At this time of year, many aphid species feed on nursery and landscape plants, sometimes in large numbers. However, because aphids generally feed in exposed locations (including terminal growth and underneath leaves), they are susceptible to a variety of natural enemies such as predators and parasitoids. Predators (which may consume all or parts of aphids) include green and brown lacewings, lady beetles, hover flies, midges, bigeyed bugs, damsel bugs, soldier beetles, and blister beetles. In many cases, both adults and larvae (or nymphs) feed on aphids. Parasitoids or parasitic wasps that attack aphids include ichneumon and braconid wasps. Female parasitoids lay eggs into aphids; the eggs hatch into larvae that consume the internal contents. Eventually, the larva pupates and becomes an adult, which chews out an emergence hole. Parasitized aphids are swollen, brown to gray, and are called mummies.
Both predators and parasitoids can maintain aphid numbers below thresholds for causing plant damage so that an insecticide application is not warranted. An effective management strategy for controlling aphids without harming natural enemies is to spray plants with a hard stream of water. Although water doesn’t have any EPA registration number and is not (currently) considered a pesticide, it knocks aphids off plants. Ground or rove beetles attack aphids that fall off and land on soil, grass, or mulch.
The use of conventional insecticides to control aphids may not only kill their natural enemies but also significantly impact those of other plantfeeding pests, such as twospotted spider mites. As a result, spider mites may become a bigger problem than the aphids. This type of phenomenon is referred to as secondary pest outbreak.
It is also beneficial to attract or retain aphid natural enemies in landscapes by incorporating plants that produce pollen and nectar that serves as a food source for many aphid natural enemies. Plants that produce an abundance of pollen or nectar include Queen Anne’s lace (Daucus carota), coreopsis (Coreopsis spp.), coneflower (Echinacea spp.), goldenrod (Solidago spp.), and sweet alyssum (Lobularia maritima).
The presence of natural enemies provides “free” aphid control, so before spraying with an insecticide, check to be sure that existing natural enemies are not already “taking care of business.”