Synergistic interactions between an exotic honeybee and an exotic weed: pollination of Lantana camara in Australia

Lantana camara, a woody shrub originating in south and central America, is among the most widespread and troublesome exotic weeds of the old-world tropics. It invades pasture, crops and native ecosystems, causing substantial economic losses and environmental degradation. In Australia alone, L. camara is currently estimated to cover c. 40 000 km(2) . In glasshouse studies we demonstrate that L. camara requires cross-pollination to set fruit, and that honeybee visits result in effective pollination. Field studies carried out in Queensland, Australia, suggest that fruit set is limited by pollinator abundance, and that the main pollinator of L. camara throughout a substantial portion of its Australian range appears to be the honeybee, Apis mellifera. Seed set was strongly correlated with honeybee abundance, and at many sites, particularly in southern Queensland, honeybees were the only recorded flower visitors. Of 63 sites that were visited, seed set was highest at five sites where only honeybees were present. Hives are frequently stationed within and adjacent to areas such as National Parks that are threatened by this noxious weed. Management of honeybee populations may provide a powerful tool for cost-effective control of L. camara that has previously been overlooked. We suggest that there are probably many other weeds, both in Australia and elsewhere, that benefit from honeybee pollination

Indirect interactions between invasive and native plants via pollinators

In generalised pollination systems, the presence of alien plant species may change the foraging behaviour of pollinators on native plant species, which could result in reduced reproductive success of native plant species. We tested this idea of indirect interactions on a small spatial and temporal scale in a field study in Mauritius, where the invasive strawberry guava, Psidium cattleianum, provides additional floral resources for insect pollinators. We predicted that the presence of flowering guava would indirectly and negatively affect the reproductive success of the endemic plant Bertiera zaluzania, which has similar flowers, by diverting shared pollinators. We removed P. cattleianum flowers within a 5-m radius from around half the B. zaluzania target plants (treatment) and left P. cattleianum flowers intact around the other half (control). By far, the most abundant and shared pollinator was the introduced honey bee, Apis mellifera, but its visitation rates to treatment and control plants were similar. Likewise, fruit and seed set and fruit size and weight of B. zaluzania were not influenced by the presence of P. cattleianum flowers. Although other studies have shown small-scale effects of alien plant species on neighbouring natives, we found no evidence for such negative indirect interactions in our system. The dominance of introduced, established A. mellifera indicates their replacement of native insect flower visitors and their function as pollinators of native plant species. However, the pollination effectiveness of A. mellifera in comparison to native pollinators is unknown

Pollinator Interactions with Yellow Starthistle (Centaurea solstitialis) across Urban, Agricultural, and Natural Landscapes

Pollinator-plant relationships are found to be particularly vulnerable to land use change. Yet despite extensive research in agricultural and natural systems, less attention has focused on these interactions in neighboring urban areas and its impact on pollination services. We investigated pollinator-plant interactions in a peri-urban landscape on the outskirts of the San Francisco Bay Area, California, where urban, agricultural, and natural land use types interface. We made standardized observations of floral visitation and measured seed set of yellow starthistle (Centaurea solstitialis), a common grassland invasive, to test the hypotheses that increasing urbanization decreases 1) rates of bee visitation, 2) viable seed set, and 3) the efficiency of pollination (relationship between bee visitation and seed set). We unexpectedly found that bee visitation was highest in urban and agricultural land use contexts, but in contrast, seed set rates in these human-altered landscapes were lower than in natural sites. An explanation for the discrepancy between floral visitation and seed set is that higher plant diversity in urban and agricultural areas, as a result of more introduced species, decreases pollinator efficiency. If these patterns are consistent across other plant species, the novel plant communities created in these managed landscapes and the generalist bee species that are favored by human-altered environments will reduce pollination services