Phenotypic variation in fitness traits of a managed solitary bee, Osmia ribifloris (Hymenoptera: Megachilidae)

Authors: Sampson, Blair; Rinehart, Timothy - Tim; KIRKER, GRANT - Forest Products Laboratory; Stringer, Stephen; Werle, Christopher

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/16/2015
Publication Date: 8/4/2015
Citation: Sampson, B., Rinehart, T., Kirker, G., Stringer, S., Werle, C. Phenotypic variation in fitness traits of a managed solitary bee, Osmia ribifloris (Hymenoptera: Megachilidae). J. Econ. Entomol. 1–10; DOI: 10.1093/jee/tov233. 2015.

Interpretive Summary: Captive bee populations are useful for modeling the genetic and ecological factors of pollinator decline. We assessed population growth trends for captive mason bees from Texas, California, and Utah. Male populations exploded during the bees' long-term captivity. No reproductive barriers exist that could explain this overproduction of male bees. Sterile males may have been produced, but their preesence only partly explains population decline. The largest costs to population growth were food availability and ensueing nest takeovers, which killed between 33%-67% of brood in effected nests. Such usurpation was the greatest source of mortality among captive inhabitants. In fact, our logistic growth model indicates that captive bee populations overproduce males to perhaps limit incidences of nest usurpation. Our success in breeding commercial pollinators from wild stock gathered from different states across the USA was possible without running the risk of oversampling local bee populations.

Technical Abstract: Captive bee populations are useful for modeling the genetic and ecological factors regulating the size of small natural pollinator populations. We assessed population trends, brood size, and mortality rates for captive generations of a mason bee, Osmia ribifloris, whose parents came from Texas (TX), California (CA), and Utah (UT). Male-biased sex ratios increased exponentially with each successive generation; the causes of which may involve genetic and ecological adaptation to long-term captivity. First, outbreeding depression resulting from an unforeseen genetic barrier was hypothesized to inflate male-biased sex ratios by reducing mating success, but was ruled out as a cause however because female fitness was similar despite thousands of kilometers of separation between source populations. Moreover, two outbred generations of O. ribifloris broods were even more male-biased than the six inbred ones. Moreover, female nesting success was independent of paternity indicating all released males produced viable sperm. In fact, female genetic backgrounds were more influential in nesting success; bees from the two most distant populations, CA and TX, produced the heaviest broods on blueberry pollen, and their male progeny shared >98% similarity in mtDNA. Second, we hypothesized that inbreeding at the single locus complementary sex determiner (sl-csd) or other autosomal loci reduces female fitness in part by the production of sterile diploid males. Indeed, there was a growing class of female-sized males, possibly diploids, which may further inflate male-bias sex ratios. These diploid males being sterile convey no viable sperm to females. Such failed matings could delay nesting thereby compelling females to usurp nests killing 33%-67% of brood in the process. Such usurpation was the greatest source of mortality among captive inhabitants. In fact, our logistic growth model indicates that captive bee populations upon reaching carrying capacity will begin overproducing males, which may be linked to occasional bouts of facultative nest usurpation. Our success in quarantining and breeding commercial pollinators from wild stock gathered from geographically isolated populations was possible without running the risk of oversampling local bee populations.