Jacques Pasteels

Salicin from host plant as precursor of salicylaldehyde in defensive secretion of chrysomeline larvae

ABSTRACT. Phratora vitellinae L. and Chrysomela tremulae F. (Chrysomelinae, Coleoptera) feed on Salix or Populus spp. (Salicaceae). Their larvae, as well as the larvae of other chrysomelines feeding on Salicaceae, secrete salicylaldehyde. In this study, we demonstrate that salicylaldehyde is derived from salicin, a phenylglucoside present in the leaves of the host plant. The concentration of salicylaldehyde in the secretion is positively correlated with the amount of salicin in the food of the larvae. The transformation of salicin into salicylaldehyde occurs in the defence glands since the β‐glucosidase activity is 4 times higher in their glands than in the gut. The larvae recover most of the glucose that results from the hydrolysis of salicin. For generalist predators, such as ants, salicylaldehyde is a more potent deterrent than saligenin or salicin.

How Trail Laying and Trail Following Can Solve Foraging Problems for Ant Colonies

One of the most striking features of an ant colony’s behaviour is its capacity for the spatial organisation of foraging activity. The use of trail pheromone to guide fellow workers in the nest to a large food source or rich foraging zone has been extensively studied (e. g. Wilson 1971) and obviously contributes to foraging efficiency. We have recently, however, been able to show that trail laying and trail following behaviour are more than just a means of communicating a food source’s location. When more than one trail is present at a time, the interactions between foragers and the trails can lead to the collective selection of the shortest path or the best food source, despite the fact that individual foragers have no means of making such choices.

Economics of chemical defense in chrysomelinae

Chemical defense in chrysomelid larvae (subtribe Chrysomelina and Phyllodectina) is reviewed. Most species secrete autogenous monoterpenes. The diversity of their secretion is interpreted as a mechanism to reduce adaptation by predacious arthropods. The consequences of a host plant shift to the Salicacae are explored. Salicin from these host plants is used as a precursor for the salicylaldehyde secreted by the larvae of many species. This offers several advantages. It provides the larvae with an inexpensive and efficient defense. The recovery of the glucose moiety of the salicin contributes significantly to the larval energy budget. Adults sequester salicin in the eggs at concentrations which are toxic to ants. Owing to this maternal provisioning, neonate larvae produce salicylaldehyde from hatching onwards, whereas other species secreting monoterpenes are not protected at hatching. The secretion of salicylaldehyde by different species is considered to be chemical mimicry reinforcing visual aposematic signals.

HOST-PLANT SWITCHES AND THE EVOLUTION OF CHEMICAL DEFENSE AND LIFE HISTORY IN THE LEAF BEETLE GENUS OREINA

Insect‐plant interactions have played a prominent role in investigating phylogenetic constraints in the evolution of ecological traits. The patterns of host association among specialized insects have often been described as highly conservative, yet not all specialized herbivorous insect lineages display the same degree of fidelity to their host plants. In this paper, we present an estimate of the evolutionary history of the leaf beetle genus Oreina. This genus displays an amazing flexibility in several aspects of its ecology and life history: (1) host plant switches in Oreina occurred between plant families or distantly related tribes within families and thereby to more distantly related plants than in several model systems that have contributed to the idea of parallel cladogenesis; (2) all species of the genus are chemically defended, but within the genus a transition between autogenous production of defensive toxins and sequestration of secondary plant compounds has occurred; and (3) reproductive strategies in the genus range from oviparity to viviparity including all intermediates that could allow the gradual evolution of viviparity. Cladistic analysis of 18 allozyme loci found two most parsimonious trees that differ only in the branching of one species. According to this phylogeny estimate, Oreina species were originally associated with Asteraceae, with an inclusion of Apiaceae in the diet of one oligophagous species and an independent switch to Apiaceae in a derived clade. The original mode of defense appears to be the autogenous production of cardenolides as previously postulated; the additional sequestration of pyrrolizidine alkaloids could have either originated at the base of the genus or have arisen three times independently in all species that switched to plants containing these compounds. Viviparity apparently evolved twice in the genus, once without matrotrophy, through a retention of the eggs inside the females oviducts, and once in combination with matrotrophy. We hypothesize that the combination of autogenous defense and a life history that involves mobile externally feeding larvae allowed these beetles to switch host plants more readily than has been reported for highly conservative systems.

Toxins in chrysomelid beetles : possible evolutionary sequence from de novo synthesis to derivation from food-plant chemicals

In the Chrysomelinae, it appears that de novo synthesis of chemicals for defense is the primitive state, and the sequestration of plant chemicals for defense the derived state. The derived state evolved through both the morphological and biochemical preadaptiveness of the homologous defensive glands. In the adults, we discuss one unique case of sequestration in exocrine defensive glands of host-plant pyrrolizidine alkaloids byOreina cacaliae. However, hypericin is not sequestered either in the glands or elsewhere in the body ofChrysolina spp. feeding onHypericum, which contradicts an earlier claim. In the larvae, we examine in more detail how the phenolglucoside salicin can be used as the precursor of the salicylaldehyde present in the defensive secretion ofPhratora vitellinae andChrysomela spp. with minimal changes in the biochemical mechanisms involved in the biosynthesis of iridoid monoterpenes in related species.

Chemical defense in the Chrysomelidae

Antipredation mechanisms exist in nearly all animals, but are of course better developed in those which appear to be most vulnerable. Among them are phytophagous insects and above all phyllophagous insects such as leaf beetles. Due to their low food conversion efficiency (Southwood 1973), phytophagous insects spend much time feeding, during which they are little mobile and poorly hidden. Leaf damage to and feces left on the foliage could give additional cues to predators, including parasitoids (Weselok 1981). Many chrysomelids are food specialists and pass all or most of their life cycle on their food plant. The patchy distribution of the food plants and the low dispersion rate of larvae and gravid females are such that leaf beetles tend to form conspicuous aggregates.

Chemical defence in chrysomelid eggs and neonate larvae

ABSTRACT. Eggs and neonate larvae of chrysomelid beetles (sub‐tribes Chrysomelina and Phyllodectina) were investigated for the presence of defensive substances.

Sequestration of plant pyrrolizidine alkaloids by chrysomelid beetles and selective transfer into the defensive secretions

SummaryOreina cacaliae andO. speciosissima (Coleoptera, Chrysomelidae) sequester in their elytral and pronotal defensive secretions pyrrolizidine alkaloids (PAs) as Noxides (PA N-oxides). The PA N-oxide patterns found in the beetles and their host plants were evaluated qualitatively and quantitatively by capillary gas chromatography/mass spectrometry (GC-MS). Of the three host plantsAdenostyles alliariae (Asteraceae) is the exclusive source for PA N-oxide sequestration in the defensive secretions of the beetles. With the exception of O-acetylseneciphylline the N-oxides of all PAs ofA. alliariae, i.e. senecionine, seneciphylline, spartioidine, integerrimine, platyphylline and neoplatyphylline were identified in the secretion. PA N-oxides typical ofSenecio fuchsii (Asteraceae) were detected in the bodies of the beetles but not in their secretion. No PAs were found in the leaves of the third host plant,Petasites paradoxus (Asteraceae). The results suggest the existence of two distinctive storage compartments for PA N-oxides in the beetle: (1) the defensive secretion, containing specifically PA N-oxides acquired fromA. alliariae; (2) the body of the beetle, sequestering additionally but less selectively PA N-oxides from other sources,e.g. S. fuchsii or monocrotaline N-oxide fed in the laboratory. The concentration of PA N-oxides in the defensive secretion is in the range of 0.1 to 0.3 mol/1, which is more than 2.5 orders of magnitude higher than that found in the body of the beetle. No significant differences exist in the ability of the two species of beetles to sequester PA N-oxides fromA. alliariae, althoughO. speciosissima, but notO. cacaliae, produces autogenous cardenolides. A negative correlation seems to exist between the concentrations of plant-derived PA N-oxides andde novo synthesized cardenolides in the defensive secretion ofO. speciosissima.

Production of cardiac glycosides by chrysomelid beetles and larvae

Cardenolides were looked for in 17 chrysomelid beetles belonging to 11 genera from three subfamilies, and they were found only inChrysolina andChrysochloa species (Chrysomelinae, Chrysolinini). The food plants of these insects are not known to produce cardenolides. TheChrysochloa and mostChrysolina species secrete a complex mixture of cardenolides, butChrysolina didymata secretes a single compound, andChrysolina carnifex, none. Several quantitative and perhaps qualitative differences were observed in the patterns of cardenolides produced by far distant populations of bothChrysolina polita andC. herbacea, collected in either France and Belgium, or Greece. These differences remain constant from one generation to the other, whatever the food plant is, and appear to be genetic. InC. polita from Greece, the pattern is unchanged after four generations bred in the laboratory onMentha ×villosa, which is known to be without cardenolides. In adults, the cardenolides are released with the secretion of the pronotal and elytral defensive glands, but in the larvae which lack the defensive glands, cardenolides are also produced. The total amount of cardenolides and the complexity of their mixture increases through the life cycle of the insects. The six main cardenolides secreted byC. coerulans were identified as: sarmentogenin, periplogenin, bipindogenin, and their corresponding xylosides.C. didymata secretes only sarmentogenin.

Influence of phenolglucosides and trichome density on the distribution of insects herbivores on willows

The effects of both trichome density and phenolglucoside content of leaves of 76 willow hybrids (Salix alba × fragilis) were measured to estimate their influence on the distribution of Phratora vitellinae (L.), Plagiodera versicolora Baly (Coleoptera: Chrysomelidae) and Pontania proxima (Lepeletier 1823) (Hymenoptera: Tenthredinidae) in a nursery at Gramont, Belgium.