The market near my home is sporting a massive pumpkin display in honor of the harvest season and the slew of upcoming holidays that incorporate gourds. The delicacies our festivities center around rely on native pollinators in order to swell to sometimes fantastic proportions. One species of native bee in particular, the squash bee (Peponapis purinosa), a specialist that only visits plants in the cucurbit family, including watermelon and pumpkin, is one of the critical players in our autumnal celebrations.
Squash bees, like 80% of all native bee species, builds it’s nests underground. It digs a tunnel with several lateral chambers off of it in which it lays it’s young. Most chambers are not too deep in the soil profile, ranging from 3 inches at the shallowest, to a few feet deep, with most around 5-10 inches. Squash bees prefer to nest near their source of pollen and nectar, which means that it nests within agricultural fields. Although the squash vines die out and get removed- the immature bees remain in their cells within the soil, waiting until next summer to emerge and find more squash blossoms. This means that any soil management can affect the young as they overwinter underground.
Katharina Ullman, a graduate student at UC Davis, has been looking at whether tilling negatively impacts squash bee pupating in agricultural fields. She experimentally added squash bees into flight cages and determined where they nested. She then disked and tilled up to 16 inches in the ground in half of these plots. The following summer (this year) she set up cages again to see whether the tilling affected the bees. She found that tilled plots had only half the number of emerging squash bees as untilled plots.
Most farmers disk every year, and till to the depth Katharina did every 3 years. This means they may be killing or disrupting up to half of the population of squash bees! No till or low till methods may help to protect bee larvae and pupae that overwinter in underground cells- helping to bolster pollinator populations year to year, and helping to maintain delicious food on our holiday tables.
Picture of a Squash Bee in a Squash Bloom from Discover Life
(This research was presented at an awesome UC Cooperative Extension Seminar that Katharina and Rachael Long coordinated in Davis last Friday. I learned a lot about neat research projects going on in the region, that I hope to write about in the future- but this seemed most relevant to this time of year)