Retrospective analysis of a classical biological control programme

Authors: Naranjo, Steven

Submitted to: Journal of Applied Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2018
Publication Date: 4/24/2018
Publication URL: http://handle.nal.usda.gov/10113/6088617
Citation: Naranjo, S.E. 2018. Retrospective analysis of a classical biological control programme. Journal of Applied Ecology. 55:2439-2450.

Interpretive Summary: The sweetpotato whitefly is a global pest of agriculture and invaded the southern tier of the USA in the early 1990s. A multi-institutional effort was launched shortly after the invasion to develop an introductory (classical) biological control program. The goal of such a program is to introduce specific natural enemies from the native range of the pest in the hopes of establishing effective and long-lasting pest control. Classical program are complex and expensive and rarely include the funding or resources to assess impact many years after introductions. A long-term series of life tables (a method to measure the rates and sources of mortality affecting populations) were conducted over a range of time that included pre and post introduction periods of several exotic parasitoids. These life tables were then used to ask what has changed in mortality dynamics and pest population abundance. I found that the exotic parasitoids simply displaced several native parasitoid species but did not supply any additional mortality compared with what was available before introductions. The lack of program success can be possibly tied to several important factors including the fact that the pest attacks a broad range of crop plants and that these crops are produced on an annual cycle. Host specific parasitoids like those introduced are simply not well adapted to such ephemeral crop environments. Instead, generalist arthropod predators that are well adapted to ephemeral crop environments and polyphagous pests have contributed significantly to biological control of the whitefly and are the central component of a highly effective pest management strategy based on predator conservation. In the future, additional consideration should be given to the nature of the pest and the environment in order to develop lower risk pest control solutions.

Technical Abstract: 1. Classical biological control has been a key technology in the management of invasive arthropod pests globally for over 120 years, yet rigorous quantitative evaluationsof programme success or failure are rare. Here, I used life table and matrix model analyses, and life table response experiments to quantitatively assess a classical biological control programme for an invasive insect pest in the western United States. 2. Life tables and matrix models were developed for populations of Bemisia tabaci (sweetpotato whitefly) on cotton in Arizona before (1997–1999) and after (2001–2010) the permanent establishment of two exotic aphelinid parasitoids. Analyses tested multiple hypotheses relative to the expected outcome of a successful programme. 3. Marginal rates of parasitism, rates of irreplaceable mortality from parasitism, total generational mortality and finite population growth (') were unchanged for B. tabaci populations before and after exotic parasitoid establishment. Prospective analyses showed that predation during the final nymphal stadium had the greatest influence on population growth rates regardless of parasitoid establishment. Retrospective LTREs showed that predation and unknown mortality contributed most to changes in ' after parasitoid establishment. 4. Marginal parasitism acted weakly in a direct density dependence fashion after parasitoid establishment, and for all 14 years combined. However, this did not translate into an association between pest population density and marginal rates of parasitism for the 10-year period following establishment. 5. Synthesis and applications. Rarely are classical biological control programme outcomes assessed rigorously. Life tables, matrix models, and life table response experiments showed that the decline in the pest status of Bemisia tabaci (sweetpotato whitefly) was not associated with the establishment of two exotic parasitoid species in the cotton system. Instead, native arthropod predators play a major role in pest dynamics. Further efforts to enhance conservation of the extant natural enemy community, with focus on increasing mortality in final stage nymphs and adults, may be the most efficient means of increasing biological control services. Analyses deployed here should be more widely applied to assessing and improving biological control generally.