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August 13, 2007

We have had several reports of aphids on trees from around the state. Aphids have been reported on yellow poplar (tulip tree, Liriodendron), sweet gum, red maple, and swamp white oak. There are about 50,000 aphid species worldwide, and most of them are somewhat host specific. An aphid species is likely to feed on only one tree species or a few closely related species during the summer. This reduces the likelihood of an aphid species spreading to other plants in the landscape. It also makes it unlikely that garden plants or other plants in the landscape are a source of infestation to trees and shrubs.

Aphids on tree leaf.

Aphids overwinter as eggs, typically on a plant other than the one where they are common during the summer. These eggs hatch into all wingless females that feed on the winter host and give birth to more aphids. Usually after two to three generations on the winter host, the aphids mature into winged adult females that fly to the summer host. On the summer host, up to 30 generations of aphids are produced, all of them being females that give birth to living young without fertilization. Generations are commonly produced weekly, resulting in huge populations building up.

Aphid nymphs in crowded conditions are frequently bumped and jostled by nearby aphids. These aphids will mature into wingless females that give birth to nymphs that will develop wings as they mature into adults, allowing them to fly to new, less infested hosts. Lengthening nights towards the end of the growing season cause females to give birth to nymphs that will mature as winged males and females that must mate to reproduce. The aphids fly to their winter host, where the females lay eggs that overwinter.

Aphids are soft-bodied, pear-shaped insects. There are many species of aphids, with adults ranging from pinhead-sized to 1/4 inch long. On trees, most are closer to pinhead-sized, but a common aphid on willow is one of the largest aphid species. Aphid species vary in color, with green being most common. Even a population of one aphid species on a plant may vary in color, apparently depending on factors such as temperature, crowding, and food quality. Aphids tend to have long, spindly legs and long, slender antennae.

Towards the posterior end of the abdomen, they have one pair of cornicles. Cornicles are elongated, tubelike structures on many aphids, although in other speciesthey occur as merely bumps. They are commonly dark-colored to black but are the same color as the rest of the body in some species. Cornicles are used to expel pheromones, externally liberated chemicals that affect other members of that species or other animal species. Alarm pheromone is commonly liberated by an attacked aphid to warn others.

Aphids feed on the sap of plants, typically tapping a phloem vessel with their piercing–sucking mouthparts. Aphids take in huge quantities of sap but have a digestive shunt that removes the nitrogen and some of the water from the sap, bypassing most of the sap around to the anus. Insects tend to have high nitrogen requirements, and plant sap is very low in nitrogen. Research has shown that many aphid species increase dramatically in numbers on nitrogen-fertilized plants. Associated research has shown that reducing nitrogen fertilization keeps aphid populations smaller. Reducing nitrogen fertilization of plants is an important aphid-control tactic in greenhouses and other situations.

The concentrated sap or light syrup substance called honeydew is exuded from the aphids’ posterior. Honeydew is highly sought by ants, parasitic wasp adults, and other insects as a food source. It is so highly valued by ants that some ant species farm aphids, moving them to uninfested areas of plants, moving them to other plants, and even carrying them underground into their nests for the winter and then bringing them out in the spring to place them on the proper plants. Perhaps you thought that only humans maintained livestock.

In the western United States, argentine ants are so aggressive in protecting their aphid “cows” that they kill and otherwise eliminate natural enemies that attack aphid infestations. In those areas, ant control is frequently needed to control the aphids. In Illinois, the common ants associated with aphids, such as carpenter ants, pavement ants, and acrobat ants, do not defend the aphids enough to seriously hamper control.

Multicolored Asian lady beetle adult.

Aphids used to be a common problem in Illinois, but large numbers of aphids on trees almost ceased with the arrival of the multicolored Asian lady beetle throughout Illinois in the mid 1990s. The multicolored Asian lady beetle is the one that bites people and enters buildings in huge numbers in the fall. It is primarily an arboreal species, its orange and blue larvae that look like tiny alligators up to 1/4 inch long being common on tree foliage. The common native lady beetles in Illinois tend to be found primarily on herbaceous plants, which perhaps should be expected in a prairie state.

Multicolored Asian lady beetle larva.

About 2 weeks after they become numerous, aphids that are exposed on the leaves and stems are usually controlled by natural enemies, making insecticide application unnecessary in most cases. Not only does the multicolored Asian lady beetle feed on aphids, but so do other lady beetle species, lacewings, syrphid flies, parasitic wasps, and other insects.

It is more common to control aphids due to their sticky honeydew coating sidewalks, cars, lawn furniture, and other objects under infested trees than to control tree damage. Aphids tend to feed near the tips of branches on the young leaves and stems. This feeding can cause the leaves to wrinkle, curl, twist, or become distorted in other ways. Similarly, young, green stems may curl or twist due to aphid feeding. If needed, contact insecticides such as insecticidal soap, summer spray oil, and pyrethroids are effective. The systemic insecticide, imidacloprid (Merit), is also effective against aphids.