Some adaptations between Danaus plexippus and its food plant, with notes on Danaus chrysippus and Euploea core (Insecta: Lepidoptera)

Eastern monarch larval performance may not be affected by shifts in phenological synchrony with milkweed

Interacting species are experiencing disruptions in the relative timing of their key life-history events due to climate change. These shifts can sometimes be detrimental to the fitness of the consumer in trophic interactions but not always.The potential consequences of phenological asynchrony for the monarch butterfly (Danaus plexippus) and its host plant (Asclepias spp.) have not been well-studied. Given that plants generally undergo seasonal declines in quality, if climate change delays the timing of the larval stage relative to the availability of younger milkweed plants, monarch performance could be negatively affected.Here, we explore the potential consequences for the eastern monarch population due to probable asynchrony with milkweed. We used field surveys around Ottawa, Canada, to determine monarch oviposition preference on common milkweed (Asclepias syriaca) plants and the seasonal availability of these plants. To determine the potential fitness consequences when females oviposit on nonpreferred plants, we conducted a field experiment to assess the effect of milkweed size on monarch larval performance (e.g., development time and final size).Preferred oviposition plants (earlier stages of development and better condition) were consistently available in large proportion over the summer season. We also found that declines in leaf quality (more latex and thicker leaves) with plant size did not translate into decreases in larval performance.Our results suggest that even if asynchrony of the monarch-milkweed interaction occurs due to climate change, the larval stage of the eastern monarch may not face negative consequences. Future studies should determine how the relative timing of the interaction will change in the region.

Host plant specificity of the monarch butterfly Danaus plexippus: A systematic review and meta-analysis

The preference-performance hypothesis explains host specificity in phytophagous insects, positing that host plants chosen by adults confer the greatest larval fitness. However, adults sometimes oviposit on plants supporting low larval success because the components of host specificity (adult preference, plant palatability, and larval survival) are non-binary and not necessarily correlated. Palatability (willingness to eat) is governed by chemical cues and physical barriers such as trichomes, while survival (ability to complete development) depends upon nutrition and toxicity. Absence of a correlation between the components of host specificity results in low-performance hosts supporting limited larval development. Most studies of specificity focus on oviposition behavior leaving the importance and basis of palatability and survival under-explored. We conducted a comprehensive review of 127 plant species that have been claimed or tested to be hosts for the monarch butterfly Danaus plexippus to classify them as non-hosts, low performance, or high performance. We performed a meta-analysis to test if performance status could be explained by properties of neurotoxic cardenolides or trichome density. We also conducted a no-choice larval feeding experiment to identify causes of low performance. We identified 34 high performance, 42 low performance, 33 non-hosts, and 18 species with unsubstantiated claims. Mean cardenolide concentration was greater in high- than low-performance hosts and a significant predictor of host status, suggesting possible evolutionary trade-offs in monarch specialization. Other cardenolide properties and trichome density were not significant predictors of host status. In the experiment, we found, of the 62% of larvae that attempted to eat low-performance hosts, only 3.5% survived to adult compared to 85% of those on the high-performance host, demonstrating that multiple factors affect larval host plant specificity. Our study is the first to classify all known host plants for monarchs and has conservation implications for this threatened species.

Migration in butterflies: a global overview

Insect populations including butterflies are declining worldwide, and they are becoming an urgent conservation priority in many regions. Understanding which butterfly species migrate is critical to planning for their conservation, because management actions for migrants need to be coordinated across time and space. Yet, while migration appears to be widespread among butterflies, its prevalence, as well as its taxonomic and geographic distribution are poorly understood. The study of insect migration is hampered by their small size and the difficulty of tracking individuals over long distances. Here we review the literature on migration in butterflies, one of the best-known insect groups. We find that nearly 600 butterfly species show evidence of migratory movements. Indeed, the rate of 'discovery' of migratory movements in butterflies suggests that many more species might in fact be migratory. Butterfly migration occurs across all families, in tropical as well as temperate taxa; Nymphalidae has more migratory species than any other family (275 species), and Pieridae has the highest proportion of migrants (13%; 133 species). Some 13 lines of evidence have been used to ascribe migration status in the literature, but only a single line of evidence is available for 92% of the migratory species identified, with four or more lines of evidence available for only 10 species - all from the Pieridae and Nymphalidae. Migratory butterflies occur worldwide, although the geographic distribution of migration in butterflies is poorly resolved, with most data so far coming from Europe, USA, and Australia. Migration is much more widespread in butterflies than previously realised - extending far beyond the well-known examples of the monarch Danaus plexippus and the painted lady Vanessa cardui - and actions to conserve butterflies and insects in general must account for the spatial dependencies introduced by migratory movements.

The metabolic fate of dietary nicotine in the cabbage looper, Trichoplusia ni (Hübner)

Cabbage looper (Trichoplusia ni) larvae are generalist herbivores that feed on numerous cultivated plants and weeds including crucifers, other vegetables, flowers, and field crops. Consuming plant material from a wide range of plant species exposes these larvae to a considerable variety of plant secondary metabolites involved in chemical defense against herbivory. The ability of the cabbage looper larvae to detoxify plant secondary metabolites, such as nicotine, has been attributed to the rapid induction of excretion via the Malpighian tubules. However, the role of metabolism in the detoxification of plant secondary metabolites in cabbage looper larvae is not well studied. We investigated nicotine metabolism in 4th larval instar cabbage looper using UPLC-MS/MS analysis to resolve the time course of nicotine metabolism, the kinetic distribution of nicotine, and the presence or absence of major metabolites of nicotine in larval tissue and excretions. The major metabolite found in our analysis was cotinine, with trace amounts of cotinine N-oxide and nicotine N-oxide. The nicotine metabolites detected are similar to those of the nicotine-tolerant Lepidopteran tobacco hornworm (Manduca sexta). The results of our study demonstrate that the 5'C-oxidation of nicotine to cotinine is the primary pathway for nicotine metabolism in cabbage looper larvae. This study showed that metabolism of nicotine and subsequent excretion of nicotine and its metabolites occurs in the larvae of the cabbage looper. Our results suggest that 5'C-oxidation in lepidopteran insects is a conserved metabolic pathway for the detoxification of plant secondary metabolites.

Poor sequestration of toxic host plant cardenolides and their rapid loss in the milkweed butterfly Danaus chrysippus (Lepidoptera: Nymphalidae: Danainae: Danaini)

Butterflies of the genus Danaus are known to sequester toxic cardenolides from milkweed host plants (Apocynaceae). In particular, Danaus plexippus efficiently sequesters and stores these compounds, whereas D. chrysippus, is considered to poorly sequester cardenolides. To estimate its sequestration capability compared with that of D. plexippus, larvae of both species were jointly reared on Asclepias curassavica and the major cardenolides of the host plant, calotropin and calactin, were analyzed in adults sampled at different time intervals after eclosion. Both cardenolides were detected in body and wings of D. plexippus. Whereas the calotropin-concentration remained constant over a period of 24 days, that of calactin steadily decreased. In the body, but not in the wings of D. chrysippus, calactin only was detected in low amounts, which was then almost completely lost during the following 8 days after eclosion, suggesting that in contrast to D. plexippus, cardenolides seem to be less important for that butterfly's defence against predators.

Milkweed butterfly resistance to plant toxins is linked to sequestration, not coping with a toxic diet

Insect resistance to plant toxins is widely assumed to have evolved in response to using defended plants as a dietary resource. We tested this hypothesis in the milkweed butterflies (Danaini) which have progressively evolved higher levels of resistance to cardenolide toxins based on amino acid substitutions of their cellular sodium-potassium pump (Na(+)/K(+)-ATPase). Using chemical, physiological and caterpillar growth assays on diverse milkweeds (Asclepias spp.) and isolated cardenolides, we show that resistant Na(+)/K(+)-ATPases are not necessary to cope with dietary cardenolides. By contrast, sequestration of cardenolides in the body (as a defence against predators) is associated with the three levels of Na(+)/K(+)-ATPase resistance. To estimate the potential physiological burden of cardenolide sequestration without Na(+)/K(+)-ATPase adaptations, we applied haemolymph of sequestering species on isolated Na(+)/K(+)-ATPase of sequestering and non-sequestering species. Haemolymph cardenolides dramatically impair non-adapted Na(+)/K(+)-ATPase, but had systematically reduced effects on Na(+)/K(+)-ATPase of sequestering species. Our data indicate that major adaptations to plant toxins may be evolutionarily linked to sequestration, and may not necessarily be a means to eat toxic plants. Na(+)/K(+)-ATPase adaptations thus were a potential mechanism through which predators spurred the coevolutionary arms race between plants and insects.

Cardenolide content and thin-layer chromatography profiles of monarch butterflies,danaus plexippus L., and their larval host-plant milkweed,asclepias viridis walt., in northwestern louisiana

This paper is the first in a series on cardenolide fingerprinting of monarch butterflies and their host-plant milkweeds in the eastern United States. Spectrophotometric determinations of the gross cardenolide content of 60Asclepias viridis plants in northwestern Louisiana indicate a positively skewed variation ranging from 95 to 432 υg/0.1 g dry weight with a mean of 245 υg/0.1 g. Butterflies reared individually on these plants contained a normal cardenolide distribution ranging from 73 to 591 υg/0.1 g dry weight with a mean of 337 υg/0.1 g. The uptake of cardenolide by the butterflies best fit a logarithmic function of the plant concentration. Female monarchs (385 υg/0.l g) contained significantly greater mean cardenolide concentrations than did males (287 υg/0.1 g). No indications of a metabolic cost for either cardenolide ingestion or storage were adduced from size or dry weight data. Thin-layer chromatograms of 24 individual plant-butterfly pairs developed in two solvent systems resolved 21 individual spots in the plants and 15 in the butterflies.A. viridis plants appear to contain several relatively nonpolar cardenolides of the calotropagenin series which are metabolized to the more polar 3'-hydroxy derivatives calactin and calotropin as well as to calotropagenin in the butterflies. The epoxy cardenolides labriformin and labriformidin were absent, although desglucosyrioside (a 3'-hydroxy derivative) appeared present in both plants and butterflies. Quantitative evaluation of theR f values, spot intensities, and probabilities of occurrence in the chloroform-methanol-formamide TLC system produced a cardenolide fingerprint clearly distinct from those previously established for monarchs reared on otherAsclepias species, supporting the use of fingerprints to make ecological predictions concerning larval host-plant utilization.A. viridis is the predominant early spring milkweed throughout most of the south central United States and may be important in providing chemical protection to spring and early summer generation monarchs in the eastern United States.

Cardenolide content and thin-layer chromatography profiles of monarch butterflies,Danaus plexippus L., and their larval host-plant milkweed,Asclepias asperula subsp.Capricornu (woods.) woods., in north central Texas

This paper is the second in a series on cardenolide fingerprinting of monarch butterflies and their host-plant milkweeds in the eastern United States. Spectrophotometric determinations of the gross cardenolide content ofAsclepias asperula plants in north central Texas indicated wide variation ranging from 341 to 1616 μg/0.1 g dry weight. The mean plant cardenolide concentration (886 μg/0.1 g) is the highest for any milkweed species on which monarch cardenolide profiles have been produced. Forty-one butterflies reared individually on these plants contained a skewed distribution of cardenolide concentrations ranging from 231 to 515 μg/0. 1 g dry weight with a mean of 363μg/0.1 g. The uptake of cardenolide by the butterflies was independent of plant concentration, suggesting that saturation occurs in cardenolide sequestration by monarchs when feeding on cardenolide-rich host-plants. Female monarchs contained significantly greater mean cardenolide concentrations (339 μg/0.1 g) than did males (320 μg/0.1 g). The mean dry weight of the male butterflies (0.211 g) was significantly greater than the female mean (0.191) so that the mean total cardenolide contents of males (675 fig) and females (754 μg) were not significantly different. Butterfly size was not significantly correlated to butterfly cardenolide concentration when differences due to sex and individual host-plant concentration were removed. Thin-layer chrornatograms of 24 individual plant-butterfly pairs developed in two solvent systems resolved 22 individual spots in the plants and 15 in the butterflies.A. asperula plants appear to contain several relatively nonpolar cardenolides of the calotropagenin series which are metabolized to more polar derivatives in the butterflies. Quantitative evaluation of theR f values, spot intensities, and probabilities of occurrence in the chloroform-methanol-formamide TLC system produced a cardenolide fingerprint clearly distinct from those previously established for monarchs reared on otherAsclepias species. Our data support the use of fingerprints to make ecological predictions concerning larval host-plant utilization.A. asperula subsp.capricornu andA. viridis Walt, are the predominant early spring milkweeds throughout most of the south central United States. Cardenolide-rich monarchs reared on these two species may be instrumental in establishing and reinforcing visual avoidance of adults by naive predators throughout their spring and summer breeding cycle in eastern North America.

Phenological Variation in Chemical Defense of the Pipevine Swallowtail, Battus philenor

Larvae of the pipevine swallowtail, Battus philenor, feed on plants in the genus Aristolochia, which contains aristolochic acids, toxic alkaloids unique to the Aristolochiaceae. Pipevine swallowtails sequester these compounds and, as a consequence, are chemically defended against many natural enemies. In California, the primary aristolochic acid present in the butterfly is aristolochic acid I. Newly eclosed adult females possess greater amounts of these sequestered toxins compared to males. However, over the course of the flight season, the aristolochic acid content of females in the population declines, whereas male aristolochic acid content remains relatively constant. Transference of sequestered aristolochic acids to eggs by females might explain the decline of these sequestered chemical defenses observed over time. We found no evidence that males transfer aristolochic acids to females via the spermatophore. The possibility that females at the end of the flight season may be automimics of males is discussed. Temporal variation in the aristolochic acid defenses exhibited by this pipevine swallowtail population is both age- and sex-dependent.

Response in nematocyst uptake by the nudibranch Flabellina verrucosa to the presence of various predators in the Southern Gulf of Maine

Aeolid nudibranchs maintain nematocysts sequestered from their cnidarian prey for protection against predators. Selection for nematocyst incorporation is a function of diet and prey choice, but ratios vary among nudibranchs feeding on a given diet, indicating that other factors may be involved. It is proposed that the presence of predators influences nematocyst incorporation. Nematocyst uptake in the nudibranch Flabellina verrucosa collected from the southern Gulf of Maine was examined in response to various potential predators, including Crossaster papposus, Tautogolabrus adspersus, and Carcinus maenas. Nudibranchs in individual flow-through containers feeding on a diet of the hydroids Tubularia spp. and Obelia geniculata were subjected to tanks containing a predator, then their nematocyst distribution was examined. Although most of the changes over the experimental period were attributable to diet, F. verrucosa responded to both T. adspersus and C. papposus by significantly increasing microbasic mastigophore incorporation. No differential uptake was seen with C. maenas. Response was evident in the nudibranchs both for predators present in the collection area and for those with which they had no previous exposure, indicating that F. verrucosa modulates nematocyst incorporation in response to the presence of predators as well as to diet. A coevolution of nudibranchs and potential predators may govern changes in nematocyst uptake.

Sequestration of defensive substances from plants by Lepidoptera

A number of aposematic butterfly and diurnal moth species sequester unpalatable or toxic substances from their host plants rather than manufacturing their own defensive substances. Despite a great diversity in their life histories, there are some general features in the selective utilization of plant secondary metabolites to achieve effective protection from predators. This review illustrates the biochemical, physiological, and ecological characteristics of phytochemical-based defense systems that can shed light on the evolution of the widely developed sequestering lifestyles among the Lepidoptera.

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.

Oviposition stimulants for the monarch butterfly: flavonol glycosides from Asclepias curassavica

The monarch butterfly, Danaus plexippus oviposits on milkweed plants, primarily within the Asclepiadaceae. Oviposition stimulants responsible for host plant recognition were isolated from Asclepias curassavica. Six flavonoid glycosides-quercetin 3-O-(2",6"-alpha-L-dirhamnopyranosyl)-beta-D-galactopyranoside, quercetin 3-O-beta-D-glucopyranosyl-(1-->6)-beta-D-galactopyranoside, quercetin 3-O-(2"-O-alpha-L-rhamnopyranosyl)-beta-D-galactopyranoside, quercetin 3-O-alpha-L-rhamnopyranosyl-(1-->6)-beta-D-glucopyranoside, quercetin 3-O-beta-D-galactopyranoside, quercetin 3-O-beta-D-glucopyranoside, and an unidentified flavonoid mixture were isolated and characterized from this plant. An additional glycoside, possibly quercetin 3-O-(2",6"-alpha-L-dirhamnopyranosyl)-beta-D-glucopyranoside, which could not be separated from the first triglycoside, was also found in some batches of plant extract. The two dirhamnosyl glycosides, the glucosylgalactose and the rutinoside were found to be active as oviposition stimulants at 0.5 g leaf equivalents.