Pollen defenses negatively impact foraging and fitness in a generalist bee (Bombus impatiens: Apidae)

Abstract

Plants may benefit from limiting the community of generalist floral visitors if the species that remain are more effective pollinators and less effective pollenivores. Plants can reduce access to pollen through altered floral cues or morphological structures, but can also reduce consumption through direct pollen defenses. We observed that Eucera (Peponapis) pruinosa, a specialist bee on Cucurbita plants, collected pure loads of pollen while generalist honey bees and bumble bees collected negligible amounts of cucurbit pollen, even though all groups of bees visited these flowers. Cucurbit flowers have no morphological adaptations to limit pollen collection by bees, thus we assessed their potential for physical, nutritional, and chemical pollen traits that might act as defenses to limit pollen loss to generalist pollinators. Bumble bee (Bombus impatiens) microcolonies experienced reduced pollen consumption, mortality, and reproduction as well as increased stress responses when exposed to nutritional and mechanical pollen defenses. These bees also experienced physiological effects of these defenses in the form of hindgut expansion and gut melanization. chemical defenses alone increased the area of gut melanization in larger bees and induced possible compensatory feeding. Together, these results suggest that generalist bumble bees avoid collecting cucurbit pollen due to the physiological costs of physical and chemical pollen defenses.

Synergism between local‐ and landscape‐level pesticides reduces wild bee floral visitation in pollinator‐dependent crops

“…Wild bees that specialize on cucurbits (e.g. squash bees) may be disproportionately impacted, since honeybees and some bumblebees are known to avoid cucurbit pollen due to fitness trade-offs (Brochu et al, 2020). When parsed from solitary bees, we found modest evidence indicating that bumblebee visitation was reduced by landscape-level fungicide hazards.…”

Phylogenetic and ecological correlates of pollen morphological diversity in a Neotropical rainforest

“…For example, it has been shown experimentally that bumble bees ( Bombus terrestris ) performed significantly less pollen foraging on plants which have large, echinate pollen grains (three Malvaceae species and Knautia arvensis (Dipsacaceae)) (Konzmann et. al., 2019), but studies of pollen foraging on Cucurbita (Cucurbitaceae) by the generalist bumble bee Bombus impatiens have shown it is likely that combinations of morphological (large size and pronounced echinae in this example), nutritional and chemical pollen traits could allow plants to selectively attract and deter particular suites of pollinators (Brochu et al., 2020). It seems unlikely that pollinators could perceive subtle differences in pollen morphology, especially those on a micrometric scale, but further experimental work could test whether the micro‐morphological landscape provided by pollen grains is functionally significant for bees and other insects during pollination.…”

“…This hypothesis provides a link between pollination ecology and pollen morphology and could be tested using foraging experiments such as those of Konzmann, Koethe, and Lunau (2019) and Brochu et al. (2020).…”

“…However, by analogy with the role of morphological diversity among flowers in pollination (e.g., Ghazoul, 2006), we hypothesize that pollen grains may contribute to the overall phenotypic diversity that is presented by plants to pollinators. This hypothesis provides a link between pollination ecology and pollen morphology and could be tested using foraging experiments such as those of Konzmann, Koethe, and Lunau (2019) and Brochu et al (2020).…”

Exposure to low doses of pesticides induces an immune response and the production of nitric oxide in honeybees

“…Although biotic and abiotic stressors may not immediately affect the survival of exposed individuals, they challenge the immune system and impair general fitness 7 , 10 , 26 . Honeybees are social insects, and may therefore compensate for their comparably small repertoire of immunity-related genes 27 , 28 and cellular immune responses 29 by developing behavioral mechanisms that limit intoxication via the avoidance and dilution of certain food sources, and the co-cultivation of beneficial microbes synergistically supporting the detoxification of plant metabolites 30 .…”