Abstract
This study evaluated foraging effectiveness of Pacific cicada killers (Sphecius convallis) by comparing observed prey loads to that predicted by an optimality model. Female S. convallis preyed exclusively on the cicada Tibicen parallelus, resulting in a mean loaded flight muscle ratio (FMR) of 0.187 (N = 46). This value lies just above the marginal level, and only seven wasps (15%) were below 0.179. The low standard error (0.002) suggests that S. convallis is the most ideal flying predator so far examined in this respect. Preying on a single species may have allowed stabilizing selection to adjust the morphology of females to a nearly ideal size. That the loaded FMR is slightly above the marginal level may provide a small safety factor for wasps that do not have optimal thorax temperatures or that have to contend with attempted prey theft. Operational FMR was directly related to wasp body mass. Smaller wasps were overloaded in spite of provisioning with smaller cicadas, while larger wasps were underloaded despite provisioning with larger cicadas. Small wasps may have abandoned larger cicadas because of difficulty with carriage.
Highlights
The effectiveness of a forager can be evaluated by comparing observed outcomes to those expected, based on an optimality model [1,2]
One approach to the study of wasps that carry their prey in flight is to compare measured masses of prey carried to a predicted optimal prey mass
All prey of S. convallis were identified as Tibicen parallelus Davis (Hemiptera: Cicadidae), for the present study, and for the hundreds of specimens examined in the course of other research projects at this site during this period
Summary
The effectiveness of a forager can be evaluated by comparing observed outcomes to those expected, based on an optimality model [1,2]. Predaceous solitary wasps have served as useful models for the examination of foraging strategies. One approach to the study of wasps that carry their prey in flight is to compare measured masses of prey carried to a predicted optimal prey mass. Some species carry less than ideally sized loads, while others carry prey that are larger than expected. Another wasp, Tachytes chrysopyga obscurus Cresson, yielded a near-optimal average but a very large variance, indicating little specialization of optimal prey mass [4]
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