Coordinator’s Corner: Spring Cleanup That Protects Pollinators
By Nate Walton, MSU Extension Consumer Horticulture Instructor and MG Coordinator
This article is a follow-up to my article about fall cleanup to protect pollinators which you can read in the September 2019 edition of The Real Dirt.
A pollinator is any animal that moves pollen from one flower of a plant to another flower of the same plant species. In Michigan, our most important pollinators of most native plant species are wild bees. For our insect-pollinated agricultural crops, wild bees contribute to pollination, but most require that large numbers of European honey bees (Apis mellifera) be placed nearby in portable hives.
Wild bees do not spend the winter in hives, they overwinter in protected locations in a state of hibernation. There are at least 465 species of wild bees living in Michigan and scientists are only beginning
to understand where most of them are located in the winter and how we can use that information to protect them. Most wild bees (about 80%) construct their nests in the soil at depths from just a few inches, to as deep as 1 m. The remainder are
called cavity-nesters and they use pithy stems (e.g. elderberry, sumac), hollow stems (wild grasses), or beetle borings in dead trees. For these soil and cavity nesting bees, conservation is a little bit easier because their nest location is also their overwintering location. With some careful observation of the bees in your yard, you should be able to see where they are nesting, make note of it, and protect that area from disturbance.
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There is probably not much that you need to do differently from your normal spring cleanup routine, to protect the soil nesting bees. On the other hand, if you left ornamental grasses or herbaceous perennial stems unpruned last fall, they may now be harboring cavity nesting bees. When you cut them back this spring, try to allow the stems to remain intact by using sharp tools. Then, you can place them in an out of the way location where the bees will be able to emerge from the nests naturally.
The compost pile is probably not the best place to place plant material that may contain overwintering bees, because there is too much moisture there and the bees may die from pathogen infections. I would recommend having a designated brush pile for all of the material that may be harboring overwintering pollinators. Keep the pile light and airy to prevent moisture accumulation. The timing of your stem cleanup can be somewhat flexible but there are a few important facts to keep in mind. First, the cavity nesting bees will be happiest and healthiest if they are allowed to emerge from the stems while they are still in their natural position (i.e. attached to the crown that they grew from originally). Second, the exact timing of natural emergence depends on the species and climate in your area. And (third) if you wait too long, they will start creating new nests in the stems. Disturbing freshly created nests is much more harmful to the bees than disturbing the bees while they are still in hibernation. Fourth, sometimes the best time to prune is when you have the shears in your hand.
Another important group of bees that I have not mentioned yet, are one of springtime’s most conspicuous pollinators: Bumble Bees. Bumble bees are social bees, which means that they have a life cycle that includes several generations of workers living together in a communal nest. However, their colonies are started from scratch each spring by solitary queen bees called foundresses. The foundress mated the previous fall and spent the winter in a sheltered location, waiting for her chance to search out a location for her new nest this spring. Scientists actually know very little about where bumble bee queens spend the winter and they would like to know more. If you are interested in helping them out, you can join the Queen Quest citizen science project.
Anecdotal reports gathered over the years, give us some idea of where you are most likely to encounter bumble bee queens before they break hibernation. They seem to seek out loose litter, such as that you might find under a pine tree or other conifer. Some of them seem to even like burrowing down a few inches below the soil under the turf in our lawns. They also seem to have a preference for slopes and for locations with an adjacent vertical barrier such a tree, shrub, or building. In other words, they may be in a lot of the areas where you are targeting your spring cleanup activities. It can be quite harmful to disturb hibernating queen bees before it is warm enough for them to fly and/or seek out a new shelter.
This is one of the reasons many advocates of pollinator protection during spring cleanup recommend waiting until the temperature has been consistently over 50 degrees for five days before cleaning up your leaf litter of disturbing a lot of soil in your garden (see “Take a Load Off” by Cheryl Gross in this month’s The Real Dirt). If you do come across an overwintering bumble bee queen during your spring cleanup activities, the best thing to do is to gently return her to where you found her and replace the layer of leaves or duff that had been protecting her.