Potato leafhopper injury: Gas exchange

The potato leafhopper, Empoasca fabae Harris, has been documented to feed and reproduce on more than 200 species of plants, including many eastern North American crops (Lamp et al. 1994). Native to the eastern half of the US and Canada, potato leafhopper is considered a key pest of alfalfa (Sulc and Lamp 2006). Population characteristics, including its high vagility, polyphagy, and high rate of population increase, result in high densities during the summer months (Hogg and Hoffman 1989). In addition, potato leafhopper is a pest because of the unique plant response to feeding injury. The symptoms of hopperburn of alfalfa are the result of injury induced by its feeding (Granovsky 1928). The leafhopper feeds on alfalfa by rapid, repeated penetration of its stylets into the vascular tissue, from which plant material is ingested (Backus and Hunter 1989, Kabrick and Backus 1990). Through a combination of mechanical and salivary stimuli, potato leafhopper feeding enhances a wound response in alfalfa that changes the vascular tissue around the feeding site (Ecale and Backus 1995a, 1995b). When this occurs, photoassimilates transported through the phloem build up around the injured site (Johnson 1934, Hibbs et al. 1964, Nielsen et al. 1990, 1999), and rates of photosynthesis are reduced (Womack 1984, Flinn et al. 1990, Lamp et al. 2004). In addition, stomatal conductance as well as internode elongation is reduced (Lamp et al. 2004), resulting in the apparent stunting of stems. Thus, leafhopper feeding initiates a cascade of changes in alfalfa (Backus et al. 2005) that is ultimately expressed as hopperburn, a characteristic yellowing of leaves (Granovsky 1928), as well as delayed plant maturity, reduced nutritive components, stunted growth, and reduced yields (Kindler et al. 1973, Hower 1989, Hutchins and Pedigo 1989).

Although its role as a disruptor of phloem function has long been recognized, recent evidence suggests that vulnerable host plants respond quickly to leafhopper injury by closing stomata and reducing photosynthetic and transpiration rates within 12 hours of the initiation of feeding (Lamp et al. in prep.). In addition, injured plants quickly synthesize starch from photoassimilates, even in energy-starved tissue such as apical meristematic tissue (Pirone et al. 2004). Thus, the initial plant response to potato leafhopper injury can be characterized as a generalized wound response. We continue to document the response of alfalfa, especially specific genotypes, with the goal of identifying tolerant forms of the crop.

Potato leafhopper economic threshold in alfalfa systems

Potato leafhopper negatively affects the value of alfalfa forage across the northcentral and northeastern US. Although new varieties are available with moderate resistance, and alfalfa-grass mixtures demonstrate some tolerance, leafhopper densities still achieve economic levels, evoking the need for insecticide applications. This pest is responsible for the majority of insecticide applied to alfalfa in this region, and patterns associated with climate change suggest that leafhoppers will have an even greater economic impact in the future. Adopting resistant varieties and alfalfa-grass mixtures are expected to decrease the damage caused by leafhopper and the amount of insecticide applied, but producers need specific guidelines when leafhopper populations in those systems rise above an economic threshold. In addition, research suggests that the leafhopper inhibits nitrogen fixation rates, thus reducing protein levels in the forage, and reducing vigor of regrowth after harvest. We will use three locations in divergent states to conduct investigations and extension leading to the adoption of improved, robust economic thresholds for potato leafhopper. Collaborators include Dr. Mark Sulc at the Ohio State University and Dr. Ken Albrecht at the University of Wisconsin. Our objectives are: 1) to quantify economic loss associated with potato leafhopper in alfalfa systems across three states, 2) to examine the effect of potato leafhopper on nitrogen fixation under greenhouse and field conditions, and 3) to develop economic threshold guidelines for potato leafhopper management and disseminate through extension outlets. Adoption of resistant varieties, grass-alfalfa mixtures, and new thresholds for leafhopper management will improve alfalfa forage yield in both organic and conventional production systems, reduce insecticide use in alfalfa, and improve persistence of alfalfa stands.

Lamp Lab research will focus on Objectives 1 and 2. For Objective 1, all three states have planted three fields representing different production systems: a susceptible variety of pure-seeded alfalfa, a resistant variety of pure-seeded alfalfa, and a susceptible variety of alfalfa co-planted with tall fescue. A randomized complete block design will be established with treatments of an insecticide at different rates to reduce potato leafhopper densities. Conducted over three years, the study will define a relationship between economic loss and leafhopper densities for each production system. This relationship will be used in objective 3 to determine appropriate economic thresholds.

For Objective 2, research suggests that foliar feeding affects nitrogen fixation rates of legumes. Potato leafhopper has long been recognized as a foliar pest of many legumes (Lamp et al. 1994), in part because of its ability to disrupt normal flow of photoassimilates in the phloem (Nielsen et al. 1990). More recently, we have shown that leafhopper injury disrupts basal flow of photoassimilates to the roots late in the growth cycle of the forage alfalfa, which is vital to the symbiotic relationship between the plant and Rhizobium symbionts on the roots (Lamp et al. 2001). In a greenhouse study, we have further shown that total nitrogen in alfalfa plants is reduced when leafhopper feeding is confined to late in the growth cycle (Lamp unpub. data). These results are especially important because integrated pest management (IPM) has focused on early injury of the leafhopper, yet late injury is overlooked and likely to affect nitrogen fixation as well as root loading of carbohydrates and proteins, both of which impacts the longevity and total productivity of the legume. Greenhouse and field cage studies are planned to measure the timing of leafhopper injury on disruption of nitrogen fixation. In connection with Objective 1 experiments, we will compare the disruption to nitrogen fixation for susceptible, resistant, and alfalfa-grass production systems.

M. truncatula Research

Herbivores with piercing-sucking mouthparts are capable of injuring their host plants, leading to disruption of normal physiological functions. Potato leafhopper is an example whose injury ultimately is responsible for economic damage to a wide range of crop and ornamental plants. Current research suggests that the negative response of exotic legume hosts to injury induced by this native leafhopper may be the result of a lack of co-evolution between the plant hosts and the herbivore. The longterm goal of our research is to determine the mechanism for tolerance to this pest using Medicago truncatula as a model system. By first focusing on the function and constituents of potato leafhopper saliva, we intend to develop key methods for investigation of the biochemical and molecular response of legumes to potato leafhopper injury. Using laboratory and greenhouse experiments, we propose: 1) to develop a standardized model system using potato leafhopper to study the effects of delivery of salivary components into legume vascular tissue, 2) to compare the results of natural leafhopper feeding injury to salivary injections using alfalfa and strains of M. trunculata exhibiting tolerance and susceptibility to leafhoppers, and 3) to characterize the biochemical and endosymbiont constituents of leafhopper saliva which are associated with physiological injury of host plants. Ultimately, we hope to provide the basis for developing economically-important plants that are tolerant to leafhopper injury.

Brown Marmorated Stink Bug

We are interested in the role of symbiotic bacteria on the feeding ecology of the brown marmorated stink bug (Halyomorpha halys). This invasive species harbors symbiotic bacteria in its midgut and salivary glands, which are important for survival and reproduction of the insect. Other researchers have noticed that different plants often develop necrotic areas around sites where H. halys has fed. We are testing whether the same symbiotic bacteria harbored by H. halys may be pathogenic in plants that are fed upon by the stink bug. H. halys feeds on a large number of plant hosts, including many important crops. Finding that H. halys vectors pathogenic bacteria to its plants hosts can help explain the extent of plant injury caused by this species.

Transgenic corn debris risk

With the widespread deployment of genetically-modified (GM) crops for the management of insect pests, leaves with plant incorporated protectants (PIPs) will enter streams and ditches through wind dispersal. Since 2005, we have been investigating the risk potential for exposure and hazard effects of Bt proteins in corn plant debris on aquatic invertebrates in running waters.

Our work on risk assessment of GM crops and non-target aquatic invertebrates started when I was standing in an agricultural stream on a farm at the time of corn harvest. I observed corn leaves blowing in the air, and wondered what would happen to stream invertebrates that might feed on leaves from Bt corn that were deposited in the stream. I discovered through a literature search that no testing had been done on the types of invertebrates that feed on decaying corn leaves in streams. After some preliminary research, we were able to get funding to support research on the question, and we are now funded for another three years of support from the USDA-BRAG program to continue the project.

We first discovered that stream invertebrates were not exposed to the Bt protein in corn leaves. Although the corn expressed Bt in the leaves at harvest, no bioactivity of Bt was found after two weeks of exposure to the ground or to the stream water (see Jensen et al. 2010). We also measured non-target effects in laboratory experiments and found no effect on two caddisfly species (which are closely related to Lepidoptera, the target of the Bt protein in corn). We did, however, find a negative effect on a crane fly species as well as an isopod species. We explained this effect as a "tissue-mediated effect", in which the presence of Bt in the leaf during the growing season altered the physical and chemical characteristics of the leaf. In a separate study, we also measured leaf decomposition rates of Bt and non-Bt corn leaf debris and found little evidence of a negative impact of Bt corn on streams (Swan et al. 2009). We have additional manuscripts in preparation to continue the story.

After the initial grant, we discovered that new varieties of corn had a form of Bt that was highly persistent--we can detect bioactivity of Bt for six months in crop debris after harvest. As a result, stream invertebrates will be exposed to Bt, so the question is now, is Bt harmful to species that will feed on Bt corn leaves?

Our current research from our recently-funded grant proposal are as follows:

1. Refine protocols and test PIPs (plant incorporated protectants) using artificial food suitable for aquatic invertebrates that are adapted for shredding plant debris. Dr. Qin Wang in the Department of Food Science is an important part of this objective. We have developed a means to adhere a potential toxin onto food to force aquatic invertebrates to feed on the toxin under water (see Gott et al. 2014). We intend to continue the development of the artificial diet using appropriate species of invertebrates that feed by shredding their food.

2. Perform a landscape-level assessment of GM versus non-GM crops on the running water ecosystems draining cropland. Our work on ecosystem-level testing has not used whole streams as experimental replicates. We have located and sampled pairs of agricultural streams (or ditches) that have not been exposed to Bt crops. In December, 2015, we will introduce Bt and non-Bt corn debris to the pairs of streams. Starting spring, 2016, and continuing for two years, we will measure invertebrate structure and function to compare whole ecosystem effects in a BACI design.

3. Measure degradation rates of PIPs across genes, varieties, and post-harvest crop management. Drs. Galen Dively and Cerruti Hooks are leading this research. We believe that how farmers handle crop debris after harvest may reduce the persistence of the Bt protein, and so reduce the potential for exposure to aquatic invertebrates. We are testing various ways to incorporate the crop debris into the ground with tillage.

Wetland-stream connectivity

Streams and wetlands commonly coexist in the landscape. A recent review of scientific literature concluded that streams are connected to wetlands physically, chemically, and biologically (U.S. EPA 2015), although the ecological connections resulting from movements of invertebrate species across wetland-stream habitat mosaics is poorly studied. Aquatic invertebrates play important functional roles in both wetlands and streams, as they largely serve as the connection between primary producers and vertebrate species in the food web. Here, we ask, do some invertebrate species rely on both wetland and stream habitat resources for persistence of populations on the Delmarva Peninsula?

Some species of aquatic invertebrates, especially insects that fly, are capable of utilizing both wetland and stream habitats and may require both habitat types for life cycle completion or population persistence. Habitat permanence is strongly correlated with aquatic insect species’ potential for dispersal and migration (Brown 1951) and could also influence niche breadth. For example, water boatmen (Hemiptera: Corixidae spp.) that use seasonal wetlands to reproduce have been observed to migrate en masse into permanent, running water habitats when wetlands dry (S. Srayko, personal communication). By field sampling in wetland-stream habitats on the Delmarva Peninsula, and using DNA barcoding to relate immatures to adults, we plan to identify invertebrate species that:

  • A) are confined to wetland habitats,
  • B) are confined to stream habitats,
  • C) reproduce in wetlands but migrate to streams when wetlands dry (habitat complementarity),
  • D) are able to reproduce in either stream or wetland habitats, with limited (typically passive) movement between different habitat types, and
  • E) are able to reproduce in either stream or wetland habitats and opportunistically colonize both habitats, with potential for long distance (active or passive) dispersal.

Once identified to species, invertebrate samples will be analyzed for stable isotopes (δ15N, δ13C, and possibly δ34S) to determine the habitat types in which they obtained their primary food resources, and estimate their trophic status. Differences in the biologically-mediated reactions that control nitrogen cycling (e.g., feeding by invertebrate grazers or predators (trophic status), denitrification) are expected to produce natural variation in the isotopic ratios of individuals originating in different habitat types (streams vs wetlands) (Diebel and Vander Zanden 2009). This variation, combined with species data, can be used to help identify the natal habitat type, habitat breadth, and movement behaviors of aquatic invertebrates in these ecosystems. Following isotope analysis, biological traits of species that have been characterized by habitat use (i.e., classified into categories A-E above) will be analyzed to identify cross-species similarities in dispersal and migration, and to determine the utility of using traits-based models to estimate wetland-stream connectivity, established via invertebrate dispersal and migration, in other wetland landscapes.

Maryland black fly project

The black fly Simulium jenningsi is a nuisance pest found throughout the Mid-Atlantic States. The adult females form large, persistent, swarms around the head and face which can disrupt outdoor activities. The Lamp lab was alerted to the presence of S. jenningsi in Maryland in 2013, when residents of southern Washington County complained of nuisance swarms that occurred in their backyards every summer. Our lab conducted preliminary sampling in 2013 to confirm the species of black fly and determine which rivers and streams in the region served as the larval source. Keeping the residents involved with and informed of our research was important for the project, so the lab established http://mdblackfly.com to track our progress. An online survey was created to assess how strongly the flies impacted the daily lives of the residents, and which areas had the largest amount of complaints. To further this community involvement, adult flies were collected by resident volunteers who were given sampling kits by the lab. All of the adult black flies collected were found to be S. jenningsi, and the primary larval source appeared to be the Potomac River.

In 2014 we expanded our work on the Maryland black fly project to determine if S. jenningsi was a nuisance in other counties, and to investigate what factors led to the swarms being denser and more bothersome in some communities compared to others. Our objectives were: 1) to locate larval breeding sites within the Potomac River and its larger tributaries, 2) to expand our knowledge of the geographic range of the nuisance problem, and 3) to determine the relationship between fly distribution and land use. We collected adult and larval black flies from locations throughout Washington and Frederick counties, and the bordering areas of Virginia and West Virginia, expanding the known range of S. jenningsi in Maryland. Of the 125 locations sampled for adult flies, S. jenningsi was found in the highest numbers in rural and forested areas, and was rarely found in the urban areas of Hagerstown and Frederick. Numbers of flies were also very high in communities near the confluence of the Shenandoah and Potomac Rivers, which led us to believe the stretch of the Potomac immediately below the confluence may be an area of high larval productivity.

In 2015 the Lamp lab was awarded an MAES Competitive Grant with the following objectives: 1) to complete a spatially-dependent model of adult black fly distribution in western Maryland and surrounding areas, 2) to compare larval densities in riffle habitats surrounding the confluence of the Potomac and Shenandoah Rivers, and 3) to use an experimental approach to test the hypothesis that S. jenningsi larval growth is improved with seston from below the confluence of the Potomac and Shenandoah Rivers in comparison to seston above the confluence. Sampling was conducted during the summer of 2015, and samples are currently being processed.

Macroinvertebrates in agricultural ditches

Since 2008, our lab has been studying the aquatic macroinvertebrate communities of agricultural drainage ditches on Maryland’s Eastern Shore. Drainage ditches are an important component of agricultural infrastructure, and help to increase crop yields by controlling soil moisture levels. Ditches also provide habitat to many different species, as they are un-cropped areas that may contain many different plant species, and provide habitats to aquatic species for at least part of the year. The main research questions our lab has been interested in are: what are the communities of aquatic macroinvertebrates present in drainage ditches, and what are some of the functional roles these communities play in ecosystem-level processes occurring across ditch networks?

Our initial studies of ditch invertebrates characterized the taxa that make up a typical ditch community, and related patterns of community composition with environmental factors related to nutrient cycling. This work was carried out primarily by Ph.D. student Alan Leslie in collaboration with the labs of Dr. Brian Needelman and Dr. Joshua McGrath in the Environmental Science and Technology Department. As an aquatic habitat that is rarely studied in the context of a source of biodiversity, gaining a basic understanding of the composition of a typical ditch community was our first goal. In streams, macroinvertebrate communities often show strong relationships with water quality factors and different land-use characteristics within the watershed. Our second goal was to determine whether the macroinvertebrate community of ditches could serve as a useful indicator of the quality of water draining from ditches. From this research, we learned that ditches provide habitat for many different species of aquatic invertebrates, and that many of the communities are typical of streams that may be degraded (Leslie et al. 2012). Patterns in macroinvertebrate communities across ditches did not show strong relationships with water quality factors that also varied between ditches. Therefore, macroinvertebrate communities may not be a good indicator of water quality in ditches. Communities were correlated with physical conditions within ditches, including the size of the ditch, and flow velocity of water within the ditch. Therefore, it is the physical structure of ditches, rather than the quality of water draining within ditches that likely determines what macroinvertebrates are present.

One key result of the initial surveys of drainage ditches was that burrowing species make up a significant proportion of the macroinvertebrate community. Bioturbation by burrowing species can affect exchanges of materials across the sediment-water interface in different aquatic and marine habitats. The movement of phosphorus from below-sediment to surface waters is of concern for Maryland, as nutrient pollution from excess phosphorus is partly responsible for seasonal algal blooms that result in anoxia across large portions of the Chesapeake Bay. Drainage ditches represent a focal point where nutrients can move from agricultural lands into regional watersheds. Our current research is focused on using laboratory experiments to determine the role of burrowing invertebrates in altering rates of phosphorus release from ditch sediments and soils to overlying water. Results from this work will determine the functional role that natural macroinvertebrate communities play in a highly human-modified aquatic system.