An Insecticidal Check Method for Measuring the Efficacy of Entomophagous Insects1

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Abstract
A new type of check-plot method is described, which utilizes treated or insecticidal checks for measurement of the efficacy of entomophagous insects. An insecticidal treatment, such as DDT, is used to eliminate or greatly reduce the populations of entomophagous insects involved, while having little or no effect on the host population. Comparison of a treated plot with one having introduced enemies or the normal complement of natural enemies shows, by the differences in the 2 host populations, just how valuable the parasites or predators are. Field expts. are described in which citrus trees infested with the long-tailed mealybug, Pseudococcus longispinus, were treated with DDT in talc, with talc alone, or were untreated. Three evaluations were made possible as a result, namely: (1) the effect of parasites alone, (2) the effect of predators alone, and (3) the effect of parasites and predators combined. Predators were shown to be the major limiting factor, since mealybug populations were kept down in their presence, but increased to a certain extent in their absence and in the presence of parasites. Other field comparisons involved citrus plots infested with the cottony-cushion scale, Icerya purchasi, which were treated with DDT, or were untreated. The untreated plots showed a remarkable build-up of the predatory vedalia, Rodolia cardinalis, and virtually the complete elimination of the cottony-cushion scale. The DDT treatments killed the vedalia present and kept them from re-entering the plots and becoming effective throughout the remaining observational period. As a result, the population of cottony-cushion scale remained very high.

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