Sept.-Oct.-Nov. ORNAMENTALS Robert P. Doss 1979 USDA-SDA-AR Ornamental Plants NORTHWEST Vol. 3, Issue 5 Research Laboratory ARCHIVES Pages 24-27 Puyallup, WA 98371 THE BASIS OF OBSCURE ROOT WEEVIL RESISTANCE IN RHODODENDRON- - SOME THOUGHTS ON THE REASONS FOR INVESTIGATING THE RESISTANCE OF PLANTS TO INSECTS1 Abstract A study of the resistance of Rhododendron species to foliar feeding by obscure root weevil has provided several interesting observations. Resistant species are more often found in the lepidote branch of the genus. Resistance is basically of the nonpreference type and may be conferred by either morphological or biochemical characteristics of individual species. Feeding stimulants have been identified in both susceptible and resistant Rhododendron species using a membrane filter feeding bioassay. Sucrose is one feeding stimulant compound. Chemical feeding inhibitors are found in some resistant varieties. The study of the resistance of plants to insects is both economically and environmentally rewarding. Study of the basis of resistance in plants should yield information useful in developing novel pest management schemes. Text The genus Rhododendron consists of roughly 1000 species and many named hybrids. Leach (1961) shows the commonly accepted Rhododendron taxonomic pattern with the genus divided into three branches. The elepidote and azalea branches lack scales whereas all the species within the lepidote branch have scales. Rhododendron taxonomy is currently undergoing modification but the modified schemes will be similar to the one described by Leach (1961). We are studying the basis of resistance to weevil feeding in Rhododendron, focusing on the obscure root weevil (Sciopithes obscurus Horn) which is a common indigenous species found from Monterrey Bay, California to Vancouver, British Columbia (Van Dyke, 1935). Bell and Clarke (1979) have tested a number of Rhododendron species and rated them according to their resistance to obscure root weevil feeding. Examination of these data using statistical tests indicates that lepidote branch contains significantly more resistant species than either the azalea or the elepidote branch. Resistance is usually of the non-preference type (i.e. the plants possess traits that make them unattractive to insect pests) and can be conferred by morphological and/or chemical characteristics of Rhododendron leaf. We have been most interested in the chemical basis of feeding behavior. In order to identify feeding stimulants, we have developed a membrane filter bioassay (Bristow et al. 1979). Using this bioassay, we have found feeding stimulants in both resistant and susceptible cultivars (Doss 1979). We have used thin-layer chromatography to examine extracts from susceptible varieties and have identified distinct bands of feeding stimulant activity. Sucrose is one feeding stimulant. In addition there are at least two other feeding stimulants in all of the susceptible cultivars that we have examined. We have also examined extracts from resistant species where the basis of resistance does not appear to involve morphological factors. With such species we can extract chemical feeding deterrents that prevent feeding on membrane filters treated with sucrose. Such filters are ordinarily fed on avidly by the obscure root weevil. Some might ask why study resistance of plants to insects. There are several answers to this question. First, the study of resistance has proven to be very rewarding economically. For example, the Council on Environmental Quality (1972) reports that federal, state and private organizations have spent approximately 9.3 million on the study of plant resistance to Hessian fly, wheat stem sawfly, European corn borer, and spotted alfalfa aphid. The return, based on information generated by this research, was $308 million per year in reduced crop losses. Thus, over a 10-year period, the benefit/ cost ratio was roughly 300 to 1. Similarly, Hanson (1961) reports that study of the spotted alfalfa aphid has yielded a 100 to 1 benefit/cost ratio and McMillan and Wiseman (1972) have reported a 20 to 1 benefit/cost ratio arising from studies of the corn earworm. It would be difficult to argue that the study of insect resistance in plants is not economically justified. Similarly, the results of study of insect resistance in plants can be used to develop environmentally sound pest control strategies. Pimental et al. (1978) report that we are currently using approximately 350 million pounds per year of insecticides. The affect of these insecticides on non-target organisms is unclear at best and at worst devastation (Carson, 1962). We should also remember that humans constitute a "non-target organism." Comar (1978) has stated that we live in a "sea of carcinogens." Although few would contend that pesticides should not be used in agriculture, it can be argued that if resistant plants are identified and used, the need for adding additional chemicals to the "sea" is reduced. The mandate for studying insect resistance in plants is thus clear. However, some might argue that to go beyond the identification, selection and utilization of resistant plants is not necessary. Coppel and Mertin (1977) point out, however, that the identification of chemical bases for resistance should also be of value. For example, deterrent compounds should be useful in behavioral modification of insects. We see an example of this in a report that extracts from the sacred Neem tree of India can inhibit Japanese beetle feeding on the usual hosts (Ladd, et al., 1978). Feeding inhibition is so great that Japanese beetles will starve rather than feed on the deterrent treated leaves. Similarly, if chemical bases for resistance can be identified, it is conceivable that screening for deterrent compounds within seedling plants could be used to speed up breeding programs for insect resistance. This would be particularly useful with woody plants where generation time is quite lengthy. 1 Based upon a talk presented by Dr. Robert P. Doss on August 25, 1979 , at the Ornamentals Northwest Seminars in Portland, Oregon. References Bell, H. T. and R. G. Clarke. 1978. Resistance among Rhododendron species to obscure root weevil feeding. J. Econ. Entomol. 71: 869-870. Bristow, P. R., R. P. Doss, and R. L. Campbell. 1979. A membrane filter bioassay for studying phagostimulatory materials in leaf extracts. Annals Entomol. Soc. Amer. 72:16-18. Carson, R. L. 1962. Silent Spring. Houghton Mifflin, Boston. Comar, C. L. 1978. Risk: a pragmatic de minimis approach. Science 203:309. Coppel, H. C. and J. W. Mertins. 1977. Biological insect pest suppression. Springer-Verlag, Berlin. Council on Environmental Quality: Integrated Pest Management. Washington, D. C.: U.S. Government Printing Office, 1972. Doss, R. P. 1979. Extraction chromatography and bioassay of obscure root weevil (Sciopithes obscurus) feeding stimulants from Thododendron 'Cynthia.' (Submitted) Hanson, C. H. 1961. Moapa alfalfa pays off. Crops and Soils 13(7):11-12. Ladd, T. L., Jr., M, Jacobson, and C. R. Buriff. 1978. Japanese beetles: Extracts from Neem tree seeds as feeding deterrents. J. Econ. Entomol. 71:810-813. Leach, D. G. 1961. Rhododendrons of the World and How to Grow Them. Charles Scribner's Sons, New York, N. Y. McMillan, W. W. and B. R. Wiseman. 1972. Host plant resistance: a twentieth century look at the relationship between Zea mays L. and Heliothis zea (Boddie). Univ. Florida Monograph. Ser. 2. Piemtal, D., J. Krummel, D. Gallham, J. Hough, A. Merrill, I. Schreiner, P. Vittum, F. Kozial, E. Back, D. Yen, and S. Fiance. 1978. Benefits and costs of pesticide use in U. S. food production. BioScience 28: 772,778-784. Van Dyke, E. C. 1935. New species of North American weevils in the family Curculionidae, subfamily Brachyrhininae, III. PanPacific Ent. 11:83-96. Pesticide Use - Due to constantly changing laws and regulations, no liability for the suggested use of chemicals in this Newsletter is assumed by the ONW Newsletter. Pesticides should be applied according to label directions on the pesticide container. Permission to Reprint material appearing in the ONW Newsletter is granted with the request that you credit the source: Ornamentals Northwest Newsletter, date, volume, issue, page numbers. Do not excerpt or reprint in such a manner as to imply the author's endorsement or criticism of a product or concept. Nondiscrimination - The information in the Ornamentals Northwest Newsletter is provided with the understanding that no discrimination is intended and that listing of commercial products implies no endorsement by the authors. Criticism of products or equipment is neither intended nor implied. .