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Aphid Attack

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Aahh...springtime. Buds are breaking, turf is awakening from dormancy and the hummingbirds return from their sojourn in warmer climes. The sun warms the soil, tender green shoots emerge - and before you know it, they're covered with aphids.

Back to reality...it's time to get to work! Or is it?

There are literally hundreds of species of aphids. Some are specialized and specific to individual plants. For example there are aphids that infest ivy (Aphis hederae), nasturtiums (Aphis nasturtii) and evening primrose (Aphis oenotherae). (Notice the similarity between the plant and insect Latin names?). On the other hand, one of the most common species, the green peach aphid (Myzuz persicae) feasts on more than 200 plants. Others, such as the potato aphid (Macrosiphum euphorbiae and M. rosae), start munching their preferred host, the rose, early in the season and move on to other delicacies such as aster, gladiolas, hollyhocks, iris and a host of other annuals and perennials later in the season. Chances are if it grows, there's an aphid that will feed on it.

Aphids are small; only about 1/8-inch long. They are somewhat pear-shaped and come in a whole range of colors. Most aphids we see are green, which acts as camouflage on plants, but they can be brown, black, red or even bright gold.

When we think of aphids, we generally think of a leaf pest. However, there are also aphids that affect roots. The Brachycaudus species attack the roots of many common ornamentals, including calendula, dahlia and primrose.

Sometimes it seems amazing that we are able to grow plants at all. Insects account for about 80 percent of the earth's fauna, with more than 850,000 species. Aphids belong to the Homoptera order; along with other plant pests such as leafhoppers, scales, whiteflies and psyllids.

If you are unsure about the particular species you are dealing with, take a sample to your local Cooperative Extension or Agricultural Commissioner's office. They should be able to tell you which aphid species are particular problems in your region and suggest control measures.

BORN TO BE PREGNANT

Insect control begins with understanding the pest's life cycle. All insects undergo a change in form as they grow, a process known as metamorphosis. Moths, beetles and ants undergo complete metamorphosis. After eggs hatch, the larvae emerge. These are the grubs and caterpillars that create so much plant damage. The larvae often molt (shed their skin) as they grow, and then turn into a pupa. The fully developed insect then emerges from the pupa.

Aphids (as well as other homoptera order members, grasshoppers, earwigs and other species) undergo gradual or simple metamorphosis. Immature insects that emerge from eggs are called nymphs. Like larvae, nymphs molt as they grow, generally three to seven times. After the final molt, they are adults and usually have wings.

The aphid's reproductive process also goes beyond simple metamorphosis. Aphids also can reproduce asexually, without male assistance. The females are "born pregnant," which accounts for the amazing rapidity with which their populations can explode. Aphid populations can increase a hundred times two weeks!

When feeding conditions are ideal and uncrowded, aphids are born wingless. However, as the food supply runs out, the insects are born with wings. This way, they can fly on to new feeding and breeding territory.

Some species of aphids have even more complex life cycles. In the spring and summer, they feed on annual plants. At the end of summer, males are produced and meet with females on trees or shrubs. Their offspring are specialized egg-laying females, which then lay eggs that overwinter on the host plant. The eggs hatch in the spring and after a couple of generations the aphids move on to annual plants to begin the cycle again.

Aphids have what entomologists call piercing/sucking mouth parts. They basically slurp up plant sap, and when abundant can weaken the plant. Some aphids produce a mild toxin to plants in their saliva, which can cause misshapen or deformed leaves.

Although aphid damage is rarely fatal (especially if the plant is healthy and growing vigorously), they can also vector viruses. For example, aster yellows, a fatal virus, can be spread from plant to plant by aphids.

Sometimes aphid damage can be deceptive. For example, citrus leaves are often deformed and misshapen when they unfurl because of aphid feeding during the early developmental stages. By the time the leaves emerge, the aphids have moved on. Needless to say, any control measures at this time would be futile.

Aphids produce honeydew, a sticky substance that can turn into a sooty mold that is unsightly and cause customers to complain. Honeydew is loaded with sugar and is a favored food of ants. Ants will literally "farm" aphids, gently moving their rotund little bodies from branch to branch and plant to plant. Given the new feeding territory, the aphids rapidly reproduce and create more honeydew, which feeds more ants, which move the aphids to new feeding territory, which means more aphids...you get the picture. Bottom line is if you can control ants, you can control the spread of aphids. (Drenching their nests thoroughly with diazinon, Dursban or any of several registered pesticides should do the trick; you can also control root aphids with soil drenches.)

With the aphid's incredible reproductive capacity, you might wonder why we're not up to our necks in them. Fortunately, Mother Nature has a system of checks and balances. In addition, with the right timing, it's relatively easy to deal with infestations.

KEEP POPULATIONS IN CHECK

Your first line of defense against aphids is to do nothing. That's right; just kick back and let nature take it's course. Aphids are a favored delicacy for many of their fellow fauna. For example, in Southern California's Huntington Botanical Gardens, roses are never sprayed because a dozen different critters find the aphids the meal of choice. Predators include parasitic wasps, lacewings, syrphid fly larvae, ladybird beetles, praying mantis, lizards and several types of birds. With all that help, why spray at all?

The standard advice in many "organic gardening" books is to blast them off with jets of water. While this can be extremely satisfying and work well on a regular basis, if you are only visiting a property once a week it might not be enough.

Aphids favor tender green shoots. "Forcing" green growth with large applications of nitrogen is like ringing the dinner bell. Use balanced, slow-release fertilizers at recommended rates to avoid quick flushes of soft growth.

If client complaints, concerns about viral infection or huge populations make spraying a necessity, there are several chemicals that can make short work of aphids.

The most benevolent and least harmful to beneficial insects is a soap solution. Although folklore says that any old soap will do, it is highly recommended that you use a botanical soap formulated for insect control to reduce any chance of phytotoxicity.

If heavier artillery is required, systemic insecticides such as Cygon or Orthene should do the trick. These work especially well when aphids are ensconced in tightly rolled new leaves, where contact insecticides might not reach them. The old standbys, diazinon and malathion also work, but keep in mind that they can also kill off beneficials, too.

For aphids that overwinter on trees and shrubs, a dormant oil spray applied in winter will control them before they hatch.

Aphids prefer somewhat moist, humid conditions. In the Southwest, once the summer sets in their numbers will drop dramatically unless the plants are overhead-irrigated regularly.

Control measures for aphids need to be tailored to each situation. Healthy, vigorous plants in out-of-the-way locations might not need more than a blast of water, or even no control at all. On the other hand, heavy infestations in areas where the plants are showcased might call for chemical controls. Always read and follow label directions to the letter.

Aphids are as much a part of springtime as the first daffodil of the season. However, armed with the proper knowledge, you can control their populations and keep your projects looking good throughout the year.


 
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