LIFE CYCLE, DEVELOPMENT, AND REPRODUCTION
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LIFE CYCLE, DEVELOPMENT, AND REPRODUCTION
Life Cycle, Reproduction, Climate Change, Phenology
How does a bee transform from a tiny white larva to a prolific flying pollinator? Well, Butterflies aren't the only insects that weave cocoons.
All insects progress through one of two major developmental pathways, which differ drastically from mammalian development.
Many students may be familiar with the concept of complete metamorphosis. In this developmental process, the adults of an insect species do not resemble the larvae— for example, in butterflies where the winged adult in no way resembles the larval caterpillar. In entomology, we refer to this process as holometabolous development. It is common in many insect orders, including moths and butterflies, beetles, and flies. Other insects, such as grasshoppers and true bugs, are hemimetabolous, meaning they undergo incomplete metamorphosis. In this process, the young insects, called nymphs, hatch from their eggs with a similar body structure as an adult, albeit much smaller and with some underdeveloped features. They grow, develop, and molt several times, beginning to resemble a developed adult more and more before reaching adulthood.
Figure 1. Typical life cycle of early-spring mason bees which overwinter as adults. (Figure: Danforth, Minckley, Neff 2019)
All members of the order Hymenoptera, including bees, wasps, ants, and sawflies are holometabolous. They transition through four major life stages throughout their development: 1) eggs, 2) larvae, 3) pupae, and 4) adults; they even spin cocoons. In this module, we cover these major stages of mason bee development, when they occur, how adults reproduce, and how climate impacts development. Upon completing this module students should be able to draw the mason bee life cycle, explain basic bee reproduction and formulate hypotheses about how changes to the local climate might impact bee development.
The Mason Bee Life Cycle
Mason bees generally produce one brood, or set of offspring, per year. Adults emerge in the spring and spend the next several weeks mating, building nests, provisioning brood cells, and laying eggs (more details on adult behavior can be found in the Foraging Behavior and Nesting and Mating Behav- ior modules). The eggs will grow and develop through the four major life stages of complete metamorphosis throughout the summer (Figures 1 and 4).
Mason bees start life as a small elongate egg, resembling a tiny rice grain in shape and color, a semi-translucent white. The mother bee lays a single egg on the pollen provision in each brood cell. Like all insect eggs, bee eggs have a thin, flexible membrane, called the chorion, which acts similar to an eggshell in birds to protect the embryo from the outside environment. A yolk rich in protein and fat provides nutrition for the bee during early development (Figures 2 and 3 panel 1).
After about a week, a first instar larva hatches from the egg and feeds on the yolk. As the mason bee develops through each of the five larval instars over a month, the larvae feed on the pollen provision, grow, defecate and molt. After consuming the provision, the fifth instar larva begins to spin a cocoon, a capsule of silk fibers produced by the salivary glands, similar to those produced by moths. In early summer, the cocooned larva, or prepupa, enters a period of low metabolic activity called prepupal dormancy, which lasts one to three months. The length of the prepupal dormancy is dependent on the ambient temperature and geographic origin of the population. The shortest dormancy occurs at approximately 24°C (75°F), with slower prepupal development at warmer temperatures. Some individuals exposed to cold or very warm temperatures fail to develop into pupae at all.
Figure 2. Opened Osmia bicornis nest reeds showing the different life stages of mason bee development with eggs in the lowest reed and fully formed cocoons in the top right. The small black pellets are feces which are excluded from the cocoons. (Image: © Gilles San Martin, Flickr)
At the end of the prepupal dormancy, the larva molts into a pupa; in a process called pupation. The new pale white pupa looks significantly different from the larva, with many recognizable adult structures, including legs, compound eyes, and small undeveloped wings. Over the next month, the pupa develops adult coloration, starting with the darkening of the eyes, before the body darkens to its final color.
Approximately a month after pupation, the pupa undergoes a final molt. The exact timing of this final molt is geographically dependent, with early flying low latitude populations, such as those from central California, reaching adulthood in September, compared to July and August for northern populations. This slightly unintuitive difference in timing results from the var- iation in the duration of prepupal dormancy, where the longer summers and warmer temperatures slow the development of the early flying, low latitude bee populations.
Figure 3. Stages of the mason bee life cycle. Panel 1: egg, panels 2-7: larva, panels 8-10: fifth instar larva springing a cocoon, panels 11-12: prepupa (cocooned larva), pupa, or adult. (Images: © entomart)
The young adults enter into diapause after a short prewintering period during which they prepare themselves to withstand cold winter temperatures. Winter diapause, a period of reduced metabolic activity, allows adult bees to maintain their energy stores and emerge healthy and active the following spring. Similar to prepupal dormancy, overwintering duration varies depending on the ambient temperature and the population's geographic origin, with higher latitude populations requiring longer over- wintering periods. In the spring, as temperatures rise, the adults emerge from their natal nest, forage (see: Foraging behavior), mate, and build their nests (see: Nesting and Mating Behavior, and Reproduction below), restarting the process for the next generation.
The fundamentals of bee reproduction are similar to that of most sexually reproductive animals. Female bees produce eggs in the ovary, which are fertilized by spermatozoa produced by males. However, there are also some striking differences; perhaps most notably, male bees develop from unfertilized haploid eggs through a process called haplodiploidy. Haplodiploidy is an intriguing nuance of bee biology and is covered more in the module on Sex Determination and Allocation.
Mating in mason bees usually occurs soon after the female bees emerge from their natal nests at the nest site or occasionally at nearby flowers (Figure 4). While mating occurs early, the newly emerged females are not ready to begin laying eggs since their ovaries have not fully developed before emergence. Instead, the female bees store the sperm in a reproductive organ called a spermatheca. Female bees will use this stored sperm to fertilize eggs during oviposition.
Figure 4. Life cycle of the red mason bee (Osmia bicornis), a species endemic to Europe. Similar to other closely related mason bees, the red mason bee develops from egg to adult over the course of the summer and overwinters as an adult. Mating only occurs during a short period of time after emergence (Figure: Seidelmann and Rolke 2019)
Although many aspects of mason bee reproductive biology are unknown, multiple matings and the storage of sperm in a spermatheca lay the groundwork for sperm competition. Males of the mason bee species Osmia bicornis, use glandular secretions to create a mating plug in the female vagina. Mating plugs (sometimes also called sperm plugs) are not unique to insects; in fact they occur across animal diversity. Particularly in species where the females may mate multiple times with multiple males, mating plugs can help to ensure paternity in the males that mate with the female early. In O. bicornis, the mating plug created within the female's vagina increases the likelihood of the first male's sperm being used to fertilize the eggs by blocking the sperm of future males from reaching the spermatheca.
Climate change and Mason Bee Phenology
The phenology of mason bees, the sequence and timing of their life cycle, is dependent on the local climate. Higher or lower ambient air temperatures may result in different durations of developmental stages. By the year 2100, climate models predict that increasing temperatures and longer summers may push adult bee emergence between half a month to over one month earlier than what was observed in the early 2000s. For Osmia in the northeastern US, this means bee emergence may occur as early as March. Moreover, the duration between male bee emergence and female bee emergence may increase from between one to three days up to ten, potentially decreasing mating success and altering the population vital rates.
Lee, He, and Park (2018) hypothesized that extended developmental time might increase parasite risk. Under warmer conditions, bees spend more time in the high-risk life stages, increasing mite infection risk. Increases in temperature by several degrees Celsius may also reduce bee survival during development on its own.
The changing climate may also alter community interactions. One study by Kehrberger and Holzschuh (2019) found that although the warmer spring temperatures can push both bee emergence and floral bloom earlier in the year, they may not advance synchronously. If flowers bloom before bee emergence, they may not be pollinated.
Chorion - The outer membrane of an insect egg
Holometabolous Development (complete metamorphosis) - A process of insect development where the adults do not resemble the early life stages
Hemimetabolous Development (incomplete metamorphosis) - A process of insect development where the early life stages resemble the adults
Instar - A developmental stage of an insect between molts
Phenology - The study of cyclic natural phenomena, including life cycles
Spermatheca – A sperm storage organ in female bees
Climate Change and Mason Bee Phenology — Data Analysis
The data for this activity are adapted from Kehrberger and Holzschuh’s 2019 study titled “Warmer temperatures advance flowering in a spring plant more strongly than emergence of two solitary spring bee species” which examined the impact of warming spring temperatures on two mason bee species, Osmia cornuta and Osmia bicornis, and a threatened buttercup flower, Pulsatilla vulgaris, native to western Europe. O. cornuta and O. bicornis are early spring emerging mason bees with similar biology to O. lignaria, a species native to the United States.
Figure 1. The locations of 11 field sites where the timing of O. cornuta and O. bicornis emergence was observed by Kehrberger and Holzschuh in 2015. Each site is colored according to the mean temperature observed at that site between early February and late March. P. vulgaris phenology was observed at sites 1-3, 7-9, and 11.
Kehrberger and Holzschuh placed O. cornuta and O. bicornis cocoons and temperature loggers at 11 grassland meadows between early February and late March 2015 near Würzburg, Germany, a city in northwestern Bavaria (Figure 1). Each site varied in sun exposure and mean spring temperature. Seven sites had populations of 50 or more P. vulgaris (sites 1-3, 7-9, 11). The date of emergence for each cocoon, and flower onset and duration were recorded as Julian dates, days since January 1st, along with the mean temperature at each site.
Data Analysis and Discussion Questions
Does mean site temperature impact mason bee emergence?
Using the data table below, graph the date of the first mason bee emergence of both male and female Osmia bicornis on one plot. Draw a line of best fit for males and another for females. Use your graph to answer the following discussion questions.
Graph the data below on a separate piece of paper.
Using the data table to the left, graph the date of the first mason bee emergence of both male and female Osmia bicornis on one plot. Draw a line of best fit for males and another for females. Use your graph to answer the following discussion questions.
1. What is the relationship between mean site temperature and the first day of mason bee emergence for male bees? What about female bees?
2. How might differences in how male and female bees respond to temperature affect reproduction?
Challenge: Does mean site temperature impact the timing of bloom differently than female bee emergence?
Create a graph which shows how site temperature affects the relationship between the timing of bee emergence and flower bloom.
Graph this relationship on a separate piece of paper.
(Hint: It is difficult to graph three variables at once. Try finding the difference between first bloom and first emergence at each site and graph this)
1. Is there a relationship between site temperature and the timing of bee emergence and bloom?
2. How might a large difference in time between bee emergence and flower bloom impact bee and flower reproduction?
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Bosch, J., Sgolastra, F., & Kemp, W. P. (2010). Timing of eclosion affects diapause development, fat body consumption and longevity in Osmia lignaria, a univoltine, adult-wintering solitary bee. Journal of Insect Physiology, 56(12), 1949–1957. https://doi.org/10.1016/j.jinsphys.2010.08.017
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Kehrberger, S., & Holzschuh, A. (2019). Warmer temperatures advance flowering in a spring plant more strongly than emergence of two solitary spring bee species. PLoS ONE, 14(6), 1–15. https://doi.org/10.1371/journal.pone.0218824
Lee, E., He, Y., & Park, Y. L. (2018). Effects of climate change on the phenology of Osmia cornifrons: implications for population management. Climatic Change, 150(3–4), 305–317. https://doi.org/10.1007/s10584-018-2286-z
Seidelmann, K. (2015). Double insurance of paternity by a novel type of mating plug in a monandrous solitary mason bee Osmia bicornis (Hymenoptera: Megachilidae). Biological Journal of the Linnean Society, 115(1), 28– 37. https://doi.org/10.1111/bij.12472
Seidelmann, K., & Rolke, D. (2019). Advertisement of unreceptivity - Perfume modifications of mason bee females (Osmia bicornis and O. Cornuta) and a non-existing antiaphrodisiac. PLoS ONE, 14(5), 1–14. https://doi.org/10.1371/JOURNAL.PONE.0215925
Strobl, V., Straub, L., Bruckner, S., Albrecht, M., Maitip, J., Kolari, E., ... Neumann, P. (2019). Not every sperm counts: Male fertility in solitary bees, Osmia cornuta. PLoS ONE, 14(3), 1–17. https://doi.org/10.1371/journal.pone.0214597