Gregariousness does not vary with geography, developmental stage, or group relatedness in feeding redheaded pine sawfly larvae.

John W Terbot,Catherine R Linnen,Ryan L Gaynor

doi:10.1002/ece3.2952

Abstract

Aggregations are widespread across the animal kingdom, yet the underlying proximate and ultimate causes are still largely unknown. An ideal system to investigate this simple, social behavior is the pine sawfly genus Neodiprion, which is experimentally tractable and exhibits interspecific variation in larval gregariousness. To assess intraspecific variation in this trait, we characterized aggregative tendency within a single widespread species, the redheaded pine sawfly (N. lecontei). To do so, we developed a quantitative assay in which we measured interindividual distances over a 90‐min video. This assay revealed minimal behavioral differences: (1) between early‐feeding and late‐feeding larval instars, (2) among larvae derived from different latitudes, and (3) between groups composed of kin and those composed of nonkin. Together, these results suggest that, during the larval feeding period, the benefits individuals derive from aggregating outweigh the costs and that this cost‐to‐benefit ratio does not vary dramatically across space (geography) or ontogeny (developmental stage). In contrast to the feeding larvae, our assay revealed a striking reduction in gregariousness following the final larval molt in N. lecontei. We also found some intriguing interspecific variation: While N. lecontei and N. maurus feeding larvae exhibit significant aggregative tendencies, feeding N. compar larvae do not aggregate at all. These results set the stage for future work investigating the proximate and ultimate mechanisms underlying developmental and interspecific variation in larval gregariousness across Neodiprion.

Highlights

Aggregations, or spatial groupings of organisms, are widespread in nature and occur across diverse taxa (Krause & Ruxton, 2002; Parrish & Edelstein-Keshet, 1999; Prokopy & Roitberg, 2001)
We introduce a potentially powerful system for investigating both the proximate and ultimate causes of behavioral variation and describe an assay for quantifying one variable behavior involved in aggregation, larval aggregative tendency
We focused on the redheaded pine sawfly, N. lecontei

Summary

| INTRODUCTION

Aggregations, or spatial groupings of organisms, are widespread in nature and occur across diverse taxa (Krause & Ruxton, 2002; Parrish & Edelstein-Keshet, 1999; Prokopy & Roitberg, 2001). We log-t ransformed (natural log) pairwise distances and used ANOVAs to determine whether aggregative tendency differed among: populations, genetic clusters, or by latitude Populations, both separately and combined by genetic cluster, were compared to the random, null model using Tukey HSD. When we condensed each video down to a single log- transformed (natural log) mean pairwise distance (for postacclimation period only), we found that developmental stage had a pronounced impact on aggregative tendency, but that its effects were partially dependent on the number of larvae in the assay (Figure 3b,d,f). By contrast, nonfeeding instars from the 5-larvae videos appeared to have greater pairwise distances than expected under the random model (Tukey HSD: p = .0001) Together, these results suggest that while N. lecontei feeding instars have a strong behavioral tendency to aggregate, nonfeeding instars either ignore or actively avoid one another.

Findings

| DISCUSSION

| CONCLUSIONS