Reconstruction of Pollinator Communities on Restored Prairies in Eastern Minnesota
Total Page：16
File Type：pdf, Size：1020Kb
Conservation Biology Research Grants Program Division of Ecological Services © Minnesota Department of Natural Resources Reconstruction of Pollinator Communities on Restored Prairies in Eastern Minnesota Final Report to the Minnesota Department of Natural Resources Nongame Wildlife Program Catherine Reed Department of Entomology 219 Hodson Hall University of Minnesota St. Paul MN 55108 September 1, 1993 Table of Contents Abstract.........................................................................................2 Objectives and Introduction..............................................................3 Materials and Methods.....................................................................7 Results and Discussion Insects per Flower by Plant Species....................................…...10 Insects per Flower by Insect Field Identification Group.........……..11 Seasonal Patterns in Insect Populations.............................……..12 Insect Species Richness of Native Prairies and Reconstructions…..………………………………………………………………………..13 Insect Species Richness of Sites.................................…..........16 Bees...................................................................................18 Diptera..........................................................................…....25 Wasps.....................….............................................……...…...26 Lepidoptera............................................................…...…......27 Attractiveness of Forbs to Insects...........................…........…....28 Forbs and Their Visitors..............………………….........................................……............29 Conclusions and Recommen…………..................................................34 Acknowledgments..............…..........................................……..........37 Voucher Specimens....................................................……..............37 References Cited...................................................….....................37 Appendix Insect Species Lists...........................................………………......40 Plant Species List.........................................…………................51 List of Flower-Visiting Insects, by Plant Species.........……………....53 Insects and Plants on Sites....................................................77 2 Abstract I observed and collected insects visiting flowers on 4 relatively undisturbed prairie sites and 4 reconstructed prairies (farm fields recently replanted to prairie plants) in south central Minnesota during the summers of 1991 and 1992. Bees, flies, wasps, Lepidoptera and beetles visited flowers. Over 1100 observations of insects per flower revealed no consistent differences between the native prairie sites and the reconstructions over all, although insects per flower varied among forb species. A total of 279 insect species were identified among the 4000 insects collected; 83 species were found on native sites only, 68 on reconstructions only, and 128 species on both native sites and reconstructions. Species richness of sites ranged from 40 to 111 flower-visiting species per site; both the highest and the lowest value were observed on reconstructions. All sites contained both specialist and generalist insects. Bee species richness of sites was related to forb species richness, not to site area, reconstruction age or total number of flowers or inflorescences in bloom. Reconstructions and native sites were comparable in their bee species richness, but bee distribution was quite patchy. Only 8 of 125 bee species were found on all 8 sites, and each site had at least one unique bee species found on no other site. These results imply that prairie reconstructions can be valuable sites for insect conservation; to increase this value, managers should plant a wide variety of forbs from several families, and include forbs which bloom early and late as well as mid-season species. Native prairie sites can support many insect species; even small remnants should be preserved. Areas of greatest interest for further research include the adequacy of pollination by various insect groups for plant reproduction; the distribution of bees over larger areas including the protected sites; and the interactions among plants, their pollinators, and the other factors which allow high pollinator diversity on some sites. 3 The objectives of the project were 1. To determine what insect pollinator species become established in prairie reconstructions, and to compare the reconstruction pollinator communities with those of relatively undisturbed native prairies. 2. To identify factors which affect pollinator density and species richness in prairie reconstructions. Rediscovery of the practical and esthetic values of native plants and animals has stimulated replanting of prairies on former agricultural lands. We must discover how to manage these reconstructions so they can become self-sustaining communities. The value of native plants in roadside land management for reduction of mowing and herbicide use is being increasingly recognized. Native insects are being considered as crop pollinators due to their resistance to parasitic mites and diseases, and due to the threat of Africanization of honeybees (Torchio 1990). Though much is known about plant establishment in prairie reconstructions, little attention has been paid to prairie insects; at present, our knowledge of pollinator species presence and functioning is inadequate for management decision making. Most prairie plants require insect pollination for seed production, so pollinators are essential for the long-term functioning of preserves and of reconstructed prairies. Yet insects, including pollinators, are prone to population decrease and extinction when their habitats are altered. If pollinator populations are maintained, prairie preserves and reconstructions can serve as reservoirs for insects which also pollinate crops and other plants outside the prairie areas. The tallgrass prairie community includes many insects which are pollination specialists (oligoleges) visiting only single plant families, genera, or even single species (Schemske 1983). The pollinator community also contains many generalists (polyleges); they visit and pollinate many plants, and shift their hosts as new flowers come into bloom (Evans 1984; Heinrich 1979). Plants vary in the number of insects they attract and support: specialists (oligotrophs) are visited by only a few insect species (which may also visit other plants). Generalists (polytrophs) are visited by many insects (which may vary in their effectiveness as pollinators), and can support many different insect species. Normally many pollinators are present in an area; bumblebees tend to be highly mobile generalists, small native bees are frequently more specialized in the flowers they visit, and honeybees utilize dense flower clusters, especially of alien plants (Ginsberg 1983). Honeybees are rarely seen on prairie plants in Minnesota (Dickinson and McKone 1992). Not much is known about flies and wasps as pollinators. Likewise, Lepidoptera are considered to be important pollinators in many cases, but definite evidence is lacking on this except for a few plant species (Courtney 1982; Wiklund et 4 al. 1982). Are pollinators common in prairie remnants? Prairie management studies have emphasized plants: recent work on insects (Opler 1989; Nekola, in press) suggests that current management techniques, especially frequent burning, may in fact be harmful to native insect populations. Panzer (1981) reports that even areas as small as two hectares can support rare prairie insects if properly managed, including restrained burning and the removal of unnatural habitat features including weedy trails. In historic times the prairie has become increasingly fragmented. Insect specialists dependent on prairie plants may have shifted their resource use to become more generalized (Janzen 1974; Feinsinger 1983; Estes et al. 1983), persisted in small populations, or become extinct. Extinction is an increasingly likely prospect as populations become smaller (Dempster 1991). Have species been lost from the prairie insect community? Can prairie species become established and increase on reconstructions? Two aspects of insect biology must be understood: the insects’ colonizing ability and their ability to become established when they reach the reconstuctions. Prairie reconstructions are early successional systems, or new areas to be colonized, from the insects’ point of view. Theory gives some clues as to expectations regarding pollinator colonization of prairie reconstructions: opportunistic generalists are likely to be more important than specialists in early successional habitats (Feinsinger 1986). Specialized pollinators may have greater difficulty than generalists both in maintaining their populations in the absence of prairie (Kevan 1975) and in colonizing new habitats even where they are present nearby (Johnson 1969). However, the prairie itself is a young habitat by geological standards; it has been colonized by insects since the last glaciers (Lemkuhl ). Can prairie reconstructions be made more attractive to colonizing insects? Alternative views of pollinator-plant interactions lead to different management implications: 1. Pollinator presence depends on specific plants and their interaction with specialist insects. Insect specialists can colonize only areas which contain their host plants. For example, the sweat bee Dufourea monardae (Halictidae) requires Monarda species (Labiatae: Monarda