Squash bees are rapidly spreading across North America

A new study led by Penn State researchers has discovered that the squash bee (Eucera pruinosa) has evolved to thrive in response to the intensification of agriculture in North America. This new research, which is the first to show the role of agriculture as an evolutionary force acting on a wild insect pollinator, could have implications for food security.

The squash bee is a species of wild bee that is an important pollinator of plants in the genus Cucurbita, including crops such as pumpkins, zucchinis, and other squashes. The bee is native to the deserts of Mexico and the southwestern United States, where its primary source of pollen was the wild buffalo gourd (Curcurbita foetidissima). 

The secret to squash bees’ success lies in its love of squashes, pumpkins, and zucchinis, which have seen a significant increase in cultivation across North America in the last 1,000 years. The researchers found that the bee has adapted to agricultural practices used in the cultivation of squashes. 

In order to explore how the E. pruinosa bee has evolved in response to the intensification of Curcurbita agriculture, the team utilized various methods. First, they sequenced the bee’s genome and assessed its genetic structure, which refers to the amount and distribution of genetic variation within and among populations. They also created a new algorithm to estimate the bee’s migration patterns and effective breeding population sizes across populations.

The squash bee historically relied on the wild buffalo gourd (Curcurbita foetidissima), a small squash found in the deserts of Mexico and the southwestern United States, as its primary source of pollen. Squash, along with corn and beans, were vital crops for Indigenous Peoples in the Americas. 

Approximately 5,000 years ago, the Eastern Woodlands Indigenous Peoples started to domesticate a relative of C. foetidissima, known as C. pepo. The resulting crops, including pumpkins, squash, and gourds, were widely cultivated around 3,000 years ago and intensified around 1,000 years ago with the introduction of maize to North American agricultural systems.

“By planting squash all over North America, humans created habitat for the squash bee, and that allowed its population to explode,” said López-Uribe. “Today, the squash bee occurs throughout the United States and southeastern Canada – far beyond the range of its original food source.”

The researchers discovered that the bee’s transition from wild host plants in deserts to temperate agricultural habitats was associated with selective sweeps resulting in substantial reductions in genetic diversity in some parts of the genome. Changes in genes associated with chemosensation were particularly linked to these selective sweeps.

“Domesticated Curcurbita plants produce floral blends that are simpler than those of the wild Curcurbita plant,” said López-Uribe. “It is likely that E. pruinosa adapted to a new sensory environment in agricultural habitats, which enabled it to expand its range and significantly increase its population size.”

The study is the first to identify adaptive processes of an insect pollinator in response to human agricultural practices. López-Uribe noted that this research highlights how domesticating plants can have important indirect effects on the organisms that pollinate those plants.

Sam Scheiner is a program director at the U.S. National Science Foundation, which partially funded the work. 

“Pollination is such an important process that impacts so much of the food we eat. Understanding how humans have and continue to impact that process and pollinators – through agriculture, urbanization and in other ways – is key to ensuring we maintain food security,” said Scheiner.

“This research highlights how domesticating plants can have important indirect effects on the organisms that pollinate those plants.”

The study, which is published in the journal PNAS, may have implications for the future of food security as the researchers suggest that agriculture may facilitate increases in the population sizes of other insect pollinators.

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