Reducing Pest Pressure, Page 4

	Crimson clover is often grown in California nut groves and orchards to provide nitrogen to tree crops. Its eye-catching flowers produce abundant nectar for bees and contain pirate bugs that prey on small pests like thrips. Photo by Ray Weil.

Create Multiple Stresses on Pests

Maximizing the impacts of many 'little hammers' takes an understanding of the life cycles of pests and of beneficial organisms. In ecologically based systems, farmers scrutinize the life cycles of pests and beneficial organisms, looking for times and places where small control measures can add up to big results. A good opportunity might arise during a pest's overwintering stage, for example, and another while it is first colonizing the crop. A beneficial organism may offer protection at one stage and need protection at another. Even in small increments, pest mortality can eventually pare a big problem to a low level.

In Sentinel Butte, N.D., cattle producer Dennis Dietz is battling a 75-80 percent leafy spurge infestation with several species of imported flea beetles after herbicides had little effect on a pest that plagues farmers in the western Plains. The flea beetles, however, are leaving their mark.

'I've seen dramatic changes in stem count and flowering,' says Dietz. 'My feeling is that the control is excellent, and it's long term.' Established in his own on-site insectary plantings, the flea beetles 'will be there forever,' he says. 'Chemical control, in my opinion, is way too expensive and it doesn't last as long.'

Not only do the adult beetles feed on the tops of leafy spurge, but flea beetle larvae burrow into the weed's roots, exposing them to a second stress: opportunistic disease organisms.

Cooperating in a 'Team Leafy Spurge' project through the USDA Agricultural Research Service in Sidney, Mont., Dietz has begun to add a third stress - sheep - to his biocontrol strategies. Their mission: graze off the tops of the plant while the beetles work on the roots.

Discourage the pests' dispersal or connection with crops. Interspersing non-host plants can hinder the movement of insect pests and crop disease organisms by altering light and humidity and jumbling the critical visual and chemical signals insect pests use to recognize their hosts. Non-host plants put distance between susceptible plants and, like fly paper, intercept spores to limit the spread of diseases.

Flea beetles that attack cabbage and other crucifers are less abundant when clover - a non-host species - is sown between cabbage rows. Crop losses to mildew are higher in pure stands of barley than in mixtures of barleys that differ in their susceptibility to disease races.

Disrupt pest populations by destabilizing habitat. When pests are adapted to some crops but not to others, rotations that include non-host crops can help with control. Placing a non-host crop in a rotation sequence often destroys the habitat a pest needs, limiting population growth. Infestations of Colorado potato beetles, for example, are more severe in continuous potatoes than in potatoes that follow winter wheat or rye. Similarly, rotating soybeans with such non-hosts as corn reduces charcoal rot.

To be suppressed by rotation, a target pest must have specialized feeding habits that restrict it to a narrow host range. Its ability to move to other locations also must be low. Finally, its dormant and resting stages must be shorter than the time gap between susceptible crops.

Rotations that include diverse crops and management practices tend to have fewer weed problems than simple rotations and crop monocultures. By continually changing the 'rules of the game,' complex rotations discourage the selection and adaptation of weeds.

Giant foxtail, for example, is less of a problem in a three-year corn-soybean-winter wheat rotation than in corn-soybean or continuous corn. Differences in the timing of germination, growth and competitiveness among the three crops, plus the suppressive effects of wheat straw, are likely reasons.

Reduce weeds' access to resources. Many crops lack canopy cover and substantial root growth when they are young, limiting the plants' ability to fully capture sunlight, water and nutrients. Instead, these crops give invading weeds a foothold. Later in the season, weeds will compete with the crop and reduce its yield. Narrower row spacings, higher crop population densities and intercropping may all rob weeds of the resources they need to grow.

On the other hand, these same weed-choking practices can increase disease levels in some crops. Denser stands of wheat, for example, are more susceptible to mildew. Use disease-resistant varieties with such practices.

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