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United States Department of Agriculture

Agricultural Research Service


Location: Pest Management Research Unit

2008 Annual Report

1a.Objectives (from AD-416)
Objective 1: Develop and coordinate biological control programs to achieve sustained suppression of Lepidium draba L. (Brassicaceae), Centaurea spp., Hieracium spp. (Asteraceae), Tamarix spp. (Tamaricaceae), Euphorbia esula L. (Euphorbiaceae), and other invasive plants by: 1a) determining the systematics and phylogeography of target species; 1b) identifying, testing and releasing new arthropods and plant pathogens alone and in synergistic combinations; 1c) assessing rates of establishment, population growth, dispersal and impacts of agents on target weeds, native plants, and associated soil microbial communities; and 1d) integrate biological control agents with chemical, cultural and other control methods to enhance the impact of weed management programs. Objective 2: Identify key factors and mechanisms affecting the success of biological control programs and plant invasions including: 2a) genetic and phenotypic traits of target weeds that determine the success of invasive plant species in distinct ecological regions; 2b) genetic and phenotypic traits of biological control agents affecting their establishment, population growth, and impact; 2c) the biotic, edaphic, and environmental factors and mechanisms affecting weed establishment and expansion, and success of biological control agents; and 2d) integrate population information from Objectives 1 and 2 with remote sensing imagery and other spatial/temporal databases to develop spatial models of weed invasion risk.

1b.Approach (from AD-416)
Exotic invasive weeds cause about $27 billion annually in economic losses in addition to environmental impacts ranging from displacement of species of conservation concern to altered ecosystem functions. Biologically-based control methods can provide cost-effective, sustainable means of limiting the adverse impacts of invasive plants over extensive rangeland and natural areas. Our studies will focus on increasing the success of biological control efforts through better scientific understanding of: mechanisms underlying both the impact of agents and patterns of weed and agent dispersal; genetic variation within target weeds and biological control agents; evolutionary relationships of target weeds and agents; synergisms between plant pathogens, deleterious microbes, and arthropod agents; direct and indirect effects of biological control introductions on nontarget organisms in a risk analysis context; environmental factors affecting weed and biological control agent dynamics and invasion risk; and follow-on effects following suppression of weeds by biological control agents. Anticipated products of our project include new biological control agents, improved systematic and phylogeographic understanding of target weeds and control agents, improved systems for assessing and monitoring weed and insect populations; elucidation of factors and mechanisms limiting the establishment and success of biological control releases. This will benefit land managers, users of public lands, the general public, and the scientific community in the fields of invasive plant ecology, systematics and biological control.

3.Progress Report
Restoration of rangeland following biological control: Native species are often used to restore rangeland after removal of invasive species. We performed research on plant diseases affecting transplanted native species in Theodore Roosevelt National Park. Studies thus far have identified soilborne fungi that are highly pathogenic to native rangeland species. Biological control using plant pathogens: A leaf spotting disease of invasive hawkweed, caused by the fungus Cercospora bizzozeriana, isolated from Europe, was identified and tested for pathogenicity and host range in 2007-2008. A site with a population of hawkweed with heavy occurrence of a rust disease was located in France, from which rust spores will be obtained for pathogenicity tests against U.S. populations of hawkweed. The first occurrence of the leaf and stem pathogen, Alternaria brassicae, found on hoary cress was also found in late 2007. Pathogenicity and comparative virulence tests of a soilborne fungus, Rhizoctonia solani, associated with galls produced by larvae of an insect specific to hoary cress, were completed and analyzed. The first occurrence of Rhizoctonia solani on a North American population of hoary cress was found in June 2008. A new pathogen of hoary cress was also found, Colletotrichum higginsianum, causing stem cankers. Pathogenicity tests documenting the occurrence of a root disease in Europe, crown gall of leafy spurge, were completed in April 2008. NP 304 Crop Protection and Quarantine, Component IX/d Biological Control of Weeds; Combining Biological Control Agents. Weed studies: Collections were made for understanding relationships of Lepidium, a genus which contains 4 important invasive species in the USA. A study of levels of hybridization in Tamarix (saltcedar) indicated that 85% of invasive USA plants are hybrids. With USGS researchers, we found that frost sensitivity appears to be a factor limiting northward expansion of saltcedar. There is strong inherited latitudinal variation in cold hardiness for both Tamarix species. Hybridization between these two saltcedar species may have introduced the genetic variability necessary for rapid evolution of the latitudinal gradient in cold hardiness. NP 304 Crop Protection and Quarantine, Component VII/b Weed Biology and Ecology; Taxonomy and Systematics. Biological control using insects: Significant progress was made in evaluating hypotheses to account for the highly variable success of Diorhabda elongata (saltcedar leaf beetle) releases for biological control of saltcedar (Tamarix spp. and their hybrids) in Montana and Wyoming. Common garden experiments have shown that beetle fitness and female oviposition preference do not vary among the broad array of genotypes found in the T. chinensis/T. ramosissima hybrid complex. However, beetle survivorship did vary with the amount of water supplied to potted saltcedar plants where survivorship was highest on plants receiving the least water. Predator censuses and exclusion experiments at field release sites showed that predation on beetle larvae is high at all sites. NP 304 Crop Protection and Quarantine, Component IX/c: Biological Control of Weeds; Field Evaluation.

1. Novel application of survival analysis with interval-censored data for assessing comparative virulence of weed biocontrol agents. Under conditions in which a necrotrophic pathogen is being tested for comparative virulence with other isolates, the loss or mortality curves can be compared without requiring the daily assessment of mortality for each plant in each treatment as required of typical survival analyses. This approach is particularly useful for conditions in which the subject plants can only be assessed for mortality or some other final state of interest and the plants cannot be assessed except at intervals. NP 304 Crop Protection and Quarantine, Component IX/a: Biological Control of Weeds; Agent Discovery and Selection.

2. Insect/pathogen synergisms in successful biocontrol affect native plant restoration. Developed data showing the prevalence of the three major soilborne plant pathogens (Rhizoctonia, Fusarium and pythiaceous fungi) that are synergistic with Aphthona as the cause of mortality of transplanted forbs in restoration plots in Theodore Roosevelt Park, ND. The fungal species that act in concert with insects to control leafy spurge which, based on earlier work, were predicted to be present in soils following biocontrol, were isolated with high frequency from dead and dying transplants of 8 major forb species. Transplanted native grasses in the restoration effort suffered little mortality. NP 304 Crop Protection and Quarantine, Component IX/d: Biological Control of Weeds; Combining biological control agents.

3. Levels of hybridization in USA saltcedars. Tamarix ramosissima and T. chinensis are native to Asia, but since introduction into the USA, have become common and invasive in many western riparian habitats. Recent molecular analysis of a single locus nuclear DNA sequence marker has shown that in their native range, the two species are genetically distinct, but within the USA invasion many of the plants (23%) are novel hybrids. Here, we used a multilocus DNA marker (AFLPs) and found a much higher incidence of hybridization (84-87%) than was revealed by the single locus marker, with USA plants forming a genotypic continuum between the two parental types. The high percentage of novel hybrids may impact classical biological control efforts or increased adaptive ability of this species to invade or tolerate cold. NP 304 Crop Protection and Quarantine, Component VII/b: Weed Biology and Ecology; Taxonomy and Systematics.

4. Genetic Variation in Invasive Saltcedar Hybrids Not Linked to Variability in Biocontrol Success: Field releases of Diorhabda elongata, the first biocontrol agent for saltcedar, have been highly variable in their success across the western U.S., ranging from failure to establish permanent populations of the agent despite large release numbers to the establishment of spectacular outbreak populations of the agent that have defoliated tens of thousands of acres of saltcedar. One hypothesis to account for the variable success of D. elongata is that genetic variation in saltcedar causes variation in D. elongata survivorship or female oviposition preference. Using common garden experiments we found that D. elongata survivorship and oviposition preference did not vary among saltcedars in the Tamarix chinensis/ramosissima complex. Saltcedars in this hybrid complex constitute the vast majority of the invasive saltcedars in the western U.S. and thus, it is unlikely that the genetic variation in saltcedar is a major cause of the differential success of D. elongata. Alternative hypotheses as to the cause of the differential success of D. elongata will be investigated. NP 304 Crop Protection and Quarantine, Component IX/c: Biological Control of Weeds; Field Evaluation.

5.Significant Activities that Support Special Target Populations

6.Technology Transfer

Number of Newspaper Articles and Other Presentations for Non-Science Audiences2

Review Publications
Ward, S., Gaskin, J.F., Wilson, L. 2008. Ecological genetics of plant invasion: What do we know? Journal of Invasive Plant Science and Management. 1:98-109.

Caesar, T., Caesar, A.J., Gaskin, J.F., Sainju, U.M., Stevens, W.B. 2008. Ecology and function of culturable microbes in soil aggregation. In: Tian-Xiao Liu, editor. Soil Ecology Research Developments. Hauppauge, NY:2007 Science Publishers, Inc. p. 121-138.

Natale, E.S., Gaskin, J.F., Zalba, S.M., Celballos, M., Reinoso, H.E. 2008. Especies del género Tamarix (Tamaricaceae) invadiendo ambientes naturales y seminaturales en Argentina. Boletín de la Sociedad Argentina de Botánica. 43(1-2):1-9.

Last Modified: 4/16/2014
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