There have been many wonderful pieces about pollinators in The Natural Farmer, including a comprehensive Summer 2017 issue that I encourage you to return to. Today, I hope to add an unexpected corner of wild bee biology: the role of forest trees. I will combine woods and a smidgen of tree climbing with pollinator conservation to explore the often-overlooked role of trees for bees, especially on farms. For now, let’s start on the ground (pun intended!) with a review of who our wild bees are.
Until very recently, a conversation about “the bees” usually meant only Apis mellifera, or European honey bees. Beekeeping connects many of us to the magic of insect communication, sociality, and pollination habits. The European honey bee has captured the public imagination—and mine!
These highly social, hive-dwelling insects arrived with Europeans in the 1620s and rapidly colonized westward. As with all livestock, people have historical and meaningful relationships with honey bees and honey, similar to how we value cows and chickens and milk and eggs. These agricultural priorities mediate the trade-offs we make as we allocate land use and resources. For example, responsible pasture management avoids the environmental degradation of overgrazing and overstocking, and riparian buffers minimize water pollution. Similarly, honey bee management decisions should take into account the surrounding wildlife and local ecology.
For example, in the high densities often reached by commercial beekeepers, there is some evidence that honey bee hives can compete for and deplete floral resources, outcompeting wild bees. Luckily, this competition exclusion is unlikely at the scale of a backyard beekeeper. However, research by Laura Figueroa, Kaitlin Deutsch, and Samantha Alger has been showing that multiple bee diseases including deformed wing virus which is vectored by Varroa mite, can be spread to wild bees at flowers. So, low hive densities and careful Varroa management may be crucial for “saving the bees” – even at the backyard scale. Finally, eastern forests in fact harbor many thriving colonies of feral honeybees. The term feral indicates that they were domesticated and then escaped, rather than were wild and free-living originally. Tom Seeley follows these bees in the Arnot Forest of NY, and Margarita Lopez-Uribe in PA found that feral honeybees in PA had higher immunocompetence than managed honeybees. Today, however, we’re going to focus on native, wild species.
Although outside the scope of this piece, I encourage interested beekeepers to look into the great research and recommendations from the Pollinator Network @ Cornell.
So, who should those conversations about “the bees” also include, if not just Apis mellifera? Which bees were here in the US before the honey bees arrived with European colonists, and are they here now?
To the best of our knowledge, at least four thousand wild bee species lived on the land currently called the United States. In New York State, where I live, there are at least 420 species. To extend our livestock analogy from the prior paragraph: the wild bees are the eagles, wrens, cormorants, chickadees, and herons to livestock’s chicken, or the moose, fox, groundhog, and prairie dog to livestock’s cow. Some researchers guess there may be an average of 50,000 wild bees/hectare in the mid-Atlantic region. Wow!
Much like the suite of wild mammals and birds I listed, each wild bee species has a unique ecology, diet, and habitat requirement. Wild bees have thousands of years of relationships to the local plants, soils, and other nesting habitats. Some bees fly in the springtime, some in midsummer, and some only in the fall. I’m particularly curious about the habitat that is overlooked — which brings us to the woods and its tall, tall trees.
WHY LOOK IN THE WOODS
From an agricultural perspective, there is consistent evidence that agricultural fields near forests, hedgerows, and woodlots have higher bee abundance and diversity. Some research in orchards and strawberries has found this same effect in orchard rows themselves—the more meters away you are from the woods, the fewer wild bees. Earlier work from our lab found that the negative effects on bee diversity metrics of higher pesticide loads were mitigated by higher amounts of nearby natural habitat, including forests.
This northeastern region, of course, used to be heavily forested, although with regular disturbances – gaps and patches of varying sizes were created by beavers dams, windstorms, tip-ups, and fire, often through intentional indigenous management. The story of settler-colonial deforestation and reforestation is not news to most readers. However, the implications for bees have left some interesting biogeographic and natural history legacies. Collecting in a Connecticut wetland, I once caught a bee named “Calliopsis nebraskensis.” Is that nebraskensis… like, Nebraska, I wondered? I asked Sam Droege at the USGS, who suggested a cool hypothesis: when settlers cleared the northeast for agriculture, some bees adapted to prairies and mid-western open areas were able to spread east. Pockets of those populations have remained even after more forest has regrown.
But those “plains bees” probably weren’t the dominant groups in the historically forested northeast. Colleen Smith and colleagues at Rutgers have found that up to a third of common northeastern bees could even be classified as “forest-associated.” So today, we can move to the woods and ask: what were the strategies and resources of bees who evolved near and in the forest?
WHERE & HOW TO LOOK FOR WOODSY BEES
I’ve already warned you that I’m going to end up looking sky-high to the canopy, but let’s climb our way up to it, and start with our feet firmly planted.
Among the best-known marvelous ankle-height interactions are the bees who love our spring ephemerals. One famous forest floor forager is the solitary mining bee named Spring Beauty Bee, Andrena erigeniae. Like other solitary species, each female is in sole charge of her own nest. For each baby bee, she creates a small bee-sized chamber off of an underground tunnel. She fills it with a gigantic pile of spring beauty pollen before laying an egg on it—enough pollen to feed it all the way to adulthood! No other pollen will do, and in fact, Spring Beauty Bee has specialized leg hairs just the right size and shape for gathering up large volumes of spring beauty pollen, and she is so efficient that barely any pollen gets brushed onto other flowers. As a result, messier less specialized bees like bumblebee queens actually do a better job pollinating the flower.
It’s always good to remember that bees are pollen eaters. They are happy herbivorous, and only incidentally pollinate. In that sense, “mutualisms” aren’t always simple (or even mutual!). And Spring Beauty Bee isn’t the only one: there are specialist bees who collect pollen from plants such as bellwort, geranium, and trout lily. Many specialists are only active for a few weeks while “their flower” is blooming. Otherwise, they slowly develop and wait out most of the year in their underground nests as prepupae, waiting for the spring weather cues to signal them just the right week to emerge.
Now, if we look away from the ephemerals and onto the leaf litter, bumblebee queens diligently represent their bumbling namesakes. They mated in the fall, hibernated all winter, and now need to establish a nest. Bumblebees then live in active colonies for many months. Unlike the spring ephemeral specialists, they can’t live on just trout lily or just spring beauty—they’d be so hungry all summer! Instead, they need to be able to switch between multiple types of flowers. Bee biologists call this being a “generalist” rather than a “specialist.” A 2020 study by Genevieve Pugesek and colleagues found more successful bumblebee nests in forests than in fields. In my own observation, bumblebees nest-searching in the woods seem to favor well-drained, sloping hillsides with some leaf litter.
Another bee you’re likely to see weaving across the forest floor in early spring is a master of disguise. Bees in the genus Nomada are cleptoparasites, meaning they don’t build nests of their own. Instead of gathering pollen, Nomada usurp Mining Bee nests by sneaking inside and laying an egg on the pollen ball. The egg hatches and the parasitoid larva t
akes over, killing the host bee. Nomada have a wasp-like aesthetic with a thickened red and yellow exoskeleton. Why? They don’t need fuzzy pollen-collecting hairs, but do need to be tough if the host bee realizes what they’re up to! It’s easy to imagine someone casting parasitoids as the villain in our story, weaving slowly across the forest flower, searching for a nest, up to no good. But researchers believe that seeing them year after year is actually a good sign—abundant parasitoids are a clue that suggests an abundant host population. Parasitism rates have been estimated at around 4-5% of a host population, so if we imagine 95 mining bees for each Nomada we see, that’s an exciting number of fabulous orchard pollinators, likely nesting somewhere in these woods! If you follow a Nomada, you might even find one….
Continuing our journey upwards, we’re finally ready to look above knee height. At this stage of the journey, one of my favorite tips is: look for shiny sparkling jewels. If it’s green and glinting, go look at it! Many forest insects are iridescent. Ecologists think that this might be a predator-avoidance strategy – as iridescent insects move between dappled sun and shade, it throws off birds and others who would eat them. And lucky for forest ecologists, it makes them beautiful. So who are our iridescent forest bees? Two of my favorites are both shiny sweat bees: Augochlora pura and Lasioglossum coeruleum. They’re small and gentle forest-associated bees that nest in rotting logs, stumps, and snags. They’re active all summer, and often prefer logs that are quite “punky” or spongy, and will usually initiate their burrows in abandoned beetle holes of 3-8mm across. Also at mid-level heights are the stem and cavity nesters, who use hollow pithy stems of plants like Rubus and Sumac for overwintering, or cracks and pre-existing holes in trees.
CANOP-BEES (Canopy Bees!)
And finally, we get up to the sky. My research has us look up, and ask, have we overlooked these resources, because they physically overlook us? Do bees collect canopy pollen? What else do they do in the canopy?
Besides being just plain out of reach, the main reason forest trees have been broadly ignored as possible bee sources is that many of them are wind-pollinated. This makes sense: it is windy, cold, and often rainy in spring. So if you’re a tall tree, it’s a grand idea to take advantage of that windiness and spread your pollen in the breeze, rather than worry about whether or not a small hungry insect will make the journey to your flowers on a cold April day. So, many spring-blooming species – poplar, ash, birch, alder, hickory, oak – aren’t trying to attract bees: their pollen is optimized for flight and the statistical hope that it lands on a receptive conspecific. But if it’s edible, that same statistical hope means that these trees make a TON of it, and it often still has lots of protein, lipids, and essential amino acids…so in our project, we were curious if bees sometimes eat it anyway. After all, remember that bees are primarily pollen eaters, but only incidentally pollinators—maybe they don’t care if the tree is wind-pollinated!
I began by following some clues left by Professor Laura Russo, who had identified lots of pollen carried by bees caught in NY apple orchards. These were all bees who were busily gathering delicious apple pollen, and yet, she found they also carried some maple, willow, ash, and birch pollen. If that was happening in May, we thought, maybe they use even more of those trees in the weeks before the orchards bloom! Although not widely studied, clues in the literature suggested that orchard pollinating mason bees and mining bees also love oak pollen, and our lovely shiny green sweat bees visit walnut. A 2018 review found 200 pollen collection records for over 100 genera of wind-pollinated plants. This is especially important when we think about restrictions on pesticide sprays. Hemp, and corn, and other grasses are wind-pollinated, but since bees collect their pollen (and they take a lot of it!), then insecticide applications should take that into account.
The first step to figuring out these canopy dynamics was just to see if bees are even up in the canopy. After three years of canopy sampling with special bee traps in eleven forests and woodlots adjacent to orchards in the Finger Lakes in New York, we found over 90 species of wild bees were actively flying the forest canopy between March and the end of May—the time period prior to apple bloom, when many forest trees are blooming). But what were they doing? First, we did some tree climbing and found that those same log-nesting iridescent green bees were also nesting in canopy branches, and I observed lots of bees foraging on wind-pollinated tree catkins.
Since then, my team and I have dissected over a thousand bees that we caught from different forest strata and orchards and found that tree pollen makes up between a quarter and 100% of individual bees’ digestive tract pollen. Oak and maple made up the vast proportion of tree pollens that bees ate, while beech, walnut, and birch were also well represented. Although we’ve known from work by Suzanne Batra and others that willow and red maple, one of the biggest surprises in our work was the enormous volume of sugar maple that many species of bees had eaten. Future research is needed to understand how healthy these pollens are for bees.
Finally, our research strongly suggests that bees active in the forests are “spilling over” into apple orchards and other crop fields later in the spring. In the mining bee family, male bees emerge first. These males were active in the woods, and then the females of the same species were active in apple trees a few weeks later. The males never left the woods in high numbers — so in order for these female orchard pollinators to reproduce, we guess that they need forests nearby in order to mate! A hidden link between habitats. Overall, we calculated the highest bee abundances in the understory a week before they peaked in the canopy and finally spiked in the orchards two weeks after that. Most of the bee species had eaten pollen from all habitats, suggesting lots of cross-habitat reliance.
Although research is increasing, so far we’re really just scratching the surface when it comes to forest bees. Over half of forest ownership in the northeast is private, so taking action on your back woodlot and private property can have huge implications. Based on the natural history we know so far, and general principles of managing for diversity, the following recommendations are worth it to help wild bees thrive:
- Try uneven-aged management. Maximize niches. Light-filled gaps created by single-tree, patch selection, or shelterwood harvests will fill to the brim with flowers. Bee foraging activity is often high in forest openings; common growth at this stage of succession include plants like raspberries, whose nests, in turn, are nest habitat for stem-nesting species. Slash can create nesting habitat and protection. Niche diversity in multi-aged stands generates homes for rodents whose burrows can later be bumble bee homes, and (non-pest) beetles whose tunnels support tunnel-nesting bees.
- Keep coarse woody debris & standing deadwood. Coarse woody debris and standing deadwood provide habitat. In a recent paper, researchers reminded us “[s]aproxylic bees and wasps are endangered due to the loss of old trees, as well as due to the removal of deadwood”. Some cool work at the Arnot Forest in NY created a “slash wall” to exclude deer–deer browse is a major threat to the regeneration of plants on which bees rely–and Kristi Sullivan and I saw huge numbers shiny green bees nesting in all of that slash!
- Manage for plant diversity. This again may require protection from deer. Tree species diversity overlaps with the above management goals, but I emphasize it again now for bee health and nutrition. Bees collect saps and resins to help waterproof their nests, and as anti-microbials, and even as self-medication when challenged by parasites (e.g. from Populus). Different compounds may be needed from different species, and a diversity of species means more consistently available saps and resins.
- Manage for vertical diversity. Bees are small, so small habitat changes can mean big changes in niche availability. Multi-age and -size trees generate a matrix of environments. These include species that provide contrasting light levels, various leaf types and textures, nearby shrubs or understory communities, different bark textures, and beetle communities that leave behind different sizes and shapes of abandoned burrows.
- Consider bees in any pesticide applications. As described, bees visit forest tree blooms for food. For targeted sprays or trunk injections, consider if it’s possible to wait until bloom is over. The concentration of the pesticide expressed in pollen or nectar should be much lower by the time the tree blooms again the next year (although the risk may not be gone). Bees also visit understory shrubs and spring ephemerals, which could be impacted by soil drenches of insecticides able to translocate and taken up by nearby roots.
- Start by supporting the bee diversity you have! Generally, adding new managed bees is not the best way to encourage wild bee health. Sometimes they compete with wild species and can even carry pests and pathogens. Instead, prioritize keeping your yard messy with habitat, your flowers abundant for nutrition, and reach out to a certified forest to add bees to your ecological, wildlife-focused forest management.