Ecological Understanding of Insects in Organic Farming Systems: Insect Populations

Factors that Affect Insect Population Size

Populations are groups of individuals of the same species. For example, all the individual the Colorado potato beetles in a potato field comprise a population. Each insect population has particular characteristics that are relevant to management: density, dispersion (the pattern of occurrence within an area), birth rate, death rate, immigration (movement into a habitat), emigration (movement out of a habitat), and age distribution (Pedigo and Rice, 2006).

Immigration, emigration, birth rate, and death rate influence the rate at which insect populations increase or decrease locally. Age distribution can also influence changes in population size and amount of damage to crops. For example, if adults are removed from the population before mating, or prevented from mating (for example, through the use of pheromone-based mating disruption), then no or few eggs will be produced. Sometimes, different life stages of insects rely on different food sources and habitat. For example, the adult moth of the black cutworm, Agrotis ipsilon, does not damage plants. However, the larvae feed on a wide variety of crop plants, often cutting seedlings off at the soil surface.

Density is the number of individuals per unit of area, for example:

• 1 beetle larva per cubic foot of soil
• 10 caterpillars per 10 feet of crop row
• 100 aphids per leaf

For some crops, insect densities that trigger therapeutic management of a pest have been developed as part of the integrated pest management (IPM) program for a crop (Pedigo and Rice, 2006). This level is sometimes called the economic injury or action threshold. Economic injury levels are dynamic and depend on:

• crop value
• management costs
• degree of injury per insect
• crop susceptibility to injury

This action threshold can be tempered by other factors, for example, the vigor of the crop, or densities of natural enemies or the level of disease in the insect population. The goal of therapeutic management should be to reduce pest population to a level below which they will not cause economic injury, rather than to completely eliminate every pest. It is important to note that IPM systems that have been developed for non-organic systems are not always transferable to organic systems. For more information, see the related eOrganic article on differences between IPM in organic and conventional systems.

The dispersion of insects describes their arrangement in space, for example:

• random
• aggregated into clumps
• uniformly dispersed in a regular pattern

Most insects are dispersed in an aggregated or clumped distribution. Clumped distributions of insects can result from the response of individuals to environmental conditions or resources, including:

• food
• water
• shelter
• potential mates
• sites for laying eggs (oviposition)

Therefore, changing the availability of a resource can affect insect populations. For example, planting a resistant crop variety or including a crop that a particular insect cannot use as a food plant in a crop rotation removes food resources. Using mating disruption with pheromones can prevent male insects from finding potential mates.

Aggregation can also result from other types of behavior. Some insects are social during part or all of their lives, and live in aggregations. Some examples include honeybees, ants, and termites.

References and Citations

• 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).
• Wikipedia contributors. Population Dynamics [Online]. Wikipedia, The Free Encyclopedia. Available at: http://en.wikipedia.org/w/index.php?title=Population_dynamics&oldid=416474225 (verified 25 April 2011).
• Radcliffe, E.B., W. D. Hutchison, and R. E. Cancelado [eds.]. Radcliffe's IPM World Textbook [Online]. University of Minnesota. St. Paul, MN. Available at: http://ipmworld.umn.edu (verified 15 March, 2010).