Aphids are small plantlice that feed on plant parts. There must be about 300 species in midwestern US, each of them having a preference for a specific family or genera of plants to feed on, and they are also specific to a certain part of the plant, such as roots, or stems, or twigs, or leaves, or flowers, or fruits, or some combination of those parts.
Aphids are characterized by tiny size (1 to 5 mm), a pair of tubular appendages located near the tip of the abdomen, antennae with 4 to 6 segments, and sometimes displaying transparent membranous wings. Body colors may be a single color, or spotted or striped, and some species have different head and/or antennae colors from the body. Many species produce honeydew, and are therefore tended and protected by ants. The recreational gardener is usually plagued with aphids, some of which must be controlled, and some of which cause no serious harm. It is convenient to describe aphids by their apparent color for convenience, however, there are so many species, that color alone cannot lead to accurate identification.
Dark blue green with green head, winged or wingless: This is probably the corn leaf aphid which can be found in large colonies on the tassels, or ear tips, but mostly in the leaf whorls of corn plants. This aphid can usually be ignored as the damage is minimal, and the corn is usually harvested before the plant is seriously weakened.
Greenish yellow with dark spots on top of the abdomen, and black appendages: This aphid can be found on a great variety of plants, including melons, cucumbers, vegetable plants, ornamental plants, and weeds. This aphid has not been much of a problem in my garden.
Green or pinkish with black antennae, head, thorax, and appendages: The rose aphid is a serious pest and must be controlled with systemic pesticides. This is one of the reasons that I don't grow roses.
Pale green with long black antennae, marked with brownish or blackish on the thorax: This is the grain aphid and is usually found on grasses, cereals, and small grains. The grain aphid has not been much of a problem for me.
Pale to dark green with stripes in the summer and pinkish in the fall; winged individuals are dark brown with a brown patch on an otherwise yellow abdomen, and having two pimplelike bumps on the front of the head: This is the green peach aphid which can be found on all types of fruit, vegetable, and ornamental plants. Perhaps the most common aphid in my garden feeding on the growing tips of apple trees, etc. Damage is not serious, probably because I have a large population of ladybugs and lacewings to feed on them. Fortunately, this aphid is not present every season.
Carmine (dark red) aphids are usually seen feeding in large colonies on Silphium perfoliatum (cup plant): this aphid appears to have a very high reproductive rate, but a short life cycle, and there is no serious damage.
Entirely black aphid colonies have been found on my Levisticum officinalis (Lovage) and on Viburnum opulus (Highbush cranberry). I usually cut off affected stems and discard with the trash (not in the compost pile).
Gold aphids are the most voracious and persistent that I have seen. They only colonize on Asclepias species (milkweed), but are quite capable of destroying the plants without close attention. Plant parts must be cut off and discarded. Some aphids will drop to the ground and climb back onto remaining stubs further weakening the plants. Standard soap solutions do not affect these critters, and if you grow any of the milkweeds you may have to treat them like roses.
Pink aphids regularly appear on the growing tips of tomato plants. These can usually be controlled by washing them off with a strong water spray. I have also seen the American goldfinch feeding on them, perhaps to feed their babies.
Very pale green aphids: This is the tansy aphid of which I am quite fond, because ladybugs like them more than any other. In fact, I grow a 4 x 8 ft patch of Tanacetum vulgare just for this purpose. In years in which this aphid finds my property, the lady bug population grows exponentially.
White aphids covering themselves with white or gray threads that make them look like wool or cotton are called woolly aphids. These feed on trees mostly, and some other woody plants.
The first line of defense is to wash off aphid colonies with a strong water spray. Usually, they do not reappear in the same places. This treatment is suitable for some species.
For more persistent species, a fine spray of soapy water solution made by mixing a teaspoon of dishwashing soap with a quart of water will interfere with their ability to breathe through pores in their abdomens.
Stronger and more persistent species will not succumb to the above treatments. These must be treated with a systemic poison, or with a spray containing malathion. I prefer to cut off and discard the colonies, but in certain years, only chemical treatment will save your plants.
Aphids which hide in curled up leaves are usually best treated systemically, but watch for the presence of ladybugs. There is no real advantage to destroying aphid colonies that are supporting ladybugs and their larvae.
Overwintering eggs of fruit tree aphids can be destroyed with the application of a dormant oil spray after fall leaf drop has been completed.
To summarize, the Illinois gardener should not be overly concerned about aphids, except for the gold ones. Observe the level of damage, and give the treatments described above in order from weak to strong as necessary.
For a key to the identification of aphids in Illinois, see Hottes, Frederick C., and Theodore H. Frison. 1931. The plant lice, or Aphiidae, of Illinois. Bulletin Illinois Natural History Survey. 19 (3): 121-447
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