Use of Cover Crops and Green Manures to Attract Beneficial Insects

Cover crops are an integral part of a sustainable vegetable system. Besides building soil fertility and suppressing weeds, they can affect a farm's insect community. Attention to the effects of cover crops on insect populations can result in improvements in insect management. Manipulation of cover crops for insect pest control is a complicated proposition. It is never as simple as attracting beneficial insects and repelling pest insects. A given cover crop can be attractive to either pest insects or their predators for several reasons. Many cover crops provide a supplemental food source to insects in the form of nectar from their flowers. Cover crops can also provide shelter for insects. Insects which obtain food or shelter from a cover crop can in turn act as a supplemental food source to predatory insects.

Insect interactions with cover crops can result in either positive or negative effects on the crop plant. Positive interactions include:

1) cover crop is more attractive to pests than cash crop (referred to as trap cropping);

2) cover crop makes cash crop more difficult to locate;

3) cover crop is attractive to predators of insect pests;

4) cover crop provides nectar or other food source which sustains predator when pest
insect is not present, allowing a higher population of the predator than possible on a food source of only the pest population.

Negative interactions include:

1) cover crop provides habitat or food source for pest insect at a time when cash crop cannot support pest population;

2) cover crop attracts predator insects away from cash crop;

Management implications are complicated by the fact that a cover crop could act as either a source, a sink, or both, for beneficial and pest insects. A cover crop which attracts pest insects away from a cash crop can cause disaster if mowed or plowed at the wrong time. This can cause the pests living in that field to be released into a neighboring crop field. Not mowing can be just as disastrous if the cover crop flowers and then senesces (Bugg 1992).

In spite of the complicating factors, some general strategies can be recommended. The simplest strategy is to provide a diverse array of vegetation so that the habitat for insects is as varied as possible. This can include selecting a set of cover and cash crops so that something is always flowering on the farm. Using more than one cover crop for a given cover crop niche is a good way to increase diversity. For instance, Sudan grass and buckwheat are both good summer smother crops that have quite different insect associations. Rather than selecting one or the other, a grower might use both on different parts of the farm. Another strategy is to plan cover crops so that they flower sequentially. One example is waiting until a stand of buckwheat has flowered before plowing in a rye/vetch stand. Growers could also extend the flowering season of particular cover crop by mowing high or by mowing or plowing in portions, so as to leave some habitat in place at all times (Bugg, 1992). Strip cropping (the practice of growing crops in adjacent strips) can do much to increase the spatial heterogeneity and to bring beneficial-attractant cover crops into closer proximity to the cash crops (Dufour and Greer, 1995). A vegetable farmer in Wisconsin had a good deal of success in reducing insect problems by mowing less frequently and higher. He also plans to add permanent hedgerows approximately every sixty feet on his farm. This same grower estimated that 20 percent of his acreage was devoted to beneficial habitats (Cicero, 1993).

Additional levels of control can be achieved for growers who learn insect identification and monitor the population dynamics on their farm. For instance, cover crops could be planted that harbor beneficial insects and that die back or are mowed at just the time a nearby cash crop is particularly susceptible to invasions by pest species (Bugg 1992). Given the vagaries of weather and the double-edged nature of releasing insects into the system, this approach may be difficult to manage.

The table below lists beneficial and pest insects that are attracted to or harbored by common cover crop types. It is important to note that beneficial and pest insect species are not absolute categories. For instance, a cover crop that supports a population of aphids can sustain predatory insects at a time when the cash crop does not provide such prey. The aphids could also migrate to and damage the cash crop. Pest insects that can sometimes provide benefits to a cash crop are described in parentheses in the table.

Insects attracted to common cover crop species:

Cover Crop Beneficial Insects Pest Insects
Buckwheat Extra floral nectaries attract parasitic wasps; ladybugs; tachinid and hover flies; and lacewings Tarnished plant bugs and aphids (aphids can act as a food source for beneficials)
Clovers (differences among the various clover species--see references for more details Parasitic wasps, big-eyed bugs, minute pirate bugs, ladybugs, tachinid flies and aphid midges Spider mites and flower thrips (flower thrips can prey on spider mite eggs and provide food for several predatory insects)
Hairy vetch Minute pirate bugs; ladybugs; predatory and parasitic wasps Tarnished plant bugs
Cereals Ladybugs Aphids

Insect associations with cover crop plants provide yet another opportunity for a grower to manage pests through careful observation and attention to the details of pest populations. The simplest strategy is to plant cover crops which favor more beneficial insects than pest insects. More complicated schemes put specific cover crops next to cash crops that are benefitted by their insect associates. A high diversity of cover crop species can also help assure that there is habitat for predatory insects at all times.

Plant species not typically used for cover crops which attract beneficial insects

There are many other plant species which attract beneficial insects. Beneficial insect populations can be positively influenced by mixing some of these other species into the cover crop mix. Maintaining permanent strips of perennials at field borders, or as dividers between sections of a field, can also be advantageous. The flowers of the Umbelliferae family are attractive to beneficials, especially parasitic wasps. Caraway, dill and fennel all are members of that family which can be sold as herbs (Poncavage, 1991). Many commercial flowers, especially those in the Compositae family (sunflowers, asters and goldenrods), are also attractive to ladybugs, pirate bugs, big-eyed bugs and spined soldier bugs. Gloriosa daisy, purple coneflower and black-eyed Susan are all examples of this family which are relatively easy to maintain in permanent beds, and which are readily salable as cut flowers (Cicero, 1993). A number of beneficial-attractant cover crop mixes are commercially available (the most complete listing of them is in Dufour and Greer, 1995). A possibly cheaper alternative to these seed mixes could be to mix some flowering plant seeds into cover crop seed.

Conclusion

Both beneficial and pest insect populations can be managed through planting cover crops and other plants attractive to insects. Maintaining a healthy diversity of flowering plants throughout the farm and throughout the season can be a successful way to reduce insect pest problems. More complicated and probably more successful strategies include strip cropping and careful attention to cover crop-insect interactions. It is probably best to make cover crop decisions based on the benefits they provide to fertility and weed control, but integrating their effects on the insect community can provide one more level of control to the vegetable grower.

Jeremy Plotkin, Student, University of Maine
Proceedings - NEVBC 1999

 
References

Bugg, RL. 1992. Using cover crops to manage arthropods on truck farms. Hortscience 27(7)741-744.

Bugg, RL. 1991. Cover crops and control of arthropod pests of agriculture. In: WL Hargrove, ed.. Cover Crops for Clean Water. Proc of an Int Conf Soil and Water Conservation Society, Ankeny, IA, pp. 157-163.

Bugg, RL and RT Ellis. 1990. Insects associated with cover crops in Massachusetts. Biological agriculture and Horticulture 7:47-68.

Cicero, K. 1993. Making a home for beneficial insects. The New Farm. February, pp. 28-33.

Dufour, R. and L. Greer. 1995. Current Topics in Sustainable Agriculture: Farmscaping. ATTRA Publication, Fayetteville, AK.

Gilkeson, L and J. Grossman. 1991. The Organic Gardening guide to important beneficial insects and mites of North America. Organic Gardening. May/June. pp. 46-53.

Poncavage, J. 1991. Beneficial borders. Organic Gardening. May/June. pp. 42-47.

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