Wild Bees for Pennsylvania Cucurbits

Pollination services in the Cucumis genus (cucumbers and melons), appears to be more sensitive to the need to achieve pollination by honey bees. Studies show a mix of both honey and wild bees in the pollination of these crops, but researchers found more than 28 species of wild bees visiting cucumber flowers in addition to honey bees in central Indiana, and we have seen similar variation in Pennsylvania.

In eastern PA and parts of NJ, researchers observed 44 species visiting watermelon flowers planted in small, diversified farms. The team developed individual-based models to simulate pollination services from data on visitation rates, along with number of pollen grains deposited per visit. Results showed that wild bees provided full pollination in ~90% of these farms, even though honey bees were also present. Honey bees alone provided pollination in ~70 to 80% of the farms. The presence of both honey bees and wild bees helped ensure resiliency in pollination services.

The degree to which different species of bees provide pollination services varies among cucurbit crops. It also changes as the season changes, and can even vary at different times of the day.

In the Cucurbita genus, the squash and pumpkin crops, wild bees may well be the dominant floral visitor, regardless of whether the field is stocked with honey bees. This has been seen in NY, MA, VA and in diversified farming operations in PA. In larger 'Gladiator' pumpkin fields in 2013 in Columbia and Lancaster Counties destined for wholesale markets, measures of 4,853 bee visits showed that 67% came from bumble bees, and 25% from squash bees. Thus, wild bees provided 92% of the visits to flowers (honey bees added an additional 6%) in that year.

Among the several bumble bee species in Pennsylvania, the common eastern Bumble Bee, Bombus impatiens, plays the key role.. Overwintered solitary queens are establishing nests in late March, April, and May. During this very sensitive time in their life cycle, queens are finding and building nests, laying eggs, keeping the brood warm with her body heat, provisioning brood with pollen sometimes mixed with nectar, and rearing the first generation of brood. She needs plentiful, diverse, and high quality floral resources in close proximity to her nest. We are experimenting with a cover cropping system, planted in September of the previous year, to provide floral resources during this time. Along with providing the spring floral resources, this mix is designed to stop flowering (or be killed) by the time that the pumpkin crop needs to be pollinated (mid July through mid August). You may have other ideas that would achieve this same purpose.

Trial Seed Mix, with a mid September Planting Date, to provide floral resources for the overwintered queen of the Common Eastern Bumble Bee (Bombus impatiens) during the time she establishes a nest and provisions the first brood in spring of the following year.

During the mid-summer, do you want to provide additional floral resources? Some would argue this would help support bumble bee, honey bee, and other "generalist" bee species (species that visit flowers from multiple plants). Others argue that these flowers would compete with the pumpkin flowers for visitation by bees. In our current experiments, we are not adding a floral resource during the July to late August time frame, and we see good numbers of bumble bees working pumpkin fields. This is also the time frame when Bombus impatiens colonies should be at their strongest, because they should have multiple broods of workers helping support the colony.

As summer comes to a close, Bombus impatiens enters a second sensitive time. Instead of developing as workers, female offspring become reproductive, and males are produced. These new females will mate, and attempt to overwinter - they are called 'gynes'. Gynes need to acquire substantial resources if they are to successfully overwinter. Thus, we are trialing seed mixes designed to flower during the time that gynes are acquiring resources for overwintering. Again, you may have additional ideas.

Trial Seed Mix, with a July 7 Planting Date, to provide floral resources for the new queens of the Common Eastern Bumble Bee (Bombus impatiens) during the time they are foraging prior to overwintering.

You might consider trialing seed mixes to see what works on your farm. What seeding rates work? What plant species mixes result in flowering at what time, and are you seeing bumble bee visitations to those flowers? When can you fit this into your cropping system?

Using cover-crop types of plant species is one option that could fit well within farming systems based around annual crops. You might also consider installing some perennial species, which have different advantages and logistical issues to consider. There are a wide range of perennial species that are visited by bumble bees, and food for thought in another article.

A listing of references used in preparing this article is available upon request from the author.