Autumn Aphid Invasion
While many different kinds of aphids can appear on different types of plants during most times of the year, cabbage aphids tend to appear on plants in the Brassica family (kale, broccoli, collard greens, cabbage, and brussels sprouts) in early fall. At this time, temperatures start to drop and natural aphid enemies such as ladybugs and syrphid flies tend to disappear. The cabbage aphids (also known as plant lice) will usually be seen in clusters, as shown below:
Why do aphids appear? Like most insects, aphids show up when there is an appealing food source. They’re able to cause an infestation and reproduce rapidly when the food source is easily accessible, such as when plant defenses are down. This can happen during a period of drought or when the plants are already weakened due to other pest damage.
Aphids use their proboscises to penetrate plant tissue and suck up nutrient-rich phloem (sap). Phloem is particularly nutrient-rich and high in nitrogen when plants are over-fertilized, rapidly growing in spring, and when plants begin senescence. During senescence, nitrogen stores in the leaves and stem re-mobilize to help the plant mature before a hard freeze sets in, or to help perennial or biennial plants store resources for next year’s growth. Many of our edible brassicas produce seed on a biennial schedule, going dormant in winter of the first growing season, and re-emerging in spring to produce flowers and seed.
As brussels sprouts mature, lower leaves turn yellow as nitrogen is remobilized from them and into the developing sprouts. As the yellowed leaves die and fall off, the sprouts are exposed, making them easier to harvest. Other plants, such as cabbage, collard greens and broccoli tend to concentrate nitrogen sources in the growing tips, usually in the top center of the plant. Extra nitrogen in the sap of areas such as the growing tips or developing sprouts makes them more appetizing to aphids.
Although healthy plants have some natural defenses against aphids, such as a thick leaf cuticle, plants that may be experiencing drought or a previous cabbage butterfly larva infestation have weaker natural defenses, making it easier for aphids to insert their proboscises into the leaves. Although ladybugs and other beneficial insect predators can come to the rescue and consume aphids during warmer months, in late fall a gardener will usually need to resort to other tactics.
In this case, the aphid clusters can be easily removed with the swipe of a gloved thumb. Strong sprays of water can also knock off the aphids, and garlic sprays are particularly effective. Examine the plants every few days – it may take more than one gloved thumb or spray to remove the aphids.
Pest control doesn’t stop with removing the pests. Help boost your plant’s natural defenses by treating whatever problem weakened them in the first place – watering regularly, mulching to help conserve moisture and stabilize soil temperatures, diligently removing summer pests, and making sure they are planted in an area to receive adequate sunlight.