Insect Profiles: Aphids
Aphids, also known as plant lice, blackflies, or greenflies, and sometimes erroneously referred to as whiteflies, are small, soft-bodied insects that feed on the sap and other liquids of various plants. Aphids are members of the superfamily Aphidoidea. Thousands of aphid species have been discovered worldwide. In temperate regions, aphids are among the most destructive insects of plants.
Most plants are vulnerable to infestation by one or more aphid species. Some plants are more sensitive to the feeding of aphids than others. Many aphids function as vectors of plant diseases. Aphids can transmit diseases within seconds of infesting a plant. Healthy plants seldom incur significant damage from aphid infestations. However, persistent infestation over a period of years can reduce plant vigor. Stressed or weakened plants are more susceptible to invasion from other insects, and disease pathogens.
Distribution & Habitat
Aphids have a global distribution. They are most abundant in temperate regions, but appear in cooler locations as well.
Aphids infest a broad range of plants. Nearly every plant that grows in landscape and forested settings is vulnerable to one or more aphid species.
Aphids vary in size, extending between 1 and 10 mm in length. They have soft pear-shaped bodies with long, thin legs, and a set of antennae comprised of up to six segments. Most aphid species feature a pair of characteristic tube-like structures called cornicles that project outward from their posterior. These cornicles are a distinguishing trait that helps differentiate aphids from other small insects. Aphids feed using mouthparts called stylets, which are enclosed in a sheath referred to as the rostrum. Mature aphids typically lack wings. When host plants are weakened, or grow overly crowded, some aphid species will produce winged offspring. Aphids may be black, brown, green, red, or yellow, depending on the species. A few aphid species secrete a waxy white or gray substance over their bodies, causing them to appear wooly.
Aphids can produce dozens of generations each year. Infestations occur when small numbers of winged aphids fly to new hosts, where they deposit several wingless aphids on the most tender foliage. The wingless aphids remain to feed, while the winged aphids depart in search of other plants to infest. The immature aphids, called nymphs, insert their mouthparts into the foliage to feed, voraciously consuming the sap and other plant liquids. As they feed, the nymphs rapidly increase in size. The nymphs pass through four instars prior to maturation. The nymphs mature in seven to ten days, whereupon they may begin producing offspring.
Most aphids are females that reproduce asexually. Some species produce males and females that propagate sexually. In fall or winter, aphids may lay eggs on an alternate host, where they will overwinter. When temperatures are warm, aphids can develop from nymphs to adults in a span of seven to eight days. Mature aphids can produce 40 to 80 nymphs in a week, causing aphid populations to proliferate rapidly. Depending on the initial population, hundreds to thousands of aphids may be born on a plant within a few weeks. When populations become too dense, or a plant is stressed, the adults give birth to winged forms, beginning the cycle anew.
Symptoms of Infestation
Low to moderate aphid populations seldom cause significant damage to plants. Large populations can stunt plant growth and promote extensive leaf discoloration. As populations increase, discoloration may become widespread throughout the plant’s crown. Some aphid species inject a toxin into plants, which causes the leaves to curl or pucker. A few aphid species promote the formation of galls on infested tissue.
As the aphids feed, they excrete a sticky exudate called honeydew. Severely infested plants can become laden with honeydew. The honeydew may turn black from the growth of a sooty mold fungus. Ants are attracted to the honeydew produced by the aphids. The ants harvest the honeydew, utilizing it as a vital resource. Masses of ants can often be observed navigating across infested plants. The ants tend to remain in close proximity to the aphids. They serve as a vanguard for the aphids, warding off many of their natural predators and parasites.
Aphids can transmit viruses between plants. Certain vegetables, such as bean, beet, bok choy, chard, cucumber, lettuce, melon, pumpkin, and squash are often infected by viruses conveyed by aphids. Viral infections can distort leaves, and stunt plant growth. Several aphid species are soil dwellers that feed exclusively on plant roots in spring and summer.
- Inspect plants for aphids at least twice a week, especially when plants are vigorous. Examine the leaf buds and the underside of new leaves to determine if aphids are present. Many species will become apparent in late spring, when temperatures are warm.
- Aphids have an abundance of natural predators and parasites that help to limit their populations. Various species of parasitic wasps lay their eggs inside the nymphs. When parasitized, the skin of the infested aphids will become dry and turn a golden brown. Crab spiders, aphid midge larvae, hoverfly larvae, ladybugs, lady beetles, lacewing larvae, soldier beetles, and syrphid fly larvae also prey on aphids.
- Aphids are susceptible to a bevy of fungal diseases. Entomopathogenic fungi such as Lecanicillium lecanii, as well as Entomophthora, Beauveria bassiana, and Metarhizium anisopliae can eradicate entire colonies of aphids when conditions are favorable. Infection occurs when the aphids make contact with the fungal spores. The spores cling to the insect’s body and penetrate the epidermis. Once the spores have invaded the insect’s body, they rapidly germinate. Over the next few days, the fungus grows within the aphid’s body, eventually killing it. Infected aphids will become enveloped in a wooly mass. The mass will grow denser until the aphid is completely obscured.
- Extreme temperature fluctuations inhibit the development of many aphid species.
- When aphids are present, ants will tend to them, protecting them from predators and parasites. To prevent ants from accumulating on infested plants, place a band of sticky material around the trunk. Avoid placing materials directly on the bark, as they can have phytotoxic effects.
- Infested plants can be doused with water, which will dislodge the ants. Ants are also repelled by citrus and vinegar, which can be combined with water, and sprayed onto infested plants. The solution will mask the scent trails created by the ants, and act as a deterrent when honeydew is present.
- A mixture composed of boric acid powder, sugar, and water can be placed in small containers, and distributed throughout the environment. The sugar will lure the ants to the tainted solution, which they will consume, and deliver to their queen. Once the queen has been exposed to the solution, she will expire, and the colony will collapse.
- Ant stakes or containerized bait may be placed on the ground to lure ants away from infested plants.
- Prior to planting, assess the surrounding area for sources of aphids, and eliminate them.
- Some aphids collect on weeds, and then migrate to nearby plants. This method of travel is particularly common in landscape settings. As such, periodically remove weeds and other invasive plants from the environment.
- When aphids are relegated to a few leaves or shoots, the infested plant material should be pruned out and disposed of.
- Periodically thin out plants to discourage infestations. Aphids thrive in overly vigorous plants, often concealing themselves in the dense upper canopy.
- Avoid the use of fertilizers with high levels of nitrogen. Nitrogen is conducive to aphid reproduction.
- When young, ensure that crops are grown under protective covers, in a greenhouse, or inside a warm building. Transplant the seedlings when they are older, and more tolerant of aphid infestations.
- When populations are low, aphids may be removed from infested plants with a strong stream of water. This method will help to cleanse the plant of honeydew as well. Douse infested foliage in the morning so that the plant has sufficient time to dry in the afternoon.
- Insecticidal soaps and oils can be utilized to control aphid populations. These products smother the aphids, preventing them from feeding. Eventually, the aphids die of starvation. When applied, thorough coverage of infested foliage is required. Insecticidal soaps and oils are not effective at contending with aphids that are enshrouded in galls. Avoid administering soaps or oils when temperatures exceed 90°F, or during extended periods of drought. On fruit trees, applications should be performed in early spring, as the eggs begin to hatch.
- Systemic insecticides can be used to manage aphids on woody ornamentals. Insecticides should be sprayed onto infested plants in early spring, with subsequent applications performed at 7 to 14 day intervals.
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Photo courtesy of Jacopo Werther CC-by-4.0