Squash Bees

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By Katharina Ullman, UC Davis Graduate Student in Entomology & Good Humus CSA Member, courtesy of Capay Valley Farm Shop

For many years now we have researchers from UC Davis ask if they can come to the farm to work on their individual projects. The first was Bob Bugg studying knotweed and the Big Eyed Bug. That was probably a good thirty years ago when we were farming in Woodland. We never tell someone they can’t come out and do their research, and many times we really don’t know what they are doing. There is a study going on in the Watermelon patch right now, that has been happening for several years, and another woman is doing a shade study in the hedgerows. We have had folks checking out the native bee populations, teaching us about native ground dwelling bees, learning that there are many different species of bees besides the European honey bee. Which is really interesting considering how hard a time the European honey bee is having right now. We used to keep bees, they are necessary for pollination in the squash, and a few of our crops. Even though I took Bee Biology in college and learned how to be a bee keeper, it turned out to be another layer of management that we were not able to carry out, so we gave the hives away. The following researcher Katharina Ullmann is a Graduate Student in the department of Entomology, and she has been showing us the native squash bees, Zach even found some nests as he was working out in the field one day this spring, after Katarina showed him what they look like. What is so exciting is that we do not have a lack of bees here, honey bees or the native bees, and so it is not necessary for us to rent hives for pollination, I think mostly because we have something for the bees to eat all year long, especially with the hedgerows. As the squash patch got going Katharina and her team started coming out, and we got to talking, so I asked her to write a piece for the newsletter about her project. Maybe Katharina will come to the Peach Party and be willing to show folks what a native squash bee looks like and its nest.

from Good Humus Produce

Squash Bees

I remember the first time I visited Good Humus Farm. It was in the summer of 2010 and the sun was hanging low on the horizon setting the hills aglow. I had been driving around the Sacramento Valley mapping squash fields in Colusa, Solano, and Yolo County. Turning into Good Humus that evening the green of the farm made me feel like I’d entered an oasis.

Good Humus is, indeed, an oasis for squash bees. Squash bees (Peponapis pruinosa) are specialist bees that only collect pollen from squash and pumpkin flowers. They bring this pollen back to their underground nests to feed their young. Squash bees, like the majority of the 4000 species of bee’s native to North America, are solitary; they do not live in large colonies. Instead, they work alone – in the squash bee’s case digging out tunnels in the ground. At the end of each tunnel they excavate a small cell which they provision with pollen and then lay a single egg. Once that’s done they close off the cell and never see their offspring again. Adult squash bees typically  live for four to six weeks, the rest of the time (up to 11 months) they are developing underground. The larva will hatch, eat up the pollen its mom left it and then overwinter as a prepupa. In the spring the bee will pupate and eventually emerge from the ground as an adult.

Squash bees are unique because they’ve evolved to have their daily cycle match that of their plant host. This means that they are up at dawn when squash flowers open and cease activity around 11am or noon when the flowers close; females will spend the afternoon excavating nests and resting while males will rest in closed flowers over night.

Over the past two years I’ve sampled squash fields all over Yolo County and Good Humus consistently has had the highest density of squash bees. Preliminary analyses suggest that these high densities are partly due to the fact that Jeff and Annie grow squash every year so every new generation of squash bees has access to food. I’m also guessing that the hedgerows and windbreaks on the farm act as refuges for ground nesting bees. Unlike the crop fields, these areas do not get tilled. So, next time you are out at the farm, take some time to look for squash bees – you might be surprised by how many you find!


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