Methylalkylpyrazines in aposematic insects, their hostplants and mimics

Abstract

SummaryAdults of forty-five aposematic insect species from four orders, and/or, in eight cases, their early stages, were examined for the presence of 2-methoxy-3-alkylpyrazines by coupled gas chromatography/mass spectrometry. The survey also included eight cryptic and related species and six unrelated mimics, twelve selected hostplants of those herbivores that had proved positive for such pyrazines, and nine well known toxic plants. Positive results were as follows: aposematic adult species, 26 (58 %); obscurely coloured species, 1 (12.5%); mimics, 2 (33%); host-plants, 5 (42%); well-known toxic plants, 4 (44%). Among the taxonomic groups, these pyrazines were found in the lepidopterän families and subfamilies Papilionidae, Danainae, Heliconiinae, Acraeinae, Zygaenidae and Arctiidae, and among the Coleoptera, in the Lycidae, Cantharidae, Endomychidae, Coccinellidae, Oedemeridae and Meloidae. Single cases of the occurrence of these pyrazines were also detected in the Hemiptera (Cercopidae) and Orthoptera (Pyrgomorphidae). Mimics were found with and without the pyrazines. Among the plants they were detected in Aristolochiaceae, Asclepiadaceae, Asteraceae, Papaveraceae, Passifloraceae, Solanaceae and Urticaceae. Some aposematie and phytophagous insects apparently sequester pyrazines from their hostplants whereas others, particularly the predatory species, manufacture these substances de novo. Alerting signals in general are discussed.

Chemical defence of the warningly coloured caterpillars of Methona themisto (Lepidoptera: Nymphalidae: Ithomiinae)

“…The presence of repellents cannot be discarded since chicks showed aversive responses to caterpillars without pecking them. Warning odours such pyrazines were found in several aposematic insects (Moore et al, 1990). That Methona themisto caterpillars are chemically defended is supported by the observation that chicks avoid eating mealworms treated with the dichloromethanic extract of 5 th instar caterpillars.…”

The relative importance of colour, taste and smell in the protection of an aposematic insect Coccinella septempunctata

“…The smell of seven-spot ladybirds is partly due to pyrazines (Moore et al 1990), chemicals described as 'alerting odours' (Guilford et .il 1987;Moore et al 1990). They enhance at least the positional learning ability of animals (Kayc et al 1989), although their effects on colour pattern learning have not yet been tested.…”

The chemistry of antipredator defense by secondary compounds in neotropical lepidoptera: facts, perspectives and caveats

“…Alkyl-substituted pyrazines are known to serve as trail-laying pheromones 98 or alarm pheromes 99 in some ants. In Lepidoptera, 2-methoxy-3-alkylpyrazines ( Figure 6, 21) were found in several taxa of aposematic butterflies and moths, and sometimes in their larval host plants 100,101 . These substances potentiate the rejection response of rats and chickens when they drink an unpalatable quinine-water solution 96,102,103 .…”