Forest Health Technology Enterprise Team TECHNOLOGY TRANSFER Biological Control ASSESSING HOST RANGES FOR PARASITOIDS AND PREDATORS USED FOR CLASSICAL BIOLOGICAL CONTROL: A GUIDE TO BEST PRACTICE R. G. Van Driesche and R. Reardon, Editors Forest Health Technology Enterprise Team—Morgantown, West Virginia United States Forest FHTET-2004-03 Department of Service September 2004 Agriculture __________________________________ ASSESSING HOST RANGES OF PARASITOIDS AND PREDATORS CHAPTER 1. INTRODUCTION PREDICTING HOST RANGES OF PARASITOIDS AND PREDACIOUS INSECTS—WHAT ARE THE ISSUES? R. G. Van Driesche Department of Plant, Soil and Insect Science: Division of Entomology, University of Massachusetts, Amherst, MA 01003 USA [email protected] GOALS FOR HOST RANGE TESTING Estimating the likely nontarget impacts of agents released to suppress invasive plants has been legally required, to one degree or another, for many decades. Similar predictions were not formally required for introductions of parasitoids or predators of pest arthropods. That is now beginning to change. This book has as its goal an exploration of how such estimates can best be made. This requires overcoming a series of problems, some logistical, some technical, some tied to an unclear theoretical framework for the activity. In this book, the editors and authors have tried to address many of these needs, in some chapters as essays on important tasks that need to be achieved, in other chapters as case history explorations of how the tasks were done in particular cases. This book will not be the final answer, but we hope it might propel the search for such an answer along. LEGAL REQUIREMENTS Whether or not predicting the host ranges of parasitoids and predators is legally required varies among countries. There is an absolute requirement for such predictions in New Zealand and Australia, but not in most other countries. In the EU, there is a developing consensus that such information will be required, but in the United States legal authority is lacking to impose such a requirement. Rather, the degree of such an assessment currently depends on the agency of employment of the person importing the natural enemy, with more stringent requirements for federal employees. Chapter 1. Introduction 1 ASSESSING HOST RANGES OF PARASITOIDS AND PREDATORS _________________________________ Regardless of the current legal status quo in any particular country, there is a trend to impose such requirements. The role of this book is in part to shape how such requirements are written, by revealing some of the complexities in the process of making such estimates and highlighting the risks of making overly sweeping assumptions about the utility of laboratory test data. PRACTICAL PROBLEMS Some of the problems posed by estimating the host ranges of candidate entomophagous biocontrol agents relative to the fauna of the receiving country are purely practical, rather than theoretical. Compared to plants, the number of species in a native biota of insects can be over- whelmingly large, with hundreds, thousands, even tens of thousands of native species in the target pest’s family in the receiving biogeographic region. Many of these are likely to have little or no information associated with museum specimens about such important matters as their biology, habitat, host plants, and so on. This double edged problem, too many species and too little information, can cripple efforts to rationally consider the impact of a new parasitoid or predator on such a group. Many species that would be desirable members of a host range test list may be impossible to find or, if found, information on how to rear them will be unavailable. Rearing difficulty is further compounded by the necessity of holding insects as reproducing colonies at great cost in labor, rather than as seeds or long-lived individuals as can be done for plants. Finally, the large numbers of species of many groups means that many other species exist that are unknown, especially if importations are being considered for tropical continental areas, and that modern molecular phylogenies of the relevant insect groups are less likely to be available than for plant groups. ISSUES OF THEORY Issues of theory also complicate the study of host ranges of entomophagous arthropods. For specialized herbivorous arthropods, host ranges seem to track plant taxonomy because that itself is often highly correlated to secondary plant chemistry, compounds often used by spe- cialists for host plant recognition. No such simple framework exists shaping the host ranges of insect parasitoids – or at least work to date has not shown this to clearly be the case. Rather, parasitoids themselves are of two potentially different sorts – idiobionts and koinobionts – each of which may be tracking different things in selecting hosts. Idiobionts being outside of their hosts need not have the high level of physiological adaptation to manipulate living hosts’ immune systems that koinobionts require. Rather, idiobionts may be freer to use a wider range of hosts and may be more shaped in their host choices by the habitats or host plants or type of plant structure (leafmine, gall, etc) in which they find hosts. Koinobionts, in contrast employ venoms, viruses and other devices to master their hosts’ immune systems and as such may find it more feasible to learn to find taxonomically related host insects on novel plants than to domi- nate novel immune systems of less related hosts on familiar plants. Sorting this framework out is the big theoretical issue in predicting host ranges of entomophagous insects. Such informa- tion is needed to make sense of actual host range test designs and test lists. 2 Chapter 1. Introduction __________________________________ ASSESSING HOST RANGES OF PARASITOIDS AND PREDATORS TECHNICAL ISSUES Many technical issues exist about which sorts of host range tests are most useful in predicting what entomophagous insects are likely to do after their release in a new region. How much weight should be placed on host finding versus host suitability? Should preference be consid- ered a factor likely to protect nonpreferred species from attack or will the biological control agents find themselves accepting or rejecting hosts without other immediately available choices? Should data from small cage studies be viewed as a reliable indicators of host choice or should tests use large cages? If so, how large? Should biological control agents used in tests be naïve (no contacts with the target pest) or experienced? Hungry or satiated? Mated or unmated? Young or old? While much has been learned in the past 40 years about how parasitoids and predacious insects are influenced in their host foraging by such factors, synthesis of this infor- mation into an approach for host testing is just beginning. PROCESS OF RESOLUTION Present in this book are essay-style chapters that try to address a number of the above men- tioned issues (Chapters 3-7). Other Chapters (8-16) present case histories in the belief that the particular stories they tell will cast light on methods, details, logical approaches that will have applications in other systems. Debate on many details of theory and practice will be needed to develop a consensus and a mature body of techniques for use in estimating host ranges of entomophagous arthropods. A new forum for such debate was begun in 2002 with the First International Symposium on Biological Control of Arthropods in Honolulu, Hawaii (USA). This series continues with the 2nd ISBCA scheduled to take place in Davos, Switzerland in September of 2005. Among the symposia to be held will be one debating issues affecting host range testing. AGKNOWLEDGEMENTS We thank the USDA Forest Service FHTET group in Morgantown, West Virginia, for funding the writing and printing of this volume, and to Mark Riffe, Intecs International, for formatting and layout. We look forward to the development of knowledge on host range testing and the eventual standard incorporation of such information into release decisions for new entomopha- gous insects. Chapter 1. Introduction 3 ASSESSING HOST RANGES OF PARASITOIDS AND PREDATORS _________________________________ CHAPTER 2. THE EFFECTS OF COMPSILURA CONCINNATA, AN INTRODUCED GENERALIST TACHINID, ON NON-TARGET SPECIES IN NORTH AMERICA: A CAUTIONARY TALE J. S. Elkinton and G. H. Boettner Deptartment of Plant, Soil and Insect Science: Division of Entomology, University of Massachusetts, Amherst, MA 01003 USA [email protected] INTRODUCTION Classical biological control has long been a principal weapon in the worldwide effort to com- bat the devastating effects of invasive species. Classical biological control involves locating natural enemies of invasive species in their native range and releasing them in the newly invaded habi- tat. The premise of classical biological control is that invasive species out-compete native spe- cies and become major pest problems in large part because they have become isolated from the suite of natural enemies that keep them in check in their native habitat. There have been many successes worldwide in the classical biological control of both invasive weeds and invasive arthropods. The advantages of classical biological control over any other approach are obvi- ous and well known: the control exerted is typically permanent; it requires little or no further intervention; it is thus highly cost effective compared to mechanical removal or use of chemical pesticides, which must typically be applied repeatedly and are often infeasible in forests or other natural habitats. Classical biological control of invasive weeds has had a long history of evaluating the host range of candidates for introduction. The obvious reason is that herbivorous natural enemies might become important pests of agricultural crops or other beneficial plants. In contrast, tra- ditionally there has been little concern about the potential impacts on non-target native insects that might be caused by the introduced natural enemies of invasive arthropods. Indeed, the ability of natural enemies to attack native non-target species was viewed by many as a positive attribute (e.g., Culver, 1919; Webber and Schaffner, 1926).