Influence of resource level on maternal investment in a leaf-cutter bee (Hymenoptera: Megachilidae)

Fisher's (1930) predictionof equal investment for each sexin a panmictic population is influenced by anumber of ecologicalfactors, among which resource availability plays a majorrole,particularly when the population exists under changing resourceavailability.Rosenheim et al. proposed a multifaceted parental investmentmodelbased on the underlying assumption that individual females determinetheirsex investment according to resource availability and oocyteavailabilityto maximize reproductive success. The model predicts that greateravailabilityof resources used for provisions will lead to (1) an increaseinthe proportion of females produced (when the female is thelarger sex) and (2)an increase in the amount of provisionsper offspring and thus an increase inoffspring size. I testedthese predictions by a controlled experiment using aleaf-cutterbee, Megachile apicalis. I presented two levels of foodresourcesto the nesting females, which were allowed to forage and nestincages. The experimental results supported these parentalinvestment model'spredictions.