Invertebrate and avian predators as drivers of chemical defensive strategies in tenthredinid sawflies

Abstract

Background: Many insects are chemically defended against predatory vertebrates and invertebrates. Nevertheless, our understanding of the evolution and diversity of insect defenses remains limited, since most studies have focused on visual signaling of defenses against birds, thereby implicitly underestimating the impact of insectivorous insects. In the larvae of sawflies in the family Tenthredinidae (Hymenoptera), which feed on various plants and show diverse lifestyles, two distinct defensive strategies are found: easy bleeding of deterrent hemolymph, and emission of volatiles by ventral glands. Here, we used phylogenetic information to identify phylogenetic correlations among various ecological and defensive traits in order to estimate the relative importance of avian versus invertebrate predation.

Results: The mapping of 12 ecological and defensive traits on phylogenetic trees inferred from DNA sequences reveals the discrete distribution of easy bleeding that occurs, among others, in the genus Athalia and the tribe Phymatocerini. By contrast, occurrence of ventral glands is restricted to the monophyletic subfamily Nematinae, which are never easy bleeders. Both strategies are especially effective towards insectivorous insects such as ants, while only Nematinae species are frequently brightly colored and truly gregarious. Among ten tests of phylogenetic correlation between traits, only a few are significant. None of these involves morphological traits enhancing visual signals, but easy bleeding is associated with the absence of defensive body movements and with toxins occurring in the host plant. Easy bleeding functions through a combination of attributes, which is corroborated by an independent contrasts test indicating a statistically significant negative correlation between species-level integument mechanical resistance and hemolymph feeding deterrence against ants.

Conclusions: Our analyses evidence a repeated occurrence of easy bleeding, and no phylogenetic correlation including specific visual signals is significant. We conclude that the evolution of chemically-based defenses in tenthredinids may have been driven by invertebrate as much as by avian predation. The clear-cut visual signaling often encountered in the Nematinae would be linked to differential trends of habitat use by prey and predators. Further studies on (prey) insect groups should include visual signals and other traits, as well as several groups of natural enemies, to better interpret their relative significance and to refine our understanding of insect chemical defenses.

Figure 1

Evolutionary interactions among trophic levels influencing chemical defensive strategies in phytophagous insects. Phytophagous insects are held in ‘ecological pincers’ consisting of top–down as well as bottom–up selective pressures in the case of host plants containing deleterious chemicals (red arrows). However, the insects may sequester plant compounds, and/or produce defensive chemicals themselves, and they can also combine chemical with non-chemical defensive traits, which are all traits eventually used upon attack by natural enemies (green arrows).

Figure 2

Relaxed molecular-clock phylogeny of the Tenthredinidae and selected outgroup taxa. The BEAST MCC tree is based on analysis of Dataset 1, which includes tenthredinids that have sequences from all three genes, and all outgroups. Numbers above branches are posterior probabilities (%) from the BEAST analysis, numbers below branches show corresponding values from the MrBayes run (clades not present in the MrBayes tree are indicated by hyphens). Grey shaded bars show 95% highest posterior density intervals for relative node ages for nodes with posterior probabilities over 50%.

Figure 3

Relaxed molecular-clock phylogeny of the Tenthredinidae, and the distribution of various larval ecological and defensive traits within the group. The BEAST MCC tree is based on analysis of Dataset 2, which includes all sequenced tenthredinids as well as representatives from three non-blasticotomid families in Tenthredinoidea. Posterior probabilities (%) resulting from analyses in BEAST and MrBayes are given above and below branches, respectively (clades not present in the MrBayes tree are indicated by hyphens). Grey shaded bars show the 95% highest posterior density intervals for relative node ages for nodes with posterior probabilities exceeding 50%. Branch colors denote host plant classes of the sawfly species (see legend) and ancestral reconstructions based on maximum-likelihood optimization across 1,000 post-burnin trees (see Additional file 4A). In the table to the right of the tree, diet breadth, plant toxicity, and defensive traits (from left to right) are coded as shown in Table 1. (?) Unknown; (x) not applicable.

Figure 4

Part of the phylogenetic tree of tenthredinids with estimated levels of traits linked to easy bleeding, and plot of independent contrasts extracted from a phylogeny that includes only species with no missing data. The tree in (A) was obtained by pruning the BEAST MCC tree in Figure 3, plots on the right-hand side of the tree show levels of integument resistance and hemolymph deterrence estimated for the included species ([40,41] and U. Schaffner, unpublished data). Species excluded from the independent contrasts test due to missing data are denoted by gray terminal branches and parenthesized names. The scatterplot in (B) shows standardized contrasts for 21 nodes on the tree that include only species that have estimates for both traits, as well as the regression line forced through the origin.