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The timing of metamorphosis in insects determines adult body size and influences aspects of fitness including flight performance and fecundity. The regulation of metamorphosis has been well studied in model insects but determining the cue that initiates the physiological shift toward metamorphosis in other insects has been challenging. In most insects, the critical weight is a cue to initiate metamorphosis. Our goal was to determine the cue for metamorphosis in the solitary beeMegachile rotundataand to characterize the physiological mechanisms initiated by this cue. Larval bees were fed ad libitum (also termed “fed”) or had all food removed (also termed “starved”). We measured the time to initiate metamorphosis, juvenile hormone (JH) titers, and the abundance of transcripts involved in the endocrine response to metamorphosis. We experimentally manipulated JH by topically applying precocene, which interrupts JH production. We found thatM. rotundatadoes not have a critical weight and that starvation through the removal of food triggered the initiation of metamorphosis. Food removal was associated with low JH titers, and the application of precocene mimicked the response to food absence. Food removal decreased transcripts associated with JH production, and increased transcripts involved in ecdysone synthesis and insulin/TOR signaling. Food absence is an ecologically relevant cue for aM. rotundatalarva, which is provisioned a finite amount of food and is unable to forage for additional food. Despite having food removal as a cue for metamorphosis, the physiological mechanisms associated with metamorphosis are similar to those found in other insects. 

Abstract Diapause initiation is determined by photoperiod and temperature in many insect species. In the solitary cavity-nesting bee Megachile rotundata (Fabricius) (Hymenoptera: Megachilidae), which undergoes facultative diapause as a prepupa, photoperiod is an important cue for diapause induction. However, it is unknown if the mother or offspring makes the decision to induce diapause or when this decision is made. Our goal was to answer these questions by manipulating photoperiod during different stages of development in M. rotundata. We subjected mothers to 3 different photoperiods as pupae and released them into the field to build nests and lay eggs when those same photoperiods occurred naturally. Offspring were collected and subjected to the same photoperiods as the mothers. We examined the effects of photoperiod on offspring diapause and the number of nests and brood cells created during each photoperiod. Our findings indicate that the photosensitive period occurs in the field, suggesting that the mother determines diapause during nest construction. We also found that reproductive output is the highest during the shortening photoperiods associated with midsummer. This study furthers our understanding of how diapause functions in Megachile rotundata and could be used to advance commercial management practices. 

Performance tends to decline with age, including muscle function and stress tolerance. Yet, performance can vary widely among individuals within the same age group, showing that chronological age does not always represent biological age. To better understand ageing, we need to examine what drives some individuals to age faster than others. In order to achieve this, first we need to be able to predict whether an individual will have a long or short lifespan. In this study, we conducted a longitudinal study tracking individual-level locomotor activity, chill-coma recovery time, and metabolic rates, and assessed whether early-life performance is linked to lifespan using the solitary beeMegachile rotundata. We found that locomotor activity and chill-coma recovery times decline in old adults. However, resting metabolic rate did not change with age. We also found low cold tolerance and low mass at emergence in early-life are linked to shorter female lifespans, showing that early-life performance can explain some of the variation in lifespan in a population. Finally, these results also show that not all traits decline with age within the same species, and shed new light on sexual dimorphism in physiological traits and ageing. 

Stress can influence lifespan in both positive and negative ways, depending on exposure intensity and duration. However, mechanisms driving positive stress effects on lifespan remain poorly understood. Prolonged hypoxia extends the lifespan of overwintering prepupal Megachile rotundata. Here, we explored telomere length and reduced oxidative stress as potential mechanisms of this extended lifespan. We hypothesized high antioxidant capacity under hypoxia reduces oxidative damage and telomere loss. We exposed prepupae to 10%, 21% or 24% oxygen for up to 9 months and measured monthly survival, telomere length, antioxidant capacity and lipid peroxidation across treatment duration for prepupae and adults. After 9 months of exposure, survival was highest in hypoxia and lowest in hyperoxia. Telomere length did not differ among oxygen treatments but increased in adults compared with prepupae. Total antioxidant capacity and lipid peroxidation showed no significant differences among oxygen treatments, suggesting compensatory responses to maintain baseline oxidative levels. 

ABSTRACT <p>Social bees have been extensively studied for their gut microbial functions, but the significance of the gut microbiota in solitary bees remains less explored. Solitary bee,<italic>Megachile rotundata</italic>females provision their offspring with pollen from various plant species, harboring a diverse microbial community that colonizes larvae guts. The<italic>Apilactobacillus</italic>is the most abundant microbe, but evidence concerning the effects of<italic>Apilactobacillus</italic>and other provision microbes on growth and survival are lacking. We hypothesized that the presence of<italic>Apilactobacillus</italic>in abundance would enhance larval and prepupal development, weight, and survival, while the absence of intact microbial communities was expected to have a negative impact on bee fitness. We reared larvae on pollen provisions with naturally collected microbial communities (Natural pollen) or devoid of microbial communities (Sterile pollen). We also assessed the impact of introducing<italic>Apilactobacillus micheneri</italic>by adding it to both types of pollen provisions. Feeding larvae with sterile pollen +<italic>A. micheneri</italic>led to the highest mortality rate, followed by natural pollen +<italic>A. micheneri</italic>, and sterile pollen. Larval development was significantly delayed in groups fed with sterile pollen. Interestingly, larval and prepupal weights did not significantly differ across treatments compared to natural pollen-fed larvae. 16S rRNA gene sequencing found a dominance of<italic>Sodalis</italic>, when<italic>A. micheneri</italic>was introduced to natural pollen. The presence of<italic>Sodalis</italic>with abundant<italic>A. michene</italic>ri suggests potential crosstalk between both, shaping bee nutrition and health. Hence, this study highlights that the reliance on nonhost-specific environmental bacteria may not impact fitness of<italic>M. rotundata</italic>.</p><sec><title>IMPORTANCEThis study investigates the impact of environmentally acquired gut microbes of solitary bee fitness with insights into the microbial ecology of bee and their health. While the symbiotic microbiome is well-studied in social bees, the role of environmental acquired microbiota in solitary bees remains unclear. Assessing this relationship in a solitary pollinator, the leaf-cutting bee,Megachile rotundata, we discovered that this bee species does not depend on the diverse environmental bacteria found in pollen for either its larval growth or survival. Surprisingly, high concentrations of the most abundant pollen bacteria, Apilactobacillus micheneridid not consistently benefit bee fitness, but caused larval mortality. Our findings also suggest an interaction betweenApilactobacillusand theSodalisand perhaps their role in bee nutrition. Hence, this study provides significant insights that contribute to understanding the fitness, conservation, and pollination ecology of other solitary bee species in the future. 

Abstract The mechanisms that underlie senescence are not well understood in insects. Telomeres are conserved repetitive sequences at chromosome ends that protect DNA during replication. In many vertebrates, telomeres shorten during cell division and in response to stress and are often used as a cellular marker of senescence. However, little is known about telomere dynamics across the lifespan in invertebrates. We measured telomere length in larvae, prepupae, pupae, and adults of two species of solitary bees,Osmia lignariaandMegachile rotundata. Contrary to our predictions, telomere length was longer in later developmental stages in bothO. lignariaandM. rotundata.Longer telomeres occurred after emergence from diapause, which is a physiological state with increased tolerance to stress. InO. lignaria, telomeres were longer in adults when they emerged following diapause. InM. rotundata, telomeres were longer in the pupal stage and subsequent adult stage, which occurs after prepupal diapause. In both species, telomere length did not change during the 8 months of diapause. Telomere length did not differ by mass similarly across species or sex. We also did not see a difference in telomere length after adultO. lignariawere exposed to a nutritional stress, nor did length change during their adult lifespan. Taken together, these results suggest that telomere dynamics in solitary bees differ from what is commonly reported in vertebrates and suggest that insect diapause may influence telomere dynamics.