Ecological Understanding of Insects in Organic Farming Systems: How Insects Damage Plants

Feeding Damage

Most damage to plants caused by insects is a result of direct feeding on above-ground and below-ground plant parts. The type of feeding damage caused by insect pests is related to the type of mouthparts of the insect (Cranshaw, 2004; Pedigo and Rice, 2006).

• Insects with chewing mouthparts, for example, grasshoppers, caterpillars, and beetles, cause feeding damage such as holes or notches in foliage and other plant parts, leaf skeletonizing (removal of tissue between the leaf veins), leaf defoliation, cutting plants off at the soil surface, or consumption of roots.
• Some insects with chewing mouthparts bore or tunnel into plant tissue. Stem-boring insects can kill or deform individual stems or whole plants. Leaf mining insects feed between the upper and lower surfaces of leaves, creating distinctive tunnel patterns visible as translucent lines or blotches on leaves.
• Insects with sucking mouthparts, such as aphids, scales, leafhoppers, and true bugs, feed by sucking sap from plant tissues. This type of feeding can cause spotting or stippling of foliage, leaf curling, and stunted or misshapen fruits.
• Some insects, for example thrips, have rasping mouthparts that scrape the surface of foliage or flower parts. Thrips suck up the spilled contents from the damaged cells.

Oviposition Damage

Insects can also cause injury to plants when they lay eggs (oviposit) into plant tissue. Heavy oviposition into stems can cause death or dieback of stems or branches on the plant. Dieback of the ends of stems or branches is often called flagging. Oviposition in fruits can result in misshapen or aborted fruits, sometimes called cat-facing. Gall-forming insects cause their host plants to grow abnormally. Depending on the insect species, the gall formation can be stimulated by feeding or by oviposition (egg-laying) into plant tissue.

Transmission of Plant Pathogens

Some insects are associated with the transmission of plant disease (Agriotos, 1997). Insects that transmit plant disease are called vectors. Because of the greater risk of economic loss from insect-plant disease associations relative to damage from insects that do not transmit disease, there is usually a very low tolerance for the presence of insect vectors. Producers that expect pests that transmit plant disease need to plan ahead—for example, using disease-resistant or tolerant crop varieties, timing planting to avoid exposure to vectors, using practices or materials like row covers to exclude insects, monitoring crops carefully, and being ready to react quickly with rescue treatments if the insect population begins to increase. Most of the plant diseases transmitted by insects involve plant viruses, but there are examples from all plant pathogen groups—fungi, bacteria, mollicutes, protozoa, and nematodes. Insects transmit plant disease in three main ways:

Accidental or Incidental Transmission

Insect damage creates an “infection court”. The plant disease organism (pathogen) gains entrance into the plant tissue through feeding or oviposition wounds caused by insects.

Phoretic or Passive Transmission

The insect carries the plant pathogen on its body from one plant to another. An example is fire blight of pears and apples, caused by Erwinia amylovora. The bacterium is picked up on the feet and mouthparts of bees and flies when they visit flowers on diseased trees, and can be carried to healthy trees.

Active Transmission

The pathogen is carried within the body of the insect and the plant is inoculated with the pathogen when the insect feeds on an infected plant and then moves on to a healthy plant to feed. Examples include many aphid-transmitted viruses of cucurbits and potatoes, and bacterial wilt of cucurbits transmitted by cucumber beetles.

References and Citations

• Agriotos, G. N. 1997. Plant pathology. 4th edition. Academic Press. San Diego, CA.
• Cranshaw, W. 2004. Garden insects of North America. Princeton University Press, Princeton, NJ.
• Pedigo, L. P., and M. E. Rice. 2006. Entomology and pest management. 5th ed. Pearson Prentice Hall. Columbus, OH.

Additional Resources

• Altieri, M., C. I. Nichols, and M. A. Fritz. 2005. Manage insects on your farm: A guide to ecological strategies. Sustainable Agriculture Network Handbook Series Book 7. Available online at: http://www.sare.org/Learning-Center/Books/Manage-Insects-on-Your-Farm (verified 25 April 2011).