Line Trapping: A Promising Method for More Accurate Pest Monitoring
It’s easy to think of a pheromone trap as a lure, putting out a scent and waiting for insects to come rolling in. But insects aren’t sharks; they don’t sense the proverbial blood in the water from great distances. They move much more randomly, and pheromone plumes don’t broadcast all that far—a couple dozen meters, perhaps, but often less.
For the past decade, researchers led by James Miller, Ph.D., at Michigan State University have been refining our understanding of how insects move and interact with pheromone traps. In particular, they’ve studied the codling moth (Cydia pomonella), a global pest of apple and pear orchards, and found the standard pheromone traps for the pest emit plumes of no more than five meters. “Rather than envisioning a codling moth pheromone trap functioning as a vacuum cleaner sucking up males from great distances,” Miller says, “we deduced that it had very weak ‘suction’ and that the males were doing most of the work in moving around randomly until arriving at that weak suction.”
The problem, then, with a pest that staggers about and a trap that is not broadly attractive is variability: one trap in a field might catch zero moths, or it might catch 30. And when a pesticide application for codling moth could cost $1,000 for a 20-hectare plot, that kind of randomness makes for a potentially costly lack of reliability in predicting the actual abundance of a pest.
Placing more pheromone traps at multiple locations throughout an orchard to get a more representative sampling, however, has been an unattractive option for pest managers for the simple added time cost of visiting multiple trap locations throughout the season. But, a new approach that Miller and colleagues have tested effectively addresses that problem: lining up multiple traps in very close proximity to each other.
In an experiment reported this week in the Journal of Economic Entomology, a team led by MSU postdoctoral researcher Christopher Adams, Ph.D., show that five codling moth pheromone traps placed in a line, each four meters apart, produce catch numbers—when averaged across all five traps per line—that are much less variable than single traps at their own location.
In their report, Adams and co-authors write that they were inspired by long-line fishing: “In pondering this problem, we were struck by its parallels to the challenge fishermen face in needing to deploy multiple baited hooks across fish habitat while minimizing travel and service time per hook. A favored solution to this optimization problem in fishing is long-line fishing, where multiple baited hooks descend from short snood lines at increments from a tow-line. This configuration dramatically raises the probability of catch by summing the reaches of plumes emanating from all bait point-sources.”
For the insect-trapping version, they conducted the experiment by pairing a single pheromone trap with a five-trap line, with 70 meters between the two, in a commercial apple orchard, and releasing 100 male and 100 female codling moths from each of four locations in the orchard. This setup was conducted in six separate zones, with each test lasing one week, and repeated six times during the 2015 growing season, for a total of 36 paired tests. They then calculated an average catch among the five traps in each five-trap line and compared that to the single catch number of the solitary trap.
The resulting numbers were clear: The mean catch of a five-trap line in 36 tests ranged between one and 12, while the catch number of solitary traps ranged more widely, from zero to 32.
Miller, senior author of the study and now recently retired from his role as distinguished professor of entomology at MSU (and also an ESA Fellow), says the study found that the five traps in a line compete only slightly with each other, which aligns with previous findings of small plume reach and the moth’s random movement. “But, most important from the practical perspective is that the troublesome variation with catch number in an individual trap is overcome by averaging the catch of five traps that can be deployed all at one site so as to keep costs low,” he says.
In 2015, Miller and Adams co-authored a book, Trapping of Small Organisms Moving Randomly, with Jeffrey Schenker and Paul Weston, that detailed the applications of their research for pest management. Miller says a diverse array of insects move randomly, meaning the line-trapping method may likely be useful for a wide range of insects, both pests and beneficials, and could mark a significant advance for the pest-management field.
“Line-trapping promises to raise pest management procedures to a new standard of precision that should increase grower profits and increase the food supply and food safety, while reducing environmental damage,” he says.
Journal of Economic Entomology