Cabbage aphids wreak havoc in the garden
Cabbage aphids (Brevicoryne brassicae) can wipe out a cabbage crop before it ever gets started. Native to Europe, this pest of cole crops is now found throughout the United States.
Like other aphids, cabbage aphids are small, teardrop-shaped, sap-sucking pests that can reproduce at an alarming rate. While soft-bodied cabbage aphids are actually grayish-green, they look powdery blue to grayish-white because of a waxy covering. Cabbage aphids are not difficult to see because they live in dense colonies that can cover stems, new leaves, and entire plants practically overnight. In our moderate climate, these pests produce live offspring year-round.
Cabbage aphids have an amazing defense mechanism. They produce an enzyme in their head and throat muscles which gets combined with defensive chemicals (glucosinolates) from their host plants to create an explosive chemical reaction. This reaction produces mustard oil. Unfortunately, this “walking mustard oil bomb” defense is particularly effective against ladybug larva.
Cabbage aphids feed on the youngest, most tender parts of new cabbage, broccoli, Brussels sprouts, kale, and cauliflower. These pests also eat the innermost parts of cabbage and Brussels sprouts heads. Large colonies can stunt or even kill young plants. Heavy aphid feeding causes leaves to curl up, providing the pests with a protective cover.
Prevention is key to cabbage aphid control. Row covers are an excellent way to protect young crops while they are getting established. Once aphids are seen you can often use a strong spray from the garden hose to dislodge them. If that doesn't work, insecticidal soaps can provide some control. Since some insecticidal soaps may be phytotoxic (meaning sunlight causes them to burn the plant), it is a good idea to apply them on a foggy day, especially for cabbage and Brussels sprouts.
Another way to make life more difficult for cabbage aphids is to remove any weeds in the mustard family from your property. Cabbage aphids hide out in the mustard and then return to your garden plants. Pesticides can be used as a last-ditch effort, but aphids are developing resistance to these chemicals—a potentially dangerous spiral.
Another problem with using pesticides against cabbage aphids is that those same chemicals also kill beneficial, predatory insects, such as lady beetles, parasitic wasps and syrphid flies (hoverflies). These helpful insects are natural predators of caterpillars, imported cabbage worms, diamondback moths, loopers and armyworms, which can cause other problems for your cole crops.
Monitor your plants every couple of days and be on the lookout for cabbage aphids!
by UC Master Gardener Kate Russell
Photo: South Valley Magazine
This article first appeared in the February 9, 2018 issue of the South Valley Magazine.