Interactions between the European red mite, Panonychus ulmi (Koch), and the predaceous mite Amblyseius potentillae (Garman) were studied under semicontrolled glasshouse conditions in two types of ecosystems: single leaves, isolated by a sticky barrier at the petioles, and eight-leaf seedlings. Three different starting numbers of the prey (one, four, and eight females, respectively) were established in each type of ecosystem, and a single adult female predator was introduced 10 days later. Predator-free controls were also included.

In the single-leaf systems, prey:predator ratios at the beginning averaged 23, 94, and 152 to I, respectively. Suppression of the Panonymus ulmi populations from the peaks occurred within about seven days after predator introductions. In some cases the prey population “escaped” control after the initial suppression, because the predators declined either to extinction or to one or two nonreproductive individuals.

In the eight-leaf ecosystems, the numerical response of the predator population was somewhat more delayed, and the prey population peaks attained before initial suppression averaged higher than those of the single-leaf systems. There was considerably more variation among replicates on these more complex systems. The experiments showed that Amblyseius potentillae had the potential for responding to and suppressing a wide range of densities of Panonychus ulmi. As much as a 14-fold increase in predator populations in two weeks was observed, and the cases of an inadequate numerical response to overtake an increasing prey population were attributed, not to inadequate reproductive capacity, but to other factors, such as failure to find prey or mates.