How Do I Control Caterpillars on My Vegetables Using IPM?

Caterpillars are voracious feeders that can strip a brassica bed in days during peak season. The large white butterfly (Pieris brassicae) and small white (Pieris rapae) are the most common culprits in UK gardens, but cabbage moth, diamond-back moth, and tomato moth are also significant in some regions. An IPM approach to caterpillars uses prevention and early detection to stop problems before they escalate, with Bacillus thuringiensis (Bt) as the most targeted spray option when numbers cross a threshold.

The good news is that caterpillar IPM is one of the most effective applications of the IPM approach — prevention alone eliminates the need for any chemical intervention in most gardens.

Prevention: Fine Mesh Netting

Covering brassicas with fine mesh (0.8mm aperture) before butterflies begin flying in April — and keeping it on until the last crop is harvested — excludes both large and small white butterflies completely. This single measure makes caterpillar spraying unnecessary on brassicas for most home gardeners. Install nets on hoops that keep fabric clear of the foliage (so butterflies cannot lay through the mesh onto leaves pressed against it), and seal all edges to the ground.

Monitoring: Check Weekly for Eggs

If you grow without netting, or if netting has gaps, monitor weekly during butterfly flight season (April through October, with peaks in May and August). Check the undersides of leaves for egg batches — large white eggs are yellow, laid in groups of 20–100 on the underside; small white eggs are pale green, laid singly. Remove egg clusters by hand immediately. A weekly check during flight season, with prompt removal of any eggs found, is often sufficient to prevent significant larval damage.

Biological Control: Bt

Bacillus thuringiensis var. kurstaki (Bt) is the most selective and safest spray for caterpillar control. It is a naturally occurring bacterium whose toxins are lethal only to caterpillars — no effect on beetles, bees, parasitic wasps, lacewings, or any other non-target insect. Apply as a spray to the foliage when young larvae (first and second instar) are detected, before they are large enough to have caused significant damage. Bt breaks down in UV light within 48–72 hours; repeat after rain. It is ineffective against large, late-instar caterpillars.

Natural Enemies

Many caterpillars are parasitised by ichneumon wasps and tachinid flies. If you find a caterpillar covered in white cocoons (braconid wasp pupae), leave it — it is already dead and the wasps are still emerging. Parasitised caterpillars behave abnormally and often fail to feed — if you observe a very slow or lethargic caterpillar, check it for parasite signs before removing it.

Handpicking and Thresholds

On small plantings, handpicking caterpillars in the evening is effective and satisfying. Drop them in soapy water. For larger plantings, set an action threshold — one to two caterpillars per plant on seedlings justifies immediate Bt application; ten caterpillars per plant on large established kale might be tolerable if the plant can outgrow the damage.