One or more sprays of Bacillus thuringiensis (Bt) before blossom will control winter moth larvae, providing temperatures are high enough for the caterpillars to be feeding actively.
- The bacteria and the crystal toxin which it produces, have to be ingested in order to act.
- The main problem is that caterpillars are often feeding in or amongst the buds or in furled rosette leaves where they are inaccessible to sprays.
- The bacterium is degraded by heat and UV light so is of short persistence.
- For these reasons, it is probable that more than one spray will be required for a high standard of control.
- Bacillus thuringiensis is harmless to bees and may be applied during blossom if necessary.