Aphids, also known as plant lice are soft-bodied, sucking insects. They feed on plant sap and excrete a sugary honeydew that attracts ants and creates the conditions for sooty mold, a type of fungus (saprophytic) that feeds on decaying organic matter.

There are several aphid species, all belonging to the insect family Aphididae, that are capable of attacking any type of vegetation. Aphids that pose the most serious problem to Wisconsin vegetable production include the green peach, melon, and potato aphids.

Appearance

All aphids are soft-bodied and pear-shaped with a pair of cornicles, or little horns, projecting from the rear end of their abdomens. Adult aphids may or may not be winged. Some of the more common species are listed below.

Symptoms and effects

Reduced plant vigor, stunting and deformed plant parts are common symptoms of aphid infestations. In some cases, it’s the production of honeydew or presence of sooty mold that alerts the gardener to an aphid outbreak. Most importantly, aphids are excellent transmitters of several viral diseases, such as the mosaic viruses, that cause leaves to shrivel and that infect a wide range of hosts. In some cases, it’s the appearance of virus symptoms that indicate aphid activity. Learn more about insect-vectored plant pathogens here.

Life cycle

It is difficult to generalize the life cycle of all aphids because of the diversity of their life habits, which can range from single to multiple hosts. One of the unique characteristics of aphids that sets them apart from all other insects is their ability to bear live young.  Aphids overwinter as eggs on a perennial host. In spring, the eggs hatch and the aphids migrate onto their summer host when it becomes available. The female aphids can then reproduce without mating and will hold the eggs in their bodies to give birth to live young. By eliminating mating and egg laying, aphids have successfully shortened their life cycle and thereby increased their reproductive capability. Throughout the summer, wingless females predominate. However, winged forms may arise when populations become too large for the available food source. In late summer, in response to the shortened daylight hours, wingless females and males are produced for the purpose of mating and laying fertilized eggs that will survive adverse winter conditions.

Scouting

Look for “hot spots” of aphid activity scattered throughout the field. Because of the spotty nature of infestations, look for aphids on a number of plants in several areas. Examine the terminals of 15 consecutive plants or sample units and rate the plants as infested or uninfested. Given the huge reproductive potential of aphids, an infestation level of 5%-10% indicates a potentially damaging infestation. Repeat checks at weekly intervals to determine the need to treat.

Management

Non-chemical control

Predators such as lady-bird beetle adults and larvae, green lacewing larvae, syrphid fly larvae and several parasitic wasps all help reduce aphid numbers. Heavy rains help dislodge aphids from the plant and, during periods of high humidity, fungal diseases may greatly reduce populations. The remarkable reproductive capacity of the aphid normally overcomes the effects of natural controls in spring when cool temperatures hinder the development of natural enemies. These natural controls most often catch up in the warmer weather of summer and fall.

Chemical control

Treat crops with an insecticide when threshold levels have been reached. One of the problems associated with the control of green peach and melon aphids results from their resistance to several insecticides. In particular, aphids have shown resistance to organophosphate insecticides.

Refer to the UW-Extension publication Commercial Vegetable Production in Wisconsin (A3422) for a list of registered insecticides and management recommendations.

Gallery

Asparagus aphid (Brachycorynella asparagi) Appearance:Small, bluish-gray in color, powdery, with small cornicles Host plant: Asparagus Description: The asparagus aphid is a recent import from Europe that causes a brooming distorted growth of the ferns of asparagus. It is bluish gray in color, and the cornicles aren’t readily apparent. Normally only isolated plants are affected and can be spot treated.

Bean aphid (Aphis fabae) Appearance: Dark green to sooty black Host plants: Artichoke, asparagus, bean carrot, corn, lettuce, parsnip, rhubarb, spinach, squash Winter hosts: Euonymous & Viburnum spp.

Cabbage aphid (Brevicoryne brassicae) Appearance: Gray-green with a powdery, waxy covering Host plants: Broccoli, Brusssels sprouts, cabbage, collards, kale, kohlrabi, radish Overwinters as eggs on host plants Cabbage aphids are gray in color and live in closely packed groups. Heavy infestations cause leaves to curl and may prevent head formation. Aphid damage is most serious on young cabbage and in the seedbed. Once cabbage heads form, this insect is difficult to control.

Corn leaf aphid (Rhopalosiphum maidis) Appearance: Bluish green Host plant: Corn No winter host. Migrates from the south annually Resources: http://entoweb.okstate.edu/ddd/insects/cornleafaphid.htm Wikipedia

Green peach aphid (Myzus persicae) Appearance: Yellowish-green with 3 dark lines on their backs Host plants: Beet, celery, cole crops, cucurbits, lettuce, pepper, potato, spinach, tomato Winter hosts: Cherry and peach Description: The green peach aphid is the most destructive and insecticide-resistant aphid in Wisconsin. Many crops are attacked including greenhouse transplants of pepper, tomato, and cabbage, along with beet, carrot, broccoli, Brussels sprouts, lettuce, eggplant, and potato. Green peach aphids are 1/8 inch long, yellowish green, peach, or dirty red in color, and can be found on the undersides of leaves. This aphid is a very efficient vector of many virus diseases. Scouting is usually done by examining the undersides of leaves and looking for aphid activity. In potatoes, remove 25 leaves per sample from the lower half of 25 different plants, with at least 10 sample sites per field. Treat if more than 10 aphids are found per 100 leaves in seed fields or more than 30 per 100 leaves in table stock or processing potatoes.

Melon aphid (Aphis gossypii) Appearance: Pale yellow to brown or nearly black with black cornicles Host plants: Asparagus, bean, beet, celery, cucurbits, okra, spinach Winter hosts: Many Description: Melon aphid is a small dark aphid that can be found building up on the undersides of leaves of cucumber, squash, pumpkin, and melon. They can produce up to 10 generations per year and cause infested plants to yellow and wilt. Low numbers are most often controlled by natural enemies.

Pea aphid (Acyrthosiphon pisum) Appearance: Pale green with black legs Host plant: Peas Winter host: Alfalfa

Potato aphid (Macrosiphum euphoribae) Appearance: Relatively large, pink to green in color, with long slender cornicles Host plants: Potato, tomato, eggplant, sunflower, peppers, peas, beans, apple, turnip, corn, sweet potato, asparagus, clover, roses Overwinter as eggs on perennial host plants Description: Potato aphids are larger than green peach aphids and come in both red and green. They attack eggplant, tomato, and potato and are most often found on young, actively growing tissue. To sample for potato aphids, remove leaves from the terminal parts of 35 plants and count the number of aphids. Repeat in at least 10 locations per field. When aphid counts exceed 20 per 100 in seed potato and 50 per 100 leaves in table or processing potatoes, control measures are suggested.

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