Super sized or minimized? Diet and tunnel diameter changes progeny outcomes for solitary bees

Article written for the Orchard Bee Association newsletter, Orchard Buzz, Winter 2020 issue (Vol. 7, Issue 1)

Written by: Courtney Grula, PhD Student, North Dakota State University

Body size is influenced by temperature, nutrition, and nest size. Nutrition is the most influential factor in determining adult body size. The amount of food the mother provides to the larval bee determines how large the bee will grow, and larva that are provided with more food will develop into larger adults. Larval bees are not capable of foraging for their own food, so the food provided to them by their mothers is very important in determining their success as adults. Nest size also influences body size be-cause there is less space for the bee to develop, as well as less space to place food. Bees that nest in larger nest holes produce larger offspring.

My goal as a researcher is to determine how body size impacts performance in two solitary bees, the blue or-chard bee and the alfalfa leaf cutting bee. I use food quantity and nest size as ways to manipulate body size in these bees.

First, I am able to control body size by manually providing different amounts of food to the larvae (see Figure 1). I remove the larvae and their food from their nests, and I put the bees into 3 different feeding treatments. One treatment has a small amount of food, the second treatment had the amount food that was provided by the mother, and the third treatment had a large amount of food, in excess of what they would be provided by their mothers. This leads to small, average, and large adult bees (see Figure 2).

Figure 1. Osmia larvae on nectar/pollen provisions.
Figure 2. Small and large adult bees as a result of low (left) and high (right) feeding treatments.

The second way I manipulated body size is in the field, by changing the nest hole diameter size. I used nest holes that ranged from 5 mm-9 mm. This led to bees creating nests of differing sizes, adapting to the size of the hole provided (see Figure 3).

Figure 3. Leaf cutting bee nests of different sizes, made by providing a range of nest diameters in the field.

The ability to fly for forage, as well as transport food and nest materials back to the nest is critical to the success of solitary bees. I measured flight performance using the size-manipulated bees. I measured their metabolic rates while flying to determine the energy efficiency of bee flight as a function of body size. The results showed that bees of differing sizes had equivalent metabolic rates, indicating they were equally metabolically efficient during flight. I also measured the size of different body parts and determined that small bees can carry heavier loads per milligram of body weight. Future studies will measure other performance outcomes based on body size including overwintering success.

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Courtney is a PhD student in Dr. Julia Bowsher’s lab studying  two species of solitary bees, Megachile rotundata, alfalfa leafcutting bee and Osmia lignaria, blue orchard bee. Her expertise is in nutrition and development of solitary bees. Currently, she is working on determining how insect nutritional quantity affects rate of development and how telomeres function in these solitary bee species. Her favorite part of the job is to ask novel questions about her favorite insect, bees!

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