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Researchers Use Simulation Model to Optimize Delimitation Trapping Surveys

McPhail trap

When an invasive insect appears, responders must quickly set monitoring traps in the area to determine the extent of the pest outbreak. For an outbreak of Mediterranean fruit flies (Ceratitis capitata), for instance, traps such as this McPhail trap might be used. Researchers at the U.S. Department of Agriculture are working to improve the design of grids for trap placement in such efforts, known as delimitation surveys, and they aim to publish new guidelines after field experiments are completed later this year. (Photo by Peggy Greb, USDA Agricultural Research Service, Bugwood.org)

By Melissa Mayer

Melissa Mayer

Melissa Mayer

The costs associated with invasive insects add up to more than $70 billion every single year—and that price tag is expected to increase as climate change, global trade, and an overall uptick in human movement help expand the ranges of exotic pests.

That expensive problem inspired a collaboration between researchers from the United States Department of Agriculture’s (USDA) Animal and Plant Health Inspection Services (APHIS) and Agricultural Research Service (ARS) as well as the Center for Integrated Pest Management at North Carolina State University. In a study published in October in the Journal of Economic Entomology, the team used simulated outbreak data to look at the design of trapping surveys for determining how far an outbreak has reached. They found that factors like grid shape and size, insect dispersal ability, trap density, and trap attractiveness all make a difference when it comes to optimizing surveys.

Finding the Boundaries

researchers

Researchers (left to right) Barney Caton, Ph.D., Hui Fang, Ph.D., Nicholas Manoukis, Ph.D., and Godshen Pallipparambil, Ph.D., hold insect traps while posing for a group picture during a week of teamwork including simulations and analysis, at the Daniel K. Inouye U.S. Pacific Basin Agricultural Research Center in Hilo, Hawaii. (Photo courtesy of U.S. Department of Agriculture)

“In the US, after an exotic pest is detected … there’s usually an early need to determine the boundaries of the population,” says Barney Caton, Ph.D., pest exclusion analysis coordinator with USDA-APHIS and the first author of the paper. “So, the delimiting survey is one tool aimed at doing that.”

This means establishing a grid over the outbreak and deploying traps to figure out how far the pests have spread. That—or visual surveys for insects that can’t be trapped—provides the data that agencies and stakeholders need to figure out the next steps in dealing with an outbreak.

The default design is a fully trapped square, using a 5-by-5-mile grid for less mobile insects or a 9-by-9-mile grid for more mobile pests. But are these ideal for the job?

Tasked with this question, Caton reached out to Nicholas Manoukis, Ph.D., a supervisory research biologist with USDA-ARS who had a model perfect for the project: TrapGrid. The team used TrapGrid to look at 30-day randomized outbreaks and calculate the mean probability of capturing individual pests using default and modified survey designs.

“No one had really done a whole lot of work on determining … how the schemes were doing—in particular whether the sizes were right or whether the densities were right,” Caton says. “So, our work was the first example of using more advanced analytics to actually go back and evaluate the traditional design.”

Early Recommendations

square vs. circle trap grid

Many current standard guidelines for delimitation surveys call for square trapping grids, but a circle design is more efficient. As this diagram shows, a pest that can reach the far corner of a square grid can easily reach beyond the grid if moving in a different direction. Conversely, a square grid large enough to cover a pest’s maximum travel radius would cover excess area—21.5 percent more than needed. (Image originally published in Caton et al 2021, Journal of Economic Entomology)

Some of the findings confirm what many experts already do in the field—like the shape of the grid. Caton says it’s intuitive to set these up as a circle, measuring the max distance the pest should move from the center of the outbreak in every direction. However, the published designs default to square grids. The corners of those squares are outside the radius, so using a circle means a 21.5 percent reduction in traps and area to service.

Optimizations like this could mean better quality data and conserved resources, which may be a big deal down the road. “If exotic species continue to be introduced and potentially established and the rates are going up, as some studies suggest, we may be doing more and more delimitation surveys over time,” Caton says. “So, doing them cheaply is going to be important to constrained budgets and [help] make this manageable.”

The study highlighted the value of updating survey designs for pests with low to moderate mobility to avoid oversizing grids. Another important factor is trap density, which may be tied to how attractive the traps are and how serious the pest is.

For traps that are super attractive to the insect—like those that use pheromone lures with irresistible cues a pest can follow some distance—a survey likely needs fewer traps. Traps that use food-based or visual lures, which only pull in pests that are hungry or close enough to notice color, may require higher densities.

“If you have a really high-risk pest, and you don’t want any to get away, then you might choose to put a higher density to facilitate that,” Caton says. “If it’s a pest but it’s not a world-ender, and your budget doesn’t allow it, then perhaps choosing a lower density will be just fine.”

Comprehensive Guidelines in the Works

The research team verified their simulation results by running empirical data from detection surveys through the model. As predicted, they found that a number of those surveys were oversized.

Now, they’re conducting field experiments in Hawaii using the Mediterranean fruit fly (Ceratitis capitata) to demonstrate how their design improvements work in the field. Eventually, they’ll make those design guidelines available for anyone interested in optimizing their own surveys.

Caton says this project is a great example of ARS and APHIS working together to bring a research tool to the field. “That’s nice when that happens—and I think that it’s likely to have big benefits for the agency and the people who respond to these things as well as the stakeholders who need us to exclude and respond appropriately to exotic pests.”

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: melissa.j.mayer@gmail.com.

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