Regional variation in body size of cicada killers, Sphecius speciosus (Hymenoptera: Crabronidae); Does prey size determine wasp size?

Big Bend National Park least visited of all National Parks Why work there?

Sphecius speciosus

S. speciosus distribution

Sphecius speciosus ground-nesting, provisioning wasp

Cicada killers are colonial wasps Nests can be solitary Colonies of 10–100 nests are common. One colony of over 5,000 nests has been documented (Hastings, et al. 2008) They are univoltine

Males are territorial Males emerge before females Defend territories in nest area Territoriality enhances mating success Large males usually win contests over territories Large males have mating advantage

Cicada killer nests; built and provisioned by females Nests up to 2 m long, 0.5 m deep 1or 2 cicadas placed in each cell One egg laid within each cell Cicada killers are mass provisioners (as opposed to progressive provisioners)

Size variation among wasp populations

Location of study sites

Results of wasp two-way ANOVA

Size variation among cicada prey

Results of cicada ANOVA

Relationship between mean cicada size and mean wasp size among locations

Female size and provisioning success Provisioning is difficult 1. time consuming 2. females are burdened (Coelho, 1997) Conflicting selection pressures 1. maximize offspring number by minimizing number of prey given to each 2. maximize provision mass given to daughters: produce daughters large enough to be successful providers Compromise: produce daughters of minimum size necessary to handle local cicadas

Alternative explanations for regional size variation in wasps Local adaptation hypothesis –Regional size variation has evolved by natural selection Genetic differences in adult size potential? Females programmed to give daughters specific provision mass –Varies among locations –Females must hunt selectively Fixed-provision-number hypothesis –Regional size differences are not evolved –Females hard-wired to give offspring fixed number of prey (1 for male offspring, 2 for female offspring) –Females are opportunistic hunters –Offspring size parallels size of available prey

The current thinking Females are opportunistic hunters; they capture whatever they find Consequently, cicadas brought to nests are representative of what is available in the local environment Qualifier: –The smallest females are constrained from carrying large prey to nest

Do females hunt selectively or opportunistically?

Important point about prey availability At most locations only one species of cicada or, at most, two species of cicadas of similar size are available for hunting Big exception to this rule: –Northern Florida; this is where we focused our research this summer

Summer 2008 Research Newberry, FL and St Johns, FL The wasps in Newberry are the smallest we have ever seen The wasps in St. Johns are the largest The 2 locations are less than 100 km apart Explanation based on current knowledge? What did we find?

Comparison of cicada populations Newberry vs. St. Johns Cicadas are very difficult to sample in Florida. Why? Based on songs we heard and what we saw: –No obvious difference between locations Based on what the wasps brought to nests: –Newberry wasps provisioned almost exclusively with 2 very small species –St. Johns wasps provisioned mainly with 2 large species

Female-prey pairs 2 locations combined

Big questions Are females opportunistic hunters or are they selective hunters? Is prey size determined by wasp size or by location? If females are selective hunters, and the same prey are available at the different locations, what accounts for the large difference in size of the wasps?

Other data sets Large samples of male and female wasp live masses and wing lengths from both study sites From focal studies of hunting females –Individual hunt times, including the species of cicada brought to nests; mean hunt times seem to vary among cicada species –Estimates of how many cicadas of which species are provided for each offspring –Long-term hunting records for individual females; individuals appear to specialize

References Coelho, J. R Sexual size dimorphism and flight behavior in cicada killers, Sphecius speciosus. Oikos 79: Coelho, J. R Sexual size dimorphism and flight behavior in cicada killers, Sphecius speciosus. Oikos 79: Grant, P. R Opportunistic predation and offspring sex ratios of cicada-killer wasps (Sphecius speciosus Drury). Ecological Entomology 31: Grant, P. R Opportunistic predation and offspring sex ratios of cicada-killer wasps (Sphecius speciosus Drury). Ecological Entomology 31: Hastings, J Provisioning by female western cicada killer wasps, Sphecius grandis (Hymenoptera:Sphecidae): Influence of body size and emergence time on individual provisioning success. Journal of the Kansas Entomological Society 59: Hastings, J Provisioning by female western cicada killer wasps, Sphecius grandis (Hymenoptera:Sphecidae): Influence of body size and emergence time on individual provisioning success. Journal of the Kansas Entomological Society 59: Hastings, J. M The influence of size, age, and residency status on territory defense in male western cicada killer wasps (Sphecius grandis, Hymenoptera: Sphecidae). Journal of the Kansas Entomological Society 62: Hastings, J. M The influence of size, age, and residency status on territory defense in male western cicada killer wasps (Sphecius grandis, Hymenoptera: Sphecidae). Journal of the Kansas Entomological Society 62: Hastings, J.M, P. J. Schultheis, M.K. Whitson, C.W. Holliday, J.R. Coelho, and A.M. Mendell. 2008a. DNA barcoding of New World cicada killers (Hymenoptera: Crabronidae). ZooTaxa 1713: 27-38Hastings, J.M, P. J. Schultheis, M.K. Whitson, C.W. Holliday, J.R. Coelho, and A.M. Mendell. 2008a. DNA barcoding of New World cicada killers (Hymenoptera: Crabronidae). ZooTaxa 1713: Hastings, J. M., J. R. Coelho, C. W. Holliday. 2008b. Mating at high population density in a colonial territorial wasp, Sphecius speciosus Drury (Hymenoptera: Crabronidae). 2008b. Journal of the Kansas Entomological Society. In press.Hastings, J. M., J. R. Coelho, C. W. Holliday. 2008b. Mating at high population density in a colonial territorial wasp, Sphecius speciosus Drury (Hymenoptera: Crabronidae). 2008b. Journal of the Kansas Entomological Society. In press. Warton, D., I. J. Wright, D. S. Falster, M. Westoby Bivariate line-fitting methods for allometry. Biological Reviews 81: Warton, D., I. J. Wright, D. S. Falster, M. Westoby Bivariate line-fitting methods for allometry. Biological Reviews 81: