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One of the first insect pests to arrive in the in spring is the aphid. This small (1/8 inch), soft-bodied insect derives nourishment by sucking plant sap. Information about the life cycle and behavior of aphids is described in Box A.
Most plants can tolerate low to moderate numbers of aphids without noticeable damage. However, when aphid numbers are high, their feeding habits can distort foliage and flowers, and stunt plant growth. Some species of aphids can also transmit plant viruses when they puncture plant tissue to feed.
|Aphids - So Many, So Fast
At least 4000 aphid species have been described, and probably more remain to be discovered and named. Within each species there may be considerable variation in color (green, gray, black, etc.), size, or other aspects of appearance, as well as response to environmental conditions or food preference. Only about 10% of aphid species are able to feed on a wide range of plant species. Most feed on only one type of plant and its close relatives. Thus, you need not assume that if aphids are feeding on your roses, they are likely to move onto your birch trees.
The remarkable life cycle of aphids helps to explain how they can quickly appear in large numbers. In temperate climates, female aphids lay their eggs in the plant bud scales or in oracles and crevices of bark. In spring, a plump, distinctive looking aphid emerges from this egg. This "stem mother" in turn gives birth to live daughters, and they to more daughters-- all without need of mating! These growing aphid colonies cluster around the stem mother and multiply long after her death. At the end of the season, males are produced, mating occurs, and eggs are again laid on bud scales or bark. (In greenhouses, female aphids usually produce live young; males, mating and egg-laying rarely occur.)
Aphids also produce "honeydew," from the sweet sap they imbibe from plants. Excess honeydew is excreted, and forms a sticky coating on leaves. Honeydew itself is harmless; however, it is soon colonized by a black fungus called "sooty mold," which makes the leaves look dirty (although it does not harm them). Some tree-feeding aphids produce so much surplus honeydew that it falls onto parked cars or sidewalks, where it creates a sticky mess.
In some areas honeydew also attracts ants which feed on the nutritious substance. To insure a continuing supply, these ants protect aphids from their natural enemies. In fact, whenever you see ants on a plant, you can generally assume that aphids (or another honeydew producer such as scales or mealybugs) are there as well. In such cases, aphid management must include ant management.
Fortunately, aphids have many natural enemies such as lady beetles, lacewings, and minute parasitoids ("mini-wasps") that often keep aphid numbers below damaging levels. These are discussed under "Biological Controls," below. In order to assure themselves of a food supply, however, these beneficial insects rarely appear on the scene until alter aphids have begun attacking plants. This "lag-time" may be as short as a day or two, or as long as several weeks if temperatures are cool.
Since control of aphids by natural enemies improves as the season progresses, it is important to learn to tolerate low to moderate numbers of aphids, so long as they aren't causing noticeable plant damage. For example, in a local rose garden, it was learned that up to 10 aphids per bud caused no appreciable plant damage. By observing your plants in early spring, you can note when aphids first appear, see how long it takes for the natural enemies to show up, and decide whether or not the amount of plant damage you observe during the "lag-time" is tolerable.
By confining any treatments to just those times and places where aphid numbers grow large enough to threaten significant damage to plants, and by using methods that limit harm to beneficial insects, you can maximize natural biological control and minimize your time and expense on pest control.
When you need to reduce aphid numbers choose among the following options:
The best way to attract natural enemies to y« garden is to plant a wide diversity of flowering plants. Many beneficial insects rely on pollen and nectar from flowers to supplement their insect diet Spring blooms will help encourage arrival of beneficial insects earlier in the season than they normally appear. Early arrival can also be encouraged by mixing 1 part sugar with 10 parts water and spraying this solution on aphid-susceptible plants. Blooms throughout the year will prolong the stay of helpful organisms.
Tolerating a few aphid-infested weeds such as annual sowthistle, Sonchus oleaus, in early spring can also encourage benefcials. The aphid species that attacks sowthistle does not feed on ornamental plants. However, its presence on weeds in the garden attracts beneficials early in the season, so they are available when laterarriving aphids threaten vegetables, fruits, or flowers.
Beneficial insects can also be ordered from commercial insectaries (see the Resources box below). We recommend ordering lacewing larvae. These tiny alligator-like predators have voracious appetites for aphids and other pesos. They are shipped in cardboard containers, and can be gently sparkled over infested plants. Aphids in greenhouses or on indoor plants can also be controlled by the larvae of a tiny gall fly called Aphidoletes.
Don't wait till aphid numbers are high before releasing beneficials. If you have an aphid emergency on your hands, first use soap or oil sprays to knock the numbers down, then release the natural enemies. This gives the beneficials time to find and kill aphids before excessive plant damage occurs.
We do not recommend ordering lady beetles also called "lady bugs." It is better to use the techniques described above to encourage their presence in the garden. Lady beetles ordered from commercial sources generally have been collected from hibernation sites, and are kept under refrigeration until the spring growing season. When they arrive by mail, the lady beetles are not hungry because they still contain body fat meant to sustain them when they emerge from hibernation in the mountains and fly to the valleys to feed. That's why mail-ordered beetles usually By away as soon as you release them.
Due to space constraints, only a few sources of beneficial insects are listecl here. For a full listing, consult BIRC's annual Directory of -IPM Products and Services, available for $6.50 from BIRC, PO Box 7414, Berkeley, CA 94707. Beneficial insects can often be ordered from local nurseries, or by mail from the following insectaries:
- Bozeman Bio-Tech, 1612 Gold Ave., PO Box 3146, Bozeman, MT 59772; 800/289-6656
Buena Biosystems, PO Box 4008, Ventura, CA 93007; 805/525-2525
- Rincon-Vitova Insectaries, Inc., PO Box 95, Oak View, CA 93022; 805/643-5407
For More Information
If you have questions about the information in this fact sheet, call BIRC 510/524-2567
Developed by the Bio-Integral Resource Center (BIRC), a non-profit institution providing education and research on leats-toxic pest control. BIRC PO Box 7414 Berkeley, CA 94707 Tel: 510/524-2567 Fax: 510/524-1758
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