Most fruit growers have used cover crops for many years as a part of the standard fruit production system. Cover crops help minimize soil erosion, improve soil structure and increase soil organic matter. Although most growers seed grasses in the alleyways, other broadleaf species inevitably make their way into the orchard floor species mix (as weeds). While some broadleaf weeds provide good habitat for certain orchard pests, they also improve the habitat for natural enemies. Where mixed species cover crops are grown, insect species diversity is greater, including beneficial insects.
Conserving predator mites
Living plants--cover crops or weeds--provide an improved habitat for predatory mites on the orchard floor compared to bare ground. This is particularly important for populations of the mite predator Amblyseius fallacis, which will build by feeding on twospotted spider mites on broad leaf plants on the orchard floor. These predators spend a large amount of time down in the ground cover, suppressing twospotted spider mites before they ever get up into the trees. Increased populations of predators can effectively suppress pest mite populations, reducing or even eliminating the need for miticides.
Careful orchard floor management can improve the effectiveness of this strategy. Orchards where a herbicide strip is kept completely bare right up to the tree have fewer predatory mites in the trees than in orchards where cover crops or weeds grow directly under the tree. Limbs hanging low into the ground cover offer multiple routes for the predators to migrate into the trees. If trees are pruned high, the only route between the ground cover and the tree is the trunk.
Delaying herbicide application in the spring allows weeds to grow up enough to provide the improved habitat the predator mites need. Once the predators have moved into the trees to feed on the pest mites, herbicides can be used.
Biological control of
Where mixed species of grasses and clovers have been planted under the trees and in alleyways, the ratio of beneficial nematodes to plant parasitic ones increases significantly. Soil quality and nutrition is improved, leading to a better habitat for the beneficial nematodes. One caveat Although cover crops can be an integral component of conservation biological control, it isn't clear what impact competition with the trees will have on long term tree growth and yields.
C. E. Edson and J. E. Nugent. 1998. Role of cover crops in orchards biological control. Crop Advisory Team Alert 13 (18): 8-9.
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