Terence M. Laverty

Plant interactions for pollinator visits: a test of the magnet species effect

SummaryFrom 1985–1987, patterns of fruit and seed set were studied in a population of mayapple (Podophyllum peltatum), a clonal, self-incompatible herb found in deciduous woods in eastern North America. Mayapple flowers do not produce nectar, but depend on infrequent visits by nectar-seeking queen bumble bees for pollination. In all years female reproductive success in mayapple colonies was influenced by colony size (number of flowers), by the distance to neighbouring colonies and by proximity to lousewort plants (Pedicularis canadensis), a prolific nectar producer heavily visited by bumble bees. In all years fruit and seed set were greater in mayapple colonies <25 m from lousewort flowers than in matched colonies which were >50 m from lousewort. In 1985 and 1987 the frequency of queen bumble bee visits to flowers in colonies close to lousewort was about four times greater than to distant colonies. In 1986 I removed about 80% of lousewort flowers to test whether the enhanced fruit and seed set in mayapples close to lousewort was pollinator mediated. Mayapple colonies close to flowerless lousewort patches did not differ in fruit or seed set from matched colonies >50 m from lousewort. In contrast, mayapples close to flowering lousewort patches had greater fruit and seed set compared with distant colonies. Over all years, a larger proportion of mayapples close to flowering lousewort patches had enhanced fruit and seed set compared with colonies close to louseworts without flowers. Though rarely documented, this type of facilitative interaction between plants that are highly attractive to pollinators (“magnet” species), and co-flowering species that are rarely visited by pollinators, may be widespread in plant communities.

The effect of purple loosestrife (Lythrum salicaria L.; Lythraceae) on the pollination and reproductive success of sympatric co-flowering wetland plants

AbstractThis study investigated the effect of purple loosestrife (Lythrum salicaria L.) on the pollination and reproductive success of co-flowering plant species (Impatiens capensis Walt., Eupatori...

Flower constancy in bumblebees: a test of the trait variability hypothesis

Pollinators often sequentially visit one flower type while bypassing other equally rewarding flower types in the process. Many explanations for this pattern of flower choice in pollinators, known as flower constancy, have been proposed; yet, a sufficient answer to the question of why pollinators are constant still remains elusive. We tested the hypothesis that flower constancy in pollinators is related to the number of traits distinguishing available flowers by measuring the floral selectivity (both constancy and preference) of bumblebees, Bombus impatiens, when artificial flower types differed in either colour only (variation in a single trait) or colour and other floral traits (variation in multiple traits). As expected, bees showed increased degrees of selectivity (constancy and preference) when available flower types differed in colour and other floral traits compared with when available flower types differed in colour only. In addition, bee foraging rates (measured in flowers visited per min) varied inversely with the number of variable floral traits added to colour but not with the number of colours. Together, these results are consistent with the idea that the mechanism underlying flower constancy in bumblebees is a limitation on their ability to effectively search for and/or remember multiple combinations of floral traits at the same time. We discuss the roles of floral-trait variation, flower constancy and pollinator cognitive limitations in the coevolution of flowering plants and their animal pollinators.

Bumble Bee (Hymenoptera: Apidae) Activity and Pollination Levels in Commercial Tomato Greenhouses

Abstract Commercial greenhouse studies were conducted to assess levels of pollination of tomato (Lycopersicon esculentum Mill.) flowers in relation to bumble bee (Bombus impatiens Cresson) colony activity and colony densities. For the assessment of pollination levels of tomato flowers, five categories were defined based on bruising levels caused by bumble bee pollination. Colony activity was measured as bee trips per ha/d using electric powered photodiode monitors inserted into the hive entrance. Levels of pollination were positively correlated with bee activity levels, up to a mean of ≈400 pollen grains per stigma per day, after which greater activity did not result in further increases in daily pollination levels. Densities of colonies in the commercial greenhouses studied ranged from 7.6 to 19.8 colonies per hectare with a mean of 11.6 ± 0.9. We found that an average activity of 2,000 bee trips per hectare per day was more than adequate to ensure sufficient pollination, and that this level of activity could be achieved with 7–15 colonies per hectare, depending on greenhouse conditions. Greenhouses requiring >15 colonies per hectare to achieve this level of pollination may be able to increase bee activity through alteration of greenhouse conditions. Across 50-m rows of tomato plants, levels of pollination decreased with increasing distance from bee colonies, suggesting that colonies should be evenly distributed throughout the greenhouses.

Effect of bumble bee (Hymenoptera: Apidae) pollination intensity on the quality of greenhouse tomatoes.

Abstract Laboratory studies were conducted to assess tomato, Lycopersicon esculentum Mill. (Solanaceae), quality in relation to the level of buzz-pollination by bumble bees. Studies were conducted in commercial tomato greenhouses in the Leamington, Ontario, area to categorize bruising of tomato anther cones by bumble bees into five levels of bruising. The number of pollen grains per stigma was determined for each bruising level, and the bruising level was found to be a good predictor of stigmatic pollen load. Experimental flowers were pollinated by bumble bees and assigned to bruising levels based on the degree of anther cone discoloration. Fruit set, tomato weight, minimum diameter, the number of days until ripe, roundness, weight, percentage sugars, and number of seeds were assessed and compared among bruising level. Fruit set in flowers receiving no pollination visits was 30.2%, whereas, 83.3, 84.4, 81.2, and 100% of the flowers set fruit in bruising levels 1, 2, 3, and 4, respectively. Minimum diameter, number of seeds, and tomato weight all increased from no bruising to different levels of bruising. There was no increase in weight or diameter above a bruising level of 1, and no increase in the number of seeds per fruit after a bruising level of 2. We found that pollination of tomato flowers greater than a bruising level of 2 (corresponding to approximately one to two bee visits) did not result in a significant increase in quality.

Recall of flower handling skills by bumble bees: a test of Darwin's interference hypothesis

Abstract Pollinators tend to visit flowers of one plant species repeatedly even when flowers of other rewarding plant species are available. Darwin proposed that such flower constancy occurs because flower handling methods learned on one plant species interfere with previously learned flower handling methods of other plant species. Darwins interference hypothesis was tested by first allowing bumble bees, Bombus bimaculatus , to learn flower handling methods on one plant species (comfrey, Symphytum officinale ), and then retesting the bees on comfrey after they had learned to handle flowers of a second species (vetch, Vicia cracca ). The same experiment was repeated with different bees and with the order of the plant species reversed. Interference effects were detected by comparing flower access times (time to insert tongue into the flower) during the retesting period with the initial flower access times. Bees relearning handling methods for comfrey returned to their initial access times after one flower visit; bees relearning handling methods for vetch took two flower visits. The interference effects detected were 0·8 s for comfrey (a 47% increase over initial access times) and 2·0 s for vetch (a 120% increase). These results indicate that learning to handle flowers of one plant species does interfere with remembering previously learned flower handling methods. However, interference effects resulting from switches between comfrey and vetch under biologically realistic conditions appear to be relatively small, and are unlikely to account fully for flower constancy by bumble bees.

`Futile cycle' enzymes in the flight muscles of North American bumblebees

SUMMARY In the flight muscles of European bumblebees, high activities of fructose-1,6-bisphosphatase (FbPase) relative to phosphofructokinase (PFK) have suggested a thermogenic `futile cycle important for regional endothermy. We find generally low activities of FbPase (0.7-19.7 units g-1 thorax) in North American Bombus species, with the exception of Bombus rufocinctus, where activity (43.1 units g-1 thorax) is comparable with that of European congeners. These data, taken with estimates of maximal rates of heat production by cycling, do not support a significant thermogenic role for the PFK/FbPase cycle. In agreement with earlier studies, both PFK and FbPase activities were found to scale allometrically with body size (allometric exponents -0.18 and -1.33, respectively). The cycle may serve to supplement thermogenesis or amplify glycolytic flux in rest-to-flight transitions, especially in smaller bees.