Bumblebees are a group of valuable pollinators that are declining all over the world. The main reason is believed to be habitat loss due to the changes in agriculture. The changes in agriculture influence the structure of the vegetation and flower abundance, which are of great importance to the abundance of bumblebees. The aims of this study were to get a better understanding of bumblebees and what preferences they might have regarding vegetation height and flower abundance in semi-natural grasslands. This study is based on data collected by a Swedish national environmental monitoring program in 2006-2010. Data on twelve bumblebee species were analysed using a generalized linear model based on the dependent variables of occurrence/ non-occurrence. The target variables used in the analysis were flower abundance, vegetation height and standard deviation of vegetation height. The species-wise analyses mostly showed no or weak positive relationships between the bumblebee occurrence and the three target variables. Grouping the species-wise results into ecologically meaningful guilds and analysing them did not increase the explanatory power of flower abundance or vegetation characteristic, but vegetation height showed significant (positive) results in relation for the forest dwelling species and the species group non-parasite and parasite. The results of this study were considerably weaker than expected. Flower abundance and variation in vegetation height could not explain the occurrence of bumblebees. However, the results of this study show that vegetation height is of importance to the occurrence of bumblebees. Bumblebees in general seem to prefer a higher vegetation height. Based on the findings of this study I would recommend a low intensity of grazing in semi-natural grasslands.

The overall aim of the study was to assess the risk of gene flow from Brassica crops by insectmediated pollen transport. I measured the viability of pollen in Brassica flowers throughout crop development and compared this with the viability of pollen transported by insects inside and outside one early- and one late-season crop. In order to evaluate the relative importance of different species in pollen transport, I measured abundance of flower visitors during crop development, and measured the foraging behaviour of five key pollinator species throughout the growing season, in relation to variation in microclimate, crop phenology and the relative abundance of other pollinator species competing for flower resources. Flower visiting insects of Brassica rapa crops were highly diverse, and their abundance and diversity changed with crop phenology. I found similar abundances at the family level for both crops studied, although capture rates were greater in the early- than in the late-season crop. Across flowering development, the greatest numbers of insects were captured at the peak of flowering for both crops. During the flowering period, Diptera was the most abundant order collected, followed by Hymenoptera. The most abundant family in Hymenoptera was Apidae which tracked crop development in both fields, with greater numbers of insects captured inside than outside the field. Standardized-count pollen loads were smaller in Diptera than in Hymenoptera. Of the five key pollinator species sampled, Lasioglossum sordidum (Hymenoptera: Halictidae), Apis mellifera (Hymenoptera: Apidae) and Bombus terrestris (Hymenoptera: Apidae) transported similar pollen loads, which were much greater than those carried by Eristalis tenax (Diptera: Syrphidae) and Melangyna novae-zealandiae (Diptera: Syrphidae). The numbers of insects captured outside of the crop were 10% and 33% of the totals captured inside for the early- and the late-season crop, respectively. The proportion of insects entering versus leaving the crop varied considerably across species, crops and trap location (i.e., whether traps were inside or 50 m outside the border of the crop). However, it is worth noting that not uncommonly more insects were attracted into the crop early in the season, staying there rather than leaving, and then when flowers started to disappear there was a massive escape of insects leaving. This research provides evidence for the influence of crop age on the foraging behaviour of key pollinators and for species-specific variation in the foraging behaviour of Brassica visitors with crop development. Temporal variation in the rate and variability of movement between flowers, and the duration and variability in time spent on each flower, throughout the growing season differed markedly between pollinator species. Flower density, plant density, and the abundance of other insects contributed to the observed variation in pollinator behavioural activity for A. mellifera, E. tenax, M. novae-zelandiae and L. sordidum. Bombus terrestris had the greatest rates and variability of movement, and the greatest rates of flower visitation among all key pollinators studied. Therefore B. terrestris might contribute to gene flow to a greater extent than other key pollinators. Additionally B. terrestris had the greatest variability in the rate of movement, increasing the risk of pollen movement over long distances. In summary, I found that (i) insect abundance and diversity changed with crop phenology and Diptera was the most abundant order collected, (ii) flower density, plant density, and the abundance of other insect pollinators were important factors explaining pollinator behaviour for all key pollinators, except B. terrestris, (iii) B. terrestris might contribute to gene flow to a greater extent than other key pollinators, because it has a greater rate of flower visitation and a greater flight distance between flowers than other pollinators, and (iv) pollen viability tended to decrease with crop development and declined sharply even just 50 m outside the edge of the crop.

Livestock management influences the extent to which grazing lands provide resources for native species. We compared how livestock species – sheep or cattle – affected floral resources and bee and butterfly communities in low-diversity, post-Conservation Reserve Program (CRP) pastures managed with patch-burning. We sampled bees and butterflies three times per season 2017-2019 and counted flowering stems within 1 m of transects. Pastures grazed by sheep had significantly fewer flowers and significantly lower floral richness than cattle pastures. Native bees were three to sixteen times more abundant in cattle pastures compared to sheep. Butterfly communities were similar between grazing treatments, because agricultural-tolerant, habitat generalists comprised the majority of the butterfly community. Grassland-obligate butterflies comprised only 2% of observations. The dearth of grassland-obligate butterfly species and low native bee abundances suggest that post-CRP fields, especially those grazed by sheep, do not provide abundant and diverse floral resources for native bees and imperiled butterfly species.

Despite evidence of pollinator declines from many regions across the globe, the threat this poses to plant populations is not clear because plants can often produce seeds without animal pollinators. Here, we quantify pollinator contribution to seed production by comparing fertility in the presence versus the absence of pollinators for a global dataset of 1174 plant species. We estimate that, without pollinators, a third of flowering plant species would produce no seeds and half would suffer an 80% or more reduction in fertility. Pollinator contribution to plant reproduction is higher in plants with tree growth form, multiple reproductive episodes, more specialized pollination systems, and tropical distributions, making these groups especially vulnerable to reduced service from pollinators. These results suggest that, without mitigating efforts, pollinator declines have the potential to reduce reproduction for most plant species, increasing the risk of population declines.

Global warming is causing a general trend of rising temperatures worldwide. Simultaneously there is also a decline in populations of pollinators all over the world. Therefore, it is important to examine the effect warming temperatures might have on different pollinator species. The focus of this study was to look at how flight phenology of hoverflies in southern Sweden is affected by rising summer temperatures using two regions differing in temperature, and 11 years of citizen science data on hoverfly observations. Summer temperature and observations of 13 species were used. Although four species had a significantly earlier first flight in years with warmer weather, there were overall no apparent trends toward earlier or longer flight periods due to temperature deviation. However, geographical location had a strong impact on flight behaviour of hoverflies in Sweden with hoverflies in Götaland having an earlier first flight compared to Svealand (located further north). This might be the result of an earlier onset of spring and summer in Götaland than in Svealand. The results of this study indicate that more factors than temperature affect flight phenology in hoverflies.

The goal of this thesis was to examine the breeding system and natural history of the Maguire primrose (Primula cusickiana var. maguirei). Maguire primrose is an endemic, threatened subspecies found only along a narrow corridor within Logan Canyon in northern Utah, USA. This plant displays distinct flower distyly, with clear distinction of pin and thrum morphologies (morphs). The timing of Maguire primrose flower blooms was disparate between upper and lower canyon populations, and the flowers experienced cool temperatures occasionally. I captured eight different species of flying insects visiting Maguire primrose flowers, and made 67 observations of insect visitation. Inter-morph outcrossing hand pollinations were the most successful hand pollinations performed, but were usually not as successful as those that were naturally pollinated. Selfing rates were quite low compared to outcrossing reproductive scenarios. Morph-specific fecundity differences were detected in my hand pollinations, but not in the naturally pollinated maternal plants. I did not observe clear fecundity differences based on the canyon location of Maguire primrose.

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In studies of plant-pollinator interactions, much attention has been paid to interspecific variation in foraging behaviors among pollinators and their consequences for plant reproduction and the stability of plant-pollinator communities. In contrast, there has been little consideration of intraspecific variation and its consequences. Specifically, male and female pollinators may forage differently due to differences in phenology, nutritional requirements, and behavior. Here, we compare male and female foraging patterns of the rare declining eastern regal fritillary butterfly, Speyeria idalia idalia. Using a 21-year dataset, which monitored the abundance, sex, and foraging of S. i.idalia across grasslands within Fort Indiantown Gap National Guard Training Center in Pennsylvania, USA, we asked whether males and females differed with respect to the diversity of nectar plants visited and the floral compositions which they visited. To better understand the behavior of individual butterflies and draw inferences about repercussions for plant fitness, in 2019 we collected pollen loads directly from male and female S. i.idalia in these same grasslands and compared the diversity, composition, and size of pollen loads between the sexes after identifying pollen grains to the lowest taxonomic unit. Both datasets point to the dominant use of three nectar species by both sexes, but also show evidence of sex specific foraging pattern. Males and females differ statistically, although slightly, in the composition of plant species visited and plant species per pollen load. Females visit a significantly greater diversity of plant species when accounting for both abundance and evenness of plants visited. In contrast, pollen loads found on males contained ~3 more species on average than female pollen loads, but this difference is not supported when accounting for evenness of pollen grains/species. Finally, we find individual male butterflies carry more pollen grains than females on average. Our results contribute to understanding pollinator resource use more broadly, with implications for pollination dynamics. For conservation of this rare species our results further emphasize the importance of nectar plants critically important for male and female members of S. i. idalia. / Biology

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The increased improvement of weed control throughout the years has led to a decrease in the amount of weed in arable fields. Weeds can be a source of pollen and nectar. Pollinating insects are dependent on a variety of plants for nectar and pollen. This study evaluated the arable field weed flower preference among pollinating insects, through mostly observational studies in July 2020 in southern Sweden. Out of the 12 weed species that were observed, 5 got many visits from pollinated insects, while 4 got almost no visits. From these visits, I calculated which preferences each observed insect group had regarding weed species. I found that Syrphidae preferred Matricaria chamomilla, Apis mellifera preferred Centaurea cyanus and Lamium purpureum, Coleoptera preferred Cirsium arvense, other Diptera preferred Cirsium arvense, and Bombus preferred Galeopsis tetrahit. This indicates that if the weed species Matricaria chamomilla, Centaurea cyanus, Lamium purpureum, Cirsium arvense, and Galeopsis tetrahit are present in a conventional arable field pollinators are benefitted. The weed species Myosotis arvensis, Capsella bursa-pastoris, Thlaspi arvense, and Fumaria officinalis got almost no visits, which indicates that it is not as important to favour these weed species to benefit pollinating insects. Out of the surveyed weed species preferred by pollinators, the species Lamium purpureum and Galeopsis tetrahit had among the highest occurrences in arable fields, which means that these species have the possibility to benefit pollinators if weed control is occasionally abstained for a year.