Bacillus thuringiensis (Bt)

How it works

A naturally occurring soil bacterium. Its crystal protein only activates in the alkaline gut of caterpillars, paralysing the gut so they stop eating within hours and die in 2-3 days. Mammals, bees, birds and beneficial insects are unaffected - their gut chemistry is wrong for it.

Recipe

Dissolve the sachet per label (typically 1 g per litre). Use the whole batch the day you mix it; the spores die in storage once wet.

How to apply

Spray at dusk onto the leaves the caterpillars are actually eating - it must be EATEN to work, so coverage of chewed foliage matters more than hitting the insect. Reapply after rain and every 7 days; ultraviolet light destroys it in 2-3 days.

Targets — pests

Cabbage white caterpillars, Tomato moth, Box tree moth, Codling moth larvae, Leek moth

Targets — diseases

Not recorded — the field is blank rather than guessed.

Supplies: Bacillus thuringiensis kurstaki, 5 L pressure sprayer

Days to harvest after spraying: 0

Best months: May, June, July, August, September

Cautions

Only works on small, actively feeding larvae - a fat late-instar caterpillar will shrug it off. Useless against aphids, beetles or anything that is not a caterpillar.

The thing people get wrong

This is the single most useful organic insecticide there is, because it is surgically specific: it removes cabbage whites without touching a single bee, ladybird or hoverfly. If you only buy one spray for the brassica bed, buy this one - and spray it while the caterpillars are still tiny.