Optimizing Environmental Benefits From Riparian Buffers in Maryland
Agricultural crops are influenced in many ways by the surrounding landscape. In Maryland, landowners have established over 40,000 acres of herbaceous riparian buffers on the edges of working agricultural lands. The primary purpose of these CREP buffers is to improve water quality and to create or enhance wildlife habitat. To date, few studies have documented the extent to which they buffers are used by wildlife or serve as propagation areas for insect populations. This SARE project focuses on three design elements of riparian grass buffers: width, vegetation composition, and timing of mowing. Our goal was to demonstrate that these elements can be modified to enhance conservation of natural enemy communities, reduce insect pest outbreaks, and provide better habitat for birds, without sacrificing the water quality functions of the buffers.
Of the 15 or more farmers and landowners involved in the project, one-third will reduce their use of endophytic grasses and two-thirds will delay the mowing of buffers to avoid removal of nectar sources for pollinators and adult parasitoids.
Twenty-four grass buffers in 2003 and 44 grass buffers in 2004 next to crop fields were sampled for invertebrate communities. Buffer sites were located on 28 farms involving the cooperation of 24 farmers/landowners. Buffers contained either cool-season or warm-season grasses, of which about half of the buffers represented each vegetation type. We predicted that warm-season grasses would foster more diverse insect populations, particularly natural enemies as a source of immigrants into adjacent crops. These grasses were thought to be more sustainable due to their structural diversity, less disruptive management regimes, and less stressed habitats during the summer. However, our results showed the opposite. Cool-season grasses provided habitat for more aerially-active beneficial insects throughout most of the growing season, although warm-season buffers supported insect communities of similar structure and diversity during late summer. Surface-dwelling communities in both grass types were similar in taxa composition and abundance, except for a few taxa. We also showed that the vegetative type may exert different influences on insect populations in adjacent crop fields. Aerial communities of natural enemies were generally more abundant in crops next to cool-season grass buffers, especially during June and July. Surface-dwelling communities of natural enemies in crops were least influenced by the adjacent buffer type.
This work was also supported by a two-year grant from the Maryland Center of Agroecology.
In 2004, arrangements were made with 12 landowners with cool-season buffers to mow only half of the filter strip after the nesting period (after August 15). The objective was to ascertain if mowing forced pest species, such as grasshoppers and spider mites, from the buffers into adjacent soybeans. Sampling in the buffers and along transects into the fields was conducted before and after mowing. Mowing did not trigger any unusual insect pest outbreaks in soybeans. We concluded that grass buffers in the CREP program were probably less likely to serve as sources of pest infestation than other grassy areas surrounding crops that could be mowed early in the season. This work was partially supported by a grant from the Maryland Soybean Board.
Additional funding was received in 2004 from USDA/NRCS to assess the use of herbaceous buffers around crop fields by breeding and wintering birds. The critical questions we addressed include: Which bird species are using herbaceous buffers in summer and winter? Which birds prefer buffers containing warm season grasses and which birds prefer buffers containing cool season grasses? How does vegetation composition and structure affect bird response? How does buffer width affect bird response? Bird surveys were completed in the summers of 2004, 2005, and 2006 and winters of 2005, 2006 and 2007 in more than 100 buffers of varying widths and vegetative types. To date, this study showed that relative bird density, species richness, and total avian conservation value were higher in grass buffers compared to field edges without buffers, but the type of vegetation is not a major determinant of use in the summer. However, narrower buffers supported higher bird density per unit area than wider buffers. Also, warm-season buffers and unmowed cool-season buffers provided shelter for more winter birds.
Numerous studies have shown that higher plant biodiversity in landscapes around crop fields provides a more sustainable habitat for beneficial insect communities. In particular, adding flowering forbs to buffer grass mixes can enhance conservation biological control by providing pollen and nectar food sources for insect predators and parasitoids. Although CREP guidelines list 25 perennials to add to seed mixes, few studies have assessed their competitiveness and potential as insectary plants. In 2004, 2005, and 2006, microplots of perennials established in an orchard grass buffer were sampled through the growing season to determine bloom period, floral area, flower structure, competitiveness, and visiting beneficial insects. Results to date showed a wide range of responses in terms of their competitiveness, attractiveness to insects, and accessibility of floral resources. Not all data have been analyzed to date, but there is evidence suggesting that certain flowering forbs if very abundant in buffers could act as a sink and drawn natural enemies away from crop fields. It is apparent from this study that flower species, intensity of floral biodiversity, and timing of floral resource production should be given careful consideration when designing grass buffers to enhance biological control.
Additional studies in 2005 and 2006 in 12 warm-season buffers (six with flowers, six without flowers) were conducted to determine if the presence of floral resources enhances beneficial insect communities in adjacent corn and soybeans. Buffers with flowering forbs clearly attracted and supported significantly more abundant and taxa-rich communities of beneficial insects, especially parasitic hymenopterans, and these same taxa were more abundant in the adjacent crop fields.
Impacts and Contributions/Outcomes
No specific outcomes to date.
Outreach efforts have included:
Presentations of project’s results were given at the following events: Dorchester County Farmers Club – 2006, Entomological Society of America, Eastern Branch Meeting – 2007, Grassland Restoration Workshop – 2005, Maryland Natural Resources Conservation Service, Planning Workshop – 2004, The Wildlife Society Annual Conference – 2006, American Ornithologists’ Union – 2005, Bioscience Research & Technology Review Day, University of Maryland – 2005, 2006, Wildlife Habitat Council’s Annual Symposium – 2004, and the Wildlife Society Annual Conference –2006.