Natural Enemies of the Spotted Alfalfa Aphid in Utah1

Abstract

Seven main groups of predators of the spotted alfalfa aphid were collected from alfalfa fields near Delta, Utah: Coccinellidae, Orius, Nabis, Geocoris, Collops, Chrysopa and syrphid flies. These predators fed largely on the pea aphid (Macrosiphum pisi (Harris)) until the spotted alfalfa aphid (Therioaphis maculata (Buckton)) invaded the area. The effectiveness of predators depended on the availability of host insects and climatic conditions. The coccinellids, as a group, proved to be the most effective predators. Within this group, Hippodamia convergens G.-M. was the most beneficial, numerous and widely distributed predator. The species of Orius and Collops were abundant enough in many cases to be of real importance in the control of this aphid. The predatory species, without question, exerted a considerable controlling effect on the spotted alfalfa aphid populations. The predators held the overwintered spotted alfalfa aphid population in cheek until insecticides were applied for lygus bug control. The chemical treatments of alfalfa fields had generally adverse effects on the predators. The decrease in the number of predators and favorable environmental conditions enabled aphids to build up to the point that the application of insecticides for their control became quite generally necessary. The use of chemicals produced a sudden but temporary reduction in the population of aphids.