Nesting Preferences of the Alfalfa Leaf-cutting Bee

Part of the Young Naturalist Awards Curriculum Collection.

by Russell, Grade 8, Utah - 2008 YNA Winner
Drilling holes in a wooden block.

March 2007 
The sun was shimmering on the not-yet-melted snow in my backyard as I walked to the edge of my house. Stacked neatly on the side was a row of white boxes. The boxes (or supers, as beekeepers call them) were filled with bees. Since it was getting warm outside, the bees had started to come out, but one of the supers didn't have any activity. I walked over to it. The snow crunched under my feet until I stopped in front. I opened the top of the box. The smell of sweet honey hit my nose, but when I looked down there weren't any bees. I quickly checked through the entire box and couldn't find a single one. The honey was there, everything else looked okay, but the bees were completely gone. With snow flying up behind me, I ran back inside my house and yelled to my mom, "An entire hive has disappeared!" 
That experience happened nearly a year ago. I didn't realize it at the time, but one of my beehives had gotten a mysterious new condition called colony collapse disorder. Since then, I have learned that the problem is destroying beehives all over the United States and the world.
I started my first beehive at the age of 10 when my dad and I ordered a box of bees through the mail. We made a super (or box) for them and put 10 frames inside. In the middle of each frame, we connected thin sheets of beeswax. The beeswax was stamped with a special impression that made it easier for the bees to start their honeycomb.
Since then, I have added several new hives to my collection but have also lost some. One of my hives was invaded by hive beetles, one was attacked by wasps, one was wiped out by pesticides, one was destroyed by wax moths, and another got chalk brood fungus.

Using a blowtorch to darken the surface.

One day I was looking up bees on the Internet and found out that there was another type of bee called a solitary bee. I learned that solitary bees live separately from each other and do not have a hive. This surprised me and started me on a new adventure with bees. I was curious and asked myself, "Since solitary bees don't live in a hive, where do they nest?" I talked to a beekeeper and a botanist and found out that solitary bees like to nest in small holes or crevices.
I decided to do a study to find out more. I hypothesized that solitary bees would like deep small holes because they would provide more shelter for their larvae. I also hypothesized that solitary bees would like dark-surfaced holes because of some things I had read on the Internet.
To do my study, I got a piece of fir wood and cut it into two-inch, four-inch, and six-inch-long blocks. I drilled 20 holes in each block. Ten of the holes were 2/8 inch wide and 10 of the holes were 3/8 inch wide. I drilled 300 holes in all. I also took a blowtorch and made half of each block dark. Then I put duct tape on the back of each block to close it up.

Wooden blocks under a metal railing in my backyard.

I decided to put the blocks under a hollow metal railing at the back of my house because my older sister said she had seen some bees flying in and out of it. I arranged the blocks so that the darkest part was in different positions. I numbered each block to make it easier to figure out which holes were being nested, so I could record the data accurately in my field journal. The following excerpts are observations I made over the next 10 weeks.

July 9 - Putting the Blocks Out 
Everything is quiet in my backyard except for the call of a mourning dove and an occasional car passing by. I am putting the wooden blocks on a cement ledge under a metal railing in my backyard. I will check the blocks in two weeks to see if anything has nested in them. 

Checking the blocks to see which holes have been nested.

July 23 - Week Two 

I can't believe it! Solitary bees have already started nesting in the wooden blocks. Fifty-one holes have been filled. I can tell because there are tiny cut-up pieces of leaves in the holes. I think the bees are making a place to put their larvae. 
As I watch the solitary bees fly around the blocks, I notice that they fly differently than honey bees. Honey bees use a set flyway (or route) to fly back and forth from their hive. The solitary bees I am watching don't seem to follow the same path each time they fly back and forth. When a bee gets to one of the blocks, it hovers before it decides which hole to go into. 
Solitary bees also seem to buzz differently than honey bees. They make a higher-pitched sound than the lower-pitched buzzing that honey bees make. I wonder if the lower drone of the honey bees is because there are more of them. 


August 6 - Week Four 
It's amazing how much progress the solitary bees are making. They have filled 76 holes, but they don't seem to like the two-inch-deep blocks. None of them have been filled. The bees are also avoiding the 3/8-inch-wide holes. A small black spider scurried out of one of the holes when I picked up a block today. It looks like the blocks make a good home for spiders, too. 

Only the 2/8-inch wide holes are being nested by the solitary bees.

I went to the Monte L. Bean Natural History Museum last week and talked to an entomologist. He told me the bees nesting in my wooden blocks are probably alfalfa leafcutting bees. Their scientific name isMegachile rotundata . They are smaller than honey bees and don't have stingers, but the females can "pinch" with the organ they use to deposit their eggs. They only do this when they are really, really scared. He also told me that the bees are putting pollen into the holes so their larvae will have something to eat when they hatch. 

Holes nested by solitary bees after four weeks.

August 20 - Week Six 

Nesting is slowing down. Only eight new holes have been filled during the past two weeks, but some of the larvae have started to hatch. I can tell because the leaf cap sealing the end of the hole has been punched through on three of the holes, and the place where the larvae had been is empty. This means the alfalfa leafcutting bees can hatch in around six weeks. 
I picked up a solitary bee today and put it in my hand. It just sat there as I studied it. I could never have done that with one of my honey bees. Alfalfa leafcutting bees are much more gentle than honey bees. I wonder if it is because they don't have a hive to defend. 

Holes nested by solitary bees after six weeks.

September 3 - Week Eight 
It's getting colder outside, and only two new holes have been nested during the past two weeks. Could the temperature dropping be the reason? Alfalfa leafcutting bees stay under leaves at night and can't survive when it gets too cold. 

Table of holes in Nesting Preferences of the Alfalfa Leaf-cutting Bee
Holes nested by solitary bees after eight weeks.

When I checked the blocks today, I found a wasp in one of the 3/8-inch-wide holes. The wasp was filling the hole with a cotton-like substance. Its abdomen was wiggling back and forth as it worked. The wasp is much bigger than the alfalfa leafcutting bees I am studying. Is that why he liked the larger hole?


Holes nested by solitary bees after ten weeks.

September 7 - First Frost 
September 17 - Week Ten - No More Activity 
Results and Conclusions 
Based on my observations, alfalfa leafcutting bees liked the four-inch and six-inch-deep holes the best. They nested in 41 of the four-inch-deep holes and 45 of the six-inch-deep holes. The bees did not like the two-inch-deep holes. None of them were nested. I think the reason they did not like the holes is because they did not provide enough shelter.

Depth of holes.

The alfalfa leafcutting bees preferred 2/8-inch-wide holes over the 3/8-inch-wide holes. They nested in 86 of the 2/8-inch-wide holes but none of the 3/8-inch. I think they liked the 2/8-inch-wide holes the best because the holes were closer to their size and offered more shelter.

The alfalfa leafcutting bees appeared not to have a preference for the surface color of the blocks. They liked both the dark- and light-surfaced holes about the same. They nested in 41 of the dark-surfaced holes and 45 of the light-surfaced holes.

A chart titled Width of Holes shows nesting preferences of alfalfa leafcutting bees. Over a 10-week period, the bees nested in 86 of the 2/8-inch-wide holes and in none of the 3/8-inch holes.
Width of holes.

Since finishing my study, I have talked to a lot of people about the importance of solitary bees. I have given several speeches on the subject and have displayed my project at the Utah County Fair. I also had the opportunity to display my project at the Smithsonian National Museum of Natural History in Washington, D.C. When I tell people about solitary bees, they are surprised to learn that there is a type of bee that lives alone and not together in a hive. They are even more surprised to learn 85 percent of the 20,000 bee species in the world are solitary.

Surface of blocks.

Bees are extremely important to the environment because they pollinate one-third of our food supply, but they are disappearing. In the past five years, over 40 percent of the beehives in the United States have died out. Because solitary bees live alone, they do not spread parasites or diseases as easily as honey bees. It is important to study solitary bees because in the future we may become more dependent on them for pollination. Who knows, solitary bees may even become a solution for the honey bee crisis.


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