Osmia taurus (Hymenoptera: Megachilidae): A Non-native Bee Species With Invasiveness Potential in North America.

Bees are important pollinators and are essential for the reproduction of many plants in natural and agricultural ecosystems. However, bees can have adverse ecological effects when introduced to areas outside of their native geographic ranges. Dozens of non-native bee species are currently found in North America and have raised concerns about their potential role in the decline of native bee populations. Osmia taurus Smith (Hymenoptera: Megachilidae) is a mason bee native to eastern Asia that was first reported in the United States in 2002. Since then, this species has rapidly expanded throughout the eastern part of North America. Here, we present a comprehensive review of the natural history of O. taurus, document its recent history of spread through the United States and Canada, and discuss the evidence suggesting its potential for invasiveness. In addition, we compare the biology and history of colonization of O. taurus to O. cornifrons (Radoszkowski), another non-native mason bee species now widespread in North America. We highlight gaps of knowledge and future research directions to better characterize the role of O. taurus in the decline of native Osmia spp. Panzer and the facilitation of invasive plant-pollinator mutualisms.

Decline of six native mason bee species following the arrival of an exotic congener.

A potential driver of pollinator declines that has been hypothesized but seldom documented is the introduction of exotic pollinator species. International trade often involves movement of many insect pollinators, especially bees, beyond their natural range. For agricultural purposes or by inadvertent cargo shipment, bee species successfully establishing in new ranges could compete with native bees for food and nesting resources. In the Mid-Atlantic United States, two Asian species of mason bee (Osmia taurus and O. cornifrons) have become recently established. Using pan-trap records from the Mid-Atlantic US, we examined catch abundance of two exotic and six native Osmia species over the span of fifteen years (2003-2017) to estimate abundance changes. All native species showed substantial annual declines, resulting in cumulative catch losses ranging 76-91% since 2003. Exotic species fared much better, with O. cornifrons stable and O. taurus increasing by 800% since 2003. We characterize the areas of niche overlap that may lead to competition between native and exotic species of Osmia, and we discuss how disease spillover and enemy release in this system may result in the patterns we document.

Species differences in bumblebee immune response predict developmental success of a parasitoid fly.

Endoparasitoids develop inside the body of a host organism and, if successful, eventually kill their host in order to reach maturity. Host species can vary in their suitability for a developing endoparasitoid; in particular, the host immune response, which can suppress egg hatching and larval development, has been hypothesized to be one of the most important determinants of parasitoid host range. In this study, we investigated whether three bumblebee host species (Bombus bimaculatus, Bombus griseocollis, and Bombus impatiens) varied in their suitability for the development of a shared parasitoid, the conopid fly (Conopidae, Diptera) and whether the intensity of host encapsulation response, an insect immune defense against invaders, could predict parasitoid success. When surgically implanted with a nylon filament, B. griseocollis exhibited a stronger immune response than both B. impatiens and B. bimaculatus. Similarly, B. griseocollis was more likely to melanize conopid larvae from natural infections and more likely to kill conopids prior to its own death. Our results indicate that variation in the strength of the general immune response of insects may have ecological implications for sympatric species that share parasites. We suggest that, in this system, selection for a stronger immune response may be heightened by the pattern of phenological overlap between local host species and the population peak of their most prominent parasitoid.

Inbreeding in Mimulus guttatus reduces visitation by bumble bee pollinators.

Inbreeding in plants typically reduces individual fitness but may also alter ecological interactions. This study examined the effect of inbreeding in the mixed-mating annual Mimulus guttatus on visitation by pollinators (Bombus impatiens) in greenhouse experiments. Previous studies of M. guttatus have shown that inbreeding reduced corolla size, flower number, and pollen quantity and quality. Using controlled crosses, we produced inbred and outbred families from three different M. guttatus populations. We recorded the plant genotypes that bees visited and the number of flowers probed per visit. In our first experiment, bees were 31% more likely to visit outbred plants than those selfed for one generation and 43% more likely to visit outbred plants than those selfed for two generations. Inbreeding had only a small effect on the number of flowers probed once bees arrived at a genotype. These differences were explained partially by differences in mean floral display and mean flower size, but even when these variables were controlled statistically, the effect of inbreeding remained large and significant. In a second experiment we quantified pollen viability from inbred and self plants. Bees were 37-54% more likely to visit outbred plants, depending on the population, even when controlling for floral display size. Pollen viability proved to be as important as floral display in predicting pollinator visitation in one population, but the overall explanatory power of a multiple regression model was weak. Our data suggested that bees use cues in addition to display size, flower size, and pollen reward quality in their discrimination of inbred plants. Discrimination against inbred plants could have effects on plant fitness and thereby reinforce selection for outcrossing. Inbreeding in plant populations could also reduce resource quality for pollinators, potentially resulting in negative effects on pollinator populations.

Resource assurance predicts specialist and generalist bee activity in drought.

Many short-lived desert organisms remain in diapause during drought. Theoretically, the cues desert species use to continue diapause through drought should differ depending on the availability of critical resources, but the unpredictability and infrequent occurrence of climate extremes and reduced insect activity during such events make empirical tests of this prediction difficult. An intensive study of a diverse bee-plant community through a drought event found that bee specialists of a drought-sensitive host plant were absent in the drought year in contrast to generalist bees and to specialist bees of a drought-insensitive host plant. Different responses of bee species to drought indicate that the diapause cues used by bee species allow them to reliably predict host availability. Species composition of the bee community in drought shifted towards mostly generalist species. However, we predict that more frequent and extended drought, predicted by climate change models for southwest North America, will result in bee communities that are species-poor and dominated by specialist species, as found today in the most arid desert region of North America.

The role of resources and risks in regulating wild bee populations.

Recent declines of bee species have led to great interest in preserving and promoting bee populations for agricultural and wild plant pollination. Many correlational studies have examined the indirect effects of factors such as landscape context and land management practices and found great variation in bee response. We focus here on the evidence for effects of direct factors (i.e., food resources, nesting resources, and incidental risks) regulating bee populations and then interpret varied responses to indirect factors through their species-specific and habitat-specific effects on direct factors. We find strong evidence for food resource availability regulating bee populations, but little clear evidence that other direct factors are commonly limiting. We recommend manipulative experiments to illuminate the effects of these different factors. We contend that much of the variation in impact from indirect factors, such as grazing, can be explained by the relationships between indirect factors and floral resource availability based on environmental circumstances.

Complex responses within a desert bee guild (Hymenoptera: Apiformes) to urban habitat fragmentation.

Urbanization within the Tucson Basin of Arizona during the past 50+ years has fragmented the original desert scrub into patches of different sizes and ages. These remnant patches and the surrounding desert are dominated by Larrea tridentata (creosote bush), a long-lived shrub whose flowers are visited by > 120 native bee species across its range. Twenty-one of these bee species restrict their pollen foraging to L. tridentata. To evaluate the response of this bee fauna to fragmentation, we compared species incidence and abundance patterns for the bee guild visiting L. tridentata at 59 habitat fragments of known size (0.002-5 ha) and age (up to 70 years), and in adjacent desert. The 62 bee species caught during this study responded to fragmentation heterogeneously and not in direct relation to their abundance or incidence in undisturbed desert. Few species found outside the city were entirely absent from urban fragments. Species of ground-nesting L. tridentata specialists were underrepresented in smaller fragments and less abundant in the smaller and older fragments. In contrast, cavity-nesting bees (including one L. tridentata specialist) were overrepresented in the habitat fragments, probably due to enhanced nesting opportunities available in the urban matrix. Small-bodied bee species were no more likely than larger bodied species to be absent from the smaller fragments. The introduced European honey bee, Apis mellifera, was a minor faunal element at > 90% of the fragments and exerted little if any influence on the response of native bee species to fragmentation. Overall, bee response to urban habitat fragmentation was best predicted by ecological traits associated with nesting and dietary breadth. Had species been treated as individual units in the analyses, or pooled together into one analysis, these response patterns may not have been apparent. Pollination interactions with this floral host are probably not adversely affected in this system because of its longevity and ability to attract diverse pollinators but will demand careful further study to understand.

Retrieval of granular bait by the Argentine ant (Hymenoptera: Formicidae): effect of clumped versus scattered dispersion patterns.

Argentine ants, Linepithema humile (Mayr), use mass recruitment foraging, with clumped prey items being retrieved more efficiently than dispersed prey. However, in prior field experiments, granular baits, whether dispensed in containers or broadly scattered, had a similar impact on Argentine ant populations. In laboratory experiments, granular insecticide bait was encountered faster by Argentine ant workers and more granules were initially returned to the colony when the granules were scattered versus clumped. After 2 h, granules from both dispersion patterns were retrieved equally. Our results suggest that Argentine ant colonies adjust their foraging patterns to resources of different quality (prey versus bait). Also, foraging activity patterns for bait in the laboratory are consistent with prior field results demonstrating no efficacy advantages to discrete granular bait placements.