Dr. Jim Cane of the USDA Agricultural Research Service has recently shared with us the following excerpt of his recent work with native bees that exclusively pollinate squash. An article by Science Daily also discusses the findings showing that squash-pollinating bees migrated with the spread of squash agriculture across North America.

Squash Pollinators of the Americas Survey (SPAS)

Peponapis pruinosa pollinating a flower of the squash family

Peponapis pruinosa pollinating a flower of the squash family.

Squash, beans, and corn are the so-called Three Sisters of pre-Columbian agriculture that together fueled Mesoamerican civilizations (Mayan, Aztec) and agrarian societies as far away as the Iroquois of the northeastern USA.  Of this trio, only the squashes (including pumpkins and gourds) continue to require a pollinator, owing to the simple fact that their flowers are unisexual, and so require a bee to move pollen from male to female flowers.  Honeybees are typically provided for commercial squash pollination, but native bees of two genera – Peponapis and Xenoglossa, the so-called “squash bees” – are the ubiquitous, often dominant pollinators of many wild New World Cucurbita (the genus that includes squashes and gourds).  Where squash cultivation has extended beyond the ranges of wild progenitors, representative species of squash bees have followed (in North America, everywhere outside the Southwest, and in South America, all of southern Brazil).  Squash bees are non-social but often gregarious ground-nesters, and all species are strict specialists for Cucurbita pollen.  They forage early in the morning, before honey bees are active, and have been shown to be excellent pollinators of zucchini.  If numerous, they thoroughly pollinate all available flowers, rendering flower visits of later-flying honey bees superfluous for pollination.  Before honey bees were introduced to the Americas by European colonists, it seems evident that squash bees were critical to the adoption, domestication, spread and production of Cucurbita by native peoples throughout the Americas.

In Tremonton Utah, a field of squash surveyed by Dr. Cane

In Tremonton Utah, a field of squash surveyed by Dr. Cane.

Today, we know from earlier intensive sampling efforts plus museum label data that squash bees are found throughout most of the US and SE Canada, and southward through Mexico to near Buenos Aires and thence across through Brazil.  But how much do they contribute to pollination of cultivated squashes and pumpkins on these continents?  To answer that question, we must know three variables.  First, we should know the pollination efficacies of squash bees at the other squash species and sub-species, although results with zucchini are probably a good first estimate.  Second, we need to know the relationship between bee abundance at flowers and satisfaction of pollination needs of the various cultivated squash species and subspecies; at some visitation intensity, full pollination is achieved and subsequent visits, as by honey bees, become superfluous.  And third, we need a wide-ranging standardized survey of abundance of squash bees at cultivated Cucurbita, to estimate how frequently, or to what degree, squash bees satisfy these estimated pollination requirements of cultivated Cucurbita.  These surveys should represent different biomes, habitats, cultivation practices and histories, and different scales of production from home gardens to big commercial fields, in order to quantify these bees’ overall contribution to New World production of pumpkins and squashes. If Peponapis and/or Xenoglossa prove to be ubiquitous, prevalent, abundant and effective, then this would be the first case for unmanaged, native non-social bees playing a key role for production of any agricultural crop.  As a practical matter, their recognition and stewardship by farmers and gardeners would translate directly into production and sales, while the need for renting honey bee colonies would be brought into question.

The zucchini below is an example of an under-pollinated squash fruit

The zucchini underneath is an example of an under-pollinated squash fruit.