Aphids on Plants: Causes, Treatment & Prevention

Symptoms

Here are the signs, from the ones you see first to the ones that take longer to appear. Aphid damage often shows first as curled or distorted leaves on the affected plant. The leaves and shoots may also turn yellow and look stunted. Look for sticky honeydew on the leaves, which often turns black as a sooty mold fungus grows on it. That black sooty fungus can discolor both fruit and leaves. Aphid-borne viruses can cause mottled, yellowed or curled leaves and stunted growth. Gall formation on leaves or stems is another sign. Lettuce plants may wilt, and a few may die. Carrot plants may suffer tearing off of the tops. When root-feeding aphids infest the roots for several years, you may see overall tree decline. Aphids are recorded on trees such as apple and pear, on vegetables such as lettuce and carrot, and on ornamentals such as rose and oleander. Virus symptoms and direct feeding damage can both appear on the same plant, since the symptom list includes both. Distorted growth, yellowing, honeydew and galls are all recorded aphid symptoms.

Causes

Several conditions help aphid numbers build up on a plant. High levels of nitrogen fertilizer favor aphid reproduction. Warm weather in the range of 65 to 80 °F (18 to 27 °C) is also listed as a cause. Ants protect aphids from natural enemies. Natural enemy populations also build up later than aphid populations do. Broad-spectrum insecticides are toxic to natural enemies, and using them is listed as a cause of aphid problems. A dense inner canopy shelters aphids on the plant. Nearby weeds such as sowthistle and mustards can act as a reservoir for aphids. The list mixes feeding practices, weather, other insects, spray choices and the plants around your garden. Because ants protect aphids from natural enemies, a band of sticky material around the trunk is used to block ants from climbing. Slow-release fertilizers, such as organic fertilizers or urea-based time-release formulations, are preferred over fast-release nitrogen fertilizers. Limiting broad-spectrum insecticides matters because they also kill natural enemies.

Diagnosis

Sticky honeydew on the leaves, often with black sooty mold growing on it, is one of the recorded aphid symptoms. Galls on leaves or stems are another recorded sign, as is curled or distorted growth. Yellowed, stunted leaves and shoots are recorded too, but yellowing and stunting appear in the virus symptoms as well. Aphid-borne viruses cause mottled, yellowed or curled leaves and stunted growth, so these signs can come from a virus rather than from the insects feeding directly. Your conditions can also help you judge the problem. Heavy nitrogen fertilizer, warm weather between 65 and 80 °F (18 to 27 °C), ants on the plant, or weeds such as sowthistle nearby all match the recorded causes. A dense inner canopy is listed as a place aphids shelter, so the center of the plant is worth checking. Root-feeding aphids are different, since their damage shows as overall tree decline after several years of root infestation. Ant activity on the trunk or stems is a clue, because ants protect aphids from natural enemies. None of the facts on this page name a look-alike problem, so the signs above and the conditions in the causes are the only checks this page can offer.

Treatments

Cultural steps come first. Prune infested leaves and stems. You can knock aphids off by shaking the plant or spraying it with a strong stream of water. Use slow-release fertilizers, such as organic fertilizers or urea-based time-release formulations, rather than fast-release nitrogen fertilizers. Silver-colored reflective mulches reduce transmission of aphid-borne viruses, and protective covers also prevent that transmission. A band of sticky material around the trunk blocks ants from climbing and protecting aphids. Naturally occurring predators and parasites can be relied on, especially in garden and landscape situations. Parasitic wasps are among the most important natural enemies of aphids. Fly and lacewing larvae and ladybird beetles are predators too. Fungal diseases that kill aphids can help under humid conditions. You can release lady beetles, but high numbers are required for control. Insecticidal soaps and oils kill aphids mainly by smothering them, and they are for when numbers are high and other methods are not effective. Summer horticultural oils give good control of various aphid species. Limit use of broad-spectrum insecticides, because they also kill natural enemies. Systemic insecticides such as imidacloprid are used for aphid management mainly on woody ornamentals, and they should not go on plants in bloom or before bloom because of the impact on pollinators. A single spray of a product containing imidacloprid is used for aphids on fruit trees. The product label is the legal instruction and must be followed for the crop and site being treated.

Prevention

Prevention comes down to the conditions listed as causes. Feed with slow-release fertilizers, such as organic fertilizers or urea-based time-release formulations, rather than fast-release nitrogen fertilizers, since high nitrogen favors aphid reproduction. Keep natural enemies in place by limiting broad-spectrum insecticides, which are toxic to them. Parasitic wasps are among the most important natural enemies of aphids. Keep ants off the plant with a band of sticky material around the trunk. Silver-colored reflective mulches and protective covers reduce transmission of aphid-borne viruses. Weeds such as sowthistle and mustards nearby can act as a reservoir for aphids. A dense inner canopy shelters aphids. Warm weather of 65 to 80 °F (18 to 27 °C) is also listed as a cause.

Plants most often affected

Aphids are recorded on apple, pear, hawthorn, cotoneaster, plum, pyracantha, ash, crape myrtle, fir, pine, spruce, cedar, hackberry, oleander, milkweed, rose and tulip tree. The list also includes pepper, spinach, tomato, carrot, lettuce, corn, potato, squash, cucumber, pumpkin, melon, bean, beet, chard, bok choy and citrus.

Aphids on Plants

At a glance