COMPETITION EXPERIMENTS FOR PRE-RELEASE EVALUATION OF THE POTENTIAL EFFICACY OF NEW BIOLOGICAL CONTROL AGENTS

Authors: Center, Ted; Van, Thai; REBELO, TERESA - FACULTY DE CIENCIA LISBOA; Dray, F Allen; Pratt, Paul; Rayamajhi, Min

Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: 10/24/2002
Publication Date: 6/30/2004
Citation: Center, T.D., Van, T.K., Rebelo, T., Dray Jr, F.A., Pratt, P.D., and Rayamajhi, M.B. 2004. Competition experiments for pre-release evaluation of the potential efficacy of new biological control agents. In: Proceedings of the XI International Symposium on Biological Control of Weeds (eds. J. M. Cullen, D. T. Briese, D. J. Kriticos, W. M. Lansdale, L. Marin, and J. K. Scott), pp. 222-223. CSIRO Entomolgy, Canberra, Australia. Meeting Abstract.

Interpretive Summary: Two factors are of concern when considering a new biological control agent: biosafety and ability to control the weed. Methods for evaluating safety are well known but scant attention has been given to assessment of the candidate's potential value. This is understandable inasmuch as the agent's performance depends on the role of regulating factors that differ between donor and recipient regions. Also, important subtle effects of seemingly benign biological control agents are not easily discerned. These, however, can become apparent when the targeted plant is subjected to other stresses, like interspecific competition. Additive series analysis (inverse linear models) of competition between the weed and a competitor as mediated by the prospective agent has been proposed for judging the value of new agents. We examined this possibility by comparing the abilities of two congeneric waterhyacinth weevils, Neochetina eichhorniae and N. bruchi, to modify competition between waterhyacinth and waterlettuce. The competition analysis revealed that, without weevils, 41 waterlettuce plants were required to produce an effect equivalent to a single waterhyacinth plant on waterhyacinth yield, i.e., intraspecific competition was 41 times stronger than interspecific competition. Exposure to weevils reduced the intraspecific to interspecific competition ratio to near unity, indicating parity between the competing species. Nonetheless, N. bruchi was more effective than N. eichhorniae, and the two combined was only slightly better than N. bruchi alone. Similar results were obtained with ramets or flowers as yield components. Nutrient limitation didn't alter relative results, although all yield components were reduced in lower nutrient environments. We conclude that important effects of these weevils act through modification of water hyacinth competitive ability. This approach could allow assessment of the value of proposed introductions by preempting the release of risky agents with little control value, while increasing the valuation of those that cause seemingly trivial damage.

Technical Abstract: Two factors are of concern when considering a new biological control agent: biosafety and ability to control the weed. Methods for evaluating safety are well known but scant attention has been given to assessment of the candidate's potential value. This is understandable inasmuch as the agent's performance depends on the role of regulating factors that differ between donor and recipient regions. Also, important subtle effects of seemingly benign biological control agents are not easily discerned. These, however, can become apparent when the targeted plant is subjected to other stresses, like interspecific competition. Additive series analysis (inverse linear models) of competition between the weed and a competitor as mediated by the prospective agent has been proposed for judging the value of new agents. We examined this possibility by comparing the abilities of two congeneric waterhyacinth weevils, Neochetina eichhorniae and N. bruchi, to modify competition between waterhyacinth and waterlettuce. The competition analysis revealed that, without weevils, 41 waterlettuce plants were required to produce an effect equivalent to a single waterhyacinth plant on waterhyacinth yield, i.e., intraspecific competition was 41 times stronger than interspecific competition. Exposure to weevils reduced the intraspecific to interspecific competition ratio to near unity, indicating parity between the competing species. Nonetheless, N. bruchi was more effective than N. eichhorniae, and the two combined was only slightly better than N. bruchi alone. Similar results were obtained with ramets or flowers as yield components. Nutrient limitation didn't alter relative results, although all yield components were reduced in lower nutrient environments. We conclude that important effects of these weevils act through modification of water hyacinth competitive ability. This approach could allow assessment of the value of proposed introductions by preempting the release of risky agents with little control value, while increasing the valuation of those that cause seemingly trivial damage.