Predator vs. the bug that ate Washington

The tiny aphid is a formidable enemy for gardeners.
The tiny aphid is a formidable enemy for gardeners. (Bigstockphoto)
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By Barbara Damrosch
Special to The Washington Post
Thursday, April 22, 2010

"Infestation" is an alarming word. Organisms, perhaps tolerable in small numbers, assemble in unnervingly large ones and overrun your garden, your home or you. Aphids, also known as plant lice, are especially good at this, ganging up on greenery and sucking out its juices before they move on, sometimes spreading plant viruses as they go. Look closely at a suffering plant, with the foliage at its growing tips yellowed, distorted or gone, and you may see hundreds of tiny aphid bodies clinging to the stems and to the undersides of the leaves.

Your next thought will be of action, but as in any grade-B creature feature, your opponent is more clever and interesting than you are in your role of action hero. Most aphids are females, and if a few of them find your plant, attracted by succulent young tip growth, they will give birth to vast numbers of live, wingless young, each of which will give birth to more of the same. So fecund is this insect that its offspring can be pregnant even before they are born. When life gets too crowded, or they run out of food, some aphids give birth to winged babies that fly off in search of better fare.

Aphids also make deals with other species. For example, some are willingly farmed by ants, who feast on the sticky-sweet "honeydew" that aphids secrete, and protect them in return.

Protection is something aphids need. Predator insects such as lady beetles and lacewings, as well as larger feeders such as birds, are always after their soft, defenseless bodies. No wonder they breed in such numbers.

To keep aphid populations in balance, you want to avoid toxic sprays that harm natural predators. When aphids enter our greenhouse in summer, the sparrows that fly in and out of the roof vents notice them before we do and take care of the situation.

Such beneficial predation can occasionally backfire. Braconid wasps can come in and parasitize aphids on our greenhouse spinach, creating little mummified capsules that are harder to wash off the leaves than the aphids are. We also find that while mature plants are not bothered by an aphid attack, small seedlings can be greatly weakened. In both cases, the problem occurs in a greenhouse where plants have overwintered in the soil. It happens because the aphids are responding to the high nitrogen levels in the greenhouse, where little irrigation is needed. The solution is to irrigate the soil thoroughly in mid-January to flush out any excess nitrogen. Nitrogen promotes growth that is especially nutritious to aphids.

In short, if you encourage a healthy biodiversity on your property, avoid poisons, use compost instead of high-nitrogen fertilizers and flush away built-up nitrogen in your soil, aphids are less likely to congregate. If they do, a forceful squirt with a garden hose is usually enough to defeat them. As I said, your role is not a dramatic one. But you'll like the happy ending.

Damrosch is a freelance writer and the author of "The Garden Primer."

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