The squash bee, a native pollinator of pumpkin, squash and zucchini

Hannah Fraser – Entomology Program Lead – Horticulture, OMAFRA; Jim Chaput – Provincial Minor Use Coordinator, OMAFRA

From ONvegetables in The Grower, 2014

At the 20th Annual Diagnostic Day in Ridgetown in July, we asked growers and consultants to guess what species is the most important pollinator of Cucurbita pepo (squash, pumpkin and zucchini) in North America. Almost everyone replied “honey bees”. A few mentioned “bumble bees” (a good guess as these are second on the list). Fewer still answered “squash bees”.

Cucurbita pepo produce both male and female flowers on the same plant. Male flowers produce both pollen and nectar, while the female flowers produce nectar. Each female flower has an ovary under the flower that resembles the fruit it will become following pollination. Poor pollination results in small, unmarketable fruit. And since these flowers bloom less than a day, there is only a small window in which pollination can occur.

The pollen of these crops is large, sticky and spiny – characteristics which make it relatively unattractive to honey bees. Surveys of farms where C. pepo are grown indicate the (hoary) squash bee, Peponapis pruinosa, outnumber honey bees, bumble bees and other pollinators by several orders of magnitude. Indeed, where squash bees are present and abundant, honey bees are essentially redundant, in terms of pollination requirements for these crops.

The squash bee is a highly-specialized native pollinator. It has co-evolved with squash and gourds to the point where its local success and survival depends on the presence of these crops: the female squash bee provisions her brood exclusively with the pollen collected from these plants.

Squash bees are ground nesters. The females excavate vertical tunnels up to 45 cm deep, with 3-5 lateral tunnels that end in a single brood cell (where the larva develop into new bees). Each cell is provisioned with a pollen ball and a single egg is laid, before being capped off. A single female may construct more than one nest over the season, depending on resources, from July through August.

The females have hairy hind legs and bodies that are very effective at picking up the pollen from male flowers, and for transfer to female flowers (Figure 1). They are active early in the day, in synch with the opening of Cucurbita spp. flowers; by the time other bees arrive, later in the morning, much of the pollination has already been accomplished!

A few other key facts about squash bees:

• they are solitary (no worker bees);
• nest entrances are about the width of a pencil (Figure 2), are typically in the crop or in uncultivated field margins and are often found in aggregations;
• most brood cells are 12 cm or deeper
• one generation per year, overwinter in the soil as a pre-pupa

One of the easiest ways to monitor for squash bees is to tear open the wilted flowers in the afternoon or early evening: males and unmated females will sleep in the flowers (Figure 3). Alternatively, one can hold the base of the wilted flowers between two fingers and see if the squash bees are buzzing inside. Don’t worry about being stung, as these bees are stingless.

Honey bees and bumble bees are cavity nesting, social insects that forage on a wide variety of pollinating plants. Because of their solitary nature, a dependence on a very limited number of crops for brood development and its ground nesting behaviour, the squash bee is particularly vulnerable to farm practices. Choice of pest control products and their time of application, soil cultivation and crop rotation can all influence squash bee populations on farms. Using no-till, cultivating to depths of 10 cm or less, and / or leaving some nesting aggregations undisturbed will encourage local populations. While squash bees are extremely good at locating their host plants, keep the distance between field locations during crop rotation as small as possible.

For more information on this hard-working native pollinator in Ontario and for tips on encouraging populations on your farm, see the publication “The Pollination System of Pumpkin, Squash, Summer Squash, and Zucchini” by Susan Chan available for purchase at www.farmsatwork.ca/library.