A recent comprehensive study of saltcedar distribution in northern Montana suggested that in certain cases, garden plants may have played a role in starting local invasions, because garden plants were found to be older than the invasions, and no other seed sources were nearby. A specific example cited was the Musselshell River, infested with an estimated 24,500 saltcedars ranging from seedlings to approximately 25 years old. Dispersal mechanisms other than localized spread of seed from ornamentals were also suggested for other areas in the study, including long distance seed movement by wind, water, earth-moving equipment, and towed boats. Earlier research, which included a very limited number of garden saltcedars from other areas in the western U.S., indicated that the garden and invasive saltcedar plants were most often different chloroplast genotypes, though the garden genotype did show up rarely within an invasion. We will use chloroplast and nuclear DNA sequence markers to compare genetic identity of garden and invasive saltcedars of the Northern Plains, and thus determine the influence that garden plants have on starting or contributing to nearby invasions.
Contributing Scientist: John Gaskin(Botanist)
An experimental release program is ongoing for Diorhabda elongata, a leaf beetle that is the first deliberately-introduced biological control agent for saltcedar (Tamarix spp. and their hybrids). The primary goals of the program are to determine rates of establishment, population growth and dispersal of the beetles as well as the impact of the beetles on saltcedar and nontarget plants. NPARL is responsible for 3 experimental release sites in Montana and Wyoming while state, university and federal cooperators have responsibility for other sites throughout the western U.S. Results to date range from complete failure to spectacular and perhaps unprecedented rates of beetle population growth and dispersal. For example, at one site in Montana, over 215,000 adult beetles have been released but this has not resulted in detectable beetle populations in subsequent generations. In contrast, at the Wyoming site, the beetles, numbering over 1,000 per plant in some cases, completely defoliated over 200 acres of saltcedar in just the second growing season after they were first released. The full impacts of defoliation by the beetles on saltcedar are not yet known as this woody shrub is extremely resilient. Ongoing studies will determine the impacts of beetle defoliation on saltcedar fitness components, how this varies with environmental factors such as precipitation and winter temperatures, and how beetle defoliation changes saltcedar recruitment rates.
A variety of hypotheses have been developed to account for the extremely variable results obtained thus far with D. elongata. Ongoing studies at NPARL include common garden experiments to determine how different saltcedar genotypes affect beetle fitness components, predator exclusion and inclusion experiments to determine how predation affects beetle establishment and population growth rates, DNA studies of the D. elongata complex to better understand species and subspecies boundaries, and field plot studies to determine how water stress in saltcedar affects beetle oviposition and feeding preferences, larval growth rates, and plant response to defoliation.
Contributing Scientist: David Kazmer(Entomologist)