Native Insect Pollinators and Their Habitats
Bruce Barrett, Division of Plant Sciences, University of Missouri Extension
James Quinn, Regional Horticulture Specialist, University of Missouri Extension
Richard Houseman, formerly of Division of Plant Sciences, University of Missouri Extension
Damon Hall, School of Natural Resources, University of Missouri
Humans and wildlife depend on insect pollinators for the production of fiber, fruits and vegetables. Insect pollinators are required for the pollination of two-thirds of the world’s plants. Bees, butterflies, flies, wasps, moths, beetles and even mosquitoes unwittingly pollinate plants while foraging for nectar and pollen on flowers. In the United States alone, more than 4,000 species of native bees, 750 species of butterflies, 30,000 species of beetles, and thousands of wasps and flies contribute an estimated $29 billion to farm income each year.
Of all the insect pollinators, bees are the only ones that collect pollen for eating. As such, bees are responsible for 90 percent of all the world’s pollination. The Food and Agriculture Organization of the United Nations estimates that of the roughly 100 crop species that provide 90 percent of food supplies for 146 countries, 71 are bee-pollinated, mostly by wild — native and nonmanaged — bees. Globally, bee pollination is the most valuable ecosystem service provided by wildlife.
Bumble bees, carpenter bees, sweat bees, metallic bees, orchard bees, digger bees, leaf-cutter bees, miner bees, cuckoo bees, mason bees, yellow-faced bees and others are as effective as or more so than honey bees in pollinating fruits and vegetables. For example, one blue orchard bee (Osmia lignaria) is more effective than 25 honey bees at pollinating almonds. And for many tomatoes, bumble bees are 40 to 60 times more effective than honey bees, yielding larger fruits. However, with a few exceptions, the contribution to pollination from native and nonmanaged bees remains unaccounted in the economy.
Butterflies, moths and flies also contribute in a noteworthy way to pollination. This publication discusses these pollinators, in addition to native bees. More information on the pollinators of Missouri — including hummingbirds, beetles, wasps and ants — can be found in MU Extension publication M402, Pollination Mechanisms and Plant-Pollinator Relationships.
Figure 1. Bumble bee (Bombus spp.).
Bumble bees (Bombus spp.) belong to the family Apidae and comprise a group of about 250 species found primarily in temperate regions throughout the world (Figure 1). They are considered to be generalist foragers and usually feed from a variety of plant species.
Adult bumble bees are typically larger and more robust than other types of bees. Their entire body is usually covered with dense black and yellow hairs, but a few species are marked with orange hairs. The tip of a bumble bee’s abdomen is rounded. The ovipositor is a modified stinger found on females. It is not barbed, so they can sting multiple times. However, bumble bees are generally not considered aggressive and only become a problem when their nests are threatened or disturbed, especially if their nests are located near areas of high human activity.
Bumble bees undergo a complete metamorphosis and have four life stages: egg, larva, pupa and adult. Like honey bees, a queen bumble bee lays fertilized and unfertilized eggs. The fertilized eggs develop into females: either workers, which are sterile females with 8- to 16-millimeter-long bodies, or new queens, which are fertile females 17- to 23-mm-long bodies. Unfertilized eggs develop into males, which are fertile drones with 12- to 18-mm-long bodies.
Note: Certain measurements in this publication are given in millimeters because small measurements are more easily expressed in millimeters than in inches. One millimeter (mm) is less than four-hundredths of an inch (1 mm = 0.03937 inch).
Like honey bees, bumble bees are eusocial organisms. They live in colonies whose members comprise different castes, or subsets of individuals, based on the tasks they perform. For example, a single queen heads a bumble bee colony. But unlike honey bees, only a mated new queen hibernates throughout the winter. In the spring, the queen becomes active and begins searching for a suitable, protected nest site. Such sites may be located in an underground cavity, such as an abandoned rodent nest or empty space under roots of decaying trees; under a mass of vegetation, such as bundles of hay or straw; or in hollows in decaying logs. After establishing a nest, the queen uses pollen paste to form a container-like structure called a cell. She lays eggs in the cell and then seals the cell with wax. The developing larvae initially feed on the pollen paste. Later, the queen feeds them through a hole in the cell. Eventually, these individuals will emerge as workers and assume the responsibilities of constructing more cells and taking care of later broods. Workers use wax, secreted by their abdomens, mixed with pollen to construct oval cells in which nectar or pollen is stored. In other cells, the queen lays eggs (several per cell), and the workers feed the developing larvae honey and pollen.
Similar to honey bee workers, bumble bee workers forage for nectar and pollen and have a large flat structure on their hind legs, called the corbicula, used to collect large amounts of pollen during their foraging trips. Workers live for about a month.
Colony size varies among species and the time of year. At its peak, a nest may contain from 50 to 400 bees.
Toward autumn, the queen shifts production from workers to reproductives, and begins to lay eggs that will produce drones and future queens, called gynes. A few days after emergence, the drones and gynes will leave the nest. Eventually, the old queen and the last of the workers die. Mating among the recently emerged reproductives varies among species. Bumble bees do not swarm like honey bees. Sometimes bumble bee mating occurs near a nest opening as males wait for emerging females. In other cases, the males may gather at various visual markers — such as a flower, rock or fence post — and pounce on any passing females, or they may fly along definite routes searching for females entering the area. After mating (often more than once), the new queen feeds heavily on pollen and nectar, storing the energy as fat inside her body, and begins searching for a site to overwinter. In the spring, she will search out a suitable area to build her nest, and the annual cycle repeats itself.
Habitats with coarse vegetation and other kinds of natural debris and rodent activity are most favorable for bumble bee populations. Their nests are found under vegetation or debris, in hollow logs aboveground, or in cavities underground. They do not dig their own burrows but seek out abandoned rodent burrows in which to build nests. Areas that appear to be unkempt are best for nesting. Relative to other bees, bumble bees have a long period of summer activity and a small capacity to store resources in their nests, so they require continual access to nearby food sources through the summer. Although they have been shown to fly as far as 10 kilometers (about 6 miles), they do most of their foraging much closer to the nest.
Like other bees, bumble bees do well in habitats containing a variety of flowering plants and a consistent source of water. As a nest is getting established, a bumble bee queen may have to visit as many as 6,000 flowers a day to get enough nectar to produce the body heat required to brood her eggs. The brood cools down while the queen is gone, so her foraging trips need to be short. For this reason, most nests are located near an area with a variety of flowering plants that persist throughout summer.
Bumble bees are better pollinators than other bees for certain kinds of plants because they collect pollen in a unique manner referred to as buzz pollination. During buzz pollination, a bumble bee rapidly vibrates its wing muscles while holding onto the pollen structures of the flower, causing them to release large amounts of pollen onto the bee’s body. These vibrations more effectively move pollen from the blossoms onto a bumble bee’s body for transport to other plants of the same variety. Some kinds of plants — including nightshade, rose and heath families — require buzz pollination for proper pollen transfer. Many crops — such as blueberry, cranberry, tomato and kiwi — belong to these groups and show improved yields from buzz pollination.
Artificial nest boxes for bumble bees are composed of two parts: the container and the nesting material. The container can be made of wood, plastic, metal or clay and can be of various shapes and sizes. It must have an entrance hole large enough for a bumble bee to enter and at least two ventilation holes to minimize the accumulation of moisture inside. A tube can be inserted into the entrance hole if the nest is going to be buried later to attract species that nest underground. The ventilation holes should be covered with some kind of fine mesh to keep other insects, especially ants, from getting in through the holes and destroying the nest in the early stages of establishment. A variety of hollow containers can be used to make artificial habitats for nesting bumble bees. For an individual nesting compartment, the recommended volume should be somewhere between 216 and 1,000 cubic inches, or 6-by-6-by-6 inches and 10-by-10-by-10 inches, respectively.
The best nesting materials to place inside the container include dried moss, upholstery stuffing, wool, horsehair or old mouse- or bird-nest materials. Synthetic fibers and cotton stuffing should be avoided because they will entangle and kill the bees. Place a wad of nesting material about the size of a tennis ball inside the container so that it does not touch the sides of the container and air movement is possible. Make a small depression in the ball so it is slightly concave on one side. Situate the ball so the concave side faces the nest entrance. Rodent droppings placed inside nest boxes are often attractive to bumble bee queens. The queen will begin rearranging things slightly when she enters the cavity.
Bumble bee boxes should be in place in suitable areas by early spring. Different species have their own preferences for nest location, but most prefer locations that are sheltered from the wind and out of the direct sun. Nest boxes can be placed under sheds, shrubs or debris, or buried in the soil. Success rates for use of artificial nest boxes each year ranges from about 3 to 30 percent, so place more boxes than you think you will need. Use of a nest box is often related to where it is placed and whether mice have used it as a nest. Being used by mice is often a good sign that a box is in a good location and that bumble bees will use it later.