Butterflies Can Spell Trouble for Your Brassicas

It all starts innocently enough: a beautiful summers day, the garden is filled with lovely white butterflies which gently lands one of your brassicas (cabbage, kale, brussels and the like). But what looks like a charming encounter can mark the beginning of devastation for your precious brassica crop. In the UK, several butterflies are notorious for laying their eggs on plants like broccoli, cabbage, cauliflower, and kale. Chief among them are the Large White (Pieris brassicae), often called the “Cabbage White”, and the Small White (Pieris rapae). While they might look delicate, their offspring are anything but.

Butterflies typically lay their eggs on the underside of brassica leaves, often in sheltered spots protected from wind and rain. The Large White lays eggs in clusters of 20 to 50—bright yellow, bullet-shaped with ridged sides—easy to spot if you know what you’re looking for. The Small White, by contrast, lays single eggs that are slightly smaller, also yellow and oval-shaped. These eggs hatch in as little as four to eight days depending on temperature, so timing is crucial. If you catch them early, you can gently rub them off with your fingers or use a damp cloth to remove them before they hatch into hungry caterpillars.

Once hatched, the caterpillars waste no time. The Large White caterpillar is yellow-green with bold black spots and fine hairs and can grow up to 4 cm. The Small White caterpillar is pale green, velvety, and less conspicuous. The Cabbage Moth (Mamestra brassicae) adds another layer of trouble: its caterpillars are dull green or brown and often hide during the day, feeding mostly at night. These larvae go through multiple moulting stages (instars), each time growing larger and more voracious.
During this feeding frenzy, the damage to your plants can be extensive. Young brassica seedlings are particularly vulnerable and can be completely stripped of leaves in just a few days. Even larger, established plants suffer from tattered leaves, reduced photosynthesis, and in some cases, stunted growth or secondary infections.
Caterpillar droppings—small, dark green or black pellets (or frass) —often collect in leaf crevices or inside tight

cabbage heads, making the crops messy and more susceptible to rot, particularly in the damp UK climate.
So, how do you stop these miniature munchers from ruining your hard work? Prevention and vigilance are key. The earlier you act, the better your chances of saving your crop. Some of the best non-chemical control methods include:

• Fine mesh netting: Use 0.8 mm mesh over hoops to stop butterflies from reaching your plants altogether. Ensure the netting is well-secured at the ground to prevent sneaky access.
• Regular inspections: Check the undersides of leaves every few days for eggs or tiny caterpillars, especially in warm weather when butterflies are most active.
• Hand-picking: Remove caterpillars and eggs by hand and drop them into a bucket of soapy water—oddly satisfying and highly effective. Too squeamish to drop them into soapy water then just re-home them as far away from your brassicas as possible!
• Encouraging predators: Birds, beetles, and parasitic wasps are natural allies in the fight against caterpillars. A wildlife-friendly garden invites these helpful hunters.
• Companion planting: Try nasturtiums, which lure butterflies away from your brassicas, or strongly scented herbs like thyme, dill, and sage, which can confuse or deter pests.
• Neem oil spray: Derived from the neem tree (Azadirachta indica), neem oil acts as a natural insect growth regulator. It disrupts the caterpillars’ ability to feed and moult by interfering with their hormones. Spray it on the leaves (especially the undersides) in the early morning or late evening, and reapply after rain. It’s best used when you first spot eggs or tiny caterpillars.
• Nematodes: Beneficial nematodes enter caterpillars through their mouth, anus, or breathing pores (Gross I know, but effective) Once inside, the nematodes release a specific type of bacteria that is lethal to the caterpillar. The nematodes then feed on the caterpillar’s body and reproduce, further spreading the infection. These nematodes are harmless to plants, humans, pets, and other beneficial insects like bees and ladybugs, making them a safe option for pest control. My top tip here is don’t spray too much oil onto young seedlings, just a smidge, you don’t want to suffocate very young growth.

One of the best defences is early detection. Make it part of your routine to lift leaves and look very closely, especially in warm spells when butterflies are most active. Remember, even tiny seedlings aren’t safe—caterpillars can completely destroy young plants before you even notice they’re there. Established plants can usually survive some damage, but yields and quality will suffer if infestations are heavy.
Creating a caterpillar-unfriendly environment doesn’t mean going to war with nature. Quite the opposite—inviting nature in can help restore balance. Birds such as blue tits, robins, and sparrows feast on caterpillars, especially if you provide feeders or nesting boxes nearby. Ground beetles and parasitic wasps also help keep caterpillar numbers down. By fostering biodiversity in your garden with flowers, wild corners, and insect hotels, you’ll increase the odds that predators outnumber pests. Even with good biodiversity, nothing beats checking regularly by hand and removing the little beasties.
In the end, a bit of vigilance, a watchful eye, and some physical barriers will all go a long way. There’s no need for harsh chemicals when nature already offers elegant solutions. Whether you’re growing kale in a raised bed or a few cabbages in the allotment, keeping on top of caterpillars is simply part of the rhythm of the gardening year. With a little effort, your brassicas can thrive—hole-free and delicious, just the way they should be.

One last piece of advice – rinse you brassicas under the tap prior to cooking. I can’t tell you the amount of times I’ve started cooking cabbage leaves only to see a boiled caterpillar float to the top of the pan. Gross I know. 

Let me know in the comments how you deal with caterpillars in your growing area.

If are a beginner gardener you might find these blog posts helpful How to Start Your First Organic Vegetable Garden or Labelling: simple but vital

Thanks for reading, I hope it was helpful.

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