The study of origins of populations is currently expanding rapidly, facilitated by novel molecular methods and advances in computational ability. Computer pro-
Tracing the Origin of Pests and Natural Enemies 107
grams for analysing population-genetic data for the study of population origins have already been developed and are available on the Internet. A number of case studies have demonstrated their usefulness. However, despite the ease of analysis, some limitations in the use of genetic data to determine population origins persist, particularly the difficulties involved in the development of molecular markers for novel taxa, as well as limited accessibility to material from potential source material worldwide. It is likely that, for both issues, coordinated interna- tional and multi-agency efforts will be needed for substantial progress.
The methods for the determination of population origins described here can be applied to the area of biological control in a number of ways, includ- ing: (i) the determination of origins to identify the pathways of invasion of non- indigenous species, as well as their subsequent spread in their new habitat; and (ii) understanding of the biology of pests in the habitats in which they are indige- nous, in order to develop effective methods of control, particularly as it relates to the use of predators and parasites. Other applications of the methodologies show some promise for the future (Roderick and Navajas, 2003). For example, by fol- lowing the fates of genetically or chemically marked individuals, it may be possi- ble to address the issue of the importance of genetic variation in the success of colonization.
One might ask, ‘Do multiple genotypes or individuals from multiple origins have a greater chance of success in new environments?’ That is, do populations with greater genetic variability have a greater chance of establishment? The importance of particular traits can also be examined by studying individuals in the field that possess those traits. For example, are particular strains of parasitoids that have been selected for increased searching ability better able to persist in novel habitats? If and when genetically modified insects are released, methods described here can be used to trace their fate (and the fate of their genes) in natural and managed habitats. Finally, extensions of the methods used to deter- mine origins can be applied in identification of cryptic species in all stages of the process of colonization and establishment (see Chapter 6, this volume).
Conclusions
1.The use of genetic and other methods for determining origins of non-indige- nous species offers much promise.
2.The typical demographic processes associated with invasions necessitate the use of non-traditional methods in population genetics.
3.It is unlikely that population-specific genetic markers can be developed rou- tinely for non-indigenous species.
4.There is little evidence that lack of genetic variation limits success in colo- nization, though more data are needed.
5.More genetic loci are nearly always more useful in tracing population histo- ries; bioinformatics approaches can be used to combine information from the work of many research laboratories.
108 G.K. Roderick
6.Tracing the fate of particular genotypes in the environment allows the study of a number of issues related to invasion biology, biological control and the use of genetically modified organisms.
Acknowledgements
I thank the editors of this volume and the symposium organizers for making both resounding successes! This work is supported by grants from the California Department of Food and Agriculture, US National Science Foundation, US Department of Agriculture, and the University of California Agricultural Experiment Station.
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Introduction
In the United Kingdom alone, 1700 arthropods have been introduced, with 1500 of them having an economic impact, and about 170 being considered alien pests.
Thirty to forty per cent of crop losses are attributed to damage caused by intro- duced pests in the UK and USA (Pimentel, 2001). More than 50,000 species of arthropods, birds, mammals, microbes and plant weeds have been introduced into the USA since North America was discovered by Columbus. Damage attrib- uted to these introductions has been estimated at US$137 billion. Worldwide it is estimated that over 400,000 non-indigenous species exist, and it has become increasingly clear that many introduced species negatively affect the ‘global eco- logical integrity’, and are major factors in the extinction of native species within the habitat (Pimentel et al., 2000; Craig, 2001; Pimentel, 2001; Ebert et al., 2002).
Most mammals, birds and plants have been introduced intentionally, whereas arthropods and microbes are typically accidental introductions (Pimentel, 2001).
As such, arthropods and microbial pathogens are amongst the most difficult to discover in a timely manner and, hence, to restrict their movement and/or erad- icate them.
In addition, upsurgent insects from both local and exotic locations are important pests and vectors of plant pathogens. Insect pests that occur unex- pectedly frequently become problematic because they have developed resistance to insecticides or have overcome genetic (introgressed) or engineered resistance in the host plant. In the case of insect vectors of transmissible plant pathogens, virus–vector specificity may shift; an introduced, or upsurgent, new vector biotype may occur unexpectedly; or new, more virulent viral or microbial vari- ants may emerge as pathogens, causing unprecedented epidemics or pandemics.
In such situations, it is essential to act quickly to provide a rapid and accurate 113
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