Squash Pollinators of the Americas Survey (SPAS)

James Cane

USDA Pollinating Insect Research Unit, UtahStateUniversityLogan, UT84322-5310

email: jcane@biology.usu.edu

14 February 2005

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 confederated Iroquois tribes of the northeastern USA.  Of this trio of crops, only the squashes (including pumpkins and gourds) continue to require a pollinator.  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 specialist bees of two genera - Peponapis and Xenoglossa, the so-called "squash bees" - are the ubiquitous, often dominant pollinators of many wild New WorldCucurbita (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, anywhere outside the Southwest; in South America, areas 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, beginning 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.

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 s 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 (see table), 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..

It is clearly impractical impossible for any one investigator to locate and concurrently survey for bees across a series of cultivated squash fields and gardens across any country. Therefore, to achieve this necessary objective, the Squash Pollinators of the Americas Survey (SPAS) was conceived and first implemented in January 2004 by Jim Cane at the USDA-ARS Pollinating Insect Research Unit at UtahStateUniversity in Logan, Utah.  SPAS consists of a network of volunteer collaborators, mostly pollination ecologists or bee biologists, drawn from universities and federal agencies from Guelph, Ontario, Canada in the north all the way south to Buenos Aires, Argentina and three states in southern Brazil.  A systematic, unbiased protocol was designed for censusing squash bees and other bees at a recorded count of squash flowers and plants.  The protocol was then tested, evaluated and refined by 16 founding members at 55 sites during 2004, mostly by collaborators in the USA.  Attendant attributes of counts and contexts are also recorded.  These include hour and date of census, species of squash, field size, years in production, organic or not, habitat surround, as these variables may be responsible for differences in counts.  Communication between participants is facilitated by a listserv and a bilingual master spreadsheet for data entry, compilation, and redistribution back to participants.  These features will continue to be refined as insights are gained.  Rather than the traditional model of a principal investigator directing subordinate contributors, it is hoped that SPAS will foster inquiry and decentralized leadership from among the participants, who are encouraged to band together in various combinations to tackle regional, conceptual or practical components of the overall program.

Initial data indicates that one or more species of Peponapis are abundant if not dominant at flowers of cultivated Cucurbita throughout the eastern US, from s Maine to s Texas, as well as at the few scattered sites sampled in the western USA and Mexico.  Population densities do not seem to diminish with larger patch size.  For instance, one 150 acre field of kabocha squash hosted one Peponapis bee at every five flowers, unbeknownst to that farmer who was spending thousands of dollars annually for rented honey bee hives.  Locations with a history of Cucurbita cultivation are typically hosting abundant Peponapis.  Some conventional farms that use pesticides judiciously in the crop or the agricultural surround are nonetheless served by abundant Peponapis at their Cucurbita flowers.  This unexpected result may reflect attributes of this particular system that are forgiving of insecticide use so long as insecticides are not systemic, and they are applied as liquids at dusk or night and dry by morning.  Peponapis was found to be absent or rare at a minority of sites for as yet obscure reasons.

Additional collaborators are being sought and recruited for the survey in the western US and especially from other countries, as large regions remain entirely unsampled.  Meanwhile, a subset of collaborators is sought to characterize the pollination needs of the other species and subspecies of cultivated Cucurbita, and to relate floral visitation intensity and bee abundance to satisfaction of pollination need.  Success or failure of SPAS hinges on the reliable and consistent contribution of participants; our first year's efforts show that this is a reasonable expectation (once on site, the survey itself is taking about 10 minutes).  For participants with no financial support, for whom even just gas money is a burden or personal contribution, modest funding to compensate their efforts is sought.  Given our first year's data, it appears that an unmanaged group of non-social native bees - the specialist squash bees- are largely responsible for the production of cultivated squashes across North America, and by extrapolation, to much of the Americas.

Selected Bibliography

Tepedino, V. J.  1981.  The pollination efficiency of the squash bee (Peponapispruinosa) and the honey bee (Apismellifera) on summer squash (Cucurbitapepo). Journal of the Kansas Entomological Society 54:359-377.

Hurd, P. D., Jr., and E. G. Linsley.  1964.  The squash and gourd bees -- genera Peponapis Robertson and Xenoglossa Smith -- inhabiting America north of Mexico (Hymenoptera: Apoidea). Hilgardia 35:375-477.

Hurd, P. D., Jr., E. G. Linsley, and T. W. Whitaker.  1971.  Squash and gourd bees (Peponapis, Xenoglossa) and the origin of the cultivated Cucurbita. Evolution 25:218-234.

Hurd, P. D. J., and E. G. Linsley.  1967.  South American squash and gourd bees of the genus Peponapis (Hymenoptera: Apoidea). Annals of the Entomological Society of America 60:647-661.

---.  1966.  The Mexican squash and gourd bees of the genus Peponapis. Annals of the Entomological Society of America 59:835-851.