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Aphids

Potato Aphid, Macrosiphum euphorbiae (Thomas)	Buckthorn Aphid, Aphis nasturii (Kaltenbach)	Foxglove Aphid, Aulacorthum solani (Kaltenbach)	Green Peach Aphid, Myzus persicae (Sulzer)

Photos above courtesy Univ. of Minnesota Extension Service

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 or "plant lice" may infest almost any plant. They are more commonly found on camellia, crape-myrtle, gardenia, hibiscus, ixora, oleander, palm, rose, as well as nearly all annual plants. Aphids have piercing-sucking mouthparts and cause damage by sucking the plant juices. However, their ability to transmit plant virus diseases may be more harmful than any direct feeding damage.

Aphids are soft bodied pear-shaped insects generally less than 1/8 inch long and usually green in color but many are black, brown, pink, yellow, blue, or white. Most aphids are wingless but when colonies become overcrowded or the host plant becomes undesirable, winged forms are produced which establish new colonies. The most distinguishing feature in the identification of aphids is the two short cornicles or tubes which extend from the end of their body. They are commonly found on the stems or undersides of leaves in clusters or colonies of individuals.

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.

Most aphids feed on the new flush of growth in the early spring, but they may be found throughout the year. Their feeding makes the leaves curl or crinkle and flower buds may become hardened, causing the flowers to be distorted.

Aphids are unlike most insects in two ways: almost all are females that reproduce without mating, and most give birth to living young instead of laying eggs. Aphids have the ability to reproduce rapidly and there are many generations per year. Each female aphid produces 50 to 100 daughters during her life span and each daughter will begin reproducing in 6 to 8 days.

Aphids as well as soft scales, mealybugs, and whiteflies excrete large amounts of honeydew which provides an excellent medium for the growth of a black fungus called "sooty mold." Besides being unattractive, sooty mold may interfere with photosynthesis and retard plant growth. Sooty mold usually weathers away following control of the insect infestation. Ants feed on the honeydew and when ants are observed, plants should be examined closely for these sucking pests.

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.

Beneficial Insects

Some examples of aphid predators are lady beetles, praying mantis, assassin bugs, ambush bugs, and aphid lions. Spiders also prey on numerous insect pests. Aphids that have a small hole in a bloated-looking body were parasitized by tiny wasps (Figure 2). If predators are present or the pests show signs of parasitism, every effort should be made to preserve the beneficial insects. Delay applying a pesticide and provide the beneficials an opportunity to control the pest populations.

Aphids on Landscape Plants

Figure 1. Aphids feeding on plant stem.                        Figure 2. Healthy (yellow) and parasitized (brown/swollen) oleander aphids. 

Credits: Eileen Buss, University of Florida 

Inspecting Plants

Examine your plants weekly during the spring, summer, and fall. Look at the undersides of a few leaves on each plant and observe the stems for aphids, especially the new growth. The use of a 10 to 15 power hand lens or magnifying glass aids in detection and identification. Learn to determine when aphids are present in damaging numbers and to evaluate the potential of the predator or parasite population. To aid in locating aphids, a sheet of white paper or cloth may be held beneath the leaves and the foliage struck sharply. The insects will fall onto the paper and can be more easily observed and identified than on the green foliage. Most plants in the urban landscape are over-sprayed, resulting in unnecessary environmental contamination and often upsetting the natural predator/parasite-pest balance. Don't apply a control measure until a pest population is present and damage is beginning to occur.

Non-Insecticidal Control

Many homeowners can remove aphids and keep populations below damaging levels by spraying their landscape plants with a forceful stream of water. Use a garden hose with an adjustable nozzle and spray undersides of leaves and stems when the aphids appear.

Soaps are available that are formulated for controlling insects and related pests. If one of the commercial soaps is unavailable, 3 tablespoons of dishwashing liquid (do not use those containing a degreaser or an automatic dishwashing soap or detergent) per gallon of water may be applied as a foliar spray to woody plants. Use 2 tablespoons for bedding, foliage and flowering plants. Repeat at weekly intervals as needed. Soap is effective in controlling aphids, safe for people and the environment.

Insecticidal Control

Usually aphids are not difficult to control with insecticides; however, plants may become re-infested from adjacent areas throughout the year. For control, apply one of the suggested insecticides (see Table 1) if aphids are beginning to damage the plants. Spray the plants to the point of run-off. Be especially careful to cover the undersides of the leaves and all parts of the twigs thoroughly. Continue to inspect the plants periodically especially new flushes of growth and apply an insecticide if plants become re-infested. 

Table 1. Insecticides labeled for use by professional applicators in residential areas to control aphids.

Natural 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. 

NOTE: Disclaimer - Pesticide recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registrations, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. 

Sources: University of Florida IFAS Extension, , University of Minnisota - Madison Extension Service, Virginia Cooperative Extension

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