An Alfalfa Leafcutting Bee’s First Meal is a Big Deal

Megachile rotundata

An alfalfa leafcutting bee (Megachile rotundata) visits an alfalfa flower. A new study on the species shows that larval diet influences whether it matures and reproduces in one season or enters winter diapause as a prepupa, reproducing the following spring. (Photo credit: Peggy Greb, USDA-ARS)

By Meredith Swett Walker

Parents often worry about what they feed their kids and how it might affect their future. But, barring genuine malnutrition, no diet or food is likely to radically change your kid’s life—unless you are an alfalfa leafcutting bee. For such bees, diet early in life can be truly life-altering.

Meredith Swett Walker

Alfalfa leafcutting bees (Megachile rotundata) take one of two paths in life. Some grow up fast, maturing and reproducing in the same year they hatch. Others grow up more slowly, overwintering as a prepupa in a state of diapause, or dormancy, and reproducing the following spring.

But diapausing alfalfa leafcutting bees are not genetically different from the fast-to-mature “non-diapausing” bees. The path that a young alfalfa leafcutting bee takes is instead determined by its diet early in life. This phenomenon, in which a given set of genes can be expressed differently (to diapause or not to diapause) depending on factors in the organism’s environment is called “phenotypic plasticity.”

In research published in July in Environmental Entomology, scientists at the Program in Ecology, Evolution, and Conservation Biology (PEEC) at the University of Illinois at Urbana-Champaign investigate how nutrition during the larval stage affects diapause plasticity and female reproductive behavior in the alfalfa leafcutting bee. Nutrition is known to regulate diapause, as well as other phenotypically plastic traits, in several insect species. But diapause in alfalfa leafcutting bees is of particular interest because non-diapausing bees are a headache for beekeepers and farmers, says lead researcher Brielle Fischman, Ph.D., visiting professor at Hobart and William Smith Colleges and an alumna of the PEEC, where she conducted this research along with co-authors Theresa Pitts-Singer, Ph.D., and Gene Robinson, Ph.D.

Alfalfa leafcutting bees are important pollinators of alfalfa as well as crops such as rapeseed and blueberry. They are reared commercially for their pollination services. However, unlike the more familiar honey bee, alfalfa leafcutting bees are solitary. Each female lays eggs in her own nest, and she alone provides for them.

Females construct nests in small holes or tunnels excavated in rotting wood, plant stems, or other substrates. Commercially kept alfalfa leafcutting bees are typically provided with “bee blocks” with multiple holes lined with paper straws for nest sites. Nests are a linear series of cells within the tunnel or straw. The female bee lays an egg in the cell, provisions it with a ball of nectar and pollen, then uses a small circle cut from a leaf to wall off the cell. The ball of nectar and pollen provided by the mother bee is the only source of food for the bee larva until it pupates and emerges from the nest as an adult bee.

Non-diapausing alfalfa bees cause trouble for beekeepers and farmers because when they emerge they chew through the cells of any diapausing siblings that lie between their cell and the nest exit. This can kill the diapausing bees and spread disease-causing fungus. In addition, non-diapausers have bad timing, from the farmer’s point of view. When they emerge later in the summer, many crops have passed peak flowering and these bees often disperse away from crop fields in order to find food.

nest board x-ray

An X-ray of alfalfa leafcutting bee nests in straws shows individual cells and allows researchers to measure the amount of nectar and pollen provided to the larvae by the mother bee. (Photo credit: Brielle Fischman, Ph.D.)

Fischman and her colleagues first examined alfalfa leafcutting bee nests collected from commercial bee blocks. They removed the paper straws containing the series of cells from the block and X-rayed them, allowing them to see inside and measure the size of the provision ball given to each larval bee. The researchers then recorded whether each larva underwent diapause or not and how much each adult bee weighed upon emergence. Larvae provided with larger provisions were more likely to undergo diapause and were generally heavier.

With another set of nests, they experimentally manipulated the size and quality of provisions by dissecting nests and rearing each larva in a small covered dish. Larvae were given provision balls with set masses, either large or small, and with varying percentages of royal jelly. (Royal jelly is a substance secreted by honey bees; when producing a new queen, they feed a few larvae large amounts of royal jelly. This special diet causes the larvae to develop into queen bees rather than workers or drones.) The researchers added royal jelly to the alfalfa leafcutting bee provision balls with the intent of increasing the quality of the provision. They then recorded how much each larva weighed, whether or not it underwent diapause, and how much it weighed when it emerged as an adult.

In general, larvae with the experimentally enlarged provision balls weighed more and were more likely to undergo diapause. But the effects of royal jelly were more complex. While the researchers added royal jelly to the larval diet in an effort to increase its nutritional quality, it appears that the jelly may have lowered nutritional value. In general, larvae that ate more royal jelly weighed less and became smaller adults than those that did not receive royal jelly. Nevertheless, royal jelly seemed to increase the likelihood that a larva would undergo diapause. The researchers believe that, rather than simply providing nutrients, chemicals in the royal jelly may have triggered the molecular mechanisms regulating diapause.

Having established that larval nutrition affects the likelihood that a bee will undergo diapause and how much it weighs as an adult, Fischman and colleagues next examined the effects of weight and diapause status on adult reproductive behavior. They assessed diapausing and non-diapausing female bees of varying weight to see how long it took them to start nesting, if they maintained ownership of their nest, and how many cells they built. In some cases, bees were provided with ample nesting holes; in others, there was a shortage of holes, forcing females to compete for nest sites.

Female alfalfa bees (marked with paint dots for identification) battle for a nest site. (Video credit: Brielle Fischman, Ph.D.)

The effect of diapause status on reproductive behavior seemed to depend on environmental conditions. In one year of the behavioral study, non-diapausing females appeared to have an advantage, while in the second year, under different weather and alfalfa bloom conditions, that advantage disappeared. In general, when there was competition for nest sites, heavier bees were better able to expel intruding females from their nests. The results of these behavioral tests suggest that “diapausers and non-diapausers represent discrete adult variants with different reproductive strategies and capabilities.”

This study demonstrates that nutrition affects probability of diapause in alfalfa leafcutting bees, but the authors caution that it is probably only one of many factors influencing this complex process. Prior to this study, a relationship between nutrition and diapause had been shown in other groups of insects but not in the bee clade.

A better understanding of how diapause is regulated in alfalfa leafcutting bees may allow beekeepers and farmers to manage their bees more efficiently. It’s impractical for beekeepers to directly manipulate larval provisions as was done in this study. However, Fischman suggests that planting other flowering plants near alfalfa fields may improve diet quality and quantity by providing variety and a reserve food source if the alfalfa bloom is low.

But better management of alfalfa leafcutting bees is not the researchers’ only motivation. They are also interested in how different bee species evolved different traits and lifestyles. “Plasticity, or the ability to express different versions of a trait … is one hypothesis for where new traits come from. The idea is that new traits start out as plastic versions of traits found in an ancestral species, but over time one version of the trait becomes fixed, so individuals in the species always express this version,” says Fischman.

Megachilidae bees, such as alfalfa leafcutting bees, are closely related to Apidae, a group that includes many eusocial species like honey bees. Understanding how diapause is regulated in a solitary bee may be an important thread in untangling the process by which eusocial insects evolved. The “diapause ground plan hypothesis” proposes that mechanisms involved in regulating diapause in solitary species were “repurposed” during the evolution of eusociality to determine caste (queen versus worker). The effects of larval nutrition documented in this study provide support for this hypothesis.

The single meal of pollen and nectar that a mother alfalfa bee provides for her offspring will impact virtually all aspects of its life. Human parents should be thankful that the occasional meal of fast food or frozen pizza we feed our kids when we’re overwhelmed does not have the same consequences.

Meredith Swett Walker is a former avian endocrinologist who now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. You can read her work on her blogs Pica Hudsonia and The Citizen Biologist or follow her on Twitter at @mswettwalker.

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