Path Meander of Male Codling Moths (Cydia pomonella) Foraging for Sex Pheromone Plumes: Field Validation of a Novel Method for Quantifying Path Meander of Random Movers Developed Using Computer Simulations.

Christopher Adams,James Miller,Paul Weston,Lawrence Gut,Jeffrey Schenker

doi:10.3390/insects11090549

Christopher Adams, James Miller + Show 3 more

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https://doi.org/10.3390/insects11090549

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Abstract

Simple SummaryMeasures of insect movement patterns are key to understanding how insects forage for resources and mating opportunities in their environment. Directly observing large numbers of these small organisms can be extremely challenging, especially for flying insects in low light conditions such as codling moth (Cydia pomonella), a key pest of apple. Here we provide a novel approach to indirectly measure the path meander of randomly moving organisms. Computer simulations were used to simulate insect movement across a wide range of possible movement patterns, measured in circular standard deviation (c.s.d.) of turn angles between track segments. For each c.s.d., the pattern of catch across a rectangular grid of traps was plotted and the resulting exponential decay constant (k) of the fitted lines were used to generate a standard curve describing this linear relationship. Using this standard curve, field data from target organisms caught in the described trapping grid can reveal the pattern of movement employed by these movers. Here we have demonstrated methodology for indirect measure of the movement patterns employed by random walkers such as C. pomonella. While we employed codling moth as our model system, we suggest this approach could prove useful in a wide range of other systems.Measures of path meander are highly relevant to studies of optimal foraging by animals. However, directly recording paths of small animals such as insects can be difficult because of small size or crepuscular activity. Computer simulations of correlated random walkers demonstrated that the rates of decay in captures across a rectangular grid of traps when movers were released at its corner can be used to produce calibration curves for quantifying path meander indirectly. Simulations using spatial parameters matching those previously documented for male codling moths (Cydia pomonella (L.)) foraging for female pheromone plumes in the field predicted that meander, as measured in circular standard deviation (c.s.d.) of turn angles between track segments, should be ca. 50° and 30° when the target population density is high vs. low, respectively. Thus, if optimized, the mean value measured for C. pomonella populations encountering an unknown target density should fall between these limits. We recorded decay in C. pomonella catch across a 5 × 5 grid of pheromone-baited traps each separated by 15 m on 39 occasions where batches of ca. 800 males were released 10 m outside the corner of trapping grids arranged in five large Michigan apple orchards. This decay constant was translated into mean c.s.d value for path meander using the standard curve generated by the computer simulations. The measured decay constant for C. pomonella males was negative 0.99 ± 0.02 (S.E.M.), which translates to a path meander of 37 ± 2° c.s.d. Thus, the measured path meander of 37° fell between the 50° and 30° values optimal for dense and sparse populations, respectively. In addition to providing a rare documented example of optimal foraging for odor plumes, this research offers proof-of-concept for a novel approach to quantifying path meander of movers that could prove useful across diverse taxa.

Highlights

A presumption of behavioral ecology is that evolution will refine animal locomotory behaviors such that the average path taken while foraging will maximize the probability of finding critical resources, while minimizing time, energy, and bodily risk [1,2,3,4,5,6,7]
A small trap was used to approximate the small pheromone plume measured for codling moth traps [19]
Not revealed in these data from our experiment exploiting the law of large numbers, is the amount of individual variation that might occur in this codling moth population

Summary

Introduction

A presumption of behavioral ecology is that evolution will refine animal locomotory behaviors such that the average path taken while foraging will maximize the probability of finding critical resources, while minimizing time, energy, and bodily risk [1,2,3,4,5,6,7]. Actual proof of this precept is rare Animals such as lions, hawks, and predatory fishes are thought to fulfill this expectation via saltatory patterns of movement involving both cruising the habitat for prey and ambushing [8], all the while relying heavily on learning and memory. An intriguing feature of biological random walks is their time-averaged symmetry in the frequency and severity of left vs right turns between locomotory steps such that the resulting frequency histogram becomes smoothly Gaussian, e.g., Figure 7a of [23], Figure 2 of [24], and Figure 2b of [17]. Some random walkers benefit by actively increasing their c.s.d. for turning (intensifying local “search”) upon detecting resource cues and narrowing c.s.d. Some random walkers benefit by actively increasing their c.s.d. for turning (intensifying local “search”) upon detecting resource cues and narrowing c.s.d. (intensifying distant “search”)

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