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Aphids, Aphids, Aphids

Posted 4/29/2008

Yes, those are aphids on your Iris. Aphids, or plant lice, are found on nearly every species of wild and cultivated plant. They are a common pest in Nevada gardens, and can be particularly troublesome during cool, moist conditions in the spring. Their feeding results in off-colored, distorted or curled leaves. Aphids obtain their food by sucking fluids from plant tissue. They feed on flowers, foliage, twigs, branches, trunks, or roots of herbaceous and woody plants. Most plants tolerate moderate numbers of aphids. Their excrement, honeydew, is sticky and often signals their presence. Aphids may live entirely on one type of plant, or they may spend part of their development on one host and then move to a different host.

Aphids are small, soft bodied insects, about one-tenth of an inch long. Aphids are typically teardrop shaped and may be winged or wingless. Their long, slender mouthparts are used to pierce plant tissues to suck plant sap. They range in color from pink, yellow, green, gray, deep blue to black. Some aphids, including the woolly aphids, produce waxy threads to cover their bodies. Most aphids are easily distinguished from other insects by the presence of two tube-like projections called cornicles that project up and out from the rear, upper side of the abdomen. As they grow, they molt and shed their skins, approximately four times, before they become adults. The first generations are wingless, but as populations increase and become crowded, winged aphids develop and leave the colony to start new ones.

Aphids tend to congregate on leaves and shoots, particularly at the tips of new shoots. They prefer the undersides of leaves and fresh, succulent new growth for feeding and protection from the sun and drying conditions. Look for aphids on the windward side of gardens and landscapes. Young, swelling leaf and flower buds are favorite targets. Small seedlings may be severely damaged or killed by aphids, but once plants have five to seven leaves, they may "grow through" an infestation. When abundant, aphids can cause serious damage to larger plants. Some species of aphids have saliva that when injected into plant cells during feeding causes abnormal, often twisted plant growth. This may be the first sign of a heavy aphid infestation. Heavy feeding usually stunts growth, deforms leaves, flowers and fruit, or forms galls on leaves, stems or roots. Aphids also transmit plant diseases, such as viruses, from plant to plant as they feed.

In addition, many aphids secrete a sticky, sweet substance called "honeydew". Honeydew falls onto leaves, patios, vehicles and sidewalks, creating a sticky nuisance. Some ants and wasps are attracted to and "harvest" the sweet honeydew. To safeguard their source of food, these insects will protect aphids from their natural enemies. Often it is the line of ants ascending a tree trunk that first alerts a gardener to an aphid infestation at the top of a tree or shrub. In order to control an aphid population, the ants may need to be controlled first.

Honeydew often supports the growth of sooty mold that gives landscape plants, fruits, and vegetables a dull, dark cast and makes them undesirable looking. This is very common in southern Nevada. The honeydew and molds can usually be washed off both plant parts and fruits. Fruits are then safe for eating. It is difficult to remove honeydew, mold and aphids from leafy vegetables and crops like broccoli, cauliflower, cabbage and brussel sprouts. Many factors influence the extent of damage caused by aphids. Many species of aphids exist. Employ integrated pest management to reduce their damage, select plants that are unattractive to aphids, release their predators or grow plants that provide predator habitat nearby, use proper cultural practices, and lastly, apply pesticides. Removing debris, weeds and plants susceptible to aphid colonization should be the first measure of control. Then select and plant species that are least susceptive to aphids. Inspect transplants of vegetables and ornamentals before bringing them into your yard and reject those with aphids. Keeping the number of aphids entering your property to a minimum is paramount. Turn leaves over to find colonies of aphids. Where aphid mummies or carcasses are present, look for and protect predators like ladybird beetles, lacewings, syrphid fly larvae, and small parasitic wasps. They will increase in number and very rapidly control the aphids. Plant nectar-producing flowers in your garden to encourage parasitic wasps and lacewings to occupy your yard. Good control is obtained by washing aphids from plants with a forceful stream of water and repeating at regular intervals. This should be done only during cool, early morning hours. The water also helps to wash away dust and increase humidity, which helps control spider mites and powdery mildew. Where only a few leaves or stems are infested, it is easy to rub and smash aphids by gently pressing stems and leaves between your fingers. Moderate pruning or leaf picking also reduces aphid numbers. Avoid over-fertilization with nitrogen fertilizers and over-watering to prevent the rapid growth of soft, nitrogen rich tissue, which is attractive to aphids.

Under most conditions, it is not necessary to use pesticides to control aphids. Should you choose to use a pesticide, keep in mind that most insecticides kill natural enemies of aphids. Spray only the plant, or portion of the plant, that is infested. Contact insecticides like insecticidal soap and horticultural oils sprayed directly on the aphids and will do a good job of controlling them. When aphids cause severe leaf curl, contact insecticides do not reach aphids hidden inside the distorted leaves. If there are only a few curled leaves, remove and destroy them or put them in a sealed bag in the trash. An alternative is to use a systemic insecticide; use only on trees and shrubs, not on vegetables or fruit trees. Always read and follow label directions on pesticide products.

The remarkable life cycle of aphids helps to explain how they can quickly appear in large numbers. In the temperate climates of spring, female aphids called "stem mothers" emerge from over wintering eggs. These plump, distinctive-looking aphids do not need to mate to reproduce. These characteristics contribute to the tendency of aphid populations to "explode". When the weather is warm, many species of aphids can develop from newborn nymph to reproducing adult in 7 to 8 days. Because each adult aphid can produce up to 80 offspring in a matter of a week, aphid populations can increase quickly.

"Stem mothers" give birth to live daughters, and these offspring give birth to more live daughters - all without the need of mating. The swiftly growing female aphid colonies cluster around the stem mother and continue to multiply long after her death. This ’family’ is what you see on your buds and tender new growth. At the end of the season, aphids begin to produce both sons and daughters. When these males and females mature, they mate and the females lay eggs on bud scales or bark to over winter and begin the cycle again. For more information on aphids, call University of Nevada Cooperative Extension (346-7215) and ask for fact sheet "Aphids and their Management in Home Gardens".

The University of Nevada Cooperative Extension is an outreach arm of the University that extends unbiased, research-based knowledge from the University of Nevada—and other land-grant universities—to local communities. Educational programs are developed based on local needs, often in partnership with other agencies and volunteers. For more information about the University of Nevada Cooperative Extension, please visit the website at www.unce.unr.edu or call (702) 397-2604 or 346-7215.

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