Monday, January 14, 2008

Aphids







Aphids, or plant lice, are small, soft-bodied insects which are common pests of nearly all indoor and outdoor ornamental plants, as well as vegetables, field crops, and fruit trees. There are hundreds of different species of aphids, some of which attack only one host plant, while others attack numerous hosts. Most aphids are about 1/10 inch long, and though commonly green and black, they may be gray, brown, pink, red, yellow, or lavender. A characteristic common to all species is the presence of two tubes, called cornicles, on the back ends of their bodies. The cornicles secrete defensive substances. In some species they are quite long, while in others they are very short and difficult to see.

Aphids feed in clusters and generally prefer new, succulent shoots or young leaves. Some species, known as woolly aphids, are covered with white, waxy filaments which they produce from special glands.

Life History: Aphids have unusual and complex life cycles which allow them to build up tremendous populations in relatively short periods of time. Most species overwinter as fertilized eggs glued to stems or other parts of plants. Nymphs which hatch from these eggs become wingless females known as stem mothers. There are no males present at this time. Stem mothers reproduce parthenogenetically (without mating), and their eggs are held within their bodies until they hatch so that young are born alive. All offspring are females which soon mature and begin to reproduce in the same manner. This pattern continues for as long as conditions are favorable. A dozen or more generations are typical in Virginia. Periodically, some or all of the young develop wings and migrate to other plants. Some species always settle on the same type of plant; others have one or more alternate hosts. With the return of autumn's shorter days and cooler temperatures, a generation appears which includes both males and females. After matting, these females lay the fertilized eggs which overwinter and eventually hatch into stem mothers the following spring. Aphids are in the order Homoptera, Family Aphididae.

Damage: Aphids feed by sucking up plant juices through a food channel in their beaks. At the same time, they inject saliva into the host.

Light infestations are usually not harmful to plants, but higher aphid populations cause leaf curl, wilting, stunting of shoot growth, and delay in production of flowers and fruit, as well as a general decline in plant vigor. Some aphids are also important vectors of plant diseases, transmitting pathogens, particularly viruses, in the feeding process.

A distinctive feature of aphids, as well as some scales and other bugs, is the production of honeydew. Honeydew is the clear, sticky dropping that lands on the leaves or anything below the plant or tree that aphids are feeding upon. A sticky glaze of honeydew may collect on lower leaves, outdoor furniture, cars, and other objects below aphid feeding sites. Honey dew coated objects soon become covered by one or more black or brown fungi known as sooty molds. Crusts of sooty mold are unsightly on man-made objects, and they interfere with photosynthesis in leaves.

Colonies of aphids are sometimes protected by certain ants. In return for this protection the ants are allowed to collect the sweet honeydew. In most cases, the ants protect aphids that have already established themselves on the plant and these aphids or their eggs and keep them through the winter in their nests. In spring, the ants transport these aphids to food plants where they protect them from enemies and at intervals transfer them to new feeding sites.

Recognition: Unthrifty or stunted plants and plants with curled or deformed leaves are likely to have aphid infestations. Feeding aphids usually occur in clusters on succulent shoots, under leaves, or in other suitable feeding sites. The presence of honeydew or sooty mold is often an excellent clue that aphids are present. Plants should be examined closely on a regular basis to detect aphids before damage is evident.

Some Common Aphids in Virginia:

White Pine Aphid. Black or gray with long legs. This is a common pest of eastern white pine. Severe infestations reduce the growth and may even kill small trees. Colonies occur most commonly on twigs and stems where the bark may be killed in patches. Needles and twigs are sometimes completely covered with sooty mold. Eggs, which are laid in lines on the needles, may hatch when infested white pines are brought indoors for use as Christmas trees.

Rose Aphid. Green or pink with black legs. A widespread and common pest of all cultivated roses, this species may also damage pyracantha. Stems, buds, and young, tender leaves are injured.

Giant Bark Aphid. Ash gray with black spots. Nearly ½ inch long including the legs, this is our largest aphid species. It attacks willow, maple, elm, birch, and several other common shade trees. It feeds on the bark of twigs and small branches. Bees, wasps, and flies are attracted to the honey-dew it excretes.

Green Peach Aphid. Pale yellow-green. This species attacks dozens of different hosts including aster, catalpa, crocus, chrysanthemum, dahlia, English ivy, iris, lily, nasturtium, pansy, rose, snapdragon, tulip,and violet, as well as many garden vegetables and some fruit trees. It is capable of transmitting over 100 different plant viruses.

Chrysanthemum Aphid. Shiny dark brown with short cornicles. Common and widespread on chrysanthemum where they cause stunted growth and slightly curled leaves.

Woolly Alder Aphid. Plump and blue-black, but completely covered with white waxy filaments. Silver maple is the primary host, but they migrate to alder in mid-summer, then return to silver maple in late fall. This aphid is not particularly injurious to either host, but it becomes a nuisance when waxy filaments accumulate under heavily infested trees.

Also, Woolly Apple Aphid, Rosy Apple Aphid, Pea Aphid, Cabbage Aphid, Bean Aphid, Mellon Aphid, Snowball Aphid, and many, many others. Some aphids cause galls. Included in this group are:

Witchhazel Cone Gall Aphid, Spiny Witchhazel Gall Aphid, and the Elm Cockscomb Gall Aphid.

Control: Natural enemies play a very important part in controlling aphid populations. Lady beetles, lacewings, damsel bugs, flower fly maggots, certain parasitic wasps, birds, and fungal diseases all attack aphids. Without them, these pests would be much more destructive. Gardeners should avoid unnecessary use of insecticides which are harmful to beneficial organisms.

Gardeners should also strive to keep their plants healthy and growing vigorously. Migrating aphids are attracted to the yellow-green color of unthrifty plants. If an infestation does develop, there are several insecticides registered for aphid control. Check the Virginia Pest Management Guides for current pest control recommendations. These guides are available through your local Extension agent. Always read the label before applying any pesticide.

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