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  • Jim Tolstrup

Counting Garden Pollinators

By Jim Tolstrup:

Picture a lean, mature gentleman with a floppy sun hat, furiously waving a large white net over the tops of shrubs and perennials on a warm summer day. This is Dr. Paul Opler, our neighbor, and a frequent visitor to our gardens at the High Plains Environmental Center (HPEC) in Loveland.

Jim Tolstrup and Paul Opler.  Photo: Evi Buckner-Opler
Jim Tolstrup and Paul Opler. Photo: Evi Buckner-Opler

An entomologist of some renown, Paul’s list of published work, academic lectures, and public education workshops is extensive and impressive. A professor of Bio Agricultural Sciences and Pest Management at the Dept of Agricultural Biology at Colorado State University, his specialty is Lepidoptera (moths and butterflies). But these days he is in pursuit of bees.

Paul says the HPEC gardens are a sort of “ecological supersite” because of the density and diversity of native plants. His ongoing pollinator study includes 111 species of woody plants and forbs (herbaceous flowers) at HPEC, plus locations in northeastern Colorado and adjacent Wyoming. He seeks clues to plant/insect associations and other information that may be helpful in managing a garden, or natural area, as pollinator habitat.

For years we’ve heard that native plants provide habitat for the pollinators and other wildlife with which they coevolved. I’ve said this myself in numerous talks. I can tell you by casual observation that I see a diversity of pollinators in our gardens, and that they appeared quickly after volunteers helped plant extensive gardens of native plants on our 4-acre site. However, we’ve never had solid data about which pollinator species were attracted and which specific plant species were attracting specific species of insects.

The data from Paul’s study is still being evaluated. Bee specimens collected at HPEC and elsewhere are being identified by a team of experts, primarily at CU Boulder. Paul estimates the total number of bee species collected at HPEC to be more than 100. Many are very difficult to identify to species, although most can be identified to genus fairly easily (for an entomologist or well-informed amateur).

Beyond identifying the genus, arriving at the exact species can be extremely difficult. Male and female bees of the same species often look quite different, and the tiny differences can often can only be observed in a microscope. Instead of having to determine which of the 946 species native to our state we might be looking at, it’s sometimes necessary to identify which of the 1892 combinations, including species and sex, they might be.

HPEC native plant demonstration gardens blaze with color throughout the season, with virtually no watering.
HPEC native plant demonstration gardens blaze with color throughout the season, with virtually no watering.

Bee Diversity in Colorado

Colorado is home to nearly one quarter of the nearly 3,500 bee species found in the United States and Mexico. Some, such as the genus Bombus, or bumble bees, nest in the ground. For this reason, keeping some areas of bare ground is a helpful practice when maintaining landscape for pollinator habitat. All of the 28 bumblebee species we see in Colorado are natives. Only 5 species of this group were found at HPEC, including the brown-belted bumblebee (Bombus griseocollis), Hunt’s bumblebee (Bombus huntii), a few Southern Plains bumblebees (Bombus fraternus), and one red-belted bumblebee (Bombus rufocinctus). The last and most exciting species found at HPEC, the American Bumble Bee (Bombus pensylvanicus), was a frequent visitor to the gardens. This species has declined over much of its range and is being considered for listing as endangered. The Nevada Bumblebee (Bombus nevadensis) was not found although it is relatively common in Larimer County.

European honeybees (Apis mellifera) are commonly found in gardens. The decline of honeybees is a global concern, not least of all because human beings are dependent on the crops they pollinate. However, honeybees are not particularly helpful for the diversity of native bees. They can compete with native bees for forage and become a vector for diseases and parasites that affect native populations.

Solitary bees, such as mason bees (Osmia sp.) and leafcutter bees (Megachile sp.) are most active in spring. They utilize manmade “bee hotels” constructed from sections of bamboo or drilled blocks of wood. Placing bee hotels near plants of the Rosaceae (rose) family is particularly beneficial for leafcutters that prefer these plants for sealing up their egg chambers.

Understanding the insect flower-visitor ecosystem

Throughout the world, plants have developed chemical means of repelling insects that may eat the plant, its fruit or seeds. Specific insects have often coevolved with the plants, developing a tolerance for these specific chemicals. This may allow some of these insects to damage the plant, but may also allow them to become better pollinators for that plant. The vast number of intricate plant/pollinator relationships is not fully known.

These plant-insect relationships are one reason introduced plants from Europe and Asia can become invasive in the North American prairie, displacing native plants, and disrupting ecosystem function. Once in their new range, invasive plant species typically do not have the natural insect predators that helped keep them in check in their native range. Operating outside of these native plant/insect agreements, negotiated within the subtle balance of coevolving species, invasives expand rampantly, often outcompeting natives. The destruction of wild plants and competition with weeds reduces forage for native insects.

Evolving together can also lead to specialized adaptations that benefit both pollinator and plant. An example is found in the symbiotic relationship between the native soapweed yucca plant (Yucca glauca) and the yucca moth (Tegeticula yuccasella.) The yucca plant is pollinated primarily by the yucca moth and depends on it for reproduction. The female moth, in turn, lays eggs in the developing flower before it develops into fruit and leaves a pheromone scent that lets other yucca moths know that eggs have been laid on this plant. When the larvae hatch they eat some (but not all) of the yucca seeds as they’re developing, allowing both species to survive for another generation.

Plants have adapted to target specific pollinators in many ways, but a common strategy is developing specific flower forms to limit which insects can access their pollen. Insects that are successful in accessing the pollen and nectar of a particular species are likely to go to other flowers of the same type. This helps to ensure fertilization and eliminates some of the waste of pollen being carried to different plant species.

Some plants can be pollinated by many different species of insects. These include many members of the Asteraceae family, with open flowers that are easy to access. Pollinators that visit many types of flowers are referred to as “generalists” or polylectic. Insects that limit associations to one genus or species of plants are called oligolectic.

Tube-shaped flowers, such as penstemons, are particularly attractive to bumble bees. The buzzing vibrations of bumble bees causes a release of pollen in these flower species; the timing rewards both the insect and the flower. Bees, butterflies, and other insects have different tongue lengths which also limit access to the flower to specific insects.

Bees that visit the same gardens repeatedly are described as “trap-lining.” This refers to insects or other organisms visiting a string of sites periodically in search of forage. In summertime, hummingbirds travel daily to sites where they have previously found nectar, do some quick collecting, and then move on. They learn a route and check it repeatedly. This is a great example of why urban and suburban gardens that are rich in native plants are important to pollinator populations that will learn your garden’s location. Once they have found forage, they will return repeatedly.

So, who won the insects’ plant popularity contest in our gardens? By far the most visited plant, by the widest range of species of insects, was Bigelow's tansy aster (Machaeranthera bigelovii). Second to that were sunflowers, (Helianthus nuttallii, H. maximiliani, H. pumila, H. annuus). Rocky Mountain beeplant (Cleome serrulata [syn. Peritoma serrulata]), and fernbush (Chamaebatiaria millefolium), were also popular with a wide variety of insects.

Many plants produce nectar in their flowers to attract pollinators, but some have “extrafloral nectaries” - glands that produce nectar elsewhere on the plant. The additional sources of nectar not only increase pollinator traffic, in some cases they also attract ants. The ants in turn defend the plants from predation by other insects.

The complete study will be posted on the HPEC website later this year.


Recommendations for Gardeners from Paul Opler’s observations at HPEC

  • Bees are attracted to intense concentrations of flowers, the bigger the area the better.

  • Bees require a succession of plants over the year, but some specialize in particular plant families so it’s good to have a succession of bloom within those families, including Asteraceae, Rosaceae, Lamiaceae, Fabaceae, etc.

  • Don't clean up dead stalks or remove old plant material until spring if possible. Pollinators may over-winter in plant stalks. Walking in the garden can also be destructive as insects may be just below the surface.

  • Encourage a bit of weediness. Plants that aggressively self-seed such as sunflowers, Rocky Mountain bee plant, and fetid marigold (a favorite at HPEC) can supply an abundance of flowers for insect forage. We also find they help fill the space and reduce competition from more problematic weeds.

  • Provide bee houses if possible.

  • Provide areas of bare ground for nesting.

  • Keep plantings in sunny areas; most bees forage mainly in sunshine. Honeybees and bumble bees are exceptions.

  • Avoid pesticides as much as possible. Know where your plants are grown and always avoid buying plants treated with systemic pesticides, particularly neonicotinoids!


Jim Tolstrup is Executive Director of High Plains Environmental Center (HPEC) in Loveland, and author of, SUBURBITAT: Restoring nature where we live, work, and play.



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