A programme of weekly sprays of Bacillus thuringiensis (Bt) (Dipel DF) throughout the egg hatch period gives fairly good control but is costly compared to other insecticides.
- Bt has to be ingested to act and is most effective in warm weather when caterpillars are feeding actively.
- The bacterium produces a crystalline toxin. The insect dies from the effects of this toxin rather than from pathogenesis due to the bacterium.
- Bt is of short persistence as it is degraded by heat and UV light.
- It is most effective against newly hatched larvae before they form leaf rolls in which they feed internally and are inaccessible to sprays.
- The first spray should be applied at the onset of egg hatch which should be determined from pheromone trap catches and egg development sums calculated from the daily maximum and minimum air temperature (see ‘Monitoring' and 'Forecasting’).
- Bt is not detected by conventional pesticide residue analysis.
- See also ‘Chemical control and Insecticide resistance’.