They may be tiny, but aphids cause widespread damage to a variety of vegetable crops. Tomato plants can tolerate heavy populations of aphids before enough damage occurs to reduce fruit yields, but gardeners should begin aphid control for tomato plants early because they can weaken the plant over time. In addition, aphids can carry certain plant viruses that can harm tomato and other garden plants.
Several species of aphids attack tomato plants. The most common tomato plant aphids are the potato and green peach aphid. The University of Kentucky make this distinction between the two species: "Although similar in size, about 1/8 inch, these aphids vary in appearance. The potato aphid is pear-shaped and may be solid pink, green and pink mottled, or light green with a dark stripe. It has a long slender pair of tail-like appendages (cornicles). The green peach aphid is pear shaped and pale yellow to green in color. The cornicles are much shorter on this species."
Aphids are sucking insects, piercing the flesh of plants to suck out the sap. Populations tend to cluster on stems and under leaves. Over time, leaves may begin to yellow and curl. Plants may become stunted and the reduced leaf shade can cause sun scald on ripening tomatoes. Aphids can spread mosaic virus, a potentially serious disease that can cause, according to Cornell University, "spectacular" damage to tomato plants, "the most characteristic symptom being shoestringlike leaf blades."
Tomatoes planted near weedy, overgrown borders that contain mustard and sowthistle are more susceptible to aphid infestation. These common weeds are favorite aphid breeding grounds. High ant populations can promote high aphid populations--ants consume the excrement left behind by aphids and in turn, protect the aphids from predators. Other sources of aphid overpopulation include unusually cold spring temperatures and the use of nitrogen-rich fertilizers.
Chemical and Mechanical Control
Treat aphid-infested food crops like tomatoes with non-toxic controls such as insecticidal soaps, neem and oil sprays. These products kill the existing aphids on contact, on the day they are sprayed; there is no residual effect. If aphids are treated early, treatment is much more effective. When the infestation becomes severe, tomato leaves begin to curl. The curled leaves protect aphids from insecticidal sprays.
Beneficial insects like green lacewings, lady beetles, syrphid flies and parasitic wasps control aphids naturally. Many of these predators occur in nature, and some can be purchased and released. Adult and larval lacewings, lady beetles and syrphid flies actively feed on aphids, piercing the body and consuming the fluids. Parasitic wasps lay eggs inside the aphid, leaving behind a "mummy." Many of these predators feed on other undesirable tomato pests including caterpillars, scale insects and mites.