Flowers that self-shade reduce heat stress and pollen limitation.

PREMISE: Plants are facing increased risk of heat stress with global climate change. Reproductive tissues are particularly heat-sensitive, which can result in lower plant fitness. Floral shading and closure are possible mechanisms to limit heat stress although most previous work on petal orientation has considered adaptations to raise temperatures. We hypothesized that floral shading could reduce temperature and increase reproductive success. METHODS: We measured floral temperatures of four species that exhibited intraspecific variation in flower closure (Opuntia ficus-indica, Oenothera elata, Convolvulus arvensis, and Romneya coulteri). We also wired newly opened R. coulteri flowers so that they were either permanently open or permanently closed; controls were not wired. RESULTS: Individual flowers of all four species that shaded their pistils were exposed to temperatures 3-8°C lower than those that remained open and unshaded. In our wiring experiment, unencumbered R. coulteri controls were 40% more likely to produce seeds than flowers that were either permanently open or closed. Without added pollen, control flowers produced 2× more seeds than flowers wired open and 8× more than those wired closed. However, pollen addition eliminated the effects of wiring and increased capsule mass and seed production. This effect of pollen addition suggests that pollen limitation was responsible for observed differences in the wiring treatments. Pollinators may prefer control flowers over those that were wired open or closed; petal shading may make flowers cooler and more attractive to pollinators. CONCLUSIONS: Petal shading may be a behavior that allows flowers to reduce heat stress and increases their chances of pollination and seed set.

Influence of delayed density and ultraviolet radiation on caterpillar baculovirus infection and mortality.

Infectious disease is an important potential driver of population cycles but must occur through delayed density-dependent infection and resulting fitness effects. Delayed density-dependent infection by baculoviruses can be caused by environmental persistence of viral occlusion bodies (OBs), which can be influenced by environmental factors. Specifically, ultraviolet radiation is potentially important in reducing the environmental persistence of viruses by inactivating OBs. Delayed density-dependent viral infection has rarely been observed empirically at the population level although theory predicts that it is necessary for pathogens to drive population cycles. Similarly, field studies have not examined the effects of ultraviolet radiation on viral infection rates in natural animal populations. We tested if viral infection is delayed density-dependent with the potential to drive cyclic dynamics and if ultraviolet radiation influences viral infection levels. We censused 18 Ranchman's tiger moth (Arctia virginalis) populations across 9° of latitude over 2 years and quantified the effects of direct and delayed density and ultraviolet radiation on proportion infected by baculovirus, infection severity and survival to adulthood. Caterpillars were collected from field populations and reared in the laboratory. Baculovirus has not previously been described infecting A. virginalis, and we used genetic methods to confirm the identity of the virus. We found that proportion infected, infection severity and survival to adulthood exhibited delayed density dependence. Ultraviolet radiation in the previous summer decreased infection severity, which increased caterpillar survival probability. Structural equation modelling indicated that the effect of lagged density on caterpillar survival was mediated through proportion infected and infection severity and was 2.5-fold stronger than the indirect effect of ultraviolet. We successfully amplified polh, lef-8 and lef-9 viral genes from caterpillars, and BLAST results confirmed that the virus was a nucleopolyhedrovirus. Our findings provide clear evidence that delayed density-dependent mortality can arise through viral infection rate and severity in insects, which supports the role of viral disease as a mechanism, among others, that may drive insect population cycles. Furthermore, our findings support predictions that ultraviolet radiation can modify viral disease dynamics in insect populations, most likely through attenuating viral persistence in the environment.

Bees just wanna have fungi: a review of bee associations with nonpathogenic fungi.

Bee-fungus associations are common, and while most studies focus on entomopathogens, emerging evidence suggests that bees associate with a variety of symbiotic fungi that can influence bee behavior and health. Here, we review nonpathogenic fungal taxa associated with different bee species and bee-related habitats. We synthesize results of studies examining fungal effects on bee behavior, development, survival, and fitness. We find that fungal communities differ across habitats, with some groups restricted mostly to flowers (Metschnikowia), while others are present almost exclusively in stored provisions (Zygosaccharomyces). Starmerella yeasts are found in multiple habitats in association with many bee species. Bee species differ widely in the abundance and identity of fungi hosted. Functional studies suggest that yeasts affect bee foraging, development, and pathogen interactions, though few bee and fungal taxa have been examined in this context. Rarely, fungi are obligately beneficial symbionts of bees, whereas most are facultative bee associates with unknown or ecologically contextual effects. Fungicides can reduce fungal abundance and alter fungal communities associated with bees, potentially disrupting bee-fungi associations. We recommend that future study focus on fungi associated with non-honeybee species and examine multiple bee life stages to document fungal composition, abundance, and mechanistic effects on bees.