Landscape Ecology of Insects and Arthropods
Gratton Lab
Insects are a critical component of agroecosystems. As pollinators and predators, they provide valuable ecosystem services, and as pests they significantly impact crop yields. We study the roles that insects play within agroecosystems and how agricultural management and land-use patterns affect the abundance and diversity of these insects. We examine how farm management decisions, including within-farm plant diversity, pesticide use, fertilizer applications, and weed control, affect insects within and surrounding the agroecosystem.
Additionally, we are interested in how land-use patterns, including habitat fragmentation and loss of natural habitat, affect insect populations and the services that they provide. Examples of our work in agroecosystems include: Examining how landscape composition surrounding a farm affects populations of wild bees and pollination rates, studying how pesticide use impacts beneficial as well as pest insects, exploring the relationship between landscape composition, biological control services, and pesticide application, and studying how within-farm plant diversity affects natural enemy communities and rates of predation or parasitism.
The snow is slowly but surely melting and we are starting to think about the upcoming field season. Here in the Gratton Lab, we do most of our field work in agricultural settings. One crop we work in is corn. For those of you who didn’t grow up on a farm, you might not have had the opportunity to run through a corn field. We want to share that experience with you. This short video gives you a glimpse into a day in the field with the Gratton Lab.
If you noticed higher prices or fewer raspberries at your local farmer’s market this fall, you’re not alone. Wisconsin raspberries faced a new pest, Spotted Wing Drosophila (Drosophila suzukii) this year and the big question is: where is it now and will it return?
Native to Southeast Asia, Spotted Wing Drosophila (SWD) is a fruit fly with a sclerotized ovipositor capable of piercing the skin and successfully laying eggs in ripening fruits. The fly was first described in Japan in 1916 and detected in Hawaii in 1980. By the mid-2000s, SWD was in central California & spread rapidly throughout the Pacific Northwest and Florida. In 2010, SWD was in the Southeastern US, Michigan, and Wisconsin, but no significant crop damage was reported in Wisconsin until August 2012 when it was detected by raspberry growers in 17 counties.
While other fruit flies rely on fermenting or damaged fruit, SWD’s ability to attack ripening fruits can cause total crop loss, making it a potential pest for raspberry, blueberry, blackberry, grapes, strawberries and cherries in Wisconsin. Other fruit varieties at risk (e.g. elderberry, aronia) have been gaining popularity, especially in the Driftless area where growers are trending towards higher value perennial crops to increase sustainability and income. Wineries, farm stands, and value-added products are major tourism draws to the Driftless area, as well as other fruit growing regions in Wisconsin such as the southern shore of Lake Superior and Door County, and these customers tend to have a zero to low tolerance for insect larvae in fruit, so minimizing infestations are critical.
SWD are generalists which means they can jump from host-to-host throughout the year depending on ripening fruit availability. As many Wisconsin vineyards and small fruit operations are part of diversified farms, they offer the perfect season-long availability of food. Together, these characteristics may create ideal conditions for high local population growth and significant crop damage in the Upper Midwest. Current recommendations are limited to culling fruit and heavy season-long pesticide sprays, which simply are not options for the many growers who use organic, IPM or no-spray practices.
How did Spotted Wing Drosophila reach Wisconsin? The two main theories are 1) summer winds blow the flies in from southern locales 2) local overwintering. Field monitoring and laboratory tests have confirmed that SWD can overwinter in California and the Columbia River Gorge in Oregon. While Wisconsin winters are usually much harsher than the Pacific Northwest’s, the ability of small protected populations to survive (in thicker-skinned wild fruits, leaf litter, soil, heated buildings) may be enough to cause or supplement annual outbreaks.
To help answer the overwintering question, the new fruit crop entomologist, Christelle Guedot and I have constructed 21 apple cider vinegar traps at 5 locations in Dane County with confirmed infestations at small fruit farms, natural areas, and raspberry high-tunnels. We will continue monitoring traps throughout the winter months to detect any flying adult presence which indicates overwintering in Southern Wisconsin. If adults are overwintering, they may fly on warm, sunny winter days, so although our traps are few, they may be one of the only attractive ‘fruits’ available in the dead of winter. Our first month of trapping (mid-December thru mid-January) only trapped two male SWD in a fall-infested high-tunnel; high-tunnels may be especially at risk for infestation due to their controlled mild temperatures and weather protection. Our second month (mid-Jan thru mid-Feb) trapped no SWD. Starting in April, in a partnership with DATCP, we will be coordinating a farmer-based monitoring program at 15+ farms statewide to track population trends.
So will we see SWD this year? We suspect a combination of benign overwintering conditions, 2012 infestation status and landscape effects will determine if any particular county or raspberry patch will face infestation. Monitoring this year will show if SWD is overwintering and help growers make early management decisions.
During the past week, the Gratton Lab members attended the annual Entomological Society of America national conference in Knoxville, TN. We had two car loads drive the 12 hours down to TN , with both presenters and non-presenters in attendance. This was the first time some of us (Maddy, Emma, Kaitlin) had been to this particular conference and thus did not present. However, it was a valuable experience simply to gauge how things worked and to see what makes a successful, and sometimes unsuccessful, presentation of science. Communicating science to a broad audience can be very difficult, and we were able to pick up some pointers by listening to the vast diversity of presentations (in addition to learning about some cool new research!)
The Gratton Lab was well represented- Claudio, Jamin, Hannah and Rachel gave talks, and Julia presented her poster. Everyone did a great job. Below are the titles of their presentations:
Claudio: To what extent do native pollinators contribute to fruit production in Wisconsin?
Hannah: Assessing the pollination requirements of a perennial crop
Rachel: Impacts of local and broad-scale landscape structure on the diversity of pollinators in Wisconsin agroecosystems
Jamin: Aquatic insects have positive indirect effects on terrestrial prey
Julia: Species-specific physiological response of common Coccinellidae to the impact of landscape composition
Heidi, David, and Erica were also at the conference, which was a nice reunion for the Gratton lab. Another benefit of going to these national conferences is to catch up with past lab members and friends. We were able to have an alumni dinner together on Tuesday night, enjoying each other’s company and the beautiful surroundings of Knoxville. All in all this was a successful conference for everyone who came! Below are some of the few photos we took…
Check out the Huffington Post article on biological control that features research by Tim and Claudio.
Here, Mitch Teich, host of the Lake Effect on Milwaukee Public Radio, talks with Tim about how characteristics of the landscape affect natural pest control by beneficial insects.
Yesterday the annual Biology 152 poster session took place in Union South, and three of the more than three hundred and sixty posters in Varsity Hall came from the Gratton Lab. Heidi, Rachel, and I have been mentoring undergraduates through independent research as part of this course. It is a lot of fun and very rewarding to work with undergraduates and help them to understand what “real” science is all about. I think that I can speak for all of us, mentors and mentored, when I say that this has been a very very rewarding experience!
Jamin
Valentine and I.
Heidi and Darin.
Rachel and Melissa.
When are multiple species better than one? This question has intrigued ecologists for over a century, and has relevance both for conservation and agriculture. Recently, Ben Werling, Claudio Gratton and their coauthors published work examining the benefits of diversifying potato agroecosystems, both at the top and bottom of the food chain. Looking down from the top of the food chain, the team asked the question: “If and when does having multiple predator species improve control of insect pests?” To answer this question, Ben worked with David Lowenstein – then fresh from the Bronx – and Dr. Cory Straub to examine the effects of predator diversity on predation of Colorado potato beetle, a notorious potato pest. Their results, recently published in the Journal of Insect Science, provide evidence that multiple predator species are better than one – but only under certain conditions. Specifically, they found that the benefits of predator diversity were greater at low prey density. They also suggest a simple mechanism for this effect. Specifically, ecologists hypothesize that competition amongst predators will be lessened in diverse communities, which contain species that all attack different types of prey, compared to lower diversity communities, where individuals are more likely to compete for the same type of prey. Given this, it is reasonable to expect that predator diversity will be more important at lower prey density, where predators are competing for scarcer resources. This suggests that conserving multiple predator species on farmland could be important for keeping pest populations that are currently at low levels from escaping control. In other words, diverse predator communities may help keep pests “down for the count.”
Entomologists have also shown that diversity at the bottom of food chains – at the plant level – also affects the severity of pest problems. Specifically, pest numbers are often lower in more diverse cropping systems compared to those dominated by a single plant species. However, the mechanisms underlying these patterns are often less then clear. On the one hand, increasing plant diversity can have direct effects on pests by making it harder to find – and easier to lose – their host plants. On the other hand, diverse plants may provide more of the resources that predatory insects need to survive and increase their ability to control pests. Ben Werling and Claudio Gratton teamed up with Cory Straub and Jason Harmon – both postdocs at the time – to examine how diversifying potato fields by planting strips of prairie grasses affect both pests and their natural enemies. Their findings – recently published in Biological Control – suggest that prairie grasses increase the abundance of spiders and harvestmen, leading to increased predation of Colorado potato beetle. However, these benefits were limited to the area immediately adjacent to grassy strips. This suggests that planting prairie grasses on farmland could increase natural pest control – but, as in opening a business – location matters. In particular, interspersing patches of crop and natural habitat within a crop field could allow natural enemies to benefit from resources in non-crop areas, while minimizing the distance they have to travel to control pests in crops.
Above, a native warm-season prairie habitat adjacent to potato field at Arlington Agricultural Research Station in Wisconsin.
At which scale should conservation of non-crop habitat take place to sustain such important ecosystem services as the control of crop pests? To help understanding the consequences of land-use decisions, Ben Werling and Claudio Gratton examined the impact of local and broad scale landscape structure on the predation of two insect pests of potatoes in Wisconsin, the Colorado potato beetle, Leptinotarsa decemlineata, and the green peach aphid, Myzus persicae.
At a local scale (meters), potato fields of different sizes were bordered by different areas of uncultivated grassy field margins. At a broad scale (kilometers), potato fields and grassy margins were set in landscapes composed of varying percentages of non-crop habitat. The Predation of both investigated species was significantly impacted by non-crop habitats, but this relationship occurred at different scales for each pest and interacted differently with habitat type. The predation of exposed egg masses of L. decemlineata was greater in field margins than in the potato crop and increased in both habitats when field margins were large relative to the area of potatoes while that predation was less affected by the amount of non-crop habitat within kilometers. In contrast, the suppression of aphid population growth by predators increased with the area of non-crop habitat within kilometers of fields, but was less affected by the field margin area.
As a potential mechanism for the differential impact of local and broad scale landscape structure on predation of these pests, Ben and Claudio suggested that the two pests are attacked by natural enemies with different dispersal abilities. Aphid predators may move across the landscape at broader scales than predators that attack L. decemlineata eggs. Alternatively, the same predators may attack both pests, but respond to landscape structure differently in June, when L. decemlineata egg abundance peak and August when Aphids are present.
Ben and Claudio quoted that the influence of non-crop habitats on predation the potato beetles is due to the movement of natural enemies between resource-providing field margins and potatoes. Consequently small field sizes could reduce the travel distance and increase the ability of organisms to access resources in non-crop habitats. The oppositional patterns of M. persicae suppression suggest that aphidophagous predators move between non-crop habitats at the scale of kilometers. Because even a single ecosystem service, such as pest predation, can be influenced by landscape structure at multiple scales, the authors emphasize that it may be necessary to conserve heterogeneity both at the levels of individual farms and entire mosaic landscapes to maintain ecosystem services.
