This research and demonstration project evaluated the effects of vegetation composition, addition of flowering forbs, and timing of mowing on pest and beneficial arthropod communities in riparian grass buffers. Our primary goal was to demonstrate that grass buffers could be modified in design and managed to enhance conservation of beneficial arthropod communities, without sacrificing their water quality functions. More than 38 farmer/landowners with a total of 40 cool-season and 79 warm-season grass buffers were engaged in several studies to address specific objectives. A major performance objective was to transition farmer/landowners to establish buffers with native, warm season grasses, which were assumed to provide more favorable habitat for beneficial arthropods than cool season grasses.
In one study, replicate buffers consisting of warm and cool season grasses and adjacent crops were sampled to determine the influence of vegetation type on beneficial arthropods and their potential prey. Results did not support our original thinking and indicated that cool season grass buffers provided equivalent, if not better habitat for arthropods than warm season grasses. In another study, 25 native perennials adapted for the Mid-Atlantic area were evaluated for their competitiveness in grass mixes and their potential for providing pollen and nectar food sources for insect predators and parasitoids. The forbs showed a wide range of plant responses in terms of competitiveness, blooming period, accessibility of floral resources, and attractiveness to insect populations. An on-farm study demonstrated that the addition of flowering forbs in warm season grass buffers attracted and supported significantly more abundant and taxa-rich communities of beneficial insects, especially parasitic hymenopterans; and populations of these same taxa were significantly more abundant in the adjacent corn fields. Several studies were conducted to assess whether mowing grass buffers forced insect pests to move into crop fields or adversely affected the build-up of natural enemy populations the following spring. Results indicated that a typical late summer or fall mowing is unlikely to cause major movements of insect pests into adjacent crops, and has minimal effects on populations of beneficial arthropods both after mowing and during the following spring.
The project has successfully made the case to NRCS that policy and CREP guidelines changes are needed in Maryland to encourage grass buffer practices that enhance beneficial arthropod communities. As a result, NRCS has revised its conservation cover practice standard to include new planting mixes with a wider variety of flowering forbs designed to encourage beneficial insects and has added a new CRP/CREP mid-contract management practice, which will allow eligible participants to receive cost-share to over-seed existing warm season grass buffers with native wildflowers.
Agricultural crops are influenced in many ways by the surrounding landscape. As part of the Conservation Reserve Enhancement Program (CREP), landowners in Maryland have established over 40,000 acres of herbaceous riparian buffers on the edges of working agricultural lands. The primary purpose of these buffer strips is to improve water quality, filter sediments from runoff, and provide wildlife habitat. To date, few studies have documented the extent to which grass buffers are used by wildlife or serve as habitat areas for insect populations. A research and demonstration project was initiated in 2003 to evaluate the influence of buffer width, vegetation composition, and timing of mowing on insect and bird communities. Our overall goal was to determine if riparian grass buffers could be modified in design and managed to enhance conservation of beneficial arthropod communities, reduce insect pest outbreaks, and provide better habitat for birds, without sacrificing their water quality functions. Funding for this project was provided by the USDA Sustainable Agriculture Research and Education Program, Maryland Center of Agroecology, Maryland Soybean Board, Marine-Estuarine-Environmental Sciences Program at the University of Maryland, and the USDA/ Natural Resources Conservation Service. Several studies of the project described below were conducted concurrently to address specific objectives related the enhancement of beneficial insect communities.
Target: Research findings on the farms of 30 farmers/landowners will support the hypothesis that wider riparian buffers of native, warm-season grasses with an appropriate level of bio-diversification can foster large multi-species populations of natural enemies as a source of immigrants into adjacent crops and also suppress pest populations that may originate from buffers.
Result: More than 38 farmer/landowners (participants) with a total of 40 cool-season and 79 warm-season grass buffers were engaged in field research or demonstration activities since the project started in spring 2004. Overall findings did not support the hypothesis that native, warm-season grass buffers are more sustainable for enhancing conservation biological control (see Results and Discussion for more details).
Target: Fifty % of the cooperators will transition to new design features of grass buffers, including warm season grass species and addition of flowering forbs, and at least 30% of all farmers/landowners in the CREP program will adopt these practices after the project is over.
Result: This performance target was not fully achieved by the end of the project for the following reasons. Most participants involved in the project already enrolled most of their highly erodible and environmentally sensitive cropland with buffers under a ten-year CREP contract; thus, there was less opportunity to add new buffers or change buffer designs and management practices that were not specified by the contract. We expect that significant changes in buffer practices will be implemented by participants if contracts are renewed and NRCS guidelines for buffer establishment and management are revised (see below).
Target: An 80% increase in over-seeding practices on existing buffers to increase bio-diversification with flowering forbs as nectar sources for natural enemies.
Result: Based on project findings, NRCS is in the process of revising their recommended conservation cover planting mixes to include a wider variety of flowering forbs. Specifically, three wildflower mixes will be recommended, each of which is adapted to different soil moisture conditions and include 10 or more species. The mixes are designed to encourage beneficial insect species and pollinators and flowering throughout the growing season. NRCS has also added a new CRP/CREP mid-contract management practice which enabled eligible participants to receive cost-share to over-seed existing buffers with native wildflowers. In the near future, these changes in the CREP guidelines instituted by NRCS will increase the floral diversity in grass riparian buffers.
Target: To change the design and management of cool-season grass buffers to avoid disruption of natural enemy communities. One third of the farmers/ landowners in the project with cool-season grass buffers will reduce their use of endophytic grasses, which can be detrimental to natural enemies, and two-thirds will delay mowing of buffers to avoid removal of nectar sources for pollinators and adult parasitoids.
Result: Seventeen of the 38 cooperators altered their mowing practices during the second and third year of the project to establish management treatments for demonstration and evaluation purposes. The findings of the mowing demonstrations indicate that the typical practice of mowing cool-season grass buffers in late August may have minimal adverse disruptive effect on natural enemies because many parasites and predators may already have emigrated due to the declining food quality of the senesced cool season grasses. Furthermore, we found little evidence that mowing in late August increases the risks of potential pest problems (particularly grasshoppers, spider mites, thrips) occurring in soybeans that may be originating from the grass buffers. Mowing non-CREP grassy areas (such as around drainage ditches and roadways) earlier in the growing season probably pose a greater risk for triggering pest outbreaks in adjacent crops. Finally, use of endophytic grasses is a non-issue in CREP-enrolled grass buffers because endophytic species are not allowed.
Target: To influence state and district soil conservation leaders to incorporate the information into policy, resulting in modified buffer design and best management guidelines.
Result: As stated above, NRCS has revised their conservation cover planting mixes and instituted a new CRP/CREP mid-contract management practice for encourage bio-diversification with flowering forbs. These guidelines will be included in the updated conservation cover practice standard (Code 327). In addition, the Maryland NRCS staff aims to increase the diversity of wildflowers in cool season buffers.
This research and demonstration project was part of larger effort initiated in 2003 to investigate the insect and bird communities associated with riparian grass buffers, with emphasis on the effects of buffer width, vegetation composition, and timing of mowing. Our goal was to determine if these buffer features could be modified to enhance conservation of beneficial arthropod communities, reduce insect pest outbreaks in adjacent crop fields, and provide better habitat for birds, without sacrificing the water quality functions of the buffers. The approaches, methods, and design of several related studies described below were conducted concurrently to address specific objectives of the overall project.
Effects of grass type. In this study, we examined the arthropod community in CREP grass buffers and the adjacent crops to address the following questions: (1) do warm season grass (WSG) buffers harbor greater numbers and diversity of arthropods than cool season grass (CSG) buffers, and (2) do crops adjacent to WSG buffers have greater numbers and diversity of arthropods than crops adjacent to CSG buffers? Specific emphasis was given to the influence of vegetation type on beneficial arthropods and their potential prey.
With help from county NRCS conservationists, grass buffer and field sites were chosen from a list of CREP participating landowners in Queen Anne’s, Talbot, and Caroline counties on the Eastern Shore of Maryland. With landowner permission and cooperation, study sites were distributed among 15 farm units. In 2003, seven buffers of each grass type adjacent to cornfields were sampled. In 2004, sites consisted of 13 WSG and 7 CSG buffers adjacent to corn, and 9 WSG and 15 CSG buffers adjacent to soybeans. Each buffer-field combination served as a replicate within each year.
Pitfall traps were used to measure the community diversity and abundance of epigeal arthropods. Yellow sticky cards were used to measure the relative abundance and richness of aerial arthropods in the crop and buffer canopy. In each buffer-field replicate, three transects of sampling sites were established perpendicular to the crop-buffer interface. Each transect consisted of three sampling sites; one in the buffer (6 m from the crop-buffer interface) and two in the crop (6 m, near site; 46 m, far site from the interface). A sticky card and pitfall trap were exposed at each sampling site for one 7 day period during the months of June and July in 2003, and June, July, and August in 2004. The vertical density and composition of vegetation at each sampling site were measured using a Robel pole and 1 m2 frame to estimate overlapping percent canopy coverage of grass species, forb species, standing dead grass, bare ground, and litter.
Evaluation of flowering perennials as insectary plants. It is generally accepted that the addition of flowering forbs to buffer grass mixes will enhance conservation biological control by providing pollen and nectar food sources for insect predators and parasitoids. Although the CREP guidelines list more than 25 perennials to add to seed mixes, few studies have assessed their competitiveness and potential as insectary plants. From this list, 25 species of flowering perennials adapted for the Mid-Atlantic area were selected and grown from seed in the greenhouse during the early spring of 2004. In late May, nine seedlings of each species were transplanted into two 1 m2 micro-plots within each of four replicate whole plots established in an orchard grass field. Each micro-plot was roto-tilled prior to transplanting to allow plants to become established. During 2004 and 2005, communities of arthropods were sampled weekly during the bloom period of each perennial species. The relative abundance of aerially-active arthropods was measured using yellow sticky cards placed in the center of each micro-plot at canopy level. The same sampling was conducted in the center of four additional plots consisting of orchard grass only to provide baseline activity of aerially-active arthropods in the absence of flowering perennials. Each micro-plot was also sampled by a modified leaf blower/suction device to collect arthropods both aerially-active and present on the disturbed plants. In 2005, when all surviving perennial species reached mature second-year growth, data were collected on the bloom period, floral area, and flower structure. In 2006, additional information was recorded on the competitiveness of each perennial expressed as percentage cover and average distance of spread beyond the plot.
Effects of flowering herbaceous plants. Additional on-farm studies were conducted in 2006 to determine if the presence of floral resources in grass buffers enhances beneficial insect communities in adjacent corn fields. Six WSG buffers with a mixture of flowering forbs which comprised 30% or more of the vegetative cover were sampled. The major forbs in these buffers were New England aster (Aster novae-angliae), heath aster (Aster pilosus), lance-leaved coreopsis (Coreopsis lanceolata), tickseed (Coreopsis tinctoria), goldenrods (Solidago spp.), lespedeza (Kummerowia spp.), and partridge pea (Chamaecrista fasciculate). All forbs were seeded as part of the original grass mix, except several species of goldenrods that were naturally seeded. As controls representing the absence of floral resources, six buffers with few flowering plants (<5% cover) were also sampled. All buffers were established for at least three years and were greater than 11 m wide and more than 150 m long. Each buffer with flowers was paired with a control buffer in the same locality (>5 km apart) as a replicate block. All study sites were located in Queen Anne’s and Talbot counties on the Eastern Shore of Maryland.
Pitfall traps and yellow sticky cards were used to measure the community diversity and abundance of arthropods in the epigeal and aerial communities, respectively. In each buffer-field replicate, three transects of sampling sites were established perpendicular to the crop-buffer interface. Each transect consisted of five sampling sites; one in the buffer (6 m from the crop-buffer interface) and four in the crop (12 m, 24 m, 36 m, and 48 m from the interface). A sticky card and pitfall trap were exposed at each sampling site for 7 day periods initiated on 2 June, 30 June, 31 July, and 21 August.
Effects of mowing. CREP requirements do not allow mowing grass buffers during the primary nesting season (15 April – 15 August) and recommend mowing on a 2 to 3 rotation (mow only 1/3 to ½ of the planting each year) to control woody growth and optimize wildlife benefits. Furthermore, the best time to mow is late winter to early spring. However, most landowners mow their grass buffers annually, particularly cool-season grasses, shortly after 15 August. We predicted that mowing at this time may force insect pests, such as grasshoppers and thrips, to move into crop fields. Furthermore, many flowering plants are in bloom in late August and thus mowing may reduce natural enemy communities by eliminating food sources and shelter and adversely affect the build-up of natural enemy populations the following spring.
To examine the effects of mowing on arthropod communities, two studies were conducted on selected grass buffers with the cooperation of the landowners who changed their management practices to allow for different mowing treatments. In 2004, eleven CSG buffers adjacent full-season or double-cropped soybeans were selected for study sites, all at least 10 m wide and 200 m long. One-half of the linear length of each buffer strip was mowed shortly after 15 August, while the remaining half was either mowed later in September or left un-mowed. This allowed for a paired comparison of the effects of mowing on both pest and beneficial insects. Sampling was conducted in the buffers and at 6, 26, and 46 m in the soybean fields along two transects (at least 75 m apart) perpendicular to the buffer-crop edge in the middle of each mowed or un-mowed section. As a control for comparison, additional samples were taken along transects positioned along a bordering edge in fields without a buffer but adjacent to the same type of habitat present along the buffer side of the field. Thrips density was estimated by counting the number of thrips on ten randomly selected soybean trifoliate leaves at each sampling site. The number of grasshoppers was estimated by sweep net sampling by walking parallel to the crop edge at each sampling site. Grasshopper abundance was also estimated by visually counting the number of adults and nymphs flushed from a 20 m2 area of plant foliage at each site.
A second mowing study was conducted in 2006 to examine the effects of late summer mowing on the increase and abundance of natural enemy populations during the following spring. In this study, three WSG and three CSG buffers were selected, all adjacent to corn fields and at least 30 m wide and 150 m long. Similar to the 2004 mowing study, a linear section of each buffer strip was mowed during late August or September, while the remaining section was left un-mowed. The abundance and diversity of arthropods was estimated by sticky cards and pitfall traps placed at three sites located 12 m from the buffer-field edge in the mowed and un-mowed sections of each buffer. Sampling was conducted over six weekly periods starting on 11 May. Sticky cards and pitfall traps were also placed 12 m from the buffer-field edge in each crop field starting on 8 June, after the corn reached the 3 leaf stage. Sampling was conducted at three sites in the corn fields adjacent to the sites in each mowed and un-mowed section of the grass buffer. This design allowed for a paired comparison of the effects of mowing on both buffer and crop communities of arthropods and the interaction with grass type.
Statistical analysis. The composition of arthropod communities was characterized by three community-related properties (number of taxa, total abundance, Simpson’s index of dominance and evenness). Tests for significant differences in community properties among grass types, crop habitats, sampling site, and sampling time were performed by mixed model ANOVA. Due to the complexity of the arthropod community data sets, multivariate techniques using redundancy analysis summarized simultaneously all taxa, thus allowing for the detection of patterns in the data and testing of buffer and crop influences on the arthropod community as a whole. For all studies, the most abundant taxa and trophic groupings (parasitoids, predators, prey, etc.) were analyzed separately using mixed model ANOVA.
As mentioned above, the intent of this project and the anticipated changes in buffer practices by participants did not materialize as planned. The basic premise that WSG buffers provide more sustainable habitat to enhance beneficial arthropod populations was not supported by the data collected in the early stages of the project. Secondly, although there was much participation by farmers/landowners, their ability to make changes to grass buffers during the project period was limited by specific conditions of their CREP contract. Thus, with additional funding from other agencies, we decided to focus research and demonstration studies on specific information gaps that would lead to improved NRCS guidelines for buffer establishment and management.
Effects of grass type. Our original thinking was that riparian buffers with native, warm season grasses provide more favorable habitat for arthropod populations, particularly natural enemies, because they are more diverse both structurally and in plant species, less stressed during the summer, and only moderately disturbed by management practices. Thus, the primary goal (milestone) was to transition farmers/landowners to establish WSG buffers to enhance conservation biological control. However, the weight of evidence did not support our original thinking and indicated that CSG buffers provide equivalent, if not better habitat for arthropods than warm season grasses.
In this study, a total of 104,926 individuals belonging to 100 families in 13 insect orders and 7 additional arthropod classes were sampled in the grass buffers. The abundance and diversity of arthropods were significantly influenced by the grass type but dissimilarities in community structure were greater for aerially-active arthropods than for epigeal (surface-dwelling) arthropods. CSG buffers supported 13% more taxa and 11% more abundant populations than warm season buffers, especially during June and July. Abundances of the majority of beneficial arthropod taxa were more closely associated with CSG buffers, particularly during June. The type of grass buffer also had a significant influence on taxa richness and abundance of arthropod communities in adjacent crops, particularly during June and July when many arthropods were colonizing fields. The total abundance of epigeal arthropods was 42% higher in cornfields next to CSG buffers compared to fields next to WSG buffers. For the aerially-active arthropods, taxa richness and total abundance were consistently higher in crop fields at sampling sites along field edges next to CSG buffers.
The weight of evidence indicated that CSG buffers provide equivalent, if not better habitat for arthropods than WSG buffers. Coupled with higher food quality, cool season grass species (i.e. orchard grass) green-up much earlier than warm season species in the spring and provide food resources for many herbivores and natural enemies. The current NRCS guidelines encourage mixes of WSG species with flowering forbs; however, the addition of flowering forbs to CSG buffers may be more effective to enhance conservation biological control by providing pollen and nectar for natural enemy populations in the early season when these food resources are most needed.
Evaluation of flowering perennials as insectary plants. Of the 25 flowering forbs evaluated, three were annual species (herb tansy, Tanacetum vulgare; sweet alyssum, Lobularia maritime; lacy phacelia, Phacelia tanacetifolia), selected because they are well-documented in the literature as proven insectary plants. Although they attracted many beneficial insects, all three species failed to re-seed and compete with the orchard grass after the first year. The perennial forbs showed a wide range of plant responses in terms of competitiveness, blooming period, accessibility of floral resources, and attractiveness to insect populations.
The strategy when planting insectary plants in grass buffer is to select competitive perennials or self-seeded annuals with overlapping bloom periods to provide floral resources throughout critical times. Bloom periods of perennials tested in this study ranged from early May to October. From midsummer until early fall, a number of flowering perennials (black eyed susan, Rudbeckia hirta; partridge pea, Cassia fasciculate; mist flower, Eupatorium coelestinum; Joe-pye weed, Eupatorium fistulosum; boneset, Eupatorium perfoliatum; New York ironweed, Vernonia noveboracensis; rough goldenrod, Solidago rugosa; heath aster, Aster pilosus; New England aster, Aster novae-angliae; and narrowleaf sunflower, Helianthus angustifolius) survived the original plantings and were competitive within the orchard grass field, as evident by the establishment of new plants within 5 meters or more around the 1 m2 microplots.
Over the two year period, a total of 300,553 individuals belonging to 115 families in 15 insect orders and arthropod classes were collected and identified from sticky cards placed in the canopy of the flowering forbs. Saprovores (mainly flies) were the most abundant trophic group captured (64.9% of total individuals), followed by herbivores (30.2%), parasitoids (3.5%), and predators (1.4%). The most abundant beneficial insects included: parasitic wasps (primarily families Scelionidae, Mymaridae, Encyrtidae, Trichogrammatidae, Pteromalidae, Braconidae, Ceraphronidae, and Aphelinidae, in order of abundance); parasitic flies (Phoridae, Sarcophagidae, Tachinidae), and predatory flies (Dolichopodidae, Syrphidae). Of the other insects that serve as hosts or prey, thrips (Thripidae), frit flies (Chloropidae), leafhoppers (Cicadellidae), aphids (Aphididae), and planthoppers (Fulgoroidea) were the most abundant.
The relative insectary value of each forb species was estimated by the adjusted weekly capture of beneficial arthropods on sticky cards during flowering, multiplied by the number of weeks in bloom. Each weekly capture was adjusted by subtracting the average capture in the orchard grass plots without flowers (control for background activity). Sticky cards placed in 14 of the 25 forb species attracted greater numbers of beneficial insects than the numbers captured in orchard grass without flowers. Forbs that provided abundant sources of accessible floral resources and consistently attracted the greatest numbers of beneficial insects included: rough goldenrod, butterfly weed (Asclepias tuberose), lance-leaved coreopsis (Coreopsis lanceolata), black eyed susan, boneset, partridge pea, gray goldenrod (Solidago nemoralis), Joe-pye weed, New York ironweed, and narrowleaf sunflower. These same forbs competed well in orchard grass, except for butterfly weed. Noteworthy is partridge pea which exhibited excellent insectary characteristics due to its extrafloral nectaries on petioles and leaflets, long bloom period, attractiveness to a diverse assemblage of insects, and competitiveness in grass mixes. Seeds of this native annual legume are also important food for bobwhite quail.
Other forbs, such as heath aster, New England aster, mist flower, purple coneflower (Echinacea purpurea), wild bergamot (Monarda fistulosa), and tickseed (Coreopsis tinctoria) attracted beneficial insects at levels that were either moderately above or below the levels of beneficial insect activity in orchard grass without flowers.
Many natural enemies emerging from overwintered stages in the spring need nectar and pollen sources to maintain their longevity and egg laying ability and thus their effectiveness in controlling pests. The spring blooming forbs (wild columbine, Aquilegia Canadensis; wild blue indigo, Baptisia australis; spiderwort, Tradescantia subaspera) in this study attracted relatively low numbers of beneficial insects and were the least competitive and unable to spread beyond the microplots. Further research is needed to find suitable perennial plants that bloom early enough to provide floral resources during this critical spring period.
Effects of flowering herbaceous plants. The on-farm study conducted in 2006 clearly demonstrated that the addition of flowering forbs in WSG buffers attracted and supported significantly more abundant and taxa-rich communities of beneficial insects, especially parasitic hymenopterans; and populations of these same taxa were significantly more abundant in the adjacent corn fields. Many families of parasitic wasps were active in the grass buffers, of which scelionid wasps (Scelionidae), fairyflies (Mymaridae) and trichogrammatid wasps (Trichogrammatidae) were the most predominant taxa. Parasitic flies (Phoridae, Sarcophagidae, Tachinidae) were less abundant as a functional group and many members of these fly families are not strict parasitoids. The overall population of parasitic arthropods in buffers with flowering forbs was 38.8% higher than populations in buffers without flowers. In corn fields, the combined abundance of parasitic taxa was 53.5% higher in fields adjacent to buffers with forbs compared to fields next to buffers without forbs.
The presence of floral resources had an even greater impact on populations of predaceous arthropods in the buffer and adjacent corn fields. The major predator taxa, including long-legged flies (Dolichopodidae), spiders (Araneida), hover flies (Syrphidae), minute pirate bugs (Anthocoridae), and lady beetles (Coccinellidae), were significantly more abundant in buffers with flowering forbs (101.3% more) and in corn fields adjacent to these buffers (75.6% more).
The ground-dwelling beneficial arthropods were spiders (primarily Lycosidae), ground beetles (Carabidae), ants (Formicidae), and rove beetles (Staphylinidae). Except for spiders, these major predators were not significantly affected by the presence of flowering forbs. The overall abundance of spiders was 89.1% higher in buffers with forbs and the adjacent corn fields, and significantly more spiders were captured in fields at sampling sites closest to the buffers. It is not surprising that epigeal predators showed little enhancement from the presence of forbs because they generally do not require floral resources for development and reproduction.
WSG buffers were selected for this study because they are more likely to include mixes of native perennial forbs than CSG buffers. The tussock-forming growth of native warm season grasses generally allows more space for forbs to compete. However, for the purpose of enhancing conservation biological control, the addition of spring blooming forbs to CSG buffers may be a better way to enhance early season build-up of beneficial arthropod populations. Also, cool season grasses green-up earlier in the spring and provide food resources for many herbaceous insects that can serve as prey or hosts for natural enemies. As mentioned above, the problem with this approach is finding a suitable perennial that bloom early enough and is competitive with cool season grasses.
Effects of mowing. The 2004 mowing study involving 11 CSG buffers adjacent to soybean fields allowed for a paired comparison of the effects of mowing on selected pest insects. Thrips are an occasional economic pest of soybean, especially during the seedling stage. As expected, there was a significant gradient in thrips density along the sampled transects (highest densities close to field edges next to the buffers), whereas densities in the non-buffer side of fields showed no consistent gradient or edge effect. Our studies showed that thrips are the major herbivores of CSG buffers in terms of abundance; thus, CSG buffers have the potential to serve as a source of thrips colonizing soybean fields. However, there was no evidence that mowing after August 15 forced additional thrips into the soybean field. Thrips populations in soybeans adjacent to the mowed and un-mowed sections were not significantly different.
Grasshopper populations were very low in 2004, so data were too variable to detect differences. However, there was a consistent gradient in grasshopper densities related to the distance from the soybean field edge (more abundant at 20 m from the field edge). Mowing significantly reduced the number of grasshoppers in the CSG buffers but there were no significant trends in densities observed across the in-field locations between mowed and un-mowed sections. Unlike our predictions, mowing did not trigger any unusual insect pest outbreaks in soybeans. Considering that mowing occurs later in the growing season when crops such as corn and soybean are less vulnerable to foliage feeding insects, CREP grass buffers are probably less likely to trigger pest outbreaks compared to roadsides and other non-CREP grassy areas around crops that are often mowed earlier in the growing season. Furthermore, the typical practice of mowing CSG buffers in late August may have minimal adverse disruptive effect on beneficial arthropods because many parasites and predators may already have emigrated due to the declining food availability as a result of the senesced cool season grasses. Thus, it is possible that mowing CSG buffers could force natural enemies into adjacent crop fields to help suppress late season pests.
A second mowing study conducted in 2006 evaluated the effects of mowing on the build-up of beneficial arthropod populations the following spring. In this study, both CSG and WSG buffers were compared, so additional data were collected on the effects of grass type on arthropod communities during the spring. In agreement with results above, aerial-active populations of parasitoids and predators in CSG buffers increased in abundance and taxa diversity significantly much earlier than populations in WSG grasses. Overall densities of beneficial taxa were 80.3% higher in CSG buffers during late May through June, particularly populations of parasitic hymenopterans. Similarly, insects that provide food for natural enemies, such as thrips, aphids, plant bugs, and fly saprovores, were also significantly more abundant in the CSG buffers (overall 128.2% higher compared to available prey or hosts in WSG buffers). The sap-sucking leafhoppers and planthoppers comprised the only major taxonomic group that was more abundant in the WSG buffers. Many of these insects are host-specific and have been associated with native prairie grasses over a long period of evolutionary time.
There was no evidence that mowing in late summer had any adverse effect on aerially-active arthropod communities during the following spring, regardless of the grass type. Spring regrowth in mowed and un-mowed sections of CSG buffers was practically the same by mid May, when insect populations began to increase. Likewise, because warm season grasses break dormancy later in the spring, there was no apparent difference in spring regrowth between mowed and un-mowed sections of the WSG buffers.
Pitfall captures of surface-dwelling spiders and predaceous beetles were also significantly higher in the CSG buffers, and these differences in densities were consistent through May and June. Also, there was a significant interaction effect between mowing and grass type for spider densities. The numbers of spiders captured in un-mowed sections of CSG buffers were 53.2% higher than captures in the mowed sections, whereas mowing had no effect on spider populations in the WSG buffers.
Altogether, results indicate that the typical late summer or fall mowing is unlikely to cause major movements in insects into adjacent crops, which could lead to economic pest problems, and should have minimal effects on populations of beneficial arthropods both after the mowing and during the following spring.
During the project period, three training events were conducted at annual planning workshops for county and district NRCS personnel to explain the objectives and outcomes of the project. More than 85 conservation planners received information on how riparian grass buffers can affect pest populations both positively and negatively, and management practices that can enhance sustainability of grass buffers as habitats to conserve beneficial arthropods. Attendees were asked for constructive feedback on the feasibility of the proposed changes in grass buffer design and management.
A three-hour workshop was conducted at the Future Harvest Annual Sustainable Agricultural Conference held in Hagerstown, MD in January 2005. Twenty-five participants, primarily organic farmers, received training on arthropod natural enemies. Though the workshop focused on the biology, identification, and practical use of natural enemies, emphasis was given to flowering annuals and perennials in non-crop habitats to provide alternate food and shelter for natural enemies.
A training presentation on ways to optimize benefits of riparian grass buffers with emphasis on conservation biological control was given in two separate sessions at the Mid-Atlantic Crop Production School in Ocean City, MD in December 2006.
Additional 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, 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.
Several meetings with state NRCS conservationists were held at the Annapolis office to present project findings and recommendations for revisions of conservation cover guidelines. Results of the project were also presented to the Maryland CREP Advisory Committee. Discussions at these meetings led to the policy changes mentioned above, which will have major impact on the design and management of riparian grass buffers in the future.
Other sources of funding were sought for and awarded to support additional studies related to the SARE riparian grass buffer project. During the first year of the project, the Maryland Center of Agroecology provided approximately $14,000 to help defray the costs of a graduate assistantship and summer technical labor. During the second year, the Marine-Estuarine-Environmental Sciences Program at the University of Maryland also provided a full-year graduate assistantship plus tuition remission for a graduate student who was primarily responsible for evaluating the effects of grass type on buffer arthropod communities. This work was published in a M.S. thesis in June 2008. Two grants, one for $100,000 and a follow-up supplemental one for $18,000, were obtained from the NRCS Wildlife Habitat Management Institute to conduct studies on the use of riparian grass buffers by breeding and wintering birds. Supported by this grant, another Ph.D graduate student has addressed the following questions: Which bird species are using riparian grass buffers in summer and winter? Which birds prefer buffers containing warm season grasses versus cool season grasses? How does vegetation composition and structure affect bird response? How does buffer width affect bird response? Outcomes of this research will include a comprehensive assessment of the bird use in riparian grass buffers, as well as management recommendations designed to meet specific bird conservation objectives. Another grant from the Maryland Soybean Board for $14,529 was provided in 2006 to determine if certain insect pest problems in soybeans are linked to riparian grass buffers.
Altogether, this SARE project has leveraged more than $175,000 of additional direct and indirect funding to support related studies that will optimize the environmental benefits of riparian grass buffers.
Additional Project Outcomes
Impacts of Results/Outcomes
The general policy in CREP planning has been to encourage mixes of native grasses and flowering forbs in riparian buffers, whereas cool season buffers are usually planted mixed only with legumes. There is abundant evidence from other studies that non-cropped grass areas can enhance biological control in agricultural crops. The most striking finding of this study was that non-native, cool season grass buffers do in fact support more abundant beneficial arthropod communities early in the season and thus may actually be more sustainable for enhancing conservation biological control. Therefore, the addition of perennial flowering forbs to cool season grass buffers may be a more effective way to enhance conservation biological control by providing pollen and nectar for natural enemy populations in the early season when these food resources are most needed.
Numerous studies have shown that higher plant biodiversity in landscapes around crop fields provides a more sustainable habitat for beneficial arthropod communities. Results of this project clearly demonstrate that adding flowering forbs to buffer grass mixes can enhance conservation biological control by providing pollen and nectar food sources for insect predators and parasitoids. However, these studies point out that the specific flower species and its ability to compete with grasses, the intensity of floral biodiversity, and the timing of floral resource production should be given careful consideration when designing grass buffers to enhance biological control.
During the project period, participating farmers/landowners made few changes in buffer management practices, besides those manipulations for research and demonstration purposes. However, more than 25 participants who manipulated practices were exposed to new ways of thinking about how their grass buffers could be modified in design and managed to enhance conservation of beneficial arthropod communities. Along these lines, the project has successfully made the case to NRCS that policy and CREP guidelines changes are needed in Maryland to encourage buffer practices that enhance beneficial arthropod communities. As a result of these studies, NRCS is in the process of revising its conservation cover practice standard code 327, which will list new planting mixes including a wider variety of flowering forbs designed to encourage beneficial insects and provide floral resources throughout the growing season. NRCS has also added a new CRP/CREP mid-contract management practice, which will allow eligible participants to receive cost-share to over-seed existing WSG buffers with native wildflowers. In addition, the Maryland NRCS staff aims to increase the diversity of wildflowers in CSG buffers, based on project findings that these buffers may be more effective in providing floral resources and prey/hosts for natural enemy populations in the early season.
As explained above, the long-term contribution of this project will be NRCS policy changes and revisions in the CRP/CREP guidelines that will ultimately increase farmer/landowner adoption of buffer conservation practices leading to enhanced biological control.
Areas needing additional study
Given the current focus on conservation practices throughout the Mid-Atlantic area, there is great opportunity to examine and implement practices that increase the environmental benefits of CREP grass buffers. In the context of conservation biological control and integrated pest management, there is a clear need for further studies to examine the movement of beneficial arthropods between grass buffer and crop habitats. Studies of arthropod communities in grass buffers will be most informative if the dynamics of population interaction with crop habitats are taken into account. With regard to mixes of flowering forbs, there is an urgent need to identify native wildflowers that provide accessible floral resources during the spring and develop management practices to sustain these forbs in both WSG and CSG buffers.