Progress 09/01/03 to 09/01/05
Outputs
4d Progress report. This report serves to document research conducted under a reimburseable agreement between ARS and Tulane University. Additional details of research can be found in the report for the parent CRIS 6240-32000-009- 00D Development of Advanced Technologies for the Screwworm Eradication Program. The screwworm fly, Cochliomyia hominvorax, is a livestock pest and human health hazard that was eradicated from the US, Mexico, and Central America southward to the Panama Canal by the sterile insect release method. In this method, sterilized flies are liberated into the environment from aircraft. These sterile flies then mate with the native fly population, inhibiting its reproduction and causing its eradication. USDA APHIS currently maintains a 300-km wide barrier zone of continuous sterile fly release in eastern Panama to prevent screwworm reintroduction into Central America and the US. Within this barrier, sterile screwworms are released from aircraft along
parallel flight lanes 2 nautical miles apart. Matlock and Skoda are currently funded by the USDA, CSREES, NRI to develop a mathematical reaction-diffusion model of this barrier zone. The goal of this model is to investigate how wide the barrier should be to prevent the spread of the fertile screwworm population from the extant side of the barrier to the zone of eradication. The model of sterile fly distribution has been solved and algebraic formulae for the spatial distribution of sterile, released screwworm flies within the barrier zone have been derived that are functions of the sterile fly release rate and barrier width. The model for the native fertile screwworm fly population is nonlinear and has no known exact analytical solution (i.e., a solution that can be written in algebraic form). Hence, the spatial distribution of the fertile fly population has been investigated by solving the model numerically using the method of lines. The model predicts that the fertile screwworm
population should expand into an empty (i.e., previously eradicated) environment as a traveling wave. If a sterile fly barrier is erected in front of this traveling wave, the barrier causes the wave to stall, provided that sterile fly density exceeds a threshold value. The threshold sterile fly density required to stall the advancing wave of fertile flies (i.e., reduce the traveling wave speed to 0) has been calculated. Numerical calculations also show that the speed of the traveling wave decreases as sterile fly density increases. These results suggest that circumscribing screwworm outbreaks with sterile fly releases will drastically slow the expansion of fertile flies in addition to leading to their eventual eradication. Although an exact analytical solution for the fertile fly distribution is not available, accurate approximate solutions have been derived by perturbation analysis and comparison methods. These approximate results show that sterile fly abundance decays very nearly
exponentially within the barrier zone, diminishing to very small densities in a relatively short distance. Numerical solutions of the model also suggest that sterile barriers substantially narrower than the 300-km barrier in Panama would be sufficient to halt the advance of fertile fly populations. This grant also supports field experiments to gather data for the estimation of model parameters and for validation of model predictions. These experiments are being conducted in Panama in collaboration with Co- PI Steven Skoda, USDA ARS, Screwworm Research Unit and USDA APHIS. In these field experiments, flies are released from aircraft along a single release line, as in Eradication Program releases, and then censused in traps to 5 km either side of the release line. The purpose of these experiments is to measure how far sterile flies disperse perpendicular to these flight lanes in order to predict sterile fly coverage within the barrier zone. These data will also be used to test model
predictions of sterile fly distribution. Because screwworm abundance differs in forested and non-forested habitats, these experiments are being replicated at two sites in agricultural landscapes, composed of patchy forest, pasture, and other agricultural habitats, and at in one site in primary forest. A total of seven sterile fly releases have been conducted at the three replicate sites, each release including both ground and aerial liberations of flies. Model parameters have been estimated for the completed replicates and for 16 other screwworm release-recapture data sets collected by other investigators (1, 10 and 5 data sets from Mexico, Costa Rica and Panama, respectively), totaling over 1,000,000 released and 50,000 recaptured flies. Model predictions suggest that screwworm density should be maximal at the flight lane and should decline exponentially to a minimum at the midpoint between lanes. The model predicts that current flight lane spacing is adequate, but that sterile fly
density at the midpoint is not far from the threshold that would permit screwworm population survival, and hence that flight lane width should not be expanded. Results of the flight tests seem to agree with model predictions: sterile fly density declines rapidly from the point of release and is quite low 1 nautical mile from the release point for both aerially and ground released flies. Manuscripts describing 1) the model of interacting fertile and sterile flies in the barrier zone and parameter estimation and 2) flight lane separation are in progress. Presentations on the model results were given by Matlock at the Ecological Society of America (August, 2004); the third Research Coordination Meeting of the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, Co-ordinated Research Programme titled "Expansion of Sterile Insect Methods for Old and New World Screwworm" (June, 2005); the SIAM (Society for Industrial and Applied Mathematics) annual meeting (July, 2005);
and further results will be presented at the Entomological Society of America annual meeting (December, 2005). The annual cost of maintaining the sterile screwworm fly barrier zone in Panama (discussed in the progress report) is approximately $10M. Because of the breadth of northern South America, there are no current plans to eradicate the screwworm south of Panama. Hence, the Screwworm Eradication Program currently plans to maintain the Panamanian barrier zone permanently. Both the planned permanence of the barrier and the annual maintenance costs provide strong incentives for optimizing the sterile release program to reduce barrier maintenance costs without compromising its impermeability to fertile screwworms. Barrier width and flight lane width set total fly dispersal rates and flight time and thus determine the flight time required to cover the barrier, an important component of program costs. The primary goal of this research is to improve the efficiency and security of the
screwworm control program. A broader goal is to derive general principles that will facilitate application of the sterile insect release method to the control other insect pests. 7. List your most important publications in the popular press and presentations to organizations and articles written about your work. (NOTE: List your peer reviewed publications below). Matlock, Jr., R.B., Edwards, P.J. 2005. The influence of habitat variables on bird communities in forest remnants in Costa Rica. In press, Biological Conservation. Matlock, Jr., R.B. 2005. The effect of prey size and vulnerability on the ability of the predatory pentatomid bug, Perillus bioculatus, to grow, survive, recruit and regulate its prey. In press, Environmental Entomology. Dyer, L.A., Matlock, Jr., R.B., Chehrezad, D., Omalley, R. 2005. Predicting caterpillar parasitism in banana plantations. In press, Environmental Entomology. Stireman, J.O. III, Dyer, L.A., Matlock, R.M. 2005. Top-down forces in managed versus
unmanaged habitats. In: Barbosa, P., Castellanos, I. (editors). Ecology of Predator-Prey Interactions. Oxford University Press. In press.
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Progress 10/01/03 to 09/30/04
Outputs
4. What were the most significant accomplishments this past year? D. Progress Report This report serves to document research conducted under a trust cooperative agreement between ARS and Tulane University. Additional details of research can be found in the report for the parent project, 6240-32000-007-00D, "Quantitative Characterization of Natural Screwworm Populations and Improvement of Sterile Fly Release Strategies." This goal of this research is to develop and parameterize a spatially explicit mathematical model of control of the screwworm, Cochliomyia hominivorax, by the sterile insect release method. The model is based on reaction-diffusion partial differential equations, which have long been used to model animal dispersal in ecology. The goal of the model is to generate hypotheses and to stimulate empirical research on four problems of practical significance to the Screwworm Eradication Program. The current barrier zone of sterile fly release in Panama is 300
km long and is maintained at a cost of roughly $7 million per year. Could this barrier be shortened to reduce costs without compromising its security? Second, could the barrier zone be made more secure by releasing flies at greater densities at the eastern barrier boundary near the Darien Gap, where the screwworm is extant, and at lower densities near the western barrier boundary, where fertile screwworm densities should be very low? Third, what spacing between aircraft release flight lanes would be optimal for control? Fourth, how fast do outbreaks expand from initial foci of infestation, and what is the recommended pattern of treatment with sterile males to contain them? Field tests of model predictions with respect to sterile fly density within the barrier zone and realized sterile density about aircraft flight lanes will also be conducted and are already in progress. These experiments will also examine the effects of wind and habitat heterogeneity on screwworm dispersal. Three
field tests of dispersal by aerially and ground-released sterile screwworm flies about flight lanes have already been completed. Optimal experimental protocols for the conduction of these release recapture experiments, which minimize parameter estimate variances have also been designed. Data from the three completed tests, along with two other tests of sterile fly dispersal in Panama (Speed and Temperature Tests, conducted by D. G. Haile) and one dataset from Mexico (collected by R. J. Brenner) have been analyzed and used to estimate model parameters. Eight additional individual, mark, release, recapture data sets for fertile flies from Costa Rica (collected by F. D. Parker and J. B. Welch) have also been analyzed and used to estimate parameters for fertile screwworm flies. Model predictions of sterile fly density for the regular release pattern currently employed by the eradication program within the barrier zone have been also been completed for two potential field conditions in
which flies 1) do and 2) do not drift with prevailing winds. Analysis of flight lane tests suggests that sterile flies do exhibit low to moderate drift with prevailing winds. Preliminary model predictions of the impact of symmetric and asymmetric sterile fly barriers on fertile fly density and of the rate of spread of screwworm outbreaks have also been completed and are being refined. A manuscript that will report results of the analysis of the 14 data sets mentioned above and model predictions with respect to the Panama barrier zone is in progress.
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