Evaluation of Zetzellia mali1 as a Predator of Panonychus ulmi2 and Aculus schlechtendali3

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Abstract
The predatory behavior and distribution on unsprayed apple leaves of Zetzellia mali (Ewing) was studied to determine its role as a predator of Panonychus ulmi (Koch) and Aculus schlechtendali (Nal.). Z. mali feed on egg stages of P. ulmi . Occasionally the predator attacked resting and nymphal stages. Adults were not attacked. Female predators consumed a maximum of 1 P. ulmi egg/day. The predator fed on active stages of A. schlechtendali . Female predators consumed a maximum of 15 A. schlechtendali adults/day. The predator occurred only on the underside of the leaf, particularly along the midrib. Therefore, the prey on top of the leaf were immune to attack. Also, Z. mali occurred primarily on short side branches or spur growth whereas A. schlechtendali occurred primarily at the tip of the branches containing terminal leaves. Segments of the prey population were always immune to attack because of their local distribution on widely scattered parts of the leaves and trees. In other words, spatial heterogeneity was too great for effective predator control of the prey. This coupled with the fact that adult ♀ P. ulmi are not attacked suggests that Z. mali alone cannot maintain the population of its prey below economic damage levels. The practical worth of Z. mali , however should be determined only in conjunction with actual spray practice and in association with other predators.

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