by Melinda Crow
Cooler temperatures, lower light levels and an abundance of young leaves and shoots in spring provide a natural wake-up call for aphids -- one of the most widespread pests known to gardeners. For almost every commonly cultivated plant, there is an aphid species that will attack it, so whether you grow vegetables, fruits or flowers, it's likely you'll deal with an infestation at some point, particularly if you live in the South.
These soft-bodied insects, ranging from 1/16 inch to 1/5 inch in length, are generally pear-shaped. They can be yellow, pink, green, gray or black. Most are wingless, but some have a pair of transparent wings. Aphids gather on tender, succulent plant parts, such as the undersides of leaves or the growing tips. They damage plants by sucking sap from them, causing wilting, deformed or stunted growth and yellowing or dropping of leaves. Aphids are also the most common means for the transmittal of mosaic viruses among plants. A heavy aphid infestation can prove fatal to young plants or those already in poor condition.
Aphids are also responsible for sooty mold, a largely cosmetic but nonetheless unpleasant fungus. Aphids excrete half- digested sap as a sticky substance called honeydew, which builds up on the leaves and stems. After a short time the powdery black fungus begins to grow on the honeydew. Sooty mold is more common on woody ornamentals, but can also appear on vegetable crops if there are a lot of aphids present. It's an attractant to ants, hornets, yellow jackets and flies, which then protect the aphids from natural predators.
There are more than 4,000 known species of aphids. Some sort of aphid is adapted for feeding on every part of the plant -- leaves, stems, bark, fruit and even roots. Some attack only one species of plant. Many of the common aphid pests in home gardens, however, attack a wide variety of plants, often rotating generations between two plant species when the population reaches a point of near saturation on the first host.
For example, the green peach aphid (a.k.a. the spinach aphid), is a pale yellowish green species that generally overwinters on the bark of stone-fruit trees, particularly peaches, hatching at bloomtime to feed on the twigs. The aphids then migrate to vegetable gardens, where they are a widespread pest of spinach and potatoes. But green peach aphids also sip the sap of lettuce, cucurbits, peppers and tomatoes and brassicas, as well as many common flowers.
There are several other species of aphids that feed on the leaves, buds and flower heads of plants in the cabbage family. The cabbage aphid, a small gray-green species with a waxy, powdery covering, attacks the largest number of these cruciferous crops. But the melon aphid, a tiny species ranging from pale to dark green to black, likes most of them, too. Besides feeding on crucifers (and close relatives like cucumber, melon, pumpkin, squash and watermelon), they also feed on asparagus, beets, beans, eggplant, okra, spinach, sunflower and citrus plants and many flowers, including sunflowers. The turnip aphid also feeds on most cabbage-family plants.
Another species commonly seen in the home garden is the pea aphid, which is larger than other species (1/5 inch long) and pale green with black legs. It overwinters on alfalfa and clover, then migrates to pea and sweet pea crops. The 1/16 inch, dark green to black bean aphid attacks asparagus, beets, parsnips, peas, spinach and Swiss chard as well as beans. It overwinters on evergreen shrubs, then migrates to these vegetable crops, as well as many flowers, including zinnias, poppies, dahlias and nasturtiums. The 1/16 inch pink and green potato aphid overwinters on roses before migrating to feeds on potato and tomato, as well beans, corn lettuce, peas, pumpkin and squash.
Some species, such as the powdery blue-green corn root aphid, feed underground on the roots of corn, beets and carrots. Insects in the woolly aphid group are capable of feeding on all plant parts. The woolly apple aphid is a common species in this group.
Life Cycle
Aphids reproduce faster than any other insect, so massive populations often appear very quickly. In spring, the first generation, which is only females, hatches as nymphs, which develop into wingless adults just in time to feed on newly emerging vegetation. Within a few days' time and without mating, they begin cranking out live young -- which are also all female. These of course begin feeding and bearing more young immediately.
Aphids prefer slightly cooler temperatures and low light levels, making spring and fall ideal times for rapid reproduction. The lower light levels in fall also trigger the birth of a male form as well. They mate and the females produce eggs that overwinter cemented in place in protected places like bark crevices near a winter bud on trees and shrubs.
Natural Predators
Because of their size, aphids are quite easily preyed upon. But even the tiny aphid has a few tricks designed to protect itself. As aphids feed, plant leaves often curl tightly around the developing colony, creating a predator-proof (and often insecticide-proof) shield. Stem- and root-attacking species often produce galls that serve much the same purpose.
The most unusual defense mechanism recently identified is the use of a warning scent. According to Texas A & M University Extension entomologist Charles Allen, when the bodies of certain species are crushed or punctured, they release a scent that causes nearby aphids to flee the impending danger. "The urge to escape is so great that they sometimes just jump right off the plant," says Allen.
But despite their elaborate defenses, aphids have many natural predators that provide good control. "It's often best just to stay out of nature's way by avoiding other controls when sufficient predators are present," says Allen.
The convergent lady beetle (Hippodamia convergens) has long been known to keep aphid populations in check. The adults often range over quite large areas, obeying a migratory urge. The lady beetle larvae, however, can't fly away and are capable of ingesting huge quantities of aphids. An even more effective natural predator is the larva of the green lacewing (Chrysopa carnea) because the adult is more likely to remain and feed in your garden, continuing to lay eggs.
Look for these ladybug and lacewing larvae, known as "aphis wolves" and "aphis lions" respectively. Both look like tiny black, purple, or gray alligators, often with red spots. Because they can devour 500 or more aphids per day, and remain in larval form for two to three weeks, one or two larvae per medium-sized plant will keep an average aphid population in check. When neither of these are found, look for the hollow aphid carcasses left behind by the braconid larvae, tiny parasitic wasps that lay eggs in the adult aphid's body.
The aphid midge (Aphidoletes aphidmimyza) is another natural predator. This tiny black fly feeds on the honeydew, laying its eggs on the plant amidst the aphid infestation. As long as there are plenty of aphids present, the midges will continue to lay eggs and multiply. The larvae of the syrphid or hover fly also prey on aphids.
You can encourage the buildup of populations of these beneficial insects. Growing plants such as clover, daisy or umbelliferous flowers like dill, fennel and yarrow will provide pollen and nectar for lacewings and syrphid flies; parasitic wasps will be attracted to a source of drinking water. Any of these predators can be also purchased and released for aphid control. Timing factors involving mating cycles of the predator involved are critical for success, however.
Other Controls
Simple cultural techniques can help prevent aphid populations from gaining a hold on your plants. Avoid use of quick-release nitrogen fertilizers that produce the succulent new growth aphids find so attractive. Laying a mulch of aluminum foil has been shown to repel winged species of aphids; the reflection causes them to lose their sense of direction.
When intervention becomes necessary, the first step in eliminating a very small aphid population is mechanical. Handpick and crush small numbers of aphids or pinch or prune off infested branches or leaves. A strong spray of water (be sure to hit the undersides of leaves), perhaps with insecticidal soap added, will literally blast them off the plant. Most species are quite limited in their mobility, so this method is usually quite effective. You can repeat this treatment every few days until the population has been eliminated.
Insecticides designed for the particular plant under attack, and labeled specifically for aphids are the last line of defense. Use these only when damage is beyond what is acceptable to you, or when plants are not yet established or in poor health. Insecticidal soap -- sprayed directly on the aphids -- is the least toxic control and quite effective. Neem is a relatively new botanical insecticide that works as a repellent and feeding deterrent for aphids without harming beneficial insects. It's currently registered in most states for use on ornamentals and last June, the neem-based product Align was approved by the EPA for use on edible crops. Pyrethrum, sabadilla and rotenone are other plant-based insecticides that are effective controls. Horticultural oils, which smother the adult and egg forms of aphids but have minimal effect on beneficials, can be sprayed during the dormant and growing season.
And keep in mind that when temperatures rise as the season progresses, most aphid infestations on plants in full or even partial sunlight will dwindle quickly.
Melinda Crow is a Master Gardener and freelance writer in Amarillo, Texas.
Copyright NGA
Reprinted with permission HouseNet, Inc.
Pest Patrol -- Aphids