Mary E. Barbercheck, Penn State University
The Benefits of Diversity
Adding plant diversity to a production system in space and time can help break pest cycles. Plants in the same family tend to have similar pests. Crop rotation, planting a series crops from different plant families in the same space in sequential seasons, helps deter the build up of pests that can occur when one crop species is planted continuously. Crop rotation, as described in Section 205.205 in the National Organic Program final rule, is fundamental to good organic management and serves several functions. As stated in the rule, the producer must implement a crop rotation including but not limited to sod, cover crops, green manure crops, and catch crops. Crop rotation provides the benefits in addition to providing pest management in annual and perennial crops, including: maintenance or improvement of soil organic matter content; management of plant nutrients; and erosion control.
Spatial crop diversity can be achieved through crop rotation and various forms of polyculture, e.g., strip cropping, multiple cropping, or interplanting of plant species or varieties. A general effect of polyculture is a spatial mixing of crops, which can slow the build-up and spread of pests during the growing season. There are two main hypotheses to help explain how plant diversity acts to keep pest populations low. The first is based on plant apparency. It has been hypothesized that when a host plant is growing among non-host plants that the host plant is less apparent to an insect and, therefore, more difficult to locate. The second hypothesis is based on natural enemies. It has been hypothesized that plant diversity in a system increases the kinds of resources available to natural enemies of insects and encourages their occurrence, and predation and parasitism of insect pests.
An interesting application of crop diversity to control insect pests is perimeter trap cropping. In perimeter trap cropping, the crop being protected (main crop) is surrounded completely by a planting of one or two rows of a trap crop. The perimeter trap crop encloses the main crop and intercepts and concentrates incoming pests, where they can be killed. The trap crop may also serve as habitat for beneficial organisms. Perimeter trap crops must be present before or at the same time as the main crop to intercept pests. If the trap crop is maintained in a vigorous condition, the pest may not leave it. However, for maximum effectiveness, the perimeter trap crop should be treated with an allowed substance as soon as pests appear or begin to build up in numbers.
References and Citations
- Barbercheck, ME. and D. Calvin. 2007. Pest management in Organic Systems. In: The Agronomy Guide 2007- 2008.. Part 1, Chapter 11. (Available online at: http://agguide.agronomy.psu.edu/cm/sec11/sec112.cfm) (verified 17 March 2010).
- United States Department of Agriculture. 2000. National organic program: Final rule. Codified at 7 C.F.R., part 205. (Available online at: http://www.ecfr.gov/cgi-bin/text-idx?SID=a6a0935ddf00e166695f4c2138bd58d8&mc=true&node=pt7.3.205&rgn=div5) (verified 17 March 2010).