Visual counts can be used effectively for direct assessment of abundance for vertebrate pollinators, such as bats and hummingbirds. Mist nets can also be used to capture these flying vertebrates, and their density can be accurately estimated because the individuals are large enough to be banded or marked by some other method. Direct assessment of insect pollinator abundance is typically conducted by observation at the flowers they visit. Pollinating insects are much more conspicuous when visiting flowers than they are in transit among foraging areas or when engaged in other activities; for example, ground-nesting bees’ nests can be difficult to find. One method is to count all visitors to a set number of flowers, generally of a single species, during a specified period (typically 10 minutes), when meteorological observations are simultaneously recorded (Kearns and Inouye, 1993). In addition to providing an estimate of pollinator visitation rate, this method allows a snapshot assessment of the dependence of insect activity on environmental factors, such as temperature, humidity, wind, and light (for example, McCall and Primack, 1992). However, one problem with assessing pollinator populations based on flower visits is that floral abundance and diversity often vary greatly as well.
An alternative method for estimating insect pollinator abundance involves counting or collecting individuals along a transect (for example, 1m 25 m, or 1 m for a fixed period), as in a Pollard walk census for butterflies (Caldas and Robbins, 2003), or in a recent survey of bumble bee populations (Knop et al., 2006). Bees that are not collected can be captured, marked, and released to distinguish individuals and prevent redundant counting (Hines and Hendrix, 2005). Netting at flowers along transects in permanent, one-hectare plots also has been used for native bees (Cane et al., 2000). Insects that readily adopt artificial nest sites—such as nest boxes for bumble bees or trap nests for solitary bees—can be monitored by placing the nests in appropriate habitats. A disadvantage of this method is that adoption rates can be low (as is often the case for bumble bees; Inouye, unpublished). Pollinators that can be manipulated by reward—euglossine bees are attracted to terpene-soaked blotter paper (Dodson et al., 1969; Roubik, 1989; Roubik and Hanson, 2004), hummingbirds will consume artificial nectar from feeders, moths fly to traps baited with fermenting fruit—are more easily monitored than are those that cannot be reliably attracted to a particular location.
Passive traps that collect insects indiscriminately are not always suitable. Entomologists have long used Malaise traps (screen tents that catch insects and funnel them up into a collecting head) because they work well for many kinds of flies. However, the traps rarely capture butterflies, moths, or bumble bees. The selectivity of pan traps for bees depends on the use of appropriate combinations of trap size, color, and number (S. Droege, Patuxent Wildlife Research Center, presentation to the committee, October 18, 2005),