Coprophagy Facilitates Horizontal Transmission of Bait Among Cockroaches (Dictyoptera: Blattellidae)

Baits offer several advantages over other insecticide formulations in the control of populations of the German cockroach, Blattella germanica (L.). However, they may fail to target certain life stages that feed only sparingly. Recently we have demonstrated that 1st instars are significantly more vulnerable to insecticidal baits when adults are present. By preventing adults or nymphs from eating bait we now conclude that adults translocate insecticide bait to the shelter, thus facilitatingahorizontaltransferoftheinsecticidetonymphs.Bytrackingbaitmovementwithatracer dye, we show that nymphs take up adult-delivered bait via coprophagy. An alternative hypothesis, that adults delivered novel food odors to nymphs thereby stimulating them to forage and eat bait, was experimentally rejected. Analysis of time-lapse video records showed that 1st instars foraged sparingly compared with 2nd instars and adults, indicating that direct ingestion of a remotely placed bait accounted for little, if any, mortality in 1st instars. The magnitude of coprophagy in 1st instars was related to the proximity of the food to their aggregation site; nymphs ate significantly more adult feces when food was far from the shelter. We conclude that aggregating 1st instars are relatively sedentary, and that they depend on conspecific foragers to deliver widely dispersed food. Innovative baiting strategies should therefore maximize forager-mediated translocation and delivery of slow- acting bait insecticides to inaccessible cockroach aggregations.