Question: We cannot believe how many of our garden plants are infested with aphids. There appear to be millions of them and they all appeared in less than a week. What's the best way to get rid of them? We would prefer to not use anything toxic.
Answer: Aphids may be found on plants at any time, but there are some situations in which aphids are more likely to thrive and multiply. They multiply rapidly in enclosed areas, such as greenhouses, in cold frames, under row covers, and on houseplants, where they are protected from their natural enemies and variations in weather. Many species are resistant to insecticides, and their populations explode when insecticides have killed off their natural enemies without harming the aphids. Aphids are also very sensitive to the physiology of their host plant. They may multiply rapidly when the host plant has been fertilized heavily with nitrogen.
Aphids have many generations each year. Most aphids reproduce asexually throughout much of the year with adult females giving birth to live offspring (often as many as 12 a day) without mating or laying eggs. Young aphids, or nymphs, molt about four times before becoming an adult. When the weather is warm, many species of aphids can develop from newborn nymph to reproducing adult in less than two weeks. Because each adult aphid can produce up to 80 offspring in a matter of a week, aphid populations can increase with great speed. In potatoes it is common for aphid populations to double every two days. This remarkable reproductive capacity has been observed and described in a more dramatic manner by the French naturalist, Reaumur, during the late eighteenth century. His calculations revealed that if all the descendants of a single aphid survived during the summer and were arranged into a French military formation, four abreast, their line would extend for 27,950 miles, which exceeds the circumference of the earth at the equator!
Low to moderate numbers of leaf-feeding aphids are usually not damaging in gardens or on trees. However, large populations cause curling, yellowing, and distortion of leaves and stunting of shoots; they can also produce large quantities of a sticky substance known as honeydew, which often turns black with the growth of a sooty mold fungus. Some aphid species inject a toxin into plants, which further distorts growth. A few species cause gall formation.
Aphids transmit viruses from plant to plant on certain vegetable and ornamental plants. Squash, cucumber, pumpkins, melons, beans, potatoes, lettuce, beets, chard and bok choy are all common hosts of aphid-transmitted viruses. The viruses cause mottling, yellowing or curling of leaves and stunting of plant growth. Although losses can be great, they are difficult to prevent through the control of aphids because infection occurs even when aphid numbers are very low; it takes only a few minutes for the aphid to transmit the virus while it takes much longer to kill the aphid with an insecticide.
QUESTION: We heard that using aluminum foil mulches will repel aphids. Does that really work?
ANSWER: Aluminum foil mulches have been successfully used to reduce transmission of aphid borne viruses in summer squashes, melons, and other susceptible vegetables. They repel invading aphid populations, reducing numbers on seedlings and small plants. However, as plants grow, aluminum foil mulches give mixed results for aphid control; they seem to repel natural enemies of aphids as well as aphids. The few aphids that do drift onto plants grow and reproduce with greater speed than those landing on plants growing in bare soil because temperatures are higher on the aluminum foil-mulched plants. Yields of vegetables grown on aluminum foil mulches are usually increased, despite higher aphid numbers, by the greater amount of solar energy reflecting on leaves.
Another way to reduce aphid populations on sturdy plants is to knock them off with a strong spray of water. Most dislodged aphids will not be able to return to the plant and honeydew will be washed off as well. Using water sprays early in the day allows plants to dry off rapidly in the sun and be less susceptible to fungal diseases.
Many pesticides are available to control aphids around the home. Check labels before using, as not all of these materials are registered for all plant types. Selective insecticides such as oils and soaps are safer to use where children and pets may be present, and may provide more effective long-term control because they do not kill the natural enemies of the aphids.