1. Ecological Principles And Applications

Understanding the interactions among pests, natural enemies, host plants, other organisms and the environment  improves pest management decisions. Understanding the ecological factors that allow pests to adapt and thrive in a particular ecosystem will help to identify weak links in the pests' life cycle and factors that can be manipulated to manage them.     

Carrying Capacity

"The law of constant final yield" (Kiraet et al. 1953) states that yield is constant over a wide range of densities for many wild plants. Does this law apply to cultivated crops like rice? Because of the application of fertilizers, the availability of essential minerals are not necessarily a limiting factor to yield as density increases. But what happens to the pests (e.g., insects) feeding on the rice as we provide nutrients? The maximum population size of a species that can be supported indefinitely in an environment is called the "carrying capacity". By adding fertilizer to the soil of the rice field we increase carrying capacity (K) of the rice environment. This also increases K of the rice field for insects, pathogens and weeds. Increasing K of rice pests also increases K of their natural enemies. If the EIL is greater than the carrying capacity, then the species is self-limiting and cannot be considered a pest

Some species have K>EIL, but have a typical abundance below EIL due to natural enemies. Such species are potential pests that may become actual pest if natural enemies are removed, so that and the rate of population increase of the pest exceeds the rate of mortality.  The initial population size (Nt), births (B), deaths (D), immigration (I) and emigration (E) determine the present pest population. Births and immigration increase pest population while deaths and emigration reduce it. The present population of a pest can be described by the equation: Nn = Nt + B - D + I - E.