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For Large-Scale Pest Insect Detection, Traps in Parallel Lines More Efficient Than Grids

Closeup of a mottled light brown and white moth perched horizontally on a green fern branch, facing toward the viewer and to the right. The moth has black eyes and tall antennae standing straight up, each slightly curved with long bristles extending outward horizontally along the length of the antennae.

Detecting pest insects across large areas means placing vast numbers of traps, with associated costs to set them up and check them regularly. Grid patterns have been the traditional choice, but a new study shows trap-placement patterns using parallel lines could be just as effective with much lower servicing requirements. Such large-scale trapping is used in detection of pests such as the spongy moth (Lymantria dispar), and the study of trap patterns used trapping data from spongy moth detection efforts in North Carolina and Ohio in 2021 to evaluate various trapping simulations. (Photo by Susan Ellis,

By John P. Roche, Ph.D.

Sampling for the presence of insect pests has traditionally used traps laid out in grid patterns. While effective, they are labor intensive to set up and monitor and thus a costly way to sample. In a new study, however, researchers at the U.S. Department of Agriculture and North Carolina State University show that alternative trap-layout designs can match grid patterns in effectively detecting pest insects with lower servicing requirements.

Because of the expense that would be involved with testing trap-layout designs in the field, the researchers used simulations with a computer model called TrapGrid. Barney Caton, Ph.D., of the USDA Animal & Plant Health Inspection Service; Hui Fang, Ph.D., and Godshen Pallipparambil, Ph.D., of the Center for Integrated Pest Management at NC State; and Nicholas Manoukis, Ph.D., of the USDA Agricultural Research Service published their findings in April in the Journal of Economic Entomology.

TrapGrid can simulate the detection of insect pests by traps arranged in different patterns in a simulated landscape. In their simulations, the research team compared the performance of traditional grid patterns with alternative designs based on transects that they called “trap-sect” designs. Building on earlier work the researchers have conducted in trap-layout models, the team’s hypothesis was that trap-sect designs would detect pests as effectively as traditional grids but with much greater efficiency.

The alternative trap-layout designs tested were crossed lines, parallel lines, and spoked patterns. (See Figure 2.) In their simulations, Caton and colleagues measured the average probability of detection of a pest and the distance traveled to service the traps. Good sampling designs would have a high probability of detecting a pest and low servicing distances.

The researchers found that many of the alternative trap-layout designs provided pest detection that was similar to that provided by full grids. Of the alternative layouts, parallel-line designs showed the greatest probability of detection, followed by spoke designs, and then crossed-line designs. With parallel-line designs, the probability of detection increased incrementally with each additional line that was added, from two lines to seven lines, as would be expected.

Full grids had the longest servicing distance, followed by spokes and crossed lines (75 percent shorter), followed by parallel lines (66–89 percent shorter). Overall, in terms of detectability and efficiency combined, the best designs were four to seven parallel lines, followed by spoked lines.

Graphic depicting several trap-placement designs, with traps indicated by blue circles. Top left shows a grid pattern, with ten rows of circles spaced apart equally both horizontally and vertically. Top right shows a grid pattern with five rows of circles close together in rows but rows spaced further apart. Second row left shows traps in three spoke patterns. Second row right shows similar spoke patterns with an empty center in each. Remaining charts show variations on a pattern each with traps placed in horizontal parallel lines at left, vertical parallel lines in the middle, and horizontal parallel lines at right. Third row left shows this pattern with two lines in each zone, third row right five lines, bottom row left seven lines, and bottom row right four lines. Each chart shows distances on the axes, 0 to 8,850 meters on the X-axis and 0 to 4,023 meters on the Y-axis.

A study of pest-insect trap-layout designs using the TrapGrid computer simulation, compared traditional grid patterns (A and B) with several alternate designs: four crossed lines (C), eight spokes with an untrapped hub (D), two parallel lines (E), four parallel lines (not pictured), five parallel lines (F), six parallel lines (G), and four parallel lines in a modified alignment (H). All designs used 250 traps, indicated by blue diamonds. Establishment positions of pests are indicated with red circles. (Image originally published in Caton et al 2023, Journal of Economic Entomology)

It makes sense that the alternative designs such as parallel lines and spoked lines were more efficient—with the shorter servicing distances of these designs, efficiency increases. But why was pest detectability in the parallel-line and spoked-line designs similar to the detectability in the full grid?

“This similarity is dependent on many things,” Manoukis says, “like the attractiveness of the traps.” With attractive traps, pests will be drawn to traps even if they are not in a full grid pattern. In addition, in the comparisons in these simulations, pest outbreaks occurred randomly in space, which might help them be detected by the alternative designs, making detectability more similar to that in the full grid.

To approximate how alternative sampling designs might work in the field, the investigators overlaid alternative designs onto actual trapping data for two pest moth species, the European grapevine moth (Lobesia botrana) in California in 2010 and the spongy moth (Lymantria dispar) in North Carolina and Ohio in 2021. In the overlay of a four-parallel-line trap design on European grapevine moth data from California, the service distance was reduced by 43 percent. In the overlay of a crossed-lines trap design on spongy moth data in North Carolina and Ohio, the service distance was reduced by 35 percent and 47 percent, respectively.

“Aligning traps in this way is a new idea,” Caton says, “but it makes sense to improve efficiency. Survey managers already have to place traps in the field; this method just has them being placed in different shapes. The basic process is unchanged.”

Four men stand a row on a grassy knoll with a greenhouse and two palm trees in the background.

arney Caton, Ph.D. (left), of the USDA Animal & Plant Health Inspection Service; Hui Fang, Ph.D. (second from left), and Godshen Pallipparambil, Ph.D. (right), of the Center for Integrated Pest Management at NC State; and Nicholas Manoukis, Ph.D. (second from right), of the USDA Agricultural Research Service tested the probability of detection and the servicing distance of several alternative trap-layout designs for pest-insect sampling and compared the results to a traditional square grid design. They found that parallel-line and spoked-line trap designs offered good detection with significantly improved servicing efficiency. (Photo courtesy of USDA)

The investigators conclude that alternative trapping designs would reduce sampling costs considerably. But there are hurdles to overcome to implement these new designs. “The ‘tried and true’ methods often have some inertia behind them,” Caton says. “So, a new approach is almost always difficult to implement. But cost-cutting is usually a significant motivator, so our hope is that managers will adopt the trap-sect approach on that basis.”

The parallel-line and spoked-line sampling patterns worked well in the simulations in the study. Pest managers could refine these strategies even more by using an adaptive approach where surveyors add traps as pests are detected. This would permit pest detection with even greater efficiency. In future research, Caton and colleagues plan to investigate dynamic strategies of sampling that adapt over time.

“The TrapGrid model really made this research possible,” Caton says. “In the field it would be very time-consuming and costly to evaluate different designs. While some field validation is likely still needed, the results were strong enough that, given the good track record of the model, we are confident that the new sampling designs should work well.”

This investigation was the first test of alternative trap placement patterns for area-wide delimitation trapping in 40 years. Additional studies, including looking at dynamic sampling strategies, should further refine this promising approach.

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John P. Roche, Ph.D., is an author, biologist, and scientific writer with a Ph.D. in the biological sciences and a dedication to making rigorous science clear and accessible. He writes books and articles, and provides writing for universities, scientific societies, and publishers. Professional experience includes serving as a scientist and scientific writer at Indiana University, Boston College, and the University of Massachusetts Medical School, and as editor-in-chief of science periodicals at Indiana University and Boston College.

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