Cockroaches as Trojan Horses for Control of Cockroach Aggregations With Baits

Abstract

Gel bait formulations of insecticides have been shown to be highly effective in managing German cockroach (Blattella germanica L. [Blattodea: Ectobiidae]) populations. Three potential reasons for this are high palatability of baits, the use of slow-acting insecticides, and their horizontal transfer within aggregations, a phenomenon known as 'secondary mortality'. Our objective was to determine whether horizontal transfer can go beyond secondary, to tertiary and quaternary effects, and to compare various gel baits with different active ingredients. We fed adult females a bait and recorded their bait consumption, moribundity, and mortality. Groups of first instars were then exposed to the dead females and their feces, secondary mortality was quantified, and a new cohort of nymphs was then exposed to the feces and dead nymphs (for tertiary mortality); this process was repeated for quaternary mortality. This design did not distinguish among the major mechanisms of horizontal transfer of insecticides, namely coprophagy and contact with feces, exposure to regurgitated fluids, and cannibalism and necrophagy of nymphs. All the tested baits caused 100% mortality of the adult females that directly fed on the bait and high secondary mortality (average of >85%) within 48 hr. Baits containing either dinotefuran, emamectin benzoate, fipronil, or indoxacarb caused tertiary mortality (average of 15-70%), but only the fipronil and indoxacarb baits caused some quaternary mortality. The relative importance of secondary, tertiary, and quaternary transfer of the active ingredient remains to be determined in field populations of the German cockroach.

Keywords: gel bait; horizontal transfer; quaternary mortality; secondary mortality; tertiary mortality.

Fig. 1.

Experimental design demonstrating the horizontal transfer of insecticides. Diagram of the experimental design illustrating primary mortality in adult females fed various gel baits, the horizontal transfer of insecticides from adult females to nymphs (secondary mortality) and from nymphs to nymphs (tertiary and quaternary mortality). Ten adult females fed on a bait, the bait was removed once all the females were dead, and primary mortality was recorded. Twenty blue-dyed first instars were added to the same jar (#1) for 4 hr and then moved to a clean glass jar (#2) and secondary mortality was recorded after 44 hr. By this time, the nymphs had lost their color and 20 new blue-dyed first instar nymphs were added to jar #2 for 4 hr. The new nymphs were moved to jar #3 and tertiary mortality was recorded 44 hr later. The process of adding 20 blue-dyed nymphs to the jar for 4 hr, then moving them to a new jar and recording mortality after 44 hr was repeated until there was no mortality.

Fig. 2.

Primary moribundity and mortality of adult females, represented by survival plots. Time-course of (A) moribundity over 24 hr and (B) mortality over 96 hr of adult females feeding on gel baits or rodent diet (control). There were 5 replicates per treatment with n = 20 females per replicate. Thus, each survival plot consists of 100 females. Moribundity and mortality were subjected to survival analysis; MSTs ± SEM and statistical analyses are shown in Table 1.

Fig. 3.

Bait consumption of adult females. Mean (± SEM) amount of various gel baits consumed by adult females in 1 and 2 hr. There were 20 replicates per treatment (n = 20 females per replicate). Different letters above the bars indicate significant differences among the treatments (ANOVA, Tukey’s HSD test, P < 0.05).

Fig. 4.

Mortality of first instar nymphs. Mean (±SEM) percent (A) secondary, (B) tertiary, and (C) quaternary mortality of first instar nymphs. Each bar represents a different gel bait (n = 20 nymphs per replicate, 5 replicates per treatment). Different letters above the bars indicate significant differences among the treatments within each panel (ANOVA, Tukey’s HSD test, P < 0.05). Statistical analyses were conducted on arcsine-square root-transformed percentages, but figures show the untransformed percentages.