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Squash Bees: an alternative to renting honeybees for pumpkin pollination

Posted: January 17, 2011

Based on preliminary findings from work funded through a 2010 Northeast Sustainable Agriculture Research and Education (NE-SARE) Partnership grant, squash bees (Peponapis pruinosa) appear to be the most important pumpkin pollinators in south-central Pennsylvania. Unfortunately, most pumpkin growers are unaware of the free services provided by squash bees and follow the standard practices of renting up to two honeybee colonies per acre of pumpkins.
A male squash bee.

A male squash bee.

Farmers that grow pumpkins every year and establish new pumpkin fields in close proximity to the previous year’s fields generally have a strong population of squash bees that are able to provide the necessary pumpkin pollination services.  In our survey, the data did not indicate that the size of the pumpkin field (0.3 or 25 acres), irrigation method (overhead or drip irrigation), or soil management (till or no till) influenced the squash bee population.

In the 2010 field surveys, honeybees did not significantly visit pumpkin fields where squash bees were abundant, even when present at the highest recommended rate of two colonies per acre. A plausible explanation for that is that in mid-summer, fertilized female squash bees start foraging at full potential about one hour earlier in the morning than honeybees do. Female squash bees are fast fliers and are highly specialized on moving pumpkin pollen and, by the time honeybees are ready to forage, they remove most of the pollen from the pumpkin flowers. Furthermore, when honeybees do start foraging in high numbers, male squash bees and unfertilized females become active as well and start competitively depleting pumpkin flowers of nectar. As a result, honeybees usually have no other option than to forage on other pollen and nectar sources and only occasionally visit pumpkin flowers.

To an untrained eye, squash bees are easily mistaken for honeybees because of similar size and coloration. This leads growers to think that honeybees are the ones doing the bulk of pumpkin pollination. This can be a costly assumption that can result in unnecessary honeybee rental fees that can add up to $120 per acre. In order to avoid this, growers need to have an inexpensive, easy-to-use, bee-non-destructive methodology that will allow them to easily identify squash bees and to assess their population density in a given pumpkin field. Such methodology would allow pumpkin and squash growers throughout the Northeastern region to accurately determine squash bee density and know when there is a need to rent honeybees.

For 2011, a grant proposal was submitted to NE-SARE for developing such methodology. The concept is based on the unique relationship between squash bees and cucurbita plants. Unlike any other species of bees, male and unfertilized female squash bees spend the night in the cucurbita flowers that have wilted during the day. In the morning, they chew their way out and start foraging and mating. At the same time, probably in an effort to attract a strong population of squash bees, cucurbita plants produce male flowers about one week in advance of starting to produce female flowers.

As result, a grower can collect, in a bucket, all the freshly wilted male flowers from five randomly selected cucurbita plants. Once this is done, the bucket will be covered with an insect screen and left in the field until the next day. In the morning of the next day, the squash bees will naturally chew their way out of the blossoms and fly towards the screening, allowing the grower to make accurate counts and release them. The data resulting from this assessment can be compared with the threshold at which squash bee can provide sufficient pollination services. This methodology should allow farmers that grow cucurbita crops to confidently determine squash bee density and make an informed decision about the need to rent or not rent honeybees before cucurbita plants start setting fruit.

In 2011, with support from NE-SARE we aim to test and fine-tune the methodology and establish the density threshold at which squash bees are able to perform adequate pollination services in squash and pumpkin crops. This information will allow us to provide pumpkin and squash growers in the region with an easy-to-use tool for assessing the squash bee population in their pumpkin and squash fields. This will aid growers to make informed decision regarding the need for pollination services that, consequently, will result in a more sustainable pumpkin and squash production system.

By Alex Surcica, Penn State Extension

A 5 page fact sheet on pumpkin pollinators is available.