Museum records indicate male bias in pollinators of sexually deceptive orchids

Abstract

Deception has evolved in a range of taxa. When deception imposes costs, yet persists over generations, exploited species typically have traits to help them bear or minimise costs. The sexually deceptive orchids, Cryptostylis spp., are pollinated by tricking male haplodiploid wasps (Lissopimpla excelsa) into mating with flowers, which offer no reward and often elicit sperm wastage. We hypothesise that by attracting haplodiploid species, orchids have a pollinator ideally suited to withstand the costs of sexual deception—and a selective advantage compared to other orchids. Haplodiploid females can reproduce with or without sperm—albeit when spermless, females can only have sons. Through orchid deception and sperm wastage, deceived haplodiploid populations could become male biased, providing enough males to share between orchids and females. In this way, pollinator populations can persist despite high densities of sexually deceptive orchids. Here, we aim to broadly test this prediction using museum and digital records of the pollinator, L. excelsa, from sites with or without orchids. For robustness, we also analyse the sex ratio of a sister ichneumonid species that occurs in the same areas but is not deceived by orchids. We found that at sites with orchids, L. excelsa was significantly more male biased than at sites without orchids and significantly more male biased than the sister ichneumonid. This survey is the first to test the population-level effects of sexually deceptive orchids on their pollinator. It supports our prediction that orchid deception can drive male-biased sex ratios in exploited pollinators.