Modelling squash bee populations

See how regional squash bee populations grow under different farm management practices by selecting from the dropdown menus to the left and then pressing the "Simulate" button. This website works best when viewed in Chrome, Safari or Internet Explorer.

Use this squash bee model to explore how farming practices can impact an important crop pollinator.

Why should I care about squash bees?

The squash bee (Peponapis pruinosa) is a wild, native bee that pollinates Cucurbita crops (e.g. squash, gourds and pumpkin). These crops depend on bee pollination to produce fruit. Many squash and pumpkin farmers rent honey bees. However, farmers may be able to reduce costs associated with rented honey bees if they manage their farm to support squash bees. Squash bees are as good as honey bees at pollinating squash and pumpkin flowers. They also have an advantage over honey bees: squash bees are active as soon as squash and pumpkin flowers open in the morning, a time when honey bees are still in their hives. This early activity suggests that if there are enough squash bees in a field they may be able to pollinate squash and pumpkin flowers before honey bees are out working the field.

In addition to their value as a crop pollinator, squash bees have an intrinsic value by being one of the 4,000 bee species found in North America. Due to their dependence on Cucurbita, they have a close relationship with humans. In fact, scientists think that this species was able to travel across North America, in part, by tracking Native American Cucurbita cultivation. As a result, squash bees are not only found in Mexico, where they originated, but are also found as far north as Vermont.

What do I need to know about squash bees?

  1. Squash bees depend on squash, pumpkin, and gourds (Cucurbita spp.).
  2. Squash bees nest in the ground where the offspring for the next season remain over the winter months.
  3. Squash bees prefer to nest under squash making them vulnerable to tillage.

Squash bees depend entirely on squash, pumpkin, and gourds (Cucurbita spp.). In many parts of the United States there are no native Cucurbita spp. growing wild and so this bee must find squash or pumpkin fields to survive. Squash bees have one generation per year. In the summer adults will fly around collecting pollen and building nests. Their offspring will spend the most of their life underground, as they wait for the next season’s crop. Like the majority of native bees, squash bees are solitary bees that nest in the ground. Their nests are a series of tunnels, at the end of which are cells filled with pollen and bee eggs. They prefer to nest underneath squash vines which make them particularly vulnerable to disturbance caused by tilling. If they are able to survive tilling and emerge as adults, they then need to be able to reach a squash field in order to reproduce. The video below summarizes the natural history of the squash bee.

What farming practices impact squash bees?

Bees need three things to survive in agricultural landscapes: food (in the form of pollen and nectar), a safe place to nest, and protection from pesticides. Farmers can potentially increase existing squash bee populations by: (1) ensuring that squash or pumpkin is planted on or near the farm every year and that this year’s field is within about a mile of last year’s field, (2) avoiding frequent deep tilling, which can destroy overwintering offspring, and (3) not using pesticides that may harm squash bees during bloom, this would include systemic pesticides. While squash bees collect pollen from Cucurbita spp. to feed their young, you can find them visiting other plant species for nectar. This suggests that wildflower plantings may benefit this species, especially before Cucurbita spp. come into bloom.

What can I learn from this squash bee model?

The model above simulates how crop rotation and tillage practices can impact regional squash bee abundance over time. The squash bee model allows you to manipulate where squash fields are placed in the landscape using the “Choose squash field pattern” tab and to modify tillage practices via the “Choose tilling practices” tab. For example, select “squash fields clustered near each other” and “no tilling”. Does the squash bee population grow or decline over time under these practices? Why might this be? Now run the simulation again, but change the field pattern to “squash fields clustered far from each other”. Does the squash bee population grow in the same way as it did when squash fields were clustered near each other? By selecting “random squash field pattern” the model will generate new squash field configurations. How does the population change under different squash field patterns? What happens to the squash bee population over time when “deep tilling” occurs in all fields? Optimal conditions for squash bees will include fields that are clustered near each other across years and where there is no deep tilling. Under these conditions, squash bees emerging from their overwintering nests can find newly created squash fields every year.

What are the uses of this model?

Farmers, extension professionals, and anyone interested in learning about squash bees can use this model to explore the impact of different farming practices on squash bee populations. However, this website was developed with extension professionals in mind. When conducting grower workshops on squash pollination or native bees in agricultural landscapes, this model can be used to demonstrate how different cropping patterns or tillage practices can impact squash bee populations.

What is behind the model?

In this model squash bee population dynamics are a function of immigration from fields that were occupied in the previous generation, a growth rate, and overwintering survival based on the field’s tillage state. Four parameters are included in the model: adult dispersal, fecundity, overwintering survival, and tillage survival. The video below describes the model.

What other bees visit squash and pumpkin?

The most common visitors of squash and pumpkin in the northern Central Valley of California are honey bees (Apis mellifera) and squash bees (Peponapis pruinosa). Less common visitors include bumble bees (Bombus spp.) and sweat bees (family: Halictidae). There is a second type of squash bee (Xenoglossa spp.) found in other parts of the United States. Bumble bees are better pollinators than squash bees or honey bees, but in many parts of the country their density in squash fields is low. The story is different in the Mid-Atlantic and Northeastern United States, where bumble bees are abundant enough that they are considered to be the best wild bee to rely on.

On-line resources

Scientific papers about squash bees

  • Hurd, P.D., Jr., and E.G. Linsley. 1964. The squash and gourd bees Peponapis Robertson and Xenoglossa Smith inhabiting America north of Mexico. Hilgardia. 35:373-477
  • Hurd, P.D., Jr., E.G. Linsley, and A.E. Michelbacher. 1974. Ecology of the squash and gourd bee, Peponapis pruinosa, on cultivated cucurbits in California (Hymenoptera: Apoidea). Smithsonian Institution Press, Wahsington D.C.
  • Julier, H.E. and T.H. Roulston. 2009. Wild bee abundance and pollination service in cultivated pumpkins: farm management, nesting behavior, and landscape effects. Journal of Economic Entomology. 102:563-573
  • Mathewson, J.A. 1968. Nest construction and life history of the eastern cucurbit bee, Peponapis pruinosa (Hymenoptera: Apoidea). Journal of the Kansas Entomological Society 41:255-61
  • Minter, L.M. and R.T. Bessin. 2014. Evaluation of native bees as pollinators of Cucurbit crops under floating row covers. Environmental Entomology. 43:1354-1363
  • Peterson, J.D., S. Reiners, and B.A. Nault. 2013. Pollination services provided by bees in pumpkin fields supplemented with either Apis mellifera or Bombus impatiens or not supplemented. PLoS ONE 8: e69819
  • Shuler, R.E., T.H. Roulston, and G.E. Farris. 2005. Farming practices influence wild pollinator populations on squash and pumpkins. Journal of Economic Entomology. 98:790-795
  • Tepedino, V.J. 1981. The pollination efficiency of the squash bee (Peponapis pruinosa) and the honey bee (Apis mellifera) on summer squash (Cucurbita pepo). Journal of the Kansas Entomological Society 54:359-377

Authors and funding

The information on this webpage, including the model, was developed by Katharina Ullmann (University of California, Davis), Eric Lonsdorf (Franklin and Marshall College), Matt Loiacono (Franklin and Marshall College), and Neal Williams (University of California, Davis). K. Ullmann received funding from Western Sustainable Agriculture Research and Education Graduate Student Grant (GW13-018) and NSFGRFP #1148897. M. Loiacono received funding from Franklin and Marshall College. E. Lonsdorf and N. Williams received funding from a USDA Specialty Crop Research Initiative, Coordinated Agricultural Project, Developing Sustainable Pollination Strategies for U.S. Specialty Crops (Award 2012-51181-20105).