INSECT MANAGEMENT
BIOLOGICAL CONTROL
Encouraging Beneficials
Increasing the populations of natural enemies of pests (beneficials) by providing shelter, food, moisture, overwintering sites, and freedom from pesticides is an important component of IPM programs. Production fields represent disturbed habitats where pest populations tend to increase faster than populations of beneficials. Maintaining stable, varied, natural habitats with a succession of diverse plants will encourage beneficials. Closeness to woody areas is likely to increase the numbers and types of predators.
In some cases, cover crops can be managed to provide habitat for beneficials. Leaving strips of cover crops allows populations of beneficials to increase throughout the season. This is true for both fall and spring cover crops, as long as the cash crop is grown soon after the cover crop reaches full size but before it dries down. If cover crops are to be mowed, mowing only part at any given time will allow for higher carryover of populations of beneficials than if the whole plot were mowed at once.
Flowering plants that provide habitat for beneficial insects and spiders include yarrow, dill, angelica, anthemis, aster, caraway, cosmos, fennel, hyssop, lovage, mint, showy goldenrod, sweet goldenrod and tansy. Most composites will attract beneficials, but some are more effective than others. Tansy, a vigorous spreading perennial, provides homes for the largest variety of insects (including beneficials) by far of all the composites. Cosmos and marigolds, on the other hand, attract fewer beneficials than most composites. Other crops and the beneficials they attract are shown in the table below. Some of these (berseem clover, cowpea, crimson clover, buckwheat, hairy vetch and white clover) can be used as cover crops.
Minimizing application of insecticides and using selective rather than broad-spectrum insecticides preserves natural enemy populations since beneficials are often more sensitive than pests to commonly used pesticides. Insecticides which kill only when eaten by the pest will not harm beneficials but some stomach poisons also act as contact poisons.
In scouting for pests, some growers also scout for beneficials and base treatment choices on the expected population balance. For example, they might choose not to treat if the number of beneficial predators or parasites seemed likely to minimize crop damage. Successful use of natural enemies for control of pests in vegetable crops may require educating buyers and consumers. A California grower reported that migrating wasps had effectively parasitized the aphids in his spinach fields, and he relied on the wasps rather than a final chemical spray to control aphids. The wasps did their job but his buyer demanded a lower price on the spinach because he could see "little black bugs" in the cartons. These were, of course, the tiny predatory wasps escaping.
Plants attracting beneficials may also attract insect pests. In making tradeoffs between damage to pests and damage to beneficials, encouraging beneficials should be a means to control pests, not an end in itself. Experiments in Michigan, for example, found increased numbers of eggs and larvae of the imported cabbage worm and the parasite Cotesia rebecula on broccoli interplanted with nectar-producing anise hyssop than when the broccoli was planted alone or beside the anise hyssop.
Table 4.1. Plants that attract beneficial insects
| Plant | Attracts |
|---|---|
| Berseem clover | Big-eyed bugs |
| Black locust | Lady beetles |
| California lilac | Hoverflies |
| Caraway | Lacewings, hoverflies, insidious flower bugs, spiders, parasitic wasps |
| Common knotweed | Big-eyed bugs, hoverflies, parasitic wasps, soft-winged flower beetles |
| Cowpea | Parasitic wasps |
| Crimson clover | Minute pirate bugs, big-eyed bugs, lady beetles |
| Flowering buckwheat | Hoverflies, minute pirate bugs, predatory wasps, tachinid flies, lacewings, lady beetles |
| Hairy vetch | Lady beetles, minute pirate bugs, predatory wasps |
| Queen Anne's lace | Lacewings, predatory wasps, minute pirate bugs, tachinid flies |
| Soap-bark tree | Hoverflies, green lacewings, brown lacewings |
| Spearmint | Predatory wasps |
| Sweet alyssum | Tachinid flies, hoverflies, chalcids |
| Subterranean clover | Big-eyed bugs |
| Sweet fennel | Parasitic wasps, predatory wasps |
| Toothpick ammi | Hoverflies, minute pirate bugs, soft-winged flower beetles, tachinid flies |
| Tansy | Parasitic wasps, lady beetles, insidious flower bugs, lacewings |
| White sweet clover | Tachinid flies, bees, predatory wasps |
| Wild buckwheat | Hoverflies, minute pirate bugs, tachinid flies |
| Yarrow | Lady beetles, parasitic wasps, bees |
Spiders are effective general predators and will readily colonize areas with a favorable habitat. Spiders prefer areas with high humidity and stable temperatures. Experiments at the University of Tennessee found a significant reduction in populations of aphids, flea beetles, blister beetles and Colorado potato beetles in plots with high spider populations.In these experiments, plots with grass mulch hay had up to ten times more spiders than bare-ground plots.
For more information on Biological Control: Cornell
bridgesj@unity.ncsu.edu