Tolerance and "soft" techniques … ways to control greenhouse pests

Chilly fall weather kills some pests, forces some into dormancy, and drives still others into paradise-your greenhouse. There they have the warmth, food, water, and shelter they need to prosper. But the greenhouse also offers a good environment for trying out relatively nontoxic (to you) controls.

We learned the techniques listed here from public conservatories around the West. To protect visitors, gardeners for these public greenhouses have to keep plants looking good without using many of the toxic chemical controls common in commercial greenhouses.

These "soft" (low-toxicity) controls can be applied in your own greenhouse to manage the eight most common greenhouse pests, shown above. Although our focus is insects and mites, we include one rodent-the mouse-because it can do extensive damage in some greenhouses by eating flowers and fruits.

The first steps: learn a little about entomology, a lot about tolerance

Never expect to eliminate plant damagers completely; even if you kill all adults, their offspring often left behind in the form of eggs or larvae-will come back to haunt you. it's more realistic simply to work toward keeping them under control.

Use the pictures above to garner some basic identification skills, so you can be constantly on the lookout for plant problems. Although the pests on our list take many forms, you need only recognize the general type.

Learn which plants in your greenhouse are especially susceptible to harmful insects. Gardeners at all the conservatories said they watch susceptible plants closely; some said they don't grow chronically infested kinds of plants (Abutilon is a good example: it harbors whiteflies).

Using insects to fight insects

As one gardener we talked with put it, "There's just one problem with beneficial insects used outdoors: they dine and dash." But in a greenhouse, natural control is a realistic option because you can close the door. It isn't perfect-insects come and go through cracks and vents-but they usually stay long enough to do real work. Some kinds will even breed in your greenhouse.

Because insects can multiply so quickly, you need to respond right away if a problem develops. Note when and where you spot them (they often show up on new growth first); the bug board pictured below is a good tool, used by the Sherman Gardens Conservatory in Costa Mesa, California.

Order predators early, allowing a week or so for shipping, then be patient while they work. Bruce Tanner at Crystal Gardens in Victoria, British Columbia, says that at first the predators may not seem to be doing their jobs. But control will come as long as the infestation isn't too advanced.

Choosing soft" or chemical controls

If you plan to use predatory insects in your greenhouse, don't spray unless pest populations seem out-of-hand or are growing so fast that they'll be that way before a shipment of predators arrives.

Almost all the gardeners we talked with agreed that it's better to spot-spray than to apply sprays over the whole greenhouse, which can kill beneficial and harmful insects alike. If a plant is overrun, it's best to remove it from the greenhouse until you've made the plant pest-free.

If you need to use a spray, try one that was popular in our survey (see chart above). Bacillus thuringiensis (BT) got high marks against caterpillars and loopers. As its name implies, it's a bacterium, not a chemical. For chemical control, try insecticidal soap and encapsulated pyrethrum (a chrysanthemum derivative; encapsulation extends the life of the active ingredients).

Gardeners at Golden Gate Park's conservatory in San Francisco have found that insecticidal soap is remarkably more effective when mixed with 110[deg] water and sprayed immediately than when mixed with cooler water. The soap can, however, damage many plants, including some acacias, cinerarias, Easter lilies, ferns, orchid flowers, and poinsettias.

Sources of controls

BT, insecticidal soap, and pyrethrum are available from most garden centers.

You can get beneficial insects from a number of mail-order sources, including Harmony Farm Supply, Box 460, Graton, Calif. 95444, (707) 823-9125 (catalog $2); Integrated Fertility Management, 333 Ohme Gardens Rd., Wenatchee, Wash. 98801, (509) 662-3177 (free catalog); and Peaceful Valley Farm Supply, Box 2209, Grass Valley, Calif. 95945, (916) 272-4769 (catalog $2).

All these sources take credit-card orders by telephone, then have insects shipped directly from insectaries, so turnaround can be fast. (TABULAR DATA OMITTED)