Seven ways to fight cabbage caterpillars
Most of us happily shrug off the odd hole in a home grown cabbage leaf, but serving poached caterpillars with the broccoli gets beyond a joke! White butterfly and the smaller diamond back moth are two major pests of brassica crops in warmer months.
Often, we can avoid the worst pest damage by being clever about the planting time; growing brassicas mainly over winter when there are no butterflies about (and the low temps make cabbages sweeter). This is easier said than done in climates where brassicas need planting early while butterflies are still fluttering about. The quick growing caterpillars can decimate young seedlings. Most hatch from eggs laid on the undersides of leaves. In controlling this pest, it’s best to act early before serious damage is done.
Covering the crop with fine insect mesh prevents butterflies laying their eggs on young leaves. Insect mesh is now available in garden centres and can be used for a number of years. Plants must be completely covered. Also, check your seedlings for any eggs that may have been laid before covering. Crop cover is not so useful for crops that need insects for pollination.
‘Digital control’ can be effective if you are vigilant. Some gardeners keep numbers down by picking the caterpillars off the leaves.
Derris Dust has been used for many years to control caterpillars. Its active ingredient is Rotenone, a naturally occurring compound that biodegrades under warm sunny conditions. It is harmful to fish and bees. It should not be applied to food crops within a day of harvest.
Pyrethrum products (Pyrethrins) offer effective and convenient control, using a naturally occurring compound derived from chrysanthemum flowers. It’s important to be aware that pyrethrums, though natural, don’t just kill the pests but beneficial insects too. Avoid spraying when bees are foraging. Toxicity is low once surfaces are dry.
Biological control is available via products which contain the naturally occurring soil bacteria, Bacillus thuringiensis var. kurstaki (Btk). Lethal only to caterpillars, Btk is safe for humans and beneficial insects. Once a caterpillar eats treated foliage, its gut is paralysed and it stops eating, although it may take 3-4 days to die and drop from the leaf. Apply Btk late in the day to avoid intense suns, which breaks it down.
New technology in the battle against caterpillars and other pests comes in products containing the active ingredient spinetorum, which is derived from soil bacteria. It has ‘translaminar action’ which means it moves into the leaf and won't be wash away in rain. Plus it’s not so critical if you miss spraying the undersides of leaves. Avoid spraying when bees are foraging. There is a short withholding period for some crops.
Companion planting supports natural predators. Members of the carrot family (carrots, parsnip, parsley, dill) attract spiders and also tiny parasitic wasps which prey on caterpillars. The herb wormwood helps repel the butterflies.
Holes in cabbage caused by the white butterfly caterpillars
Mesh / net crop cover