Bee Visitation on Flowers in Maine, United States, Reveals the Relative Attractiveness of Plants Through Space and Time: Part I.

Bee reservoirs can be effective in agricultural and habitat restoration projects, but the relative attractiveness of plants is not fully understood. To improve plant selection with better knowledge of spatial, temporal, and competition aspects, we tested up to 90 plant subjects from 2012 to 2015 at four sites in Maine. We recognized Apis mellifera L., Bombus ternarius Say, 1837, 'Most Bombus' (except B. ternarius), 'Halictidae' and 'Other Bees' (collectively the so-called 'bee groups') on open flowers in three 1-min periods per site and day, with numerous repeated observations per plant taxon. In 14,311 observations, we recorded 17,792 bees in 61 species. Most-visited plants included Asclepias tuberosa, Borago officinalis, Clethra alnifolia cv. Hummingbird (especially by A. mellifera), Melilotus officinalis, Origanum vulgare, Rosa palustris (especially before 1400 hours), Spiraea alba var. latifolia, and taxa in the family Asteraceae. Early-flowering shrubs were visited, especially by 'Other Bees'. Bee groups each ranked plants uniquely, with some overlap, and differed in most-visited of six plant taxa that we had included in all 4 yr and sites. For 'All Bees' among 84 plant taxa, the most-visited plants were M. officinalis (June), A. tuberosa (July), and C. alnifolia (August). Indicator Species Analysis revealed low bee fidelity to host plants for all but a few plant taxa. Apis mellifera differed from native bees in plants it visited intensively, with some overlap (e.g., A. tuberosa), and was associated with increased visitation on seven plant taxa by 'Most Bombus' and B. ternarius.

Plant Origin and Other Attributes Impact Bee Forage Patterns in a Common Garden Study in Maine, United States; Part II.

In a common garden study in Maine from 2012 to 2015, we used two bee species (Apis mellifera L. and Bombus ternarius Say (1837)) and three field-recognizable bee categories ('Most Bombus', 'Halictidae', and 'Other Bees') plus an 'All Bees' data aggregation to compare 17 native and 68 introduced plant taxa. Data were from three 1-min timed periods per flowering plant taxon on a given day at a site. We observed 17,792 bees and found that their response varied by bee species or group. Using mixed models to analyze our data, we found that native bees had higher visitation rates on native plants, while A. mellifera visited both native and introduced plants. Most groups visited native late-flowering and native mid-late-flowering plants at higher rates. 'All Bees' were attracted to native perennials (vs annuals and shrubs) and to tall plants, both native and introduced; A. mellifera was attracted to introduced perennials, to introduced tall plants, and to lower-growing native plants. Asclepias tuberosa L. elicited a strong response from B. ternarius. In only two of six pairs of wild types and cultivars, bees visited wild types more. Plants with long bloom periods and with small, densely arranged white flowers attracted higher bee visitation than did other configurations (e.g., Origanum vulgare L., one of our most attractive taxa). A general linear model showed that linear combinations of flower density, floral resource height, flower corolla depth, and flowering duration explained significant variation in visitation rates for each of the different bee taxa groups.