Measures of Bacillus thuringiensis persistence in the corn whorl

Experiments were conducted to determine the longevity of B. thuringiensis spores applied directly into the maize whorl, determine the reliability of the number of live larvae in the whorl to predict centimetres of tunnelling, and determine the insecticidal activity of the B. thuringiensis crystal toxin within the whorl. Pioneer 3541 hybrid maize, susceptible to leaf feeding by Ostrinia nubilalis, was used in the experiments. B. thuringiensis was applied to all plants on 7 June 1988 with a hand-held compressed air sprayer to the centre of the whorl of each plant. Approximately 50 neonates of O. nubilalis were applied to the centre of the whorl of each plant within a subplot on the appropriate infestation date (day 0, 1, 2, 4, 8 and 12) with a hand held applicator. At the time of infestation, whorls from 5 plants per subplot were analysed for the presence of B. thuringiensis spores. The virulence of B. thuringiensis crystal protein against neonates of O. nubilalis was evaluated using the droplet assay. The results revealed significant reductions in the number of live larvae and centimetres of tunnelling, as well as significant increases in the number of B. thuringiensis spores recovered and larval mortality in B. thuringiensis-treated and control plots. Significant differences in these parameters were observed between days of infestation. The mean number of live larvae in the whorl of 5 plants was a reliable indicator of centimetres of tunnelling at 40 days. The mean number of B. thuringiensis colony forming units recorded was a reliable indicator of the percent larval mortality in the bioassay. The loss of insecticidal activity over time was indicated by the decrease in larval mortality in the bioassay and an increase in the number of live larvae recorded in the plant whorl. Mean larval mortality was 38 and 51% on day 0 and declined to 3 and 5% on day 12 for the low (6.6×109 IU/ha) and high (8.7×109 IU/ha) B. thuringiensis treatments, respectively. The mean number of live larvae per treatment increased from 8.0±3.0 and 7.5±3.0 on day 0 to 26.0±4.8 and 33.3±1.9 in day 12 for the low and high B. thuringiensis treatments, respectively. Larval mortality in the control ranged from 2 to 5% and spore counts ranged from 20 to 864 over the 12-day period.