A comparison of carabid beetle assemblages between grassy field margin habitats and potatoes found that these predators of crop pests were more abundant and diverse in grassy habitats. At broader scales beetle communities in landscapes that included large amounts of natural habitat were not as dominated by common crop species as those sampled from agricultural landscapes. Predation on green peach aphids in potatoes also increased as the amount of natural area within 1 km increased. These results suggest that conservation of natural areas adjacent to crops may enhance biological control of crop pests by natural enemies.
In the last 50 years agriculture has dramatically increased yields and the ability to support an ever-increasing human population. However, this increased capacity has relied in large part on significant human inputs including synthetic fertilizers and pesticides. This intensification of agriculture has come at a significant cost. For example, declines in biodiversity have been linked to modern farming practices. This erosion of biodiversity has resulted in the disruption of ecosystem services, such as natural pest suppression by arthropod predators and parasitoids (natural enemies) (Altieri, 1999). Our ability to increase the long-term sustainability of our agricultural practices will be dependent on our ability to modify farming practices so that natural pest control makes high external inputs of pesticides unnecessary. Increasing the amount of natural land surrounding crop fields, a form of “conservation” biological control, may be one way to enhance natural pest control.
Natural areas adjacent to crops (such as grassy field margins) may enhance pest control because they can provide important resources needed by natural enemies that are not available within the disturbed, simplified environment of crop fields (Wratten et al., 1998). This has shown to be the case in many instances and has led to the adoption of “habitat management” strategies as a form of conservation biological control (Landis et al., 2000). For example, “beetle banks” are perennial grassy strips added to fields that enhance predatory ground beetle (carabid) abundance and diversity. These predators then disperse into adjacent crops and consume insect pests such as aphids. (Wratten et al., 1998; Collins et al., 2002).
In addition, a number of studies have found that natural enemy populations are influenced by the abundance and diversity of natural habitats at scales much larger than individual farms and their immediately adjacent lands (Elliot et al., 2002; Menalled et al., 2003; see Tscharntke & Brandl, 2004 for other examples). For example, parasitism by natural enemies of pests of oilseed rape shows a strong positive correlation with the amount of natural habitat present within 1.5 km of the crop (Thies & Tscharntke, 1999; Thies et al., 2003). These findings suggest that large-scale, cooperative habitat management efforts by growers may be more effective than uncoordinated, individual efforts.
A unique opportunity to use natural areas to enhance biological control for insect pests now exists in Wisconsin. Potato growers in the state have recently adopted an intensive bio-IPM program that has reduced pesticide applications by 20% through the use of economic thresholds and pest monitoring tactics. In addition to this, a number of growers have also begun restoring natural habitats adjacent to fields. Growers are currently seeking advice on how to restore natural habitats so as to increase biodiversity on their lands. However, the current state of knowledge is inadequate to understand the role that natural habitats play in agricultural landscapes and the spatial scales at which they enhance pest control. We do not understand the degree to which pest control can be enhanced by conserving natural areas. Furthermore, it is unclear whether growers should focus on conserving habitats immediately adjacent to fields (“field margin” habitats) or at a landscape (kilometer) scale to enhance pest control. The research described here aimed to determine if natural habitats (created or naturally-occurring) influence native biodiversity of natural enemies and if this diversity results in improved pest control. We also conducted research at a variety of spatial scales to determine the scale at which habitat conservation should be practiced. Filling in these important knowledge gaps will be needed in order to make conservation biological control a more reliable and effective strategy.
The goal of our research was to determine whether grassy field margin habitats adjacent to potato fields (Objective 1) and natural areas in the broader landscape (Objective 2) support natural enemies that control potato pests. Grassy field margin habitats are found along the edges of field access roads and ditches and in the corners of fields where irrigation pivots cannot reach. Plant communities vary widely in these areas but are dominated by grasses ranging from non-natives like smooth brome grass, Bromis inermis, to native prairie grasses like big bluestem, Andropogon gerardii. Examining natural enemy communities in these areas enabled us to contrast results at a small spatial scale (crop and adjacent habitat) with larger, landscape-scale analyses (crop and habitats within kilometers distance) to provide information on the scales at which conservation biological control tactics are effective. Thus, our research was designed to determine: (1) whether conservation of natural areas can improve biological control and (2) the scales at which such strategies are effective.
Towards these ends, our research was designed to meet the following objectives and test their associated hypotheses:
Objective 1: Evaluate the role that grassy field margins play in supporting populations of natural enemies that control potato pests.
Hypothesis 1: We hypothesized that grassy field margins serve as reservoirs of a diverse and abundant natural enemy assemblage that can control potato pests. Thus, we expected that natural enemies are more abundant and diverse in grassy field margins compared to crops.
Objective 2: Ascertain if the amount of natural habitat in the landscape surrounding fields influences predation on insect pests in potatoes.
Hypothesis 2: We hypothesized that a greater abundance and diversity of natural enemies in fields surrounded by extensive natural habitat results in increased pest suppression within these fields.
Sampling natural enemies:
We sampled natural enemy populations in 2005 in 19 potato fields and their adjacent grassy field margins. Fields were chosen with a stratified random sampling scheme so that an equal number were set in landscapes (here defined as the area within 1 km of sample sites) composed of the following percentages of natural habitat: (1) 0-20%, (2) 21-40%, (3) 41-60%, (4) 61-80% and (5) 81-100% natural habitat. Landscape analyses were conducted with the aid of a GIS using satellite-derived land cover data with a 30m resolution (National Land Cover Dataset, 1991). Natural areas were defined as terrestrial, non-agricultural and non-urban habitats such as forests, grasslands and wetlands. Land classifications were assigned using ground-truthed National Land Cover Dataset land cover categories. Sampling grassy and potato habitats in these varying landscapes allowed us to use natural enemy data to simultaneously evaluate the role of immediately adjacent natural areas (grassy habitats, Objective 1) and natural areas in the broader landscape (Objective 2) in supporting natural enemies.
We focused on sampling populations of ground beetles (Coleoptera: Carabidae) because they are diverse and abundant and can be important natural enemies of potato pests (Hough-Goldstein et al., 1993). Carabids were sampled using pitfall traps consisting of 16 oz cups filled with propylene glycol and sunk level with the ground. To census carabid populations, four pitfall traps were located along three randomly selected edges of each potato field. Along each edge, we placed two pitfall traps in grassy field margin habitat and one in the edge and center (100m away from the edge) of the adjacent potato field (for a total of 12 traps per field). These were emptied every other week beginning on 28 June and ending on 30 August 2005. Carabids were then counted and identified to species.
All carabid data presented here consist of abundances averaged across the entire season for each site. We quantified diversity as the number of species encountered at each site during the season (species richness) and as the average Shannon diversity of ground beetles captured in the different locations. Shannon diversity is reported because it is a simple, univariate statistic that quantifies the evenness of communities in addition to their species richness. Furthermore, it highlights trends in our data that mirror results of the more comprehensive but complex multivariate analysis we will present in publication. Finally, we also constructed rarefaction curves to compare species richness between grassy and potato habitats while correcting for differences in the numbers of beetles caught between these areas. Rarefaction curves were constructed by plotting the mean number of species found for samples of size 1 to n, where n indicates the total number of pitfall samples taken during the season. We then transformed the X-axis by replacing sample size with the mean number of individual beetles found in a sample of size n (Means were from randomized sampling of data). These plots allow comparison of species richness between locations while correcting for bias in species richness estimates due to differences in the number of individuals captured (Gotelli & Colwell, 2001).
Natural areas and predation on potato pests:
To meet our objectives, we also determined the ability of natural enemies from natural habitats to reduce populations of potato pests. To accomplish this, we quantified predation on “sentinel” pest prey in grassy habitats and potatoes in 2005 and 2006. In both years, fields were chosen with the same stratified sampling scheme described above so that the landscapes surrounding fields ranged from being completely dominated by agriculture to being dominated by natural habitat. This allowed us to determine if predation in crops increases as more natural areas are included in agricultural landscapes (Objective 2).
To accomplish this, we measured predation on two of the main pests of potatoes, Colorado potato beetle, Leptinotarsa decemlineata, and green peach aphid, Myzus persicae, in grassy and potato habitats. To evaluate predation, we deployed potted plants (see below) infested with each pest in the potatoes and grassy field margins at each site. A portion of these plants were covered with 23 cm x 46 cm high mesh bags constructed of No-Seeum tent mesh to exclude natural enemies. After two days, plants were collected and the number of prey remaining was used as a measure of predation. Mortality of pests on bagged plants was used as a covariate in all analyses to factor out mortality due to factors other than predation.
Predation on Colorado potato beetle (CPB) egg masses was evaluated in 23 potato fields from 2-14 June 2006. To evaluate egg predation, we infested 18 cm-tall potato plants, Solanum tuberosum, with one CPB egg mass containing a known number of eggs. Egg masses were collected from a laboratory colony, were 24-48 hours old, and were punched out of potato leaves with a 1.5 cm diameter cork borer. The resulting leaf disc was glued to the underside of a leaf on the middle of each plant using Elmer’s glue. Six of these infested plants were placed along two randomly selected sides of each potato field in the grassy margin and the edges and centers (50 m from the edge) of the adjacent potato crop (for a total of 18 plants per field side and 36 plants per field). Eggs were exposed to predation for 48 hours and then returned to the laboratory where survival and mortality was quantified by measuring (1) the proportion of the initially deployed eggs that hatched into CPB larvae in the lab and (2) the proportion of eggs eaten by predators with sucking mouthparts. Predation by fluid-feeding predators such as hemipterans can be quantified because, in contrast to chewing feeders, they leave behind a collapsed eggshell after feeding so that individual predation events can be quantified. Thus, it represents an easily quantified subset of the predation events that may have resulted in hatch rates differing between fields.
Predation on green peach aphid (GPA) was evaluated in 16 potato fields during 15-22 August 2005 and 8-21 August 2006. In 2005, we deployed ten, 20 cm-tall potted cabbage plants (Brassica oleracea var. ‘Michihili’) infested with 25 aphids in each field. Cabbage plants are one of the many hosts of GPA and are ideal hosts for rearing these aphids. Four unbagged cabbage plants and one bagged plant were placed in the grassy areas and potatoes at each site. The number of aphids remaining on bagged and unbagged plants after 48 hrs was then recorded and used as a measure of predation. A similar method was used to assess aphid predation in another 16 fields in 2006, except that we placed 5 bagged and unbagged plants in the grass and potatoes at each site and used 18 cm-tall potted potato plants in place of cabbages. We used potato plants in 2006 because they were hardier than cabbages under field conditions.
To test our hypotheses, carabid abundance and diversity were compared between grassy areas and potatoes (Objective 1) and carabid community structure and predation were correlated with the amount of natural habitat within 1 km of each potato field (Objective 2). For landscape level analysis, we calculated the percentage of the landscape within 1 km of each field that was composed of natural habitat and determined if this was positively correlated with biological control. Multiple regression models were used to test how different independent variables (grassy vs. potato, and %natural area), influenced the dependent variables (natural enemy populations and predation). Analyses were performed in JMP 5 (SAS Institute, 2003). For analysis of natural enemy communities and CPB predation, a Bonferroni correction was used to control experimentwise error at alpha = 0.10 because we tested for the significance of treatment effects on multiple response variables calculated from the same data. Finally, we used the BEST feature of PRIMER community analysis software to determine which carabid species were most affected by landscape structure (Clarke & Warwick, 2001). This application ranks sample sites for the abundance of each species and determines which species are most associated with an environmental variable (in this case, landscape structure) using Spearman rank correlation coefficients.
Carabid diversity in Wisconsin potatoes:
We found 117 species of carabids in pitfall traps including two that were new records for the state of Wisconsin. The most common species included small (about 1 cm long) predatory species such as Bembidion quadrimaculatum oppositum and B. obscurellum, and slightly larger (about 1.5 cm long) omnivores such as Poecilus lucublandus lucublandus and Agonum placidum. All of these are capable of flight and typical of open habitats such as grasslands, cultivated fields and riverbanks (Larochelle & Lariviere, 2003).
Grassy field margin habitats as reservoirs of natural enemies:
Overall, our data suggest that grassy field margin habitats may be important in maintaining ground beetle diversity in the Wisconsin potato agroecosystem. Significantly more carabid species were supported by grassy compared to potato habitat. Accordingly, rarefaction curves show that this effect is not due to differences in the number of individuals caught in these two habitats. Shannon diversity of carabids was also greater in grassy habitats, which suggests that, in addition to supporting more species, grassy areas also support communities with fewer dominant species. Overall, these data suggest that these grassy field margin habitats may play a significant role in maintaining carabid diversity in the Wisconsin potato agroecosystem.
Landscape composition and biological control:
We found some evidence that the amount of natural habitat in the 1 km landscape surrounding fields influences the structure of carabid communities in potatoes. In particular, the Shannon diversity of carabid communities increased within potatoes as the amount of natural habitat surrounding fields increased. This, and the fact that species richness of carabids did not increase, suggests that the carabid communities in potatoes became less dominated by superabundant species in landscapes with more natural area. We found that abundance of two of the most dominant carabids, B. quadrimaculatum oppositum and B. obscurellum, was negatively associated with the amount of natural area surrounding crops, while another species, Amara aenea, showed the opposite trend. A. aenea climbs plants to feed on seeds in addition to being a predator (Larochelle & Lariviere, 2003). Thus, availability of seed-bearing plants in off-crop natural areas could be important in maintaining its populations in potatoes. The other two species are typical of open lands and are primarily predators (Larcochelle & Lariviere, 2003). Therefore, landscapes containing large amounts of cropland may support large populations of these species. This suggests that landscape structure at this scale may be important for pest control if key predators depend on resources in natural areas. However, we do not currently understand the role of these carabid species in regulating populations of potato pests, making it difficult to assess the importance of these changes for pest control. We did not find any evidence that overall abundance of carabids changed with landscape structure at 1 km.
We found evidence that landscape structure affected predation rates on GPA in 2005, but found little evidence that it influenced predation on CPB eggs during 2006. Predation on aphids in 2005 increased as the amount of natural habitat surrounding fields increased. The percentage of aphids recovered from plants significantly decreased as the amount of natural area in the 1 km surrounding fields increased (F1,27 = 4.35, P = 0.05). However, this relationship was not significant in 2006 (F1,24 = 0.16, P = 0.69). Hatch rates of CPB eggs and predation on eggs by fluid-feeding predators were not significantly associated with landscape structure at this scale (P > 0.20 for landscape main effects and interactions).
Overall, our current analyses suggest that the amount of natural habitat within 1 km of fields can influence natural enemy communities and predation under certain conditions. However, this association was not as consistent or strong as the differences in carabid populations we observed between grassy and potato habitats. It is possible that these near-field habitats may be more important as reservoirs of natural enemies than natural areas more distant from fields. In order to evaluate this, we will complete this analysis by determining if the amount of grassy habitats surrounding fields is correlated with natural enemy abundance, diversity and pest control in potatoes.
The results of our landscape-scale analysis may also be modified upon its full completion. We are currently creating an up-to-date land cover map for the study region based on 2005 air photos, which will provide more accurate landscape variables than the 1991 data used for the preliminary analyses reported here. Secondly, we will also correlate natural enemy community structure and predation in crops to the amount of natural habitat surrounding fields at a variety of scales. Differences in the strength of these correlations across different scales have been found in other studies (Thies & Tscharntke, 1999; Thies et al., 2003). We may also find that landscape structure at distances less than or greater than 1 km (the scale analyzed here) influences pest control.
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Educational & Outreach Activities
Werling, B.P. and C. Gratton. 2004. Non-crop land and ecosystem management. World Wildlife Fund/Wisconsin Potato Vegetable Growers Association/University of Wisconsin Collaboration Executive Committee Meeting, Madison, WI.
Gratton, C. and B.P. Werling. 2005. Landscape structure and arthropod diversity in potato agroecosystems. Wisconsin Potato Vegetable Growers Association Grower Education Conference, Stevens Point, WI.
Werling, B.P. and C. Gratton. 2006. Contribution of natural areas to pest control in Wisconsin potatoes. Annual Meeting of the North Central Branch of the Entomological Society of America, invited presentation, Bloomington, IL.
Werling, B.P. and C. Gratton. 2006. Grassy field margins as reservoirs of natural enemies of Wisconsin potato pests. Annual Meeting of the Entomological Society of America, Indianapolis, IN.
Publications in preparation
Werling, B.P. and C. Gratton. 2007. Landscape structure affects the composition of carabid communities in crops at multiple scales. Agriculture, Ecosystems and Environment.
The research described here has been conducted on the farms of Wisconsin potato growers. After each summer, I have completed a brief summary of our results, distributed it to participating growers, and had face-to-face discussions about their implications. I have also had informal conversations with them throughout each summer. These have provided excellent opportunities to get feedback on the work we have conducted and disseminate its results. In addition to these informal outreach efforts, we also plan on publishing our results in the trade magazine of the Wisconsin Potato and Vegetable Grower’s association.
Our work has made Wisconsin potato growers aware of how off-crop natural habitats can influence pest control. In our discussions with them, we find that many growers are interested in this and the possible positive or negative effects of conserving natural areas near to their crop. Because of this, the work reported here has helped to expand how they think about managing their land. Completion of this research and my other dissertation work will allow us to make more formal recommendations in regards to the appropriate scale of management of off-crop natural areas for pest control.
No economic analysis conducted.
No farmer adoption information conducted.
Areas needing additional study
There are a number of specific areas of study that would greatly aid commercial growers in their decision to conserve off-crop natural areas. Firstly, it will be important for researchers to determine how in-field management practices influence movement of natural enemies between natural areas and crops. Natural areas may support large populations of natural enemies, but if they do not enter crops to feed on pests their impact on pest populations will be minimal. Secondly, it will be important to ascertain how pests use off-crop habitats. It will also be important to document other benefits of off-crop natural areas to encourage their conservation. Finally, we will need to investigate the socioeconomic factors influencing conservation so that any strategies that are designed are adopted and have a lasting impact.