Squash bees

Every summer, squash and gourds of all sizes and shapes to begin their annual takeover of local gardens and farmer’s markets. While squash and gourds are well-known and much loved, the native bees that specialize in pollinating these plants are less well-known.

All the different types of squash, pumpkins, and many commonly-grown gourds belong to the genus Cucurbita. Plants in the genus Cucurbita are native to North, Central, and South America. As a group, they are called cucurbits. Cucurbits were some of the first plants to be domesticated in the Americas. Domesticated cucurbit seeds appear in archeologic sites along the Mississippi Valley, including Kentucky, as early as 5,000 years ago. Cucurbits played a vital role in the development of agriculture in the eastern U.S.

Because cucurbits are native plants, that means they must have native pollinators. (Honey bees aren’t native to North America.) The primary native pollinators for squash, pumpkins and their relatives are bees in the genus Peponapis and the genus Xenoglossa. There are only about 20 species of bees in these two genera and most of them are found in the southwestern U.S., Central America, and South America. Peponapis pruinosa is the most common species in the eastern U.S.

Peponapis and Xenoglossa bees are commonly called squash bees because of their unique relationship with squash, pumpkins, and related plants. Squash bees are cucurbit specialists. Pollen from cucurbits like squash, pumpkins, and gourds is the only food that squash bee larva eat. Squash, pumpkins, and other cucurbits are as critical to their survival as milkweed is to the survival of monarch butterflies.

Biology:
Squash bees are solitary, ground nesting bees. Females dig nesting tunnels in bare dirt near cucurbit plants and often underneath the plant’s leaves if suitable habitat is available. Several females may dig tunnels in the same general area, but only one female maintains each nesting tunnel. In some ways, this is similar to how many people may live in the same neighborhood, but maintain individual homes.

The female squash bees gather pollen from squash and other cucurbit flowers. The female takes the pollen back to her nesting tunnels where she deposits the pollen and lays an egg. The pollen provides a ready meal for the larva when it hatches out of the egg. The female then creates a wall in the tunnel so the egg is located in its own “room” or cell. Each nesting tunnel can hold several eggs, each partitioned off from its siblings. Each tunnel can be a couple feet deep, but is often much shallower.

Squash bees only live about a year. The adults emerge in the late spring or early summer and die in the fall. Most of their life is spent underground. When the egg hatches, the larva eats the pollen that its mother left for it. The larva spends the winter as what’s called a prepupa. In the spring, it pupates into an adult and emerges from its underground home.

When the adult bees emerge, the males head to the squash / cucurbit flowers. The males spend most of their adult lives in the flowers. After emerging, the females search for nesting sites, begin excavating their tunnels, and preparing to mate.

Squash flowers and other cucurbit flowers open very early in the morning. They can open up to half an hour before sunrise. Male squash bees hang out in the flowers waiting for females to visit the flower to gather pollen. Mating occurs in the flower. During the middle of the day, squash flowers will often close. When the flower closes for the day, the male squash bee may stay inside and wait for it to open the next day.

Squash bees can be more efficient pollinators of squash, pumpkin, and gourds than honey bees. Research shows that when both honey bees and squash bees are present, the honey bees don’t significantly increase pollination of cucurbit vegetables over just the squash bees alone. Part of the reason is that honey bees aren’t active in the dawn and pre-dawn hours. They are most active during the middle of the day. However, because squash bees are cucurbit specialists, they have evolved to be active during those very early morning hours when the flowers first open.

What can you do?

One way to promote squash bees on your property is simply to plant lots of squash, pumpkins, gourds, and other members of this family. Studies indicate that squash bee populations are highest in areas where cucurbits have been planted the longest. If you are like me and love eating these plants, then you are probably already doing this.

Also, try not to dig or till your garden more than necessary. A graduate student at UC Davis recently did a study showing that tilling to a depth of 16 inches reduces the squash bee population in the tilled area by approximately half. Remember, squash bees spend most of their life underground so digging or tilling destroys their nests and kills them.

If you have to till your garden, provide a suitable nesting site of bare ground nearby. Although squash bees prefer to nest close to their food source, they don’t have to be right under it. We have many species of native ground nesting bees so providing small patches of bare ground throughout your property can benefit more than just the squash bees.