Site-to-site differences in pollinator visitation patterns in a Louisiana iris hybrid zone

We studied pollinator visitation rates and movement patterns in experimental arrays of irises at two sites within a Louisiana iris hybrid zone. Arrays contained single-flowered stems of red-flowered Iris fulva, blue-flowered I. hexagona, and purple-flowered F-1 hybrids. At one site, where I. hexagona was the only wild-growing iris, queen bumble bees were the most common pollinator, and the rank order of pollinator visit rates was I. hexagona > F-1 > I. fulva. At a second site, where I. fulva predominated in the wild, hummingbirds were the most common pollinators. and this order was reversed: I. hexagona < F-1 < I. fulva. Thus, at both rites, the naturally occurring iris was visited most frequently and F-1 flowers were visited more frequently than the non-native species. Pollinators used both long-distance and short-distance cues to discriminate among flowers, and differences in visit rates seemed to be based on differences in the ease of access to pollen and nectar records on flowers of different size. Analysis of movement patterns between flowers showed that pollinators were more likely to make parental-F-1 transitions than heterospecific transitions. These data suggest that pollinator behavior acts as a partial barrier to initial hybridization, although this barrier largely breaks down after F-1, formation. Our results help to explain the existence of many introgressed populations of Louisiana iris, despite the fact that F(1)s are extremely rare in the wild.