Other species

Buckwheat Cinquefoil

Carum carvi Coriandrum sativum Anethum graveolens Foeniculum vulgare Ammi majus Daucus carota Ammi visnaga Pastinaca sativa

Gaillardia spp.

Echinacea spp. p. 52

Coreopsis spp. Cosmos spp. Solidago spp.

Helianthus spp. p. 41

Tanacetum vulgare Achillea spp.

Medicago sativa Vicia grandiflora Vicia fava

Vicia villosa p. 75

Melilotus officinalis

Aurinium saxatilis Berteroa incana

Brassica spp. p. 39

Lobularia maritima p. 37

Barbarea vulgaris Brassica kaber

Fagopyrum esculentum p. 39

Potentilla spp.

Using weeds to manage insect pests could create more problems than it solves. Start with small test areas that you can monitor regularly. Observe flowering activity and prevent weeds from going to seed. See additional precautions in this section.

less apparent to their prospective attackers. In other cases, it is because the alternative resources provided by weeds support beneficials.

Unquestionably, weeds can stress crops, but substantial evidence suggests that farmers can enhance populations of beneficials by manipulating weed species and weed-management practices. A growing appreciation for the complex relationships among crops, weeds, pests and natural enemies is prompting many of today's farmers to emphasize weed management over weed control.

Using weeds in your biological control program will require an investment of time and management skills. First, define your pest management strategy precisely, then investigate the economic thresholds that weeds should not exceed. If you choose to work with weeds in your biodiverse farming system, consider the following management strategies:

■ Space crops closely.

■ Limit weeds to field margins, corridors, alternate rows or mowed clumps within fields, rather than letting them spread uniformly across fields.

■ Use species sold in insectary plant mixtures.

■ Prevent or minimize weed seed production.

■ Mow weeds as needed to force beneficial insects into crops.

■ Time soil disturbances carefully — for example, plow recently cropped fields during different seasons — so specific weeds can be available when specific beneficials need them.

■ Except in organic systems, apply herbicides selectively to shift weed populations toward beneficial weed species.

Scott Bauer, USDA ARS

Scott Bauer, USDA ARS

Dandelions are an important early-season source of nectar and pollen for beneficial insects.

Enhance plant defenses against pests. The first line of defense against insect pests is a healthy plant. Healthy plants are better able to withstand the onslaught of insect pests and can respond by mobilizing inbred mechanisms to fight off the attack. You can enhance natural defenses by improving soil and providing the best possible growing conditions, including adequate (but not excess) water and nutrients.

As plants co-evolved with pests, they developed numerous defenses against those pests. Some of those defenses have been strengthened over time through plant breeding, while others have been lost. Some plant defenses — spines, leaf hairs and tough, leathery leaves — are structural. Others are chemical:

■ Continuous, or constitutive defenses are maintained at effective levels around the clock, regardless of the presence of pests; they include toxic plant chemicals that deter feeding, leaf waxes that form barriers, allelopathic chemicals that deter weed growth and other similar defenses.

■ Induced, responses are prompted by pest attacks; they allow plants to use their resources more flexibly, spending them on growth and reproduction when risks of infection or infestation are low but deploying them on an as-needed basis for defense when pests reach trigger levels.

The most effective and durable plant defense systems combine continuous and induced responses. Under attack by a plant-eating insect or mite, a crop may respond directly by unleashing a toxic chemical that will damage the pest or obstruct its feeding. It may also respond indirectly, recruiting the assistance of a third party.

Many plants produce volatile chemicals Excess nitrogen that attract the natural enemies of their at- fertilizer may ham-

tackers. To be effective, these signals must per cotton's ability to send a be identifiable and distinguishable by the chemical ca" for help. predators and parasites whose help the crop is enlisting. Fortunately, plants under attack release different volatile compounds than plants that have not been damaged. Crops can even emit different blends of chemicals in response to feeding by different pests. Different varieties of the same plant — or even different parts or growth stages — can release different amounts and proportions of volatile compounds.

Joe Lewis, USDA ARS

Joe Lewis, USDA ARS

In response to attack by insect pests, cotton emits a chemical signal calling beneficial wasps to the rescue.

Leaves that escape injury also produce and release volatiles, intensifying the signaling capacities of damaged plants.

For example, when a beet armyworm chews on a cotton plant, the plant releases a specific chemical signal blend into the air. Female parasitic wasps pick up this signal and use it to locate the armyworm. They sting the army-

worm and lay their eggs inside it, Plant varieties are causing an immediate and dramat-LAUIIUN! not equal in their ic reduction in armyworm feeding. abilities to defend themselves: some This greatly reduces damage to the modern varieties lack the defenses of plant that originated the signal. In-their native predecessors. terestingly, inappropriate levels of added nitrogen can change the ratio of the molecules that comprise the chemical signal, thereby changing the signal and rendering it unnoticeable by the wasp.

Plant breeding — though overwhelmingly beneficial in the short term — can have unforeseen consequences that unravel the best-laid plans of plant geneticists. Since the focus of plant breeding for pest resistance is often limited to a specific plant/pest interaction, selecting for one resistant gene could inadvertently eliminate other genes affecting other pests or genes that play a role in attracting the natural enemies of the pest.

In addition, newly developed varieties may stand better or yield more at the expense of natural defenses that are often unintentionally sacrificed for those other qualities. Selecting for one trait such as height could mean selecting inadvertently against any one of the many inborn plant defenses against pests. For example:

■ Scientists in Texas found that nectar-free cotton varieties attract fewer butterfly and moth pests, as their developers intended; however, as a consequence, these varieties also attract fewer parasites of tobacco budworm larvae and are thus more susceptible to that pest.

■ According to USDA Agricultural Research Service scientists in Gainesville, Fla., today's higher-yielding commercial cottons produce volatile chemical signals at only one-seventh the level of naturalized varieties, impairing their ability to recruit natural enemies.

Fortunately, our knowledge of plants' roles in their own defense is steadily expanding. This knowledge can be used to breed and engineer plants whose defenses work harmoniously with natural systems. More research as well as plant breeding programs that focus on enhancing natural defenses are needed. Such programs might emphasize open-pollinated crops over hybrids for their adaptability to local environments and greater genetic diversity.

Horticulturist Philip Forsline examines hybrid grapes developed in a uSDA breeding program.

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