Take a look deep into the yellow squash flowers and there is a good chance the stripped abdomens of squash bees (Peponapis pruinosa) will be pointing skyward, with heads buried and long tongues extended, they partake from the squashes nectar wells. These fuzzy yellow honey bee sized insects have formed a time tested relationship with plants from the squash (Cucurbita) family. Squash flowers invite other bees, such as bumble bees and honey bees, to join the feast of nectar and pollen but squash bees are specialists of Cucurbita plants and uniquely adapted in effectively pollinating their flowers. There are 13 species of squash bees with one species, Peponapis pruinosa, native to the eastern United States.
The unique relationship between bee and squash dates back 30,000 years when mastodons, elephant like creatures, roamed the continent. A new study has revealed Cucurbita seeds in their dung. It turns out these large herbivores not only transported and fertilized the seeds but created the necessary disturbed habitat for them to grow. (1) In these mastodon gardens bees were important pollinators and the Peponapis spp. in particular favored the large sticky pollen produced by squash flowers. Shortly after the arrival of humans 12,000 years ago the mastodons went extinct. The indigenous squash to North America rapidly disappeared from most of its range, with many species going extinct. Only the squash in Mexico remained where Native Americans discovered the bitter gourds were well suited for utilitarian uses such as containers. The plant had discovered a new partner to ensure it survival and once again was thriving in human gardens. The squash once again began to expand its range and so did the squash bee, at least one species did, Peponapis pruinosa.
Native Americans domesticated other wild plants including a wild grass known as Balsas teosinte the ancestor of Zea Mays and the legume known as beans, Phaseolus vulgarism L.. These three plants known as the three sisters are a classic example of companion planting in early agriculture. The three sisters had a long journey through time and distance from their homeland in Mexico to reach the land of the Lenape and Iroquois in the American Northeast. Passing from hand to hand and tribe to tribe the three sisters began their territorial expansion but they did not all reach present day New York at the same time or together. It took time for the plants to acclimate and adapt to various climates that varied greatly from the desert southwest. Not surprising, squash was the first of the three sisters to arrive (return) to New York around 6,000 BC as the bitter version of its modern incarnation. Corn (Zea Mays) arrived about 270 BC, the main type of corn we know today. Lastly, bean arrived, as a type of lima bean or butter bean in 1300 AD, just prior to Europeans arrival.(2) It is reasonable to believe Peponapis pruinosa was busy at work pollinating squash flowers on the arrival of each sister.
Floral specialists are not always the best cross pollinators as they are to efficient at collecting the pollen of their preferred plant leaving little pollen for successful pollination, but as it turns out squash bees are excellent pollinators. Upon emerging from their underground nests, the females spend up to two weeks drinking nectar before building their nesting chambers and laying eggs. Male squash bees may in fact be better at pollination than the females as they have no need for pollen. In both cases their fuzzy bodies are scattering pollen between flowers. Squash bees alone can successfully provide all the necessary pollination of their host plants without the help of most other native bees or honey bees. Male squash bees sleep in the squash flower as it shrivels up around them during the heat of the day. The next day they chew themselves out of the flowers and join the females in the darkness of a pre dawn soiree following the scent of the new days flowers. (3) Squash bees are pollinating long before other bees wake up.
These solitary females don’t travel far to build their nests. Staying close to their host plant they dig a deep tunnel under the protection of the large prickly leaves. Even though they dig down deep, up to a foot before laying eggs, their nests can be susceptible to tillage. It has been found that on no-till farms squash bee populations can be triple that of farms that use traditional tilling methods. (3) If farms use herbicides or other chemical treatments it can be assumed this can also have a detrimental impact on squash bee populations. Despite these dangers, squash bees tend to be found wherever there are squash plants. Squash farmers can avoid extra expenses associated with renting honey bee hives by taking steps to encourage healthy squash bee populations.
As a floral specialist, squash bees made a evolutionary choice to depend on one genus of plants, such risky propositions seldom have happy endings. The squash plants dependence on the mastodon nearly led to a triple extinction. In the absence of the mastodon gardens, squash developed a new strategy for survival and established a new and successful partnership with humans. In human gardens squash quickly diversified into the tender summer squashes: yellow crookneck and zucchini and the hardy winter squashes: butternut, acorn and the unmistakable pumpkin. Take a long look into the glowing eyes of the Jack-o-Lantern and rest assured the Cucurbita family has solidified its perpetual existence and with it the squash bee.
(1) Logan Kistler, Lee A. Newsom, Timothy M. Ryan, Andrew C. Clark, Bruce D. Smith and George H. Perry. (2015) Gourds and squashes (Cucurbita spp.) adapted to megafaunal extinction and ecological anachronism through domestication. Proceedings of the National Academy of Sciences. Vol. 112. no. 49. 15107-15112. http://www.pnas.org/content/112/49/15107
(2) Pritchard, Evan. Indigenous Thoughts on the History of Agricutlure in the Hudson Valley. A paper submitted June 13th 2014. Farm Hub Retreat, Mohonk Center. pp. 1-10.
(3) Wilson, Joseph, Carril M. Olivia. The Bees in Your Backyard. Princeton University Press. 2016. pp. 224-225.
Paul D. Hurd, Jr., E. Gorton Linsley and Thomas W. Whitaker. Squash and Gourd Bees (Peponapis, Xenoglossa) and the Origin of the Cultivated Cucurbita. Evolution. Vol. 25, No. 1 (Mar., 1971), pp. 218-234.