It depends on the pollinator in question. The dominant plants in the region, certainly in terms of biomass, are the coniferous trees that make up our forests. As with grasses, conifers are wind-pollinated, and winds can certainly be present during the winter months! However, I suspect this question is asking about the myriad animals that pollinate the great diversity of flowering plants found here.

The vast majority of pollinators are insects, most importantly bees, butterflies, wasps, flies, beetles, and moths. This is an incredibly diverse group, and as such, they display a wide variety of overwintering strategies, though a given species typically only adopts one. Wasps generally hibernate as adults, most often as fertilized females, but different species may hibernate at different life stages. Many hoverflies and flower flies hibernate as pupae or sleep away the winter months as adults. The diversity of beetles that visit flowers is staggering, and different species have evolved to overwinter as eggs, caterpillars, pupae, or adults. In every case, these insects must seek a sheltered space to wait out the cold season, and many will produce sugars to act as antifreeze to protect
their delicate tissues.

Painted ladies retreat to breed in southern deserts for the winter. The next generation of adults begins migrating back north through Tahoe in March, often in huge numbers. They can be seen streaming past the highest peaks and ridges when the weather is mild. Photo by Evan/iNaturalist

Our local butterflies also may overwinter as any of the life stages, depending on the species. For those that hibernate as adults, like the hardy species Mourning Cloaks (Nymphalis antiopa), a couple of weeks of warm weather can stir them from their sleep. You can occasionally find adults flitting over the snow, looking for a taste of tree sap, animal feces, rotten fruit, or other such gross substances in mid-winter. When the weather starts to turn again, they quickly retreat to a crevice behind peeling bark or find similar shelter, to await the return of spring. Some of our butterflies (most notably monarchs, Danaus plexippus and California tortoiseshells, N. californica) and quite a few of our moths migrate to lower latitudes or elevations, often to overwinter as adults in milder climates. Other species, like painted ladies, Vanessa cardui, migrate from Tahoe to reproduce in a warmer climate, with spring migrants passing back through before the snow has melted.

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Bees are arguably the most important pollinators, central to the planet’s biodiversity (and human food security), and the only group of insects that actively collects pollen for transfer (most animal pollinators are after nectar or directly consume the pollen). This too is a diverse group (about 1,500 species in California), with a likewise diversified array of overwintering strategies. Bumble bee (Bombus sp.) queens hibernate as fertilized adults in a shallow hole in the ground, often under a root or on a gentle slope. Bumble bees fatten up for the winter like bears do, filling a specialized honey stomach with nectar that, when full, can account for 90% of the bee’s body weight.

Most solitary bee species overwinter in a nest chamber, either as pupae or fully-developed adults that have not yet emerged. Depending on the kind, these nests can be down in the soil (an estimated 70% of all bee species), in a hollow plant stem, or perhaps a hole chewed in the siding of your home. Honeybees are not native to this continent, but they are nonetheless widespread either as cultivated or feral colonies. The colonies form a winter cluster around the queen, going nearly dormant, and occasionally shivering to keep warm.

One of the most conspicuous pollinators during July and August is the Rufous Hummingbird (Selasphorus rufus). During these months, you can watch the tiny birds visiting successions of red, pink, or orange flowers of the crimson columbine (Aqiulegia formosa), paintbrushes and owl-clovers (Castelleja), gilias (Ipomopsis), and penstemons. Interestingly, these birds do not breed at Tahoe, but represent southbound migrants returning from the Cascades and northern Rockies, with the first males arriving
by mid-June.

Given their small bodies and high metabolisms, mountain-dwelling hummingbirds have evolved to put themselves into a deep state of torpor to make it through each night. As you likely know, Truckee often registers the lowest temperatures in the nation during summer nights. Males also may make a nightly migration to higher elevations, to escape cold air that sinks, associated with temperature inversions. Females of our locally-breeding calliope hummingbird (S. calliope), the smallest bird in the United States, have to sit tight on their eggs, so nightly “mini-hibernations” are a critically important survival strategy. However, hummingbirds cannot hibernate through the winter and migrate to warmer climates, where flowers and insects are available year-round. Our Anna’s Hummingbirds (Calypte anna) simply move to lower elevations, and I have had a male return to my yard in Glenshire as early as Valentine’s Day — during a snowstorm! The other species commit to a longer migration, heading to Mexico and Central America.

You can greatly enhance overwinter pollinator survival in your yard by being an untidy gardener and leaving any old bark, logs, or rocks where they lie, brush piles unburned, cane fruit and other hollow-stemmed shrubs unpruned, soil undisturbed, and especially any piles of leaves untouched, until late spring. This last one is less important with regards to the pine needles that pile deeply in Tahoe/Truckee yards, but deciduous leaf cover is so vitally important to overwintering pollinators, especially moths and butterflies, that the Xerces Society has a created a “Leave the Leaves” campaign.

For more information on what you can do to help local pollinators throughout the calendar year, give us a call at the Tahoe Institute for Natural Science or visit tinsweb.org!

~ Will Richardson, executive director of Tahoe Institute for Natural Science

Compiled by Mayumi Elegado

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