A Most Familiar Insect Pest.
I believe winter is a good time to become acquainted with an insect that you are likely to see a few months from now, just as leaves and buds begin to expand and new growth commences. That way, when you observe them, you'll have a good idea of what's going on.
Aphids, sometimes known as "plant lice", are members of the order Homoptera. There are nearly 4000 species found worldwide. Evolutionary biologists believe they have been around for 250 million years. (Not, of course, 4000 species. Maybe a couple of dozen, back then.) Anyone who has ever grown a plant has very likely encountered aphids. They are common, persistent and troublesome. For nearly every species of plant life known, there is at least one species of aphid that will feed on it. The highest reproductive potential of all insects belongs to the winged adult aphid!
Aphids are soft-bodied insects generally from 1/16 to 3/16 inches long, somewhat pear shaped, with fairly long antennae. They feed by thrusting sharp, hollow stylets from their beaks through the epidermis into the plant cells where they suck the sap juices from the stems and leaves. The plant becomes depleted of food and weakened substantially. The aphids can inject toxic salivary fluids at the same time they feed, and, worst of all, a single feeding aphid can transmit a deadly virus disease to an otherwise healthy plant or tree.
Their types and habits and sizes differ widely, as do their colors (yellow, green, pink, red, purple, brown, and black).
Another aspect that differs widely among aphids is their life and reproductive cycle. Most species of aphids overwinter as eggs. In spring these eggs hatch to produce a generation of all females (stem mothers), and all are pregnant. They very quickly, generally in a week or so, give birth to living young, normally wingless and also pregnant females, which a week later produce another generation, and so on.
One can easily understand, when no aphids are seen on a plant one week, why hundreds and hundreds are seen only a couple of weeks later. As the plants become crowded, some of these females develop wings and fly to either plants of the same kind, or, in certain species, always to a different kind of plant known as a summer host. A succession of generations is started anew, just as before.
Eventually, as day length grows shorter, the first appearance of males comes as cold weather approaches. The winged females return to the original plants and give birth to nymphs. These are the true wingless females that cannot reproduce unless they mate with the males. After mating, the true females lay one to four or more large fertilized eggs in a sheltered place on the plant and die.
From these eggs arise the stem mothers of the next spring. Of all of the many generations of aphids produced, only these have both male and female parents. This is one standard life cycle, but there are many variations.
Aphid feeding damages fruits, vegetables and ornamental trees and shrubs. In addition to leaf curling, distortion reduced vigor, the victims suffer from unattractive appearance, diminished flavor and market value. Besides this direct damage to the plants, there is another equally unwelcome side effect.
Feeding aphids secrete a sweet, shiny, sticky waste product known as honeydew. This consists mainly of excess sap ingested by the insect and passed through the body. A black fungus called sooty mold rapidly colonizes this substance. It covers the honeydew, further interferes with photosynthesis, and makes the host plant unsightly.
Another curiosity about these prolific insects is worth reporting. Ants are very fond of the honeydew as a source of food because of its high sugar content. Ants and aphids share a symbiotic (mutually beneficial) relationship. Ants will gather aphid eggs and tend them over the winter. Often ants will carry them to roots where they can continue to feed or will overwinter. In spring ants will then carry them back up to stems and leaves. Ants will also transport wingless aphids to other plants.
Populations of ants that do no direct harm to the plants themselves usually accompany serious infestations of aphids. The ants serve to protect the aphids, often engaging in battle with aphid predators. Therefore, controlling aphid problems often means controlling ant numbers as well.
Which brings us to the management of this notorious pest. Whether in the home, the greenhouse, the flower or vegetable garden, or the backyard landscape, the first step is monitoring. This is the careful and frequent inspection of plants from the very beginning of an aphid population buildup. Yellow sticky cards are very useful tools. Early population control can often eliminate the need for later spraying measures.
It is important to recognize that aphids have many natural enemies. As aphid numbers increase, often beneficial predator populations rise, too. Some of the most important beneficial predatory insects include lady bird beetles (thirteen different species), ground beetles, rove beetles, soldier beetles, flower beetles, flower bugs, big eyed bugs, damsel bugs, assassin bugs, green and brown lacewings, hover or syrphid flies, aphid midges, and spiders (which are not insects, but are related). Add to this list a variety of beneficial wasps that parasitize aphids. There is also a naturally occurring fungus that is pathogenic to aphids. We can all benefit by learning to recognize these "good" insects.
Back to monitoring and control measures. Aphids will begin to make their appearance early in the growing season - probably early to mid-May. The white pine aphid will be out even in April in our area. If you recall that many of the early generations of aphids are wingless, one of the best control practices is to wash them off your plants with a forceful spray of water. On the ground they will be easy prey for a host of hungry predators. Closely monitor their presence and repeat the water sprays frequently.
Look for populations of lady beetles and get to know the other beneficial insects. You may be able to avoid chemical sprays altogether. Remember, many chemical sprays kill beneficial predators, too, so use some discretion. If aphid populations become very great and no lady beetles or their larvae are seen, you might consider using insecticidal soaps and horticultural oils. Always check the label for the plants you wish to protect and for the recommended dosages. Both of these pest control products are benign with regards to most beneficial insects.
There are several chemical insecticides recommended for aphid control on a variety of plants. Check the label for restrictions and the number of days you must wait to harvest (Days to harvest) before applying any pesticide to edible crops. The longer and warmer the season, the more generations of aphids. In the South, 15 generations is not unusual. Most chemical insecticides do not discriminate between aphids and their natural enemies, so think twice about using them. There are other options.
A handful of flowering plants are especially attractive to the aphid's natural enemies for their nectar. Dill, fennel, and yarrow grown in and around the area will bring in a substantial supply of lady beetles, lacewings, hover flies and parasitic mini-wasps. The larval stages of lady beetles and lacewings are more voracious than the adults, so it is good to know what these look like.
In recent aphid history, it's important to note that soybean aphid, Aphis glycines, arrived in America in 2000. They are all over the country now. They are native to China and Japan.
On May 13th, 2005 the pink biotype of the pea aphid was discovered in an alfalfa field in Fresno County, CA. It occurs in France, the east coast of the U.S. and some western states. But, now, it's in California.
Last summer I noticed a nearby milkweed covered with many hundreds of golden aphids. I had not seen these before. Aphids hitch rides on winds and storms due to their miniscule size and weight. A hurricane can move a species to a new area in a single day. A strong breeze can move some from one garden to a neighbor's garden in a jiffy.
So, "Coming Soon to a Neighborhood near You" may well be the promo for a new sci-fi movie, or, just maybe, a sci-non-fiction occurrence or event in your back yard. Be on the lookout.
From The Garden of Ed. Submitted for publication in The Towne Crier on February 21, 2007
© 2005 Ed Mues. All Rights Reserved.