Aphids…So Small, So Many, So Annoying
Aphids are small, soft-bodied insects with long slender mouthparts that they use to pierce stems, leaves, and other tender plant parts and suck out fluids. Almost every plant has one or more aphid species that occasionally feed on it. Many aphid species are difficult to distinguish from one another; however, management of most aphid species is similar.
Aphids produce large amounts of a sugary liquid waste called “honeydew”. The honeydew that drops from these insects can spot the windows and finish of cars parked under infested trees. A fungus called ‘sooty mold’ can grow on honeydew deposits that accumulate on leaves and branches, turning them black. The appearance of sooty mold on plants may be the first time that an aphid infestation is noticed. The drops can attract other insects such as ants, that will feed on the sticky deposits.
Some aphids are important conveyors of plant viruses. However, it is seldom possible to control these diseases by attempting to kill these infected aphids with an insecticide. Aphids carrying viruses on their mouthparts may have to probe for only a few seconds or minutes before the plant is infected. Resistant varieties or varying plantings may be helpful in reducing problems with some viruses that attack annual plants.
Identification
Aphids have soft pear-shaped bodies with long legs and antennae and may be green, yellow, brown, red, or black depending on the species and the plants they feed on. A few species appear waxy or woolly due to the secretion of a waxy white or gray substance over their body surface. Most species have a pair of tubelike structures projecting backward out of the hind end of their body. The presence of these tubules distinguishes aphids from all other insects.
Generally adult aphids are wingless, but most species also occur in winged forms, especially when populations are high or during spring and fall. The ability to produce winged individuals provides the pest with a way to disperse to other plants when the quality of the food source deteriorates.
Although they may be found singly, aphids often feed in dense groups on leaves or stems. Unlike leafhoppers, plant bugs, and certain other insects that might be confused with them, most aphids don’t move rapidly when disturbed.
Life Cycle
Aphids have many generations a year. Most aphids in mild climates reproduce asexually throughout most of the year with adult females giving birth to live offspring—often as many as 12 per day—without mating. Young aphids are called nymphs. They molt, shedding their skin about four times before becoming adults. There is no pupal stage. Some species produce sexual forms that mate and produce eggs in fall or winter, providing a hardy stage to survive harsh weather and the absence of foliage. When the weather is warm, many species of aphids can develop from newborn nymph to reproducing adult in seven to eight days. Because each adult aphid can produce up to 80 offspring within a week, aphid populations can increase with great speed.
From the Beginning
Infestations generally result from small numbers of winged aphids that fly to the plant and find it to be a suitable host. They deposit several wingless young on the most tender tissue before moving on to find a new plant. The immature aphids or nymphs that are left behind feed on plant sap and increase gradually in size. They mature in 7 to 10 days and then are ready to produce live young. Usually, all of them are females and each can produce 40 to 60 offspring. The process is repeated several times, resulting in a tremendous population explosion. Less than a dozen aphid “colonizers” can produce hundreds to thousands of aphids on a plant in a few weeks. Aphid numbers can build until conditions are so crowded, or the plant is so stressed, that winged forms are produced in search of new hosts and the process is repeated.
Damage
Low to moderate numbers of leaf-feeding aphids are not usually damaging in gardens or on trees. However, large populations can turn leaves yellow and stunt shoots; aphids can also produce large quantities of a sticky exudate known as ‘honeydew’, which often turns black with the growth of a sooty mold fungus. Some aphid species inject a toxin into plants, which causes leaves to curl and further distorts growth.
Aphids may transmit viruses from plant to plant on certain vegetable and ornamental plants. Squash, cucumber, pumpkin, melon, bean, potato, lettuce, beet, chard, and bok choy are crops that often have aphid-transmitted viruses associated with them. These viruses mottle, yellow, or curl leaves and stunt plant growth. Although losses can be great, they are difficult to prevent by controlling aphids, because infection occurs even when aphid numbers are very low; it takes only a few minutes for the aphid to transmit the virus, while it takes a much longer time to kill the aphid with an insecticide.
Management
Although aphids seldom kill a mature plant, the damage they cause and unsightly honeydew they generate sometimes warrant control. Consider nonchemical controls discussed below, as most insecticides will destroy beneficial insects along with the pest.
- Monitoring
Check your plants regularly for aphids—at least twice a week when plants are growing rapidly—to catch infestations early, so you can knock or hose them off or prune them out. Many species of aphids cause the greatest damage in late spring when temperatures are warm but not hot (65°- 80°F). For aphids that cause leaves to curl, once aphid numbers are high and they have begun to distort leaves, it is often difficult to control these pests, because the curled leaves shelter aphids from insecticides and natural enemies.
Aphids tend to be most prevalent along the upwind edge of the garden and close to other infested plants of the same species, so make a special effort to check these areas. Many aphid species prefer the underside of leaves, so turn leaves over when checking for aphids.
Ants are often associated with aphid populations, especially on trees and shrubs, and frequently are a clue that an aphid infestation is present. If you see large numbers of ants climbing your tree trunks, check higher up the tree for aphids or other honeydew-producing insects that might be on limbs and leaves.
- Biological Control
Natural enemies can be especially important for controlling aphids, especially in gardens not sprayed with broad-spectrum pesticides (e.g., organophosphates, carbamates, and pyrethroids) that kill natural enemy species as well as pests. Usually, natural enemy populations do not appear in significant numbers until aphids begin to be numerous.
Among the most important natural enemies are various species of parasitic wasps that lay their eggs inside aphids. The skin of the parasitized aphid turns crusty and golden brown, a form called a mummy. The generation time of most parasites is quite short when the weather is warm, so once you begin to see mummies on your plants, the aphid population is likely to be reduced substantially within a week or two.
Many predators also feed on aphids. The most well-known are lady beetle adults and larvae, lacewing larvae, and soldier beetles. Naturally occurring predators work best, especially in garden and landscape situations.
In some situations, ants tend aphids and feed on the honeydew aphids excrete. At the same time, ants protect the aphids from natural enemies. There are materials you can apply to trees and woody plants to retard ants from climbing up. Prune out other ant routes such as branches touching buildings, the ground, or other trees.
- Cultural Control
Before planting vegetables, check surrounding areas for sources of aphids and remove these sources. Where aphid populations are localized on a few curled leaves or new shoots, the best control may be to prune out these areas and dispose of them. In large trees, some aphids thrive in the dense inner canopy; pruning out these areas can make the habitat less suitable.
High levels of nitrogen fertilizer favor aphid reproduction, so never use more nitrogen than necessary. Instead, use a less soluble form of nitrogen and apply it in small portions throughout the season rather than all at once. Slow-release fertilizers such as organic fertilizers or urea-based time-release formulations are best.
- Chemical Control
When considering whether to apply insecticides for aphid control, remember that most larger plants can tolerate light to moderate levels of aphids with little damage. Larger aphid populations often rapidly decline due to biological control or when hot temperatures arrive. Often a forceful spray of water or water-soap solution, even on large street trees, when applied with appropriate equipment, will provide sufficient control.
If insecticides are needed, insecticidal soaps and oils are the best choices for most situations. Oils may include petroleum-based horticultural oils or plant-derived oils such as neem or canola oil. These products kill primarily by smothering the aphid, so thorough coverage of infested foliage is required. Soaps, neem oil, and horticultural oil kill only aphids present on the day they are sprayed, so applications may need to be repeated. Always read the instructions for specific directions and best methods for application. Although these materials can kill some natural enemies that are present on the plant and hit by the spray, they leave no toxic residue, so they do not kill natural enemies that migrate in after the spray.
Many other insecticides are available to control aphids in the home garden and landscape, including foliar-applied formulations of malathion, permethrin, and acephate (nonfood crops only). While these materials may kill higher numbers of aphids than soaps and oils, their use should be limited. These insecticides also kill natural enemies that provide long-term control of aphids and other pests and are associated with bee kills and environmental problems. Repeated applications of these materials may also result in resistance to the material.
Insecticides such as oils and soaps are also safer to use when children and pets may be present. Formulations combining insecticidal soaps and pyrethrins may provide slightly more knockdown than soaps alone yet have fewer negative impacts on natural enemies than malathion, permethrin, and acephate, because pyrethrins break down very quickly. Whenever applying pesticides to your garden, remember children may play in these areas. Also, bear in mind that if you are growing vegetables or herbs, these insecticides may not be safe for consumption. An easy rule of thumb…leave pesticide application to your friends here at Millette Pest Control.