In summer I don’t worry about aphids, the little sucking insects often called plant lice. Aphids have plenty of natural predators, and a low level of aphids helps balance insects by providing beneficial species with food. But then autumn comes, and super-talented cabbage aphids emerge as a familiar nuisance. When you see clusters of gray-green aphids on kale, broccoli, cabbage, or Brussels sprouts, you’re looking at cabbage aphids (Brevicoryne brassicae), one of the weirdest pests in the vegetable garden.
Males often don’t appear until late summer, when they are needed to help produce fertile, overwintering eggs. Until then, females pop out clones of themselves when conditions are good, leading to the fast formation of dense colonies of daughters, and their daughters, and so on, sometimes for 30 or more generations.
Tiny braconid wasps are willing helpers with cabbage aphid control, but ladybird larvae and other aphid eaters avoid this species because their bodies accumulate bitter mustard compounds as they feed on cabbage family crops. Some researchers have called them “walking mustard oil bombs”. Of equal importance is the fact that cabbage aphids simply outlast their predators, often surviving well into winter. The Brussels sprout plant shown below is frozen solid, but the cabbage aphids on it will resume feeding as soon as it thaws.
Cabbage Aphids on Broccoli
Earlier this fall, I was alarmed when I cut what looked like a good head of broccoli, only to find every crevice gummed up with cabbage aphids. You can’t really clean aphids from broccoli, so I gave the head to the chickens and assumed the remaining plants would be infested, too. To my surprise the broccoli that matured later was aphid free, but this is not unusual. Research has shown that tall-growing broccoli plants that mature early attract many more cabbage aphids, so staggering planting dates of different varieties can reduce aphids on broccoli in the garden.
Controlling Aphids on Kale
Plant height seems to influence cabbage aphids on kale, too, with tall, older plants much more likely to be fed upon compared to smaller, younger ones. Clip off badly infested kale leaves and immediately dispose of them in an active compost pile. Without a live plant host, they will soon perish. Follow up with a botanical spray (like those described below), insecticidal soap, or a light horticultural oil. Don’t waste your time with a strong spray of water with cabbage aphids, which can meld their waxy coatings with those of kale leaves so they become a water-resistant mass.
Sprays made from many plants have been investigated as possible cabbage aphid controls, and it appears that both peppermint and lantana have repellent properties. Spritzing plants with a strong mint tea made sticky with a few drops of dishwashing soap is pleasant duty that has few environmental repercussions, but some lantana extracts have been found to be potent pesticides that impact numerous insects. Scientists in India found that sprays made from citronella or patchouli oils repelled more cabbage aphids than mint, but here it might be easy to make a mistake and end up with over-fragranced greens.
Cleaning Up After Cabbage Aphids
Cabbage aphids can harbour and transmit more than a dozen viruses, so limiting how many survive winter is a smart strategy. Where winters are cold, cabbage aphids overwinter as tiny black eggs hidden in plant debris, cabbage stumps, or other hiding places, where they are cold hardy to at least 5°F (-15°C). Eggs or resting adults can be present in woody old stalks of kale, cabbage or Brussels sprouts, which is a good reason to pull up plants you don’t need, and cut them into pieces before composting them. To keep aphids from finding younger, more vigorous plants that you do want to keep through winter, simply cover them with horticultural fleece suspended on hoops.
Keep Notes on Resistant Varieties
Genetic resistance to cabbage aphid feeding has been found in several rustic cabbage cousins that are too bitter to eat, but you may notice that some refined varieties are less bothered than others. In my garden, I have never seen an aphid outbreak on compact, smooth-leaved ‘Copenhagen Market’ cabbage, while big crinkly savoy cabbages seem to be easy targets. Collards are rarely bothered, while kale often looks like aphid candy. Keeping notes on aphids encountered on different varieties can yield useful information for cabbage aphid control in your one-of-a-kind garden.