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COG Organic Field Crop Handbook

1.8 Pest and Disease Management


Organic farmers do not usually have major problems with insects and plant diseases in field crops. There are two factors working in the organic farmer’s favor. Plant and insect diversity is greater in the more complex agro-ecosystem, and plants fertilized by slow-release compost are more resistant to insects and disease. Problems with insects and disease can usually be traced to either the health of the soil or inappropriate crop rotation.



1. A diverse ecosystem is the first line of defense

A crop grown as part of a rotation will not be as vulnerable to pests. Growing the same crop, or those susceptible to the same disease, in the same place, results in the progressive build-up of disease organisms in the soil. Rotations break the life cycle of pest species. For example, corn rootworm is only a problem if corn is grown in the same place two years running.

The vast majority of insects cause no economic damage, and the best form of pest control is to have a balance of pests and predators on the farm. Providing a range of habitats to encourage a diverse population of wildlife on the farm, including spiders, birds, frogs and toads, will help to control pest species.

A range of shelter and food can be provided for the predators and parasites by intercropping (having two crops grow together like grains and legumes, or corn and beans), strip cropping (e.g., strips of row crops alternating with forage species), keeping hedgerows (rows of natural bush surrounding the fields) and even allowing some weeds grow in the field.


[photo: 8.1: Strip cropping]

Some weeds can have a positive effect on the dynamics of beneficial insects; parasitic wasps for example, are attracted to weeds with small flowers. Research has shown that outbreaks of certain crop pests are more likely in weed-free fields. Obviously, care is needed to avoid weed competition with the crop, and some weeds could favor the pest, so each situation needs to be looked at separately. In some cases, mowing weeds at the field edge results in beneficials moving into the crop where they are needed.

Some plant species act as attractants for certain insects, and growing that plant together with the production crop, or around the edge of the field or along the fence rows can benefit your production crop. For example, sawflies prefer to lay their eggs in bromegrass rather than spring wheat, and as the bromegrass also harbors many of the native parasites of the sawfly, few sawflies survive. In cases where a crop is disliked by a pest insect, the crop can be used as a barrier. Planting yellow sweet clover around the edge of a wheat field has been suggested as a means of preventing grasshoppers from moving into the wheat.



2. Maintaining a balance of nutrients in the soil

The importance of a balanced source of fertilizer on the health of the soil cannot be over-emphasized. There is a great deal of anecdotal evidence that organic farms have a lower incidence of disease. One explanation for this is that the slow release of nutrients from compost and green manures maintains the biological balance in the soil.

A disturbed mineral balance in the soil and subsequently in the plants can lead to metabolic disorders in livestock. For example, grass tetany in livestock occurs most frequently when grazing heavily-fertilized pastures. It is caused by low blood magnesium, resulting from deficiencies of magnesium in the forage brought about by high levels of nitrogen and potassium in the soil.

It is important to match crops to the fertility level of the soil. After a high nitrogen plowdown, do not grow oats; plant a crop that is a heavy feeder. Weak crops or crops lush from too much nitrogen are more attractive to pests and fungal disease. The reproductive rates of most pest insects are proportional to the supply of certain amino acids in their diet. Excess fertility increases the supply of these amino acids in plant tissue and the pest numbers increase too rapidly for natural enemies to control. Low levels of soil nitrates reduce the incidence of pest outbreaks.

Research at Tunworth Farm in Nova Scotia demonstrated how the interactions between soil, weeds and insects on an organic farm are important for the natural control of pests. Weeds in fababeans provided habitat for lady bugs and other natural enemies of the aphid and also helped to lower soil nitrogen. Low levels of soil nitrate minimized luxury uptake of N by the fababeans, keeping reproductive rates of the aphids low so infestations were effectively controlled before the pod-fill stage when they are most damaging to the crop. Yields were higher than in weed-free plots. If there were high levels of soil nitrogen weeds reduced yields.


[Illustration fababeans or ladybug line drawing]



3. Environmental conditions

Adverse environmental conditions can put crops under stress, making them more vulnerable to pests and diseases. Proper management can modify these conditions or prevent them from arising, making the environment more amenable to crops.

Select the appropriate field for the crop. Moisture -- too little or too much -- can be a source of stress. If crops are drought-stressed, they are more vulnerable to insects. On the other hand, if alfalfa is planted in poorly drained soil, it will be vulnerable to root rot such as Phytophthora.

Environmental management can provide optimum growing conditions. Plant windbreaks to modify air flow, help snow retention and conserve moisture. Research has shown that yields increase 10 per cent in corn and 20 per cent in soybeans in the lee of windbreaks up to a distance of 12 times the height of the barrier on the downwind side and 5 times on the windward side.


[photo 8.2: Farm windbreak]


4. Preventative management

Other management practices will help in pest control.

• Change the planting date to avoid certain problems. For example, barley yellow leaf is transmitted by aphids. Late planting of winter wheat and early seeding of spring grains will help avoid infection.

• Use sound, clean, high-quality seed from disease-free fields. Seed cleaning is crucial because debris associated with seed, like plant residue and weed seeds, can carry diseases.

• Seeding disease-resistant varieties is helpful but not the final answer. Disease-resistant varieties for many of the rusts and smuts are available.



5. Grain storage

Insects and fungi can multiply rapidly in moist grain but if the grain is dry and temperatures are low there should be no problems. Insects do not develop in grain with temperatures below 18C; however, a temperature of 5-10C is best for long term storage. Aeration will help dry and cool down grain. Bins should be thoroughly cleaned with a good vacuum cleaner before storing new grain.

Moulds are more troublesome in moist grain and therefore if the grain has more than14 per cent moisture, it will need aeration. As the air outside the bins drops in temperature, condensation may develop on the inside wall of the bins, creating an environment favorable for moulds. Aeration is necessary if this happens. A perforated drainage pipe with the end capped can be stretched across the bin and around the perimeter, and then connected to a fan on the outside which will circulate air in the bin. A fan with a one-half horsepower motor will work well.

Diatomaceous Earth (DE) is used for insect control by some organic farmers when storing grain. Empty bins can be treated by lightly coating walls and floor, especially around the circumference and at the door, or it can be added to the grain as it is being stored. Products containing DE are registered for use in grain handling and storage areas but present regulations prevent the adding of any powder to grain destined for export or to be delivered into the commercial grain elevator system. There are beneficial insects that will control insect pests in stored grain but none of these biological controls are registered for use in Canada.



Further reading

Altieri, M., Agroecology: The Scientific basis of Alternative Agriculture, 2nd ed., Boulder, CO, Westview Press, 1987, 160 pp.

Hanley, P. ed., Earthcare: Ecological Agriculture in Saskatchewan, Wynward, Sask., Earthcare Group, 1980, 236 pp.

Kock, H., "Shelterbelt agriculture uses trees to protect soil and water resources", Sustainable Farming, Summer 1990, pp. 18-19

Patriquin, D., "The Ecology of Transition", COGNITION, Vol. 12, No. 4, October 1988, pp. 8-13

Patriquin, D., "On-Farm Research Reveals Links between Nitrogen, Intercrops, Weeds and Pests", REAP Newsletter, Vol. 2, No. 1, Summer/Fall 1988


Copyright 1992 Canadian Organic Growers. Inc

Reprinted with permission. All rights reserved.


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