Endogenous toxins and the coupling of gregariousness to conspicuousness in Argidae and Pergidae sawflies

Abstract

Phytophagous insects tend to be either cryptic and solitary, or brightly colored and gregarious, as a defense against vertebrate predators. Here, we tested whether potent defensive chemicals produced de novo by larvae of Argidae and Pergidae sawflies have influenced the evolutionary relationship between larval appearance and levels of gregariousness. Phylogeny-based correlation analyses indicated only a weak trend for solitary species to be cryptic, and for gregarious ones to be conspicuous. Numerous Argidae were cryptic–solitary or conspicuous–gregarious, whereas most Pergidae were conspicuous–gregarious. Both families also included not truly gregarious but aggregated species, i.e. with individuals more evenly distributed on the host plant. By considering two specific morphological traits, predominant body coloration and contrasting spots on body, each one was (weakly) associated with appearance but none with gregariousness, which reflects the functional relevance of appearance as a whole. Furthermore, Argidae can display alternate appearances during successive larval instars. Finally, an independent contrasts test showed no obvious correlation between two major toxic peptides. Our results point towards diversely combined patterns of linked ecological traits in these insects. By assuming that warning coloration is more warranted against vertebrate than invertebrate predators, we suggest that the occurrence itself of toxins allowed this diversity via differing predator guilds and environmental factors, to which these insects were confronted during evolution.

Behavior and body size modulate the defense of toxin-containing sawfly larvae against ants

“…Still another function is the visual impact of the body movements on predators hunting by sight and that can be frightened at distance 22 , especially when gregarious insects simultaneously raise their abdomens. Argidae and Pergidae larvae are commonly aggregated or truly gregarious, and grouped individuals are better defended than single ones against vertebrate and invertebrate predators 10 .…”

“…Hepta- and octapeptides were isolated and identified from the sawfly species, and toxicological tests showed that they can kill vertebrates 5 , 8 . From an eco-evolutionary perspective, however, the toxic peptides that occur commonly among these two sawfly families were admittedly driven not by livestock but other organisms 9 , 10 .

Figure 1 Pictures showing larvae of most of the species used in the study.

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“…Argidae and Pergidae larvae differ in visual appearance and level of gregariousness from one species to another 10 . Differences in appearance can also occur during ontogeny, from one larval instar to another, in Argidae 17 , 19 – 21 .…”

“…Differences in appearance can also occur during ontogeny, from one larval instar to another, in Argidae 17 , 19 – 21 . In sawfly larvae as in other insects, appearance is expected to be primarily directed against birds by functioning as a visual signal warning of the presence of a defense 22 , whereas gregariousness would influence interactions with vertebrates and invertebrates, because grouped individuals are better chemically defended than single ones 10 , 23 .…”

Sawflies out of Gondwana: phylogenetics and biogeography of Argidae (Hymenoptera)

“…Larvae of Argidae s.s. Feed on a diverse variety of angiosperms. Although the larvae of most argids remain unknown, they also present various degrees of gregariousness, chemical defence, feeding behaviour and morphological adaptations to their host plants (Smith, 1992;Schmidt et al, 2000;Boevé et al, 2018). Contrary to the known Pergidae and Argidae s.s., the larvae of Zenarge feed on gymnosperms, including the native Australian Callitris spp.…”