Role of social aggregation in the fitness cost of pyrethroid‐resistant German cockroaches

Abstract

AbstractLiving in aggregations likely gives fitness benefits to resistant German cockroaches. We first investigated the trade‐offs in life‐history traits between six field pyrethroid‐resistant German cockroach strains and a laboratory susceptible strain (SUS‐1). Second, each strain was placed in groups of 2, 10, 20, 30 and 40 individuals with an equal sex ratio in each group to investigate the effect of social aggregation on fitness and overall population parameters. All resistant strains exhibited reduced nymphal survival (50%–83%) and a prolonged development period in both male (49–63 days) and female (47–62 days) nymphs when compared to SUS‐1 individuals (nymphal development period: 39 ± 2 days for male and 40 ± 3 days for female). Such fitness costs were minimized when the resistant cockroaches were in aggregations. Specifically, male (54 ± 3 days) and female nymphal development periods (52 ± 3 days) of German cockroach significantly decreased to 44 ± 1 days in the largest group. The nymphal survival was substantially enhanced (more than 80% survival) in German cockroaches with elevated cytochrome P450 monooxygenases, or both cytochrome P450 monooxygenases and esterases activities in the largest group. The effect of pyrethroid resistance on the life‐history traits of adult German cockroaches was limited; only adult male longevity was significantly prolonged. Group living did not significantly extend adult longevity and increase the number of offspring per ootheca. Contrary to expectations, the number of nymphs produced by a female and the oothecal hatchability were inversely proportional to the number of individuals in aggregations. Moreover, a lower proportion of viable oothecae were produced in German cockroaches when living in aggregations. Because of low fecundity, population parameters of German cockroaches living in aggregations (net reproductive rate and intrinsic rate of increase) were significantly lower than the parameters of those living in pairs.