The Role of Weather and Entomophagous Species in the Natural Control of Insect Populations12

Abstract

Weather sets the basic framework within which certain average but fluctuation insect population densities may be obtained in the absence of other possible regulatory factors. These average population densities may be very high or very low. Weather regulates insect population only by being of sufficient severity to restrict the size, quality (including food and favorability), and/or numbers of inhabitable spots in a given area—that is, its effect is obtained through interaction with micro environments. If overlying weather severity in a given spot or area is not relatively great, then, theoretically, populations will be able to increase indefinitely over a period of time unless limited by other factors. Illustrations of population regulation by weather and methods of proof of such regulation are discussed. The other principal factors capable of regulating insect population densities are (1) natural enemies (parasites, predators, and pathogens), (2) food (quantity and quality), (3) inter specific competition (other than natural enemies), and (4) intra specific competition. The last three factors usually do not control populations at low (uneconomic) levels and hence are not generally important factors in applied insect ecology. Natural enemies may regulate insect populations at low levels even if all other factors are exceptionally conducive to increase. Several experimental methods for proving the extent of regulation of incest populations by entomophagous species are given. Thirteen examples of adequate proof of control of low density population regulation of the California red scale by its natural enemies is given. It si shown that “upsets” of natural enemy effectiveness by adverse factors can result in various differing average population densities of the California red scale (Aonidiella aurantii (Mask.), even on adjacent citrus trees.