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, thus 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 injury: Nitrogen fixation

Recent research suggests that foliar feeding affects nitrogen fixation rates of legumes. Potato leafhopper, Empoasca fabae, 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.

Plant-herbivore coevolution and risk of injury

Are introduced (exotic) plant species more susceptible or more resistant to herbivory than their closely-related native plant species? Theoretical and empirical perspectives can be used to argue for either answer. For example, introduced plants lack their coevolved natural enemies, have novel defensive compounds, and have increased competitive ability. These hypotheses suggest introduced plants are less likely to be injured compared to their native counterpart. On the other hand, recent findings using molecular techniques have demonstrated that plants are capable of recognizing their co-evolved herbivores and responding to defend themselves. Also, a recent meta-analysis concluded that exotic plants are especially susceptible to novel, generalist herbivores. Thus, introduced plants do not recognize their new, non-coevolved herbivores, and may respond inappropriately to protect themselves. Here, I approach this question using a set of three closely-related legume species, two exotic and one native to North America. For the herbivore, I used potato leafhopper, Empoasca fabae: a generalist, phloem-feeding herbivore, native to North America. Based on the known reaction of plants to potato leafhopper feeding injury, I hypothesized that the co-evolved, native plant would recognize and not respond immediately to the leafhopper feeding, whereas the introduced plants would not recognize the leafhopper and respond in a generalized way to protect themselves. For the native plant, I used Astragalus canadensis L., Canadian milkvetch, and for introduced plants, I used two European/Asian species, Astragalus cicer L., cicer milkvetch, and Securigera (=Coronilla) varia (L.)Lassen, crownvetch. In greenhouse experiments, with equal levels of injury, the native legume demonstrated no effect of injury while the closely-related, exotic legumes had significant reductions in gas exchange rates. Although the leafhopper survived well during the course of the experiment on all plant species, the long-term performance is unknown and is the subject of current research. Results suggest the lack of coevolution between the exotic host plant and the native herbivore is associated with greater susceptibility of the plant to injury. If true, the inclusion of recognition genes into introduced crop plants, such as alfalfa, may lead to tolerance to injury.

M. truncatula Research

Herbivores with piercing-sucking mouthparts are capable of injuring their host plants, leading to disruption of normal physiological functions. Potato leafhopper, Empoasca fabae, 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.

Transgenic corn debris risk

With the widespread deployment of genetically-modified crops for the management of insect pests, leaves with the plant incorporated protectants (PIPs) will enter streams through the movement of wind. The activity and impact of PIPs in flowing water systems is largely unknown. Our current research demonstrates that Bt in senesced corn leaves loses its biological activity within two weeks of exposure to aquatic environments. However, some invertebrate consumers remain negatively impacted by feeding on Bt corn leaves in comparison to non-Bt isolines. Our overall goal is to develop methods to assess the potential nontarget risk of corn leaves from genetically-modified cultivars that become deposited in streams following harvest. Our objectives are to quantify the loading of corn debris into streams, to develop an aquatic medium for testing PIPs on detritivores, to improve screening techniques for aquatic detritivore taxa for both sublethal and behavioral responses, to compare environmental degradation of Cry proteins using methods based on ELISA tests and target bioassays, to test the tissue-mediated response hypothesis for observed sublethal responses to Bt corn, and to develop a list of common taxa found in agricultural ditches on Maryland's Eastern Shore. Field studies will involve sampling within agricultural streams of the Maryland Piedmont and Coastal Plain regions. Our research is designed to evaluate the overall potential risk of the deployment of Bt corn on stream ecosystems. In addition, new protocols will be developed for estimating future risks to the biota and their functions within stream ecosystems.

Jackson Lane Preserve wetlands

Restored wetlands are generally thought to aid in the reduction of nutrient losses in agriculturally dominated watersheds while enhancing biodiversity in the region. However, relatively little information has been collected regarding the effect of restored wetlands on nutrient transport rates both through surface and subsurface flow paths. In addition, little is known of the impact of restored Delmarva wetlands on local plants and animals. We hypothesize that large scale restoration of farmed wetlands on Maryland's Eastern Shore, using ecologically appropriate methods, can: 1) significantly reduce nutrient inputs into ground and surface waters of local watersheds; 2) reestablish functional native wetland plant and animal communities; and 3) provide new ecosystem services that mitigate or reduce cumulative impacts of ongoing agricultural activities at the watershed scale. A cooperative project initiated in 2003 involving The Nature Conservancy, US Fish & Wildlife Service, MD Department of the Environment, and USDA's Natural Resource Conservation Service provides a unique opportunity to assess the environmental and ecological benefits of restoring 210 acres of agricultural land in northern Caroline County to seasonal forested and open-canopy wetlands and forested uplands. Our continuing objective is to document improved ecological structure and function (physical, chemical, and biological) of restored seasonal wetlands compared to similar natural wetlands. In addition, these wetlands provide a setting for a number of student-based research projects in wetland ecology.

Macroinvertebrates in agricultural ditches

Agricultural ditches on the Eastern Shore of Maryland serve as a major pathway for linking water and nutrients between fields and the Chesapeake Bay on the Delmarva Peninsula. Current projects on ditches focus on management practices to improve nutrient retention and processing. Our goal is to determine if the macroinvertebrate community can be used to assess the beneficial ecosystem services provided by ditches. Our objectives include the identification of macroinvertebrate community structure within ditches, to relate the communities to water and soil/substrate conditions, and to develop an appropriate sampling procedure for rapid bioassessment. By combining expertise in aquatic entomology and soil science, the project will specifically address key aquatic processes within agricultural ditches, leading to improved ditch management practices.