The sight of a cabbage white butterfly, hovering over you’re your brassica plants such as kale, broccoli and cauliflower, strikes panic in every vegetable gardener because once the eggs turn into caterpillars they devour the foliage and ruin the crop. There are very few natural predators, or parasitoids, and many more white butterflies are flying in from fields of oilseed rape so they’ve become more of a problem in recent years.
Recognising the real villain - the large cabbage white
The worst pest of all is the large cabbage white butterfly (Pieris brassicae), because the females lay clusters of 30 - 50 bright-yellow eggs end on end, rather like a series of small rugby balls. The eggs are always on the undersides of the foliage and quite easy to see. These larger butterflies, which are about the same size as a peacock butterfly, have brilliant-white wings with black tips on the forewings, extending down to the wing edge. Females have an additional two black spots on their forewings, and these are not present in males. The undersides of both are a creamy-white with two spots.
A lesser threat, the small cabbage white butterfly
The small cabbage white (Pieris rapae) looks like a smaller version of the large white, but it’s less devastating because the female butterflies lay their pale greenish yellow eggs singly. However, your lone green caterpillar then heads straight into the middle of a cabbage where it does rather a lot of damage and leaves its calling card. The caterpillars are quite hard to spot.
Cabbage white predators
You have to manage cabbage white butterflies yourself because they do not have many predators. Nothing much eats them because as the caterpillars reach full size their bodies fill with mustard oil they’ve ingested from the brassicas they taste very unpalatable to predators.
The cabbage white has one parasitoid in Britain. It’s a tiny parasitic wasp called Cotesia glomerate. It lays eggs in the emerging caterpillars just as they come out of the egg case. The caterpillars grow to full size, but eventually lots of sulphur-yellow cocoons develop inside each caterpillar, although this doesn’t seem to deter the caterpillars from eating your cabbage leaves!
Towards the end of summer, these now poorly caterpillars head off and shortly afterwards the caterpillar’s body ruptures and lots of cocoons spill out of the body. The cocoons are held in a mass by a sticky substance that looks rather like sulphur-yellow cotton wool. You may see this bright-yellow fluffy ‘cotton wool’ in sheds and greenhouses over winter. If you do, leave it in situ because they will turn into next year’s parasitic wasps.
Using social wasps against cabbage whites
I have seen teams of social wasps cutting fully grown caterpillars up into sections and taking them back to their nests. It proves that for most of the year wasps are extremely useful predators. It’s said that the wasps from one nest can collect up to 3k of pests in one season. They should not be targeted and numbers of large social wasps are in decline.
Spraying cabbage white butterflies
Spray young caterpillars as they hatch with Bacillus thuringiensis, a naturally occurring bacterium found in the soil. This is toxic to a range of insect species particularly caterpillars, cabbage looper and tomato hornworm, but allegedly is harmless to birds, fish, mammals and gardeners. It comes in a powder or solution and you water it on using a watering can. Young caterpillars also dislike garlic sprays. However, these kill across the board and see off a lot of beneficial insects in your garden.
Netting against cabbage white butterflies
Netting is the most effective way of keeping the butterflies away from brassica foliage, although small whites seem to be able to do a dam buster act and drop their eggs from a height with some success. Harrods Horticultural sell soft mesh butterfly netting that’s perfect at keeping them away. Small pots upended on canes are the best way to support your net, but there needs to be a twelve-inch gap (30 cm) between the plants and the netting all the way round.
August is generally the worst month and butterfly netting is the best solution. Keep it there all year, for as the caterpillars disappear the pigeons often take over.
Butterflies will find any weak spots in your netting. If you see them underneath clap your hands together and squash them!
Check the undersides of the leaves regularly for eggs, or tiny caterpillars, and cut away any clusters and destroy them. A big boot helps.
The butterflies will continue to fly right through autumn, so be vigilant. I have picked caterpillars off plants on Boxing Day!
If you’re on an allotment, netting is less useful because as the caterpillars denude one plot, they march through the netting and eat the brassicas on your plot.
Collecting caterpillars up
Often the last resort, but it does work. Be gentle, if the caterpillars suspect they’re under attack they drop to the ground.
Using decoy plants
Cabbage whites often lay their eggs on spring and summer flowering plants when there are no cabbages about. The following members of the brassica family act as an early-warning system.
Nasturtium (Tropaeolum majus)
These frost-tender and very colourful annuals have round green leaves. Sow in early May and keep an eye out for caterpillars.
Sea stock (Matthiola incana ‘Alba’)
The very fragrant white flowers of this sun-loving sea stock flower in May. The rosettes of grey foliage are a winter feature and the foliage is often targeted by larger cabbage white caterpillars.
Dame’s violet (Hesperis matronalis)
Another large cabbage white caterpillar favourite, with a mixture of silver-white, or mauve or white flowers. Highly fragrant and the long pods can be a little too generous with their seeds.
Honesty (Lunaria annua)
This biennial, famous for its penny-like papery seedpods can also be attacked by cabbage white butterflies, although they don’t seem as fond of this.
Interested in companion planting? Read our guide to choosing companion plants for the vegetable garden.
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