Native Pollinators in Local Forestlands

This article was written by Jen Gorski at OSU Extension Forestry/Natural Resources.

Bumblebee (Photo: Oregon State University)

Aren’t pollinators usually relegated to farms? What kind of insects are pollinators and what’s their place in the forest?

Dr. James River, who does pollinator research through OSU’s College of Forestry, says there is not enough known about forest systems to say exactly which species are present there. However, there are many pollinator species that have been identified on agricultural lands in the Willamette Valley and west Cascade foothills.

Which Native Pollinators Live in Our Forests?

Local native pollinators include bumblebees, leaf cutting and mason bees, flies, wasps, beetles, butterflies, moths, hummingbirds, and bats. These creatures can be fascinating to watch and they are vital for forest health. Did you know that bumblebees vibrate a flower at such a frequency that causes the pollen to shatter and scatter all over the bee?

Many of these insects provide a food source for birds, fish, amphibians, and small mammals. When there is an adequate assortment and quantity of them for pollination, they increase the amount of seed and its genetic diversity. Some species act as predator insects (beetles, lacewings, and parasitic wasps) that prey upon harmful pests. Overall they provide an essential role that sustains forests.

Native pollinators forage earlier in the season than European honey bees (whose numbers are struggling), and they can survive in the colder, wetter weather unique to our local conditions. But species like the native bumblebees are also in decline due to habitat loss, pesticides, climate change, and disease. Learn more on how to reduce poisoning due to pesticides.

Leafcutter Bee (Photo: Bernhard Plank)

Leafcutter bees harvest small bits of leaves to build their nests and butterfly larvae (caterpillars) munch on leaves. Insecticide residues can harm them all. Aerial insecticide sprays can kill adults and ground nests; dusts and insecticide coatings can adhere to hairs on pollinators. In some instances, these coatings are the same size as pollen and are taken to insect nests as food. Eggs, larvae, pupae and adults can be poisoned. This Xerces booklet reviews how landowners can protect and provide habitat for pollinators.

What Habitat Requirements do Native Pollinators Need?

Adult pollinators need nectar and pollen from flowers and their larvae need leaves to eat. Some pollinators rely on specific host plants (like Monarch butterflies on milkweed), and other pollinators can use a variety of hosts. Because Oregon native plants and insects co-evolved with one another, it is ideal to continue to combine them. However, it’s best not to eliminate all non-native plants because they could be providing food sources while new plantings of natives get established and begin flowering. It can be best to phase non-natives out over time to provide a continuous food source.

A diverse mix of native flowering plants consisting of trees, shrubs, herbaceous flowering plants, and bunchgrasses can supply a diversity of food choices. Clump plants of one species in 3’ wide patches along a connected strip or hedgerow so the insects can easily find the nectar and pollen. Strategically place windbreaks for cover. Oftentimes, pollinators require sun and a water source to thrive. Provide undisturbed ground and dead wood for shelter.

It is ideal to provide at least three different species of native flowering plants and one species of native bunchgrass for all seasons from very early spring through fall. Plant spacing should measure the mature width of the plant or a little less to reduce the possibility of invasive weeds that can infiltrate when desirable plants are getting established. The Natural Resource Conservation Service (NRCS) produced an excellent set of tables for plant species with their bloom season and site condition needs. (Please note that Major Land Resource Areas (MLRA) listed in the table refer to geographical areas; use 2 for the Willamette Valley and 3 for foothills of the Cascades.)

Forest Management Practices Can Enhance Pollinator Survival

Mowing or fire management is best limited to scattered patches that make up less than 30% of the acreage, so there are always surviving insects to repopulate the area. The best times to implement these practices are late fall to winter (when flowers have died back), and when adult pollinators are not active or are dormant. Keep fire to low intensity and keep mowing height 12-16” high. Use mowing speeds less than 8 miles/hr and use a flushing bar which is mounted at the front end of the tractor to warn wildlife that the mower is coming so they can get out of the way. This video demonstrates how the flushing bar works.

If using herbicides, limit spraying and mowing beyond the shoulders of roads. Avoid broadcast spraying because it can eliminate non-target plants that can be a food source; spot treat undesirable weeds. Scout the area for ground nests of native bees, mark and protect them from sprays.

Grazing animals that are closely monitored and kept to a low-medium stocking rate can reduce invasive weeds. Care must be taken to avoid damage to the different life stages of pollinator species. Seasonal rotations and attention to timing is important.

Get to Know What the Adult and Larval Stages of Species in Your Forest Look Like

Collect samples of the insects (keeping them contained of course!) and bring to your local Extension office to have Forestry or Horticulture staff identify.

Pollinators can add beauty as well as functionality to your woodlands. Common butterflies you are likely to see in your forest are western tiger and pale swallowtails. Enjoy the hunt!

Western tiger swallowtail
Host plants: big-leaf maple, willow, aspen, cottonwood
Nectar plants: rhododendron, honeysuckles, milkweed, mock orange, asters

Pale swallowtail
Host plants: buckbrush, cherry, cascara, oceanspray
Nectar plants: oceanspray, columbine, penstemon, asters