Ecologically Based Systems, Page 2
|A beneficial stinkbug preys on a Colorado potato beetle larva, helping this potato plant retain leaves. |
Photo by Eleanor Groden.
More than 100,000 species of insects, plants, vertebrates, nematodes and microorganisms inhabit any given farm. Only several dozen are potential problems. Fewer still - less than a dozen - will feed on or crowd out crops in a given year.
Understanding a Pest's Strengths and Weaknesses
Pests generally succeed because they have adapted to the resources - food, water, shelter and light - they find. They explode into major problems when the factors that normally keep them under control are limited or missing. By recognizing the needs and abilities of a pest, and by designing a system that works against its preferences, producers can reduce pest numbers and pest-inflicted damage.
'The laws of nature demand that we look at the whole system,' says John Teasdale, a weed scientist with USDA's Agricultural Research Service in Beltsville, Md. 'To control any individual organism, one needs to understand how it relates to the ecosystem in which it operates.'
Many pests have impressive abilities to reproduce often and disperse widely. Although these pests may face competition from other organisms or attacks by enemies, they thrive by rapidly colonizing new habitats before their competitors or antagonists arrive. Summer annual weeds such as redroot pigweed, insect pests such as aphids and many diseases share such characteristics. Annual monoculture cropping systems - subjected to the repeated disruptions caused by tillage, planting, herbicide applications, cultivation and harvest - open many inviting habitats for such 'hit-and-run' pests.
Other pests tend to be 'stand-and-fight' types. Better adapted to the difficulties of competition and to withstanding attacks by their enemies, they thrive in long-term perennial systems. These pests, such as perennial weeds, often live for a long time. Pests like the soybean cyst nematode go through dormant stages and wait for the right opportunity to establish. While they may produce fewer offspring than the 'hit-and-run' pests, 'stand-and-fight' pests invest more energy into the care of those offspring. Expect a 'stand-and-fight' weed to have large seeds, tubers or rhizomes. In addition, like quackgrass, they compete vigorously, squelching their opponents' growth in one-on-one competition.
What Makes a Plant Susceptible to Pests?
While you cannot change a pest's basic character, you can change your management practices to decrease a crop's vulnerability. Understanding what makes a crop susceptible to pest attack is critical to devising management strategies that reduce crop losses, pesticide use and associated costs.
Monoculture plantings are more susceptible to pest pressure than mixed stands. Specialized disease-causing organisms and plant-feeding insects are less likely to bother crops that grow amid other types of plants. Not only does a pest find it more difficult to locate its preferred host in a mixture, but the pest's natural enemies are often more abundant or effective in mixed plantings.
Conversely, large fields of single crops create an ideal environment for pest attack. When crops are genetically uniform, as most modern varieties are, the opportunity for pest damage is greater still.
Plants under stress from drought, a lack of nutrients or other factors are more vulnerable to pests. Some plants are actually better food for their pests when under stress. Aphids, in particular, take advantage of the nutrients available in stressed plants. Recent research has shown that when fed on by certain insects, some plants emit chemical signals that call natural enemies to their aid.