Relative Significance of Direct Ingestion and Adult-Mediated Translocation of Bait to German Cockroach (Dictyoptera: Blattellidae) Nymphs

Abstract

A novel experimental design that selectively excluded feeding of adults or nymphs on insecticidal baits was used to distinguish mortality caused by ingestion of bait from mortality caused by horizontal transfer of insecticide by foraging to nonforaging cockroaches. In large cage laboratory assays and in apartments, exposure of Blattella germanica (L.) to an insecticidal bait containing hydramethylnon resulted in high mortality in adult females and 1st instars. However, exclusion of adult females from feeding on the bait resulted in a significant decline in mortality among nymphs, suggesting that neonate mortality was caused primarily by adult-mediated horizontal toxicant transfer through feces. A reciprocal experiment provided support for this hypothesis: Adult females with access to bait transferred insecticide to neonates that were prevented from feeding on bait, resulting in high mortality in both groups. Conversely, mortality among 2nd instars was high and significantly less dependent on adult foraging, suggesting a shift to active foraging (i.e., direct ingestion of bait) during the 2nd stadium. We conclude that horizontal toxicant transfer is a key factor in suppression of cockroach pest populations. Small nymphs, especially 1st instars, which forage infrequently and are therefore least vulnerable to direct contact with insecticides, are most susceptible to this type of insecticide translocation. Horizontal toxicant transfer should be optimized to deliver insecticides and pathogens to nonforaging stages of B. germanica.