Strategies to Enhance Beneficials

One of the most powerful and long-lasting ways to minimize economic damage from pests is to boost populations of existing or naturally occurring beneficial organisms by supplying them with appropriate habitat and alternative food sources. Beneficial organisms such as predators, parasites and pest-sickening "pathogens" are found far more frequently on diverse farms where fewer pesticides are used, than in monocultures or in fields routinely treated with pesticides.

The following characteristics are typical of farms that host plentiful populations of beneficials:

■ Fields are small and surrounded by natural vegetation.

■ Cropping systems are diverse and plant populations in or around fields include perennials and flowering plants.

■ Crops are managed organically or with minimal agrichemicals.

■ Soils are high in organic matter and biological activity and — during the off-season — covered with mulch or vegetation.

To conserve and develop rich populations of natural enemies, avoid cropping practices that harm beneficials. Instead, substitute methods that enhance their survival. Start by reversing practices that disrupt natural biological control, such as insecticide applications, hedge removal and comprehensive herbicide use intended to eliminate weeds in and around fields.

Even small changes in farming routines can substantially increase natural enemy populations during critical periods of the growing season. The simple use of straw mulch provides humid, sheltered hiding places for nocturnal predators like spiders and ground beetles. By decreasing the visual contrast between foliage and bare soil, straw mulch also can make it harder for flying pests like aphids and leafhoppers to "see" the crops they attack. This combination of effects can significantly reduce insect damage in mulched garden plots.




Michigan State University scientists have evaluated orchard-scale ground cover experiments in established commercial orchards and in a new tart cherry orchard at the Northwest Horticultural Research Station. They studied orchard floors covered with compost, mulch or cover crops such as crimson clover, berseem clover, white clover, white Dutch clover, Michigan red clover, crown vetch, indigo vetch, alfalfa, rye, annual ryegrass, hard fescue and Buffalo grass. So far, findings include:

Season-long populations of beneficial mites were attributed to the use of a red clover cover crop.

Season-long, vegetation-free strips using either herbicide or mulch increase pest mite populations.

Orchards with ground covers—irrigated but not treated with herbicides to manage weeds— had fruit yields that were not significantly lower than conventional practices over a five-year period. Note the irrigation may be critical in this system to prevent the ground cover from competing with the fruit trees for water. Adding mulch, cover crops and/or compost increases soil organic matter, populations of beneficial soil microbes and amounts of active soil carbon and nitrogen available to trees. Fewer beneficial nematodes, more plant-parasitic nematodes and more nitrate leaching were associated with lower-quality conventional-system soils. Hay or straw mulch, applied 6 to 8 inches deep, improved tree growth and yields despite higher pest mite populations. Nitrate leaching—greatest in spring and fall—was substantially reduced by vegetation growing under trees during these periods.

■ In-row soil population densities of beneficial nematodes, mycorrhizae and earthworms were greater under an organic production system.

■ Young trees benefited from adding mulch or compost but can be severely stunted by competition with groundcover plants for moisture and nutrients.

■ Trees with heavy mulches produced soft fruit in two of seven years.

The scientists also are examining the impact of mixed-species hedgerows on insect pest movement into and out of orchards. In addition, they are evaluating insect pheromone mating disruption, mass trapping of plum curculio,14-inch groundcover bands around mulched center lines, and intercropping with such income-generating woody species as sea buckthorn and Siberian pea.

Orchards offer advantages over annual row crops in biological pest control, says MSU IPM tree fruit integrator David Epstein. Because they do not undergo major renovation every year, orchard systems can be developed to let beneficials get established. "Ground covers can be used to encourage beneficials to build up their populations and remain in the orchard throughout the year," he says.

How much the beneficials actually reduce pests, however, depends on weather, pest populations and the effectiveness of growers' monitoring programs. "To say that if you plant red clover you'll never have to spray for mites again would be erroneous," says Epstein. "But if you know what's out there—what levels of pests, predators and parasit-oids you have—then you can make an informed decision as to whether or not you can save a spray." (For more information about this project, see http://www.

Charles Edson, Mich. State Univ.

Charles Edson, Mich. State Univ.

Season-long populations of beneficial mites were attributed to the use of a red clover cover crop.

As with most strategies described in this book, multiple benefits accrue from diversification. For example, carefully selected flowering plants or trees in field margins can be important sources of beneficial insects, but they also can modify crop microclimate, add organic matter and produce wood or forage. Establishing wild flower margins around crop fields enhances the abundance of beneficial insects searching for pollen and nectar. The beneficials then move into adjacent fields to help regulate insect pests. As an added benefit, many of these flowers are excellent food for bees, enhancing honey production, or they can be sold as cut flowers, improving farm income.

Valerie Berton, SARE

Valerie Berton, SARE

Straw mulch provides hiding places for such nocturnal predators as spiders and ground beetles.

Increase the population of natural enemies. To an insect pest, a fertilized, weeded and watered monoculture is a dense, pure concentration of its favorite food. Many have adapted to these simple cropping systems over time. Natural enemies, however, do not fare as well because they are adapted to natural systems. Tilling, weeding, spraying, harvesting and other typical farming activities damage habitat for beneficials. Try instead to support their biological needs.

To complete their life cycles, natural enemies need more than prey and hosts; they also need refuge sites and alternative food. For example, many adult parasites sustain themselves with pollen and nectar from nearby flowering weeds while searching for hosts. Predaceous ground beetles — like many other natural enemies — do not disperse far from their overwintering sites; access to permanent habitat near or within the field gives them a jump-start on early pest populations.

caution Uorrr populations of spiders and ground beetles only works if the pests attacking your crops are prey for those predators.

Provide supplementary resources. You can enhance populations of natural enemies by providing resources to attract or keep them on your farm. In North Carolina, for example, erecting artificial nesting structures for the red wasp (Polistes annularis) intensified its predation of cotton leafworms and tobacco hornworms. In California alfalfa and cotton plots, providing mixtures of hydrolyzate, sugar and water increased egg-laying by green lacewings six-fold and boosted populations of predatory syrphid flies, lady beetles and soft-winged flower beetles.

You can increase the survival and reproduction of beneficial insects by allowing permanent populations of alternative prey to fluctuate below damaging levels. Use plants that host alternative prey to achieve this; plant them around your fields or even as strips within your fields. In cabbage, the relative abundance of aphids helps determine the effectiveness of the general predators that consume diamondback moth larvae. Similarly, in many regions, anthocorid bugs benefit from alternative prey when their preferred prey, western flower thrips, are scarce.

Another strategy is to augment the population of a beneficial insect's preferred host. For example, cabbage butterflies (a pest of cole crops) are the preferred host for two parasites (Tricho-gramma evanescens and Apanteles rebecula). Supplemented with continual releases of fertile females, populations of this pest escalated nearly ten-fold in spring. This enabled populations of the two parasites — both parasitic wasps — to buildup earlier in spring and maintain themselves at effective levels all season long. Because of its obvious risks, this strategy should be restricted to situations where sources of pollen, nectar or alternative prey simply can't be obtained.

Manage vegetation in field margins. With careful planning, you can turn your field margins into reservoirs of natural enemies. These habitats can be important overwintering sites for the predators of crop pests. They also can provide natural enemies with pollen, nectar and other resources.

Many studies have shown that beneficial arthropods do indeed move into crops from field margins, and biological control is usually more effective in crop rows near wild vegetation than in field centers:

Jack Kelly Clark, UC

Jack Kelly Clark, UC

Select flowering plants that attract beneficial insects, such as this adult syrphid fly.

If you are tolerating or enhancing pest or host populations in order to provide continuing resources for beneficial organisms, be sure to monitor these populations carefully as they can build to economically-damaging levels.

In Germany, parasitism of the rape pollen beetle is about 50 percent greater at the edges of fields than in the middle.

■ In Michigan, European corn borers at the outskirts of fields are more prone to parasitism by the ichneumonid wasp Eriborus terebrans.

■ In Hawaiian sugar cane, nectar-bearing plants in field margins improve the numbers and efficiency of the sugar cane weevil parasite (Lixophaga sphenophori).

■ In California, where the egg parasite Anagrus epos (a parasitic wasp) reduces grape leafhopper populations in vineyards adjacent to French prunes, the prunes harbor an economically insignificant leafhopper whose eggs provide Anagrus with its only winter food and shelter.

Lady beetles follow food sources from field margins into cash crops over the course of the season.

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