Planting Food for Bees…in the Forest!

by Kass Urban-Mead, PhD, Pollinator Conservation Biologist, Forests & Forestry, The Xerces Society for Invertebrate Conservation

I signed up to join Wild Woods Restoration Project (WWRP) for three planting days in a row – Sept 27, 28, and 29 – and had the distinct delight of imagining the beloved pollinating bees who will benefit from this important work at Bye Preserve in NY, at Closter Nature Center in NJ, and at Fahnestock State Park in NY.  

When I introduce myself as a pollinator biologist, most people assume that I spend my time in meadows and gardens. They’re not entirely wrong: flower-filled fields certainly provide an important summer-long profusion of nectar, pollen, and nesting habitat. 

But there is more to the story – the oft-hidden, magical world of bees in the woods.

A queen two-spotted bumble bee, Bombus bimaculatus, freshly emerged from spring hibernation, gathering the pollen and nectar offered by forest understory forbs – in this case, common blue violet (Viola sororia) – in order to have the energy and food to start a new bumble bee colony in the spring. Photo: Heather Holm. 

In the early spring, when sun still reaches the forest floor, before the trees put on their crowns of leaves, a different type of “meadow” is feeding the first bees. The sweet and delicate blooms of understory flowers – violets, trout lilies, spring beauties, Dutchman’s breeches, bellworts and more – offer pollen and nectar to the bumble bee queens waking under the soil from a long winter’s hibernation. The maple, oak, cherry, linden, and other flowers in the canopy are a true meadow in the sky, offering nigh-uncountable blooms which are far out of reach to those of us lacking wings. The spicebushes, viburnums, blueberries, serviceberry flowers, dogwoods, and winterberry hollies offer shrub-layer nutrition to beloved butterfly larvae and pollen-seeking wild bees alike.

These resources feed a group of species that together researchers call “the forest bees.” These are some of the bees which have lived for thousands of years amongst the trees of the northeastern United States, and many of them have life cycles quite unlike the familiar European honey bee. Most live solitarily, nesting alone in wood tunnels, stems, or small holes in the ground, and none make honey. 

Of the 600-ish species of bees living in the northeastern United States – 350 in New Jersey, ~460 in New York – we now understand that a full third can live their full life cycles completely in mature forest habitats: mating, nesting, foraging, and overwintering. Another third of these species use forests for at least part of their life cycles. This is important for agriculture too – many forest bee species are also highly effective pollinators of crops. They are particularly good at pollinating spring-blooming orchard crops: research repeatedly shows orchards have more bees and better pollination services when there is forest nearby. 

Unfortunately, forest-associated bees also face the ecological pressures which so challenge our forests today: invasive species, climate change, excessive and imbalanced levels of deer browse, and rapidly arriving pests and pathogens. Each causes cascading effects through these complex ecological systems. As a result, when I first learned about Linda Rohleder’s work with the Wild Woods Restoration Project (WWRP), I knew I had found something special. 

Staff at Fahnestock State Park planting native pollinator plants inside a deer exclosure where invasive species have been removed, with the dominant cover of barberry –which deer do not like to eat – smothering the understory in the background. Photo: Kass Urban-Mead.

It would take many weeks to discuss each of the pollinator relationships we might anticipate being positively affected by the work WWRP is doing. But, a few particular relationships seem worth highlighting, as each illustrates a broader important idea in forest bee conservation: 

I. Violets at Fahnestock State Park & Geraniums at Bye Preserve 

Of our native, wild, free-living bees, none live in hives. Instead, most nests are run by solitary females who alone provision the food store upon which each egg must feed until adulthood. Once the pollen ball is made – often deep in an underground tunnel – the female seals off the chamber in which the new baby bee will emerge, and moves on to the next one. Unlike a honey bee queen, she will never meet her offspring. Most species have only a single generation per year, and adults fly for only a few weeks. The larvae will develop over the rest of the growing season months, and not emerge as an adult until the same time next year, when her mother is long gone. 

This brief adult activity period, closely tied to a specific season, can result in forest-associated bees who end up “specializing” on a flower or group of flowers which blooms in that same window. It also means that if those flowers are threatened, they may be unable to switch to novel pollen resources. These “specialist” relationships are magical and vulnerable inter-dependencies hidden beneath our feet. 

Andrena distans, the solitary mining bee whose larvae rely solely on Geranium pollen, (Photo: Heather Holm).

Andrena geranii, the specialist bee whose larvae rely on waterleaf (Hydrophyllum sp.) pollen, (Photo: Heather Holm).

Andrena erythronii, the specialist bee whose larvae have a strong preference for trout lily (Erythronium sp.) pollen, (Photo: Nevin Cullen). 

Two of the familiar species we planted with WWRP are hosts to specialist bees in the genus of “mining bees,” Andrena: Geranium maculatum and several species of Viola. The Geranium Mining Bee’s Latin name is Andrena distans, and the Violet Mining Bee is named after her host plant: Andrena violae! Only some bees’ latin names match their host plants. For example, similar to the violet, the Trout Lily Mining Bee is named Andrena erythronii after the Trout Lily, Erythronium americanum. In contrast, you’d reasonably think that Andrena geranii is a geranium specialist, but she instead prefers the pollen of Virginia waterleaf, Hydrophyllum virginianum!  Other understory flowers which host specialist bees include bellwort (Uvularia sp.), Jacob’s ladder (Polemonium sp.), and spring beauty (Claytonia sp.)! You can find an introductory guide to pollinator gardening for specialist bees here.

One of the common signs of excessive deer browse was sadly on display at Fahnestock, where a sea of barberry stretched out as far as the eye could see, under the otherwise beautiful trees. In addition to shading out the tree seedlings that make up the future tree canopy, shrub monocultures only offer flowering resources for the brief window of time during which they bloom. However, not all forest bees are active while barberry flowers bloom. And, even if they are, they may not be nutritionally balanced for most species. Finally, if you’re a specialist bee or butterfly, you won’t be able to switch hosts to either collect pollen from or rear your larvae on these flowers. 

II. Asters at Closter Nature Center, NJ

Unlike the dense thicket of monoculture shrubs at Fahnestock, the Closter Nature Center in New Jersey demonstrated the other common picture of deer-ravaged understories: its barren understory, profusion of Japanese stiltgrass (Microstegium vimineum), and nearly complete lack of pollen and nectar. The park is clearly loved by inspiring, dedicated volunteers whose energy has established tough deer fencing, and completed several rounds of enhancement plantings. 

Left: The forest at Closter Nature Center, NJ, with hardly any understory vegetation except invasive stiltgrass, Microstegium vimineum. Right: an amazing crew of WWRP volunteers planting hundreds of native, pollinator-supporting plants in one of the deer exclosures at Closter. Photos: Kass Urban-Mead.

As we have discussed, spring is important in eastern hardwood forests because many of the spring flowers and the short-lived bees which depend on them completely finish their life cycles before the leaves fill out and the understory darkens. However, we do have some native social bees whose colonies are active all growing season long, including throughout the autumn months! The most charismatic and familiar of our native social bees are bumble bees. In contrast to honey bees, bumble bees are solitary in the winter and social throughout the growing season.  So, I was thrilled to see goldenrods, asters, and other fall-bloomers included by WWRP. 

Autumn is a crucial time for a bumble bee queen, freshly hatched by her parent colony. She is the only member of a bumble bee colony who lives through the long, cold months of winter, and she does this by hibernating. The hibernation is one of the most challenging periods of a bumble bee colony’s annual cycle: will the queen survive the long winter in order to start the colony anew in the spring. 

How does a bumble bee queen do this challenging task of getting ready for a long, cold winter’s hibernation? By using the nutritious pollen and nectar of aster flowers, of course! The resources of understory goldenrods– such as zigzag goldenrod (Solidago flexicaulis) and blue-stem goldenrod (S. caesia) – and many native asters which thrive in the shade – including blue heart-leaved aster (Symphyotrichum cordifolium) and white wood aster (Eurybia divaricata) – were well-represented in our planting lists at all sites. Although they may seem unassuming, many of these asters are the primary resources available in the fall. Queens must seek them out either in the forests themselves, or in nearby meadows and wetlands where queens may also fly to forage, before returning to the forest to hibernate and overwinter. 

Bumble bee queens’ choices of overwintering sites are still poorly understood. However, we do know that they bury themselves just a few inches below the leaf and soil level, and settle in like a big black bear for the long cold months. At Tufts, researchers have found that queens of the Common Eastern Bumble Bee, Bombus impatiens, will nest just a few meters away from the nest in which they were reared. Other research suggests that queens prefer overwintering on north-facing slopes, likely due to the temperature moderation provided by the colder north-facing temperatures. On a warmer slope, they have a higher risk of trying to become active early, risking burning through precious fat reserves. 

Finally, to complete the seasonal arc, when she wakes back up in the spring the understory flowers of forbs and shrubs must be there to welcome her as she begins the hard work of establishing the new colony. You are now familiar with the importance of spring flowers from a whole new angle: they are also crucial for our beloved bumble bee colonies! 

Left: A Common Eastern Bumble bee queen taking care of two of her important autumn tasks at once: mating with a male (smaller, behind her), and filling up on goldenrod pollen and nectar, which will allow her to survive the long winter months of hibernation. Right: A Two-Spotted Bumble Bee queen in the spring, having survived hibernation, using Dutchman’s breeches (Dicentra cucullaria) as important spring fuel while she jump-starts the new season’s colony. Photos: Heather Holm.

Left: Blue-stemmed goldenrod, Solidago caesia, from an earlier planting by WWRP inside the deer exclosures at Fahnestock were blooming. Photo: Heather Holm. Right: Shiny green sweat bee, shown here foraging on Solidago flexicaulis, are forest-associated bees which nest in rotting logs and other woody material in forests. Photo: Kass Urban-Mead.

I was thrilled to see a shiny green sweat bee, Augochlora pura, one of our beloved iridescent native bees, visiting some blue-stemmed goldenrods at Fahnestock State Park. 

These shade-loving goldenrod had been planted in 2023, and were well established and flowering during our current planting – a great success! I also frequently see the shiny green sweat bee visiting jumpseed, Polygonum virginianum, another species from which Linda had the chance to collect seed during our visit to Bye Preserve in Pound Ridge. 

III. A quick note on woody material 

Just one final important way in which pollinators are supported by woody habitats is in their use of nesting materials aboveground: in stems, in tunnels and cavities left by wood-boring beetles, and in rotting logs. Just as in lawns we remind people to “leave the leaves,” so too in the woods do we remind everyone to “leave the logs!”

Bumble bee (left) and shiny green sweat bees (right) both nest in punky and rotting logs, emphasizing the importance of large and multi-aged pieces of woody debris in the forest for bee nesting habitat. Photos: Heather Holm.

It was a joy and a practice of hope to join the plantings at WWRP, and thanks for joining me on a bee-focused meander through the experience. I am particularly moved by the care and attention to plant sourcing taken by Linda, Natalie, and the rest of the thoughtful team at WWRP. It is so important to have native, local ecotypes, seed sourcing, community engagement, and attention to detail in this crucial restoration work. So – restore the understory, save the bees!

“Bee” well, and hope to see you in the woods!

Kass