Social interactions as an ecological constraint in a eusocial insect

The ecological constraints model (ECM) was formulated to predict how different ecological factors affect cooperative-breeding vertebrates. Within these groups some individuals help raise nondescendent young because of constraints that limit their own reproduction (e.g. limited foods, reproductive opportunities and nesting sites). Despite similarities between cooperative-breeding vertebrates and facultatively social insects, few studies have explicitly and experimentally determined how the ECM applies to social insects. In this study, we examined the potential effects of predation, density and nest site limitation as well as differences in nest site quality on the alternative nesting tactics of the paper wasp Mischocyttarus mexicanus. Accurate field estimates of density were possible because multiple nests are initiated in isolated Sabal palmetto trees. Results from a 17-month census showed more solitary nesting when density was low and nest sites were plentiful. In addition, females appeared to prefer nests initiated on unoccupied east-facing fronds. We then examined the potential effect of density and nest site limitation in a field experiment by adding or removing fronds from S. palmetto and forcing all females to renest. The percentage of haplometrotic females decreased when fronds were removed (increased density treatment), increased when fronds were added (decreased density treatment), and showed a mixed response when frond number remained unchanged (control density treatment). Nest site limitation did not appear to affect renesting females: the percentage of available unoccupied fronds was not affected by treatment. Mischocyttarus mexicanus females responded primarily to their social environment within palm groves and to variation in nest site quality, instead of constraints that typically affect cooperative-breeding vertebrates.