Methamidophos Application Effects on Pasimachus elongatus (Coleoptera: Carabidae): An Update

Abstract

Population densities of a predatory ground beetle, Pasimachus elongatus LeConte (Coleoptera: Carabidae), were examined at the Central Plains Experimental Range, Colorado. Using distance sampling theory, I analyzed density estimates dating from before an application of the insecticide methamidophos (1988) and three beetle generations afterward (1994) to address the following questions. Have population densities of P. elongatus recovered to prespray levels? Are current population densities of P. elongatus in areas exposed to methamidophos in 1988 equal to those not exposed in 1988? How do population densities of P. elongatus vary with habitat type in methamidophos exposed and nonexposed areas? This work updates earlier findings that P. elongatus population densities decreased after methamidophos applications. Lingering differences in P. elongatus densities were found among areas differing in methamidophos exposure; these differences were also related to habitat type. Higher population densities were found in methamidophos exposed than in control areas; higher densities were also found in shrub dominated than in grass dominated sites, regardless of past methamidophos exposure. Because potential prey species were found to be equally abundant in methamidophos exposed and nonexposed areas despite the greater abundance of the carnivorous Pasimachus in treatment areas, the P. elongatus density patterns seen are discussed in relation to possible long-term disruption of the arthropod prey community by methamidophos. Other explanations for the observed patterns (including competitive release and physiological resistance) are also discussed.