Variation in cardiac glycoside content of monarch butterflies from natural populations in eastern North America

Cardenolide storage in Danaus chrysippus (L.) with additional notes on D. plexippus (L.)

Danaus chrysippus was reared in the laboratory from stock obtained from Kampala (Uganda), Nairobi (Kenya), Dar-es-Salaam (Tanzania) and Freetown (Sierra Leone) and wild-caught samples from Nigeria, southwest Africa and Tanzania were also analysed. D. plexippus was reared on the same plants for comparison. It was found that the adult D. chrysippus is a poor and inconsistent storer of cardiac glycosides compared with D. plexippus , and contained, chiefly, highly polar cardenolides. Populations in East Africa are, on the whole, more efficient storers than those from West Africa, a factor which may contribute to the dearth of mimics in West Africa. Furthermore there appears to be a genetic element in the storage capacity of these butterflies, not merely a ‘mirror’ effect, depending on the cardenolide content of their food plants. Differences in storage capacity were shown between morphs alcippus from both Sierra Leone and Dar-es-Salaam and aegyptius from Tanzania and Dar-es-Salaam, reared side by side on the same tested food plants. In both cases aegyptius was the better storer but in other broods from Kenya, reared on Asclepias rich in cardenolides, this morph was negative for these substances. During the investigation strains of Asclepias curassavica were found which contained calotropin, but lacked calactin. D. plexippus reared on these plants also lacked calactin, but sequestered and stored it when fed on Gomphocarpus fruticosus which contained both substances. The methods for analysing cardenolides of this type are described. Maps are presented showing the distribution of the three principal morphs of D. chrysippus and the form albinus in Africa.

An experimental investigation of the effects of selective predation by birds and parasitoid attack on the butterfly Danaus chrysippus (L.)

Danaus chrysippus is a danaid butterfly commonly found throughout Africa. It feeds, in its larval stage, on species of milkweed (fam. Asclepiadacae). If the milkweed species has a high cardenolide content, then the larva as well as its pupa and the subsequent adult will be toxic to birds. If, however, the larva has fed on a species of milkweed with little or no cardenolide content, then it will be palatable to birds and so will the pupa and the adult. All the three stages (larval, pupa and adult), whether palatable or not, are subject to attack by birds. The larvae, on the other hand, also act as hosts to a large number of parasitoids. In this paper a theoretical model is described which incorporates the following three assumptions that have been verified experimentally: (i) female butterflies prefer to oviposition the species of plants on which they had developed; (ii) birds are able to distinguish between externally identical larvae from the shape of the plant on which the larvae are feeding; (iii) female parasitoids select, for oviposition, D. chrysippus larvae feeding on plants toxic to vertebrate predators. The simulations using the theoretical model predicts that parasitoids will prefer to lay their eggs in toxic larvae that are less likely to be predated by birds. This predation makes it advantageous for larvae to feed on non-toxic plants. This larval advantage is countered at the adult stage by bird predation. As one type of butterfly becomes too common, the other type will be at an advantage. The polymorphism for palatability is maintained primarily through selection at the larval stage and not by bird predation on the adult butterflies. The experimental data supporting the above assumptions are discussed. Computer simulation predicts the behaviour of birds and parasitoids as it affects the relative frequency of toxic and non-toxic butterflies.

Host plant influence on chemical defense in conifer sawflies (Hymenoptera: Diprionidae)

Host diet affects the defensive efficacy of Neodiprion sawflies. In laboratory assays with wood ants (Formica obscuripes), secretions from larvae reared on Pinus banksiana were the most repellent, while those from P. resinosa feeders were the least so. This was explained diterpene resin acid content, but not total monoterpene content. The terpene content of regurgitant generally reflected dietary concentrations. Compounds were sequestered nonselectively by larvae. Host-based differences in defense persisted at the behavioral level. P. banksiana feeders regurgitated greater volumes of fluid and were less likely to be disabled or killed by ants in one-on-one interactions than were larvae fed on P. resinosa. The defensive advantages of host diet conflicted with developmental requirements. N. sertifer reared on P. banksiana (the best diet for defense in all cases) had lower cocoon weights (a correlate of fecundity) than those reared on other diets, and had prolonged larval development compared to insects fed P. sylvestris. No such tradeoff was detected in N. lecontei. Larvae of both species strongly preferred P. banksiana over P. resinosa in feeding choice assays.

Cardenolide and Neutral Lipid Biosynthesis from Malonate in Digitalis lanata

DIGITALIS LANATA plants were grown on water culture in a controlled environment and in the young, growing leaves free sterols (0.335 micromol/g FW), triacylglycerols (0.97 micromol/g FW) and cardenolides (1.82 micromol/g FW) were the major apolar and polar lipids. The cardenolide-containing fraction from these tissues was separated into 26 cardenolides by HPLC. The 5 major components (accounting for 60 % of the occurring glycosides) lanatosides A and C, acetyldigoxin, acetyldigitoxin, and glucoevatromonoside were identified by FABMS. Incorporation experiments with [2- (14)C]-acetate, [2- (14)C]-malonate, [2- (14)C]-mevalonate, and [U- (14)C]-sucrose (absorbed by excised, young, growing leaves) showed the labelling of all the occurring cardenolides after a 3 day incorporation period (as judged by HPLC). Comparing the simultaneous synthesis of labelled sterols and triacylglycerols, malonate could be considered as the most effective precursor in cardenolide synthesis, reaching an incorporation value of 4.1 % in a 4 day incorporation period. A time-course experiment revealed a temporary accumulation of (14)C in glucoevatromonoside, which may play a role as an intermediate in cardenolide production of DIGITALIS LANATA.

Red Cochineal Dye (Carminic Acid): Its Role in Nature

Carminic acid, the well-known red dyestuff from cochineal insects (Dactylopius spp.), is a potent feeding deterrent to ants. This deterrency may be indicative of the natural function of the compound, which may have evolved in cochineals as a chemical weapon against predation. The behavior of an unusual predator is described-the carnivorous caterpillar of a pyralid moth (Laetilia coccidivora)-which is undeterred by carminic acid and feeds on cochineals. The animal has the remarkable habit of utilizing the ingested carminic acid for defensive purposes of its own.

Reexamination of tetradecenyl acetates in oak leaf roller sex pheromone and in plants

The chemistry of the oak leaf roller sex pheromone is shown by means of microozonolysis and computerized gas chromatography-mass spectrometry to be dominated by an approximate 70 to 30 ratio of E- and Z-11-tetradecenyl acetates. Tetradecenyl acetates are undetectable in highly purified oak leaf, apple leaf, and corn extracts analyzed by gas chromatography-mass spectrometry. These results reflect negatively on previous reports and on the hypothesis that plant components might govern insect chemical communication systems.

The genetics of the mimetic butterfly Hypolimnas bolina (L.)

Hypolimnas bolina is a Nymphalid butterfly having a west to east distribution from Madagascar to Easter Island, and a north to south one from Japan to Australasia. It is highly migratory in some areas. In much of the western part of its range the female is both monomorphic and a mimic of Euploea. Further east it is frequently polymorphic with the majority of the forms being non-mimetic. The polymorphism is sex-limited to the female and controlled by two unlinked loci, one with two allelomorphs, E and e, determining the extent of the dark pigmentation, the other with three allelomorphs, P, Pn and p, determining the presence and distribution of orange-brown. Only butterflies of the genotypes EEpp and to a lesser extent Eepp are satisfactory Batesian mimics of their Euploea models. The details of the mimetic pattern are under multifactorial control, following those of their local model, as is much of the variation within the non-mimetic forms, particularly with regard to the distribution of white and blue scaling.

Evidence for origin of insect sex pheromones: presence in food plants

Compounds identified as sex attractant pheromones in a number of phytophagous insects were found in a variety of host plants. These agents vary in chemical composition in different plant species, which suggests that dietary factors may provide an evolutionary mechanism for diversification of certain insect species. A theoretical framework to explain this phenomenon is postulated on the basis of experiments with the oak leaf roller moth.

Localization of Heart Poisons in the Monarch Butterfly

The cardiac glycosides that monarch butterflies sequester from milkweed plants during the larval stage differ remarkably in their emetic potency and are concentrated to different degrees in the various parts of the body as well as in the two sexes (Fig. 1). The very high concentrations of these compounds in the wings probably facilitate learned taste rejection in predators and account for the relatively high frequency of Danaid butterflies with beak-marked wings in natural populations. The cardiac glycosides in the abdomen have a much higher emetic potency than those in the rest of the body. Consequently, naive, extremely hungry, or forgetful birds which capture and peck off the wings but eat the abdomen discard the least emetic glycosides and ingest the most emetic, and thus again experience emesis. The nonrandom distribution of cardenolides in the wings, abdomen, and thorax, together with the fact that monarch males not only contain lower concentrations of cardiac glycosides than females but also contain cardenolides that are overall less emetic than those in females, is interpreted as evidence that these poisons are incorporated at a physiological cost. This cost, balanced against the benefits of protection from predation, provides a selective basis for the occurrence of both emetic and nonemetic individuals in natural populations. Since birds can discriminate emetic from nonemetic monarchs on the basis of taste, it is not necessary to invoke theories of kind of group selection to explain the evolution of this kind of unpalatability.