Fighting insects with living mulches

Bu Joel Grossman

Living mulches or cover crops' primarily used for soil conservation and weed control, but they also reduce pest insect populations in a number of crops. Competition with the main crop may lead to a reduction of yields. but this problem can be overcome by careful choice of plants and cultural techniques. Success is likely to depend on soil type. increased levels of soil nitrogen and water and may be site specific. Under optimal conditions, some crops can be grown with no loss of yields and with reduced insecticide applications.

Cover crops are grown prior to the main crop to protect soil from erosion, Improve soil and water quality, reduce nematode and soil pathogens and suppress insects and weeds (Hargrove 1991). According to Graham and Crabtree (1987), Ma living mulch Is a cover crop that Is grown simultaneously with the main crop In a reduced tillage system." It may also be considered an intercrop or a "smother , crop." Living mulches can reduce soil erosion and weed invasion and may lead to improved water infiltration and soil structure. They can also be used to deter insect pests and encourage natural enemies (Flach 1988). Since living mulches may actually be sown before, simultaneously with, or after the main crop, the terms cover crop, living mulch, and living cover crop are often used interchangeably.

Phases of development

Research on living mulches seems to proceed through three phases of development. First, a mulch with desirable characteristics that complement the main crop is found. Then, effects on soil erosion, weeds, and insects are documented while an economic crop is produced. Use of the living mulch usually depresses yields due to competition with the cash crop for nutrients and other resources, and thus the third phase of research is finding ways to restore yields.

Success with Living Mulches

Michael Costello (1992), an entomologist at the University of California (UC), successfully went through these three phases to find a clover-based living mulch system to produce broccoli without yield losses while controlling insects and weeds. In an attempt to provide as much protection against colonizing aphids as possible, he used a living mulch that provided 83% soil cover. Weeds and insects were suppressed, but his early experiments had unacceptable yield reductions because the soil there was a clay loam which held water and nutrients but had a restrictive pore space, and thus limited plant rooting ability. Less pore space led to increased competition for oxygen between broccoli and the living mulch, resulting in lower yields. This problem was eliminated when he moved to a light, well drained, and well aerated sandy loam soil

To further reduce competition between crop and mulch, Costello resorted to early season suppression of the mulch by mowing To give the crop an advantage in the living mmulch he provided extra irrigation water, and used applications of synthetic fertilizers. When compost was applied instead of soluble fertilizers, yields were reduced. Apparently, at the application rate used, the amount of available nitrogen in the compost was too low to ! counter the competitive effects of the mulch, and the compost acted like a low nitrogen fertilizer. By finding the proper soil type, providing sufficient irrigation water and fertilizer and mechanically suppressing the cover crop early in the season, Costello was able to achieve broccoli yields in the living mulch that were equivalent to yields in clean cultivated broccoli. Evolving the perfect system, how ever, took four years of work. Costello's experience shows that living mulch techniques can be optimized by proper choice of crop site, crop, and match. Yield reductions can be overcome by extra work and resources without the use of herbicides. Herbicides can be avoided by corrective management practices such as strip tillage, mowing the living mulch and careful timing of plantings.

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