Mary E. Barbercheck, Penn State University
Insect populations are dynamic; the number of individuals in a population may change from day to day, season to season, or year to year as a result of interactions with the environment (Pedigo and Rice, 2006). The environment itself is variable, and can provide different availability of resources that insects need to survive. The amount of resources available can affect the size that an insect population can attain. This concept is sometimes referred to as the carrying capacity of an environment (Schowalter, 2006). Resources that provide food or shelter are sometimes called “bottom up” factors. Populations can also be limited by natural enemies or infection by insect disease-causing organisms, sometimes called “top down” factors. Planting a non-preferred crop plant through rotation or resistant varieties, and conserving natural enemies combines both bottom-up and top-down factors to lower insect pest populations (Tscharntke and Hawkins, 2002).
The ability of some factors to keep pest populations in check can be influenced by the density of the pest population (Pedigo and Rice, 2006). These are called density dependent factors. For example, a parasitoid wasp may be able to attack a large proportion of insect pests when the pest population density is low, but may be overwhelmed by high pest densities. This is one reason that it is important to scout a crop to assess if pest population densities are increasing, staying the same, or decreasing. If pest densities exceed the ability of natural enemies to regulate the population, remedial control measures using another tactic may be necessary.
Pest populations can also be changed by environmental factors that are independent of the density of an insect population. These are called density independent factors. For example, weather conditions that create temperature or moisture conditions that exceed the tolerance of an insect can negatively affect survival, growth, reproduction or dispersal, or even eliminate local populations. Examples include droughts or a late-spring hard freeze. A management example is the use of flaming to control Colorado potato beetles. Regardless of the amount of food resources available or the numbers of Colorado potato beetles, flaming kills all stages by creating very localized, lethal high temperature and low moisture conditions. The use of an allowable insecticide that kills 90% of the pest insects regardless of their density is another example of a density independent mortality factor.
Pedigo, L.P., and M.E. Rice. 2006. Entomology and pest management. 5th ed. Pearson Prentice Hall. Columbus, OH.
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