyard manure (FYM) for first instar grubs. In two replicated bioassays with first instar grubs, 10 mm diameter (D) carrot discs dispensed on one side with six doses viz., 1, 3, 6, 9, 12 and 15 μg of toxin per disc were fed to individual grubs and their mortality was recorded at 24 h intervals. In tests with second and third instar grubs, carrot discs of 30 mm dia. were similarly contaminated with 25, 50, 75, 100, 125, 150 and 175 μg of toxin per disc and fed to grubs. Percentage carrot disc area consumed was assessed visually to represent feeding rates and infer on feeding inhibition. In the two replicated trials against first instar, grubs suffered mortality 1 day after treatment (DAT) and mortality levels increased with days of exposure. Mean mortality rates at different doses were significantly higher than that in control and they increased with time reaching a peak on 3 rd DAT. In tests with second and third instar grubs, feeding inhibition manifested on 1 st DAT but inhibition rates decreased at a decreasing rate as feeding progressed. Mean feeding rates at different doses were significantly lower than that in control and they peaked on 4 th to 5 th DAT. The results of the laboratory tests and the prospects of Bt 62 in white grub control are discussed.