Bumblebee Densities Research Articles

Habitat quality of field margins in a Finnish farmland area for bumblebees (Hymenoptera: Bombus and Psithyrus)

Of all non-arable habitats, field margins are the most important ones for bumblebees in agricultural environment, especially as they are wide-spread and offer food resources, but they are also places for reproduction and overwintering. This study was made to investigate bumblebee abundance in the agricultural environment and to obtain information on the use of southern Finnish field margins by bumblebees. Bumblebees and cuckoo bumblebees were studied in a farmland area in Lammi, southern Finland, in 1996. Line-transect counts in field margins were made to compare habitat availability of patches of farmland and qualities of field margins within the patches. Landscape variables, dicotyledonous flower abundances and ecological and morphological characteristics of flowers were used as factors to explain variation in bumblebee assemblages and their frequencies of visits to the field margins. Bumblebee species richness and total density were higher in patches of farmland where field margins had more dicotyledonous flowers rather than being grassy. Abundance and flowering phenology of a limited number of flower species during the bumblebee breeding season were the most important factors explaining bumblebee visits in field margins. The most important flower species was zigzag clover ( Trifolium medium). The width of field margins was significantly related to the total density of bumblebees and cuckoo bumblebees, but not species number or diversity.

Density dynamics of bumblebees in subalpine meadows: competition and resource limitation

Densities of Bombus flavifrons and B. rufocinctus were measured over one summer in 23 and 11 discrete subalpine meadows, respectively. I assessed the relation between observed bee densities and meadow size, elevation, floristics, temperature, and time of day by multiple regression. Multivariate regression solutions for each week of observation accounted for 38 to 61% of the variation in density for B. flavifrons and 47 to 87% for B. rufocinctus. Temperature or time of day consistently accounted for the most variation in density of the two species. In early summer meadow size or elevation also accounted for a large proportion of explained variation in B. flavifrons density; as the summer progressed meadow floristics increasingly contributed to the explanation of variation in both species.In mid‐ to late summer a statistically significant portion of the variation in densities not explained by habitat characteristics was accounted for by densities of congeners: i.e., densities of B. flavifrons and B. rufocinctus were negatively related in eight meadows where they co‐occurred. In experimental meadows where all Bombus but one of the two target species were removed. B. rufocinctus underwent significant positive density compensation but B. flavifrons did not. To the extent that B. flavifrons appeared to limit the density of B. rufocinctus these statistical and experimental results are consistent with the hypothesis that competition limits local bumblebee densities.

Detecting Interspecific Competition in the Field: Testing the Regression Method

Crowell and Pimm, and Schoener have independently suggested an indirect method to measure interaction coefficients in the field. We tested the method in the field by comparing the interaction coefficients estimated by the indirect method with those measured directly in manipulation experiments of bumblebee densities. The method failed to detect the considerable negative effect of the density of B. flavifrons on B. rufocinctus which was observed in the manipulation experiment. The present study questions the validity of the method and advises ecologists not to use it before testing it more rigorously.

Influences on the density and dispersion of bumble bee nests (Hymenoptera: Apidae)

To determine some of the influences on nesting densities of bumble bees I examined the dispersion and occupancy of 35 nests in an old field on Amherst Island, Ontario, Canada and 39 nests found by Cumber (1953) in England. Densities up to 1 nest per 200 m2 were observed. Overall, Cumber's nests were randomly dispersed, but nests on the ground surface were aggregated. Nests of surface‐nesting species were consequently more aggregated than expected at random, whereas surface‐nesting and underground‐nesting species seldom nested near one another. This distribution of nests probably reflects the nesting habits of the rodents that originally constructed the nests, rather than interactions between bees. On Amherst Island, abnormally wet spring weather apparently hampered normal colony development, particularly of underground‐nesting bees based on a low frequency of underground nests, scarcity of workers of two underground‐nesting species compared with the relative abundance of spring queens, and delayed appearance of workers of all species relative to the previous year. The nests of two common species were randomly dispersed, but those of a third species were regularly distributed, suggesting that intraspecific interactions may limit nesting density for this species.

THE EFFECT OF AERIAL INSECTICIDE SPRAYING ON HYMENOPTEROUS POLLINATORS IN NEW BRUNSWICK

AbstractThe effects of aerially sprayed fenitrothion (210 g/ha) and aminocarb (70 g/ha) on several hymenopteran taxa in New Brunswick are reported. Exposure cage experiments showed fenitrothion to cause as high mortality among solitary bees and vespid wasps as that previously reported for bumble bees. Exposure cage experiments under aminocarb sprays revealed no detectably increased mortality for bumble bees, but a significant effect on solitary bees. Experiments in which whole bumble bee colonies were set out in fenitrothion and aminocarb spray blocks a few days before spraying gave comparable results to those from the exposure cage experiments.A series of comparisons of bumble bee densities in sprayed and unsprayed areas confirmed our earlier finding that populations are greatly reduced after fenitrothion application. Population recovery during the rest of the season appears to be influenced by meteorological conditions. The ecological replacement of early-emerging by late-emerging Bombus species in fenitrothion spray blocks is further documented. No relationship between bee densities and aminocarb application was found.A method for retrospective assessment of pollinator activity by measuring nectar levels in Clintonia borealis was successfully carried out on a small-scale basis.

Increasing bumble bee densities in red clover seed production areas

Abstract Spring introduction of 115, 85, and 114 queens of Bombus ruderatus and B. subterraneus in three successive years resulted in a threefold increase in peak population densities (from 2000 to 6000 bees/ha) at a red clever seed production site in the Mackenzie Country in the South Island of New Zealand. There was no carry-over effect from year to year, and in the season after queen introduction ceased, density dropped to 3000 bees/ha.