T'ai H. Roulston

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.

A Comparison of Pan Trap and Intensive Net Sampling Techniques for Documenting a Bee (Hymenoptera: Apiformes) Fauna

Recent interest in pollinator sampling, often motivated by concerns about pollinator decline, has led to the increased use of standardized sampling protocols based on passive insect traps. Compared with netting insects at host plants, these protocols have the potential to limit some types of sampling bias, such as those associated with a researchers observational and netting skills. The most commonly deployed recent protocol is the use of colored pan traps (bowls) filled with soapy water (Aguiar and Sharkov, 1997; Calbuig, 2001; Cane et al, 2000; Leong and Thorp, 1999; Toler et al, 2005). Insects approach the bowls, land on the water, and drown. The method is particularly good at catching numerous species of bees, but can also be effective for capturing various flower-visiting flies, skippers and a wide range of other insect taxa. As a bee sampling device, pan traps have several known biases: they catch bumble bees, honey bees and bees in the genus Colletes much less frequently than expected by their perceived abundance (e.g., Toler et al, 2005). Pan traps are especially good at catching halictid bees (Family Halictidae). Because of these biases, researchers sometimes use a modest amount of net collecting on flowers to accompany pan traps. Other potential biases remain to be studied, such as whether the effectiveness of pan traps is inversely related to flower abundance, a bias suspected by several researchers but not yet studied. Based on their pan-trapping study, Toler et al (2005) concluded that the predominant flower color in the plant community did not influence the relative attractiveness of particular pan trap colors, but they did not study the effect of floral abundance per se. Depending on a researchers interest in deploying pan traps, these biases may or may not generate concern. Pan traps seem very well suited to testing for the presence of particular bee species in the community (Leong and Thorp, 1999), including many parasitic bee species that are seldom caught at flowers. They also provide a very valuable tool for augmenting other collecting methods, especially when there are few host plants to sample (e.g., early spring). Work by Cane et al. (2000) included a comparison of pan traps versus intensive netting to sample the very diverse and well known flower-visiting insect fauna of creosote bush. These workers concluded that pan trapping was of limited use in detecting the creosote bush fauna because of numerous species caught by netting but not by pan traps. Pan traps did catch many bee species not netted at creosote bush, however. Because netting in that study was restricted to creosote bush, there was no basis to compare the relative effectiveness of netting versus pan trapping for sampling a local bee fauna in the wider flower-rich community. In 2002 in northern Virginia, we carried out an intensive netting/observational survey of floral visitors within one hectare plots at Blandy Experimental Farm, an ecological experiment station of the University of Virginia. The sampled habitat was an open field and the sampling protocol comprised net sampling on the six most prominent plant species in the community from 8:00-16:00 hrs Eastern Daylight Time. Each plant species was surveyed for a total of 2.25 hrs, spread equally across the sampling period. Each of three researchers sampled all plant species in succession, one species at a time, three times across the sampling period (8:00-10:00, 11:00-13:00, 14:00-16:00). Survey periods lasted 15 mins per plant species per researcher, with researchers moving through the entire plot during that period. Bee species that could be recognized on the wing (e.g., Apis mellifera) were noted but not captured. To compare the effectiveness of pan trapping to such an intensive netting protocol at one of our sites, we placed a line of 30 pan traps (6 oz Solo bowls painted fluorescent blue, fluorescent yellow, or left white) in a diagonal line across the plot,

Wild Bee Abundance and Pollination Service in Cultivated Pumpkins: Farm Management, Nesting Behavior and Landscape Effects

ABSTRACT Recent declines in managed and feral honey bee populations have greatly increased interest in the current and potential role of wild pollinators in agricultural pollination. Pumpkin (Cucurbita pepo L.) has great potential to be served by wild pollinators because of a reliable and widespread group of bee species that are commonly associated with their flowers, including bumble bees (Bombus spp.), and, in the Americas, two genera of specialist ground nesting bees (Peponapis and Xenoglossa). We examined the effects of several key farm management practices and landscape variables on bee abundance in pumpkin on 20 farms in Virginia and Maryland during summer 2006. We evaluated bee abundance with respect to tillage, irrigation practices, soil properties, natural habitat (forest and grassland), flowering crop, and disturbed areas. Additionally, we examined nest site preference (within or outside of crop areas) of Peponapis pruinosa (Say) at one farm and in a large screenhouse. We found P. pruinosa nesting preferentially within crop areas and near the vines and leaves of their host plant. Although these bees typically place some of their brood cells within tillage depth, we did not find a tillage effect on their abundance at flowers. We found a negative effect of soil clay content (R2 = 0.24, P = 0.03) and a positive effect of irrigation (F1,15 = 12.2; P < 0.001) on P. pruinosa abundance. Using published data on pollinator visitation requirements, we found wild bee densities were sufficient to fully pollinate all pumpkin flowers on 13–17 of the 20 farms studied.

Patterns of parasite infection in bumble bees (Bombus spp.) of Northern Virginia

In recent decades, several North American bumble bee (Bombus spp.) species have undergone precipitous declines. It is suspected that a parasite or pathogen may be responsible, yet few studies have examined the extent of parasitism and the ecology of host–parasite relationships in U.S. bumble bee populations. A season‐long survey of bumble bees in seven grassland meadows of the northern Shenandoah Valley and Piedmont regions in Virginia was conducted in 2011 to ascertain the local prevalence and predictors of parasitism by the internal parasites Nosema and Crithidia, and by parasitoid conopid flies. In total, 835 bumble bees representing six species were examined. Using visual detection methods, we determined that 25% of bees were infected with parasitoid larvae, 17.4% with Crithidia, and 7.3% with Nosema. Nosema infections were more prevalent and intense in locally rare than locally common species, with the two rarest bumble bees [B. fervidus (Fabricius) and B. auricomus (Robertson)], newly suspected to be in decline, having the highest frequencies of infection (11–17.8%). Crithidia was generally more prevalent in common bumble bee species (11–35%). With fewer than 5% of individuals infected, the two rarest species had the lowest frequencies of Crithidia. Conopid fly larvae were more prevalent in common species. Body size significantly influenced the probability of parasitism by conopids and Crithidia. Smaller bees were more likely to be parasitised by Crithidia. Larger bees were more likely to be parasitised by conopid flies, although the largest bee species (B. auricomus) was not infected by conopids in this study.

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.