An ecological study of the spider miteOligonychus punicae(Hirst) and its natural enemies

Abstract

The population dynamics of the avocado brown mite, Oligonychus punicae (Hirst), and its natural enemies was studied over a four-year period in six avocado orchards located in two different climatic areas in southern California. Annual peak levels of abundance of O. punicae ranged from 4 to 84 adult females per leaf. The seasonal increase usually began in early summer, with a peak in late summer, followed by an abrupt decline. The increase was sometimes delayed until late summer, with the peak in fall or early winter. Abrupt declines were commonly associated with biotic factors: predation, intraspecific competition, or both. The most abundant natural enemies were a lady beetle, Stethorus picipes Casey, and two phytoseiid mites, Amblyseius hibisci (Chant) and A. limonicus Garman and McGregor. The beetle appeared to be the key predator. When it responded numerically to population increases of spider mites before they reached high numbers, control resulted before serious leaf damage occurred. A. hibisci usually failed to show a numerical response soon enough to overtake an increasing population of O. punicae. Possible factors limiting the effectiveness of A. hibisci were a relatively low reproductive rate when feeding on spider mites, failure to congregate and oviposit on infested leaves, and inability to gain access to prey that was well protected beneath silken webbing. A. limonicus was found to be a more effective predator than A. hibisci, since it could suppress relatively high populations of O. punicae, but it occurred only in orchards near the ocean. Other insect predators reported, in larval or adult form, are the six-spotted thrips, Scolothrips sexmaculatus (Pergande); a cecidomyid fly, Arthrocnodax occidentalis Felt; a staphylinid beetle, Oligota oviformis (Casey); the green lacewing, Chrysopa carnea Stephens; and dusty-wings (Coniopterygidae). None of these was observed to keep pace with rapid increases of the prey, even though some species were present early in the season, when the prey density was still extremely low.