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, Colorado.
Paul is an entomologist of some renown. His list of published work, academic lectures, and public education workshops are both extensive and impressive. He is 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 our gardens at HPEC are a sort of “ecological supersite” because of the density and diversity of native plants growing here. His ongoing study has included 111 plant species of woody plants and forbs (herbaceous flowers). Aside from HPEC, the study includes locations in northeastern Colorado and adjacent Wyoming, seeking clues to plant/insect associations and other information that may be helpful in managing a garden, or natural area, as a pollinator habitat.
For years we’ve heard people say that native plants provide habitat for the pollinators and other wildlife with whom they coevolved. I’ve said this myself in numerous talks and on garden tours. 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 to plant extensive gardens of native plants on our 4-acre site. However, we have never had solid data about which pollinator species were attracted to our gardens 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 species collected at HPEC to be more than 100 species. Many of the bees are very difficult to identify to species, although most can be identified by genus fairly easily (for an entomologist or well-informed amateur).
Beyond identifying the genus, arriving at the exact species can be extremely difficult. Paul notes that male and female bees of the same species often look quite different, and the differences in species can be so tiny that they often can only be observed in a microscope. The differences in sex within each species can be almost to the degree that instead of having to determine which of the 946 species native to our state we might be looking at, it’s necessary to identify which of the 1892 combinations, including species and sex, they might be.
Colorado is home to nearly one-quarter of the approximately 3,500 bee species found in the United States and Mexico. Some bee species, 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 bumblebees we see in Colorado, a total of 28 species, are native bees. 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. Honeybees can compete with native bees for forage, as well as being 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. These are the bees that utilize manmade “bee hotels” constructed from sections of bamboo or drilled blocks of wood. Placing bee hotels near plants of the Rosaceae family is particularly beneficial for leafcutters that prefer these plants for sealing up their egg chambers.
Lisa Mason, Horticulture Agent at Colorado State University Extension, Arapahoe County, suggests keeping the following in mind with bee hotels and nesting boxes:
Different species need different diameters of tunnels. Megachile bees need tunnels that are approximately 5-6mm, while Osmia can vary between 6mm-9mm.
· All tunnels should be a minimum of 5 – 6 inches long, and the tunnels need to have a back (only one entrance in).
· The nest box should have a frame or roof that slopes to protect the nest box from rain.
· Typically, the nest box should be anywhere from 3-6 feet off the ground and mounted firmly.
· The nest box needs sunlight throughout the day. The bee nest box should generally face south to southeast to maximize sunlight.
· Maintenance is a big consideration in bee nest boxes/bee hotels. Over time, nest boxes can build up mold, fungi, pollen mites, and other pests/pathogens. Depending on what the nest box is made of, the next box should be replaced every year or two or designed in a way that tubes can be replaced every year. Wood blocks should be cleaned with a bleach solution.
· Since nest boxes need to be cleaned and replaced, it’s hard to know exactly when to clean or replace them since different species of bees emerge at different times. For this reason, having two nest boxes is recommended. Place one in the garden all season, and leave throughout the winter. When spring arrives, add the new nest box. As soon as all the solitary bees and wasps exit the previous nest box, remove it, and leave the new box until the following spring. Repeat with a new or cleaned box each year.
Throughout the world, plants have developed chemical means of repelling insects that may eat the plant, its fruit, or its seeds. Often, specific insects have coevolved with the plants, developing a tolerance for these specific chemicals. While this tolerance may allow some of these insects to damage the plant, it may also allow them to become better pollinators for that plant. The vast number of these 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, invasive species 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 pollinators and plants. An example of a mutually beneficial symbiotic relationship between native plants and insects can be found in the relationship between the soapweed yucca plant (Yucca glauca) and the yucca moth (Tegeticula yuccasella.) The yucca plant is pollinated primarily by the yucca moth and depends on this moth to reproduce. The moth, in turn, lays its eggs in the developing flower before it develops into fruit. The female moth leaves a pheromone scent that lets other yucca moths know that eggs have already been laid on this plant. When the larvae hatch, they eat some (but not all) of the yucca seeds as they’re developing, and both species survive for another generation in the process.
There are many ways in which plants have adapted to target specific pollinators, 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 of flower 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 plants 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 cause a release of pollen in these flower species, the timing of which 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. Hummingbirds travel to sites daily in the summertime where they have previously found nectar; they do some quick collecting and then move on. These individual organisms 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. These species 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.
Among the most popular plants were many that have extrafloral nectaries. Many plants produce nectar in their flowers to attract pollinators, but some flowers have “extrafloral nectaries.” These are glands that produce nectar elsewhere on the plant. The additional sources of nectar on these plants not only increase pollinator traffic but, in some cases, 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, http://www.suburbitat.org, later this year.
Here are some recommendations for gardeners that Paul observed or confirmed at HPEC.
1. Bees are attracted to intense concentrations of flowers; the larger the area, the better.
2. 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.
3. 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.
4. Encourage a bit of weediness. Plants that aggressively self-seed, such as sunflowers, Rocky Mountain bee plants, and fetid marigolds (a favorite at HPEC), can supply an abundance of flowers for insect forage. We also find that these plants help to fill the space and reduce competition from more problematic weeds.
5. Provide bee houses if possible.
6. Provide areas of bare ground for nesting.
6. Keep plantings in sunny areas; most bees forage mainly in the sunshine. Honeybees and bumble bees are exceptions.
7. Avoid pesticides as much as possible. Know where your plants are grown, and always avoid buying plants treated with systemic pesticides, particularly neonicotinoids!