Mother Nature's Montrose Garden: Garden Pollinators: Pruinose Squash Bee

The Pruinose Squash Bee (Peponapis pruinosa) pollinates large-flowered cucurbits

The word ‘pollinator’ conjures up images of Honey Bees for many people. But the world of pollinators is far more complex. Our Garden Pollinators series features some of the many interesting pollinators seen in Western Colorado gardens. For more on pollinators in general – and tips for attracting pollinators to your garden – see: https://mothernaturesmontrosegarden.blogspot.com/2020/06/life-friendly-gardening-planning-for.html

If you grow any of the large-flowered Cucurbits (squash; pumpkins; gourds) you’ve likely noticed some energetic bees who begin their day quite early. These are likely Pruinose Squash Bees, a species with an interesting history tied the American Southwest. While now found throughout the U.S. and southern Canada Canada – where ever squash and pumpkins are grown - Peponapis pruinosa has migrated from the native origin of the Cucurbits, in the Southwestern U.S. and Central America. [1] For an interesting video on the spread of this bee see reference 2, below.

Cucurbits like squash, pumpkins and gourds have large, open flowers

Peponapis pruinosa is sometimes called the Eastern Squash Bee, Hoary Squash Bee or Eastern Cucurbit Bee; and other Bees are sometimes also referred to as ‘squash bees’. But Peponapis pruinosa is the most important squash and pumpkin pollinator, both in agriculture and in home gardens. [3, 4] You can see this bee anytime from June through September, whenever the squash are in bloom. We can attest to the pollination potential of these bees. This year we’ve eaten, given away to neighbors and donated over 50 pounds of summer squash (so far) from just two plants!

Pruinose Squash Bees are solitary bees in the tribe Eucerini, the long-horned bees. You likely attract other long-horned bee species if you grow any sort of sunflower. But the Pruinose Squash Bees are specialist bees that depend to a large extent of the pollen and nectar of cucurbits. While they mostly visit cultivated cucurbits, they also pollinate wild cucurbits such as the Cucurbita foetidissima (commonly known as the buffalo gourd, calabazilla, chilicote, coyote gourd, fetid gourd, fetid wild pumpkin, Missouri gourd, prairie gourd and stinking gourd). [4]

Pruinose Squash Bee (Peponapis pruinosa): common view

The Pruinose Squash Bee is slightly smaller than the European Honey Bee and the females have hairy legs (Honey bees have smooth legs). See reference 5, below to learn how to tell Squash Bees from Honey Bees.

The thorax is usually orange, the abdomen is black with whitish bands and it’s covered in yellowish hairs.[4] Branched hairs (scopae) on the females hind legs help to carry the large, coarse pollen of cucurbits. This pollen is the sole food eaten by offspring while in the nest. For some good pictures of Peponapis pruinosa see references 4, 6 and 7, below.

The entire life of the Pruinose Squash Bee is tied to the squash plant and its large flowers. Females build their nests in the ground near squash plants. The nests can reach 1 ½ ft. deep. Females produce one new generation a year. They lay their fertilized eggs in cells in the nest and provide stored squash pollen provisions for the larvae to eat. The offspring over-winter as larvae in the nest, emerging as adult males and females in early summer. [8]

Pruinose Squash Bee (Peponapis pruinosa): well adapted for pollinating
cucurbits distinctive floral anatomy

Females gather pollen in the morning, when squash flowers are open. In fact, squash bees can actually begin pollinating when it’s still quite dark – likely one reason why they are such good pollinators. Squash bees can actually see in the dark! [8] And squash bees are very active, since they must complete their work before the flowers close mid-day. Males forage for food and mate with females in the open flowers. The males sleep in the closed flowers in the afternoon, often several males per flower. Females work in their nests in the afternoons.

Pruinose Squash Bee (Peponapis pruinosa): in summer squash blossom

Peponapis pruinosa faces several threats in the agricultural setting. Any activity that disturbs the soil, such as plowing, harrowing or discing may destroy the underground nests. In addition, commonly used agricultural chemicals can pose a risk to any ground-nesting bee species. [9] In the home garden, limiting both ground disturbance and the use of pesticides are good ways to safeguard the ground-nesting bees.

Why enjoy the Pruinose Squash Bee in your garden? Well, they’re interesting to observe, particularly if you’re a morning person. And if you want a good crop of Cucubits, this docile pollinator is truly your best friend. So, welcome this Southwestern native bee into your garden with an open heart. You’ll be well-rewarded!