Palatability of aposematic queen butterflies (Danaus gilippus) feeding onSarcostemma clausum (Asclepiadaceae) in Florida

Multimodal Aposematic Defenses Through the Predation Sequence

Aposematic organisms warn predators of their unprofitability using a combination of defenses, including visual warning signals, startling sounds, noxious odors, or aversive tastes. Using multiple lines of defense can help prey avoid predators by stimulating multiple senses and/or by acting at different stages of predation. We tested the efficacy of three lines of defense (color, smell, taste) during the predation sequence of aposematic wood tiger moths (Arctia plantaginis) using blue tit (Cyanistes caeruleus) predators. Moths with two hindwing phenotypes (genotypes: WW/Wy = white, yy = yellow) were manipulated to have defense fluid with aversive smell (methoxypyrazines), body tissues with aversive taste (pyrrolizidine alkaloids) or both. In early predation stages, moth color and smell had additive effects on bird approach latency and dropping the prey, with the strongest effect for moths of the white morph with defense fluids. Pyrrolizidine alkaloid sequestration was detrimental in early attack stages, suggesting a trade-off between pyrrolizidine alkaloid sequestration and investment in other defenses. In addition, pyrrolizidine alkaloid taste alone did not deter bird predators. Birds could only effectively discriminate toxic moths from non-toxic moths when neck fluids containing methoxypyrazines were present, at which point they abandoned attack at the consumption stage. As a result, moths of the white morph with an aversive methoxypyrazine smell and moths in the treatment with both chemical defenses had the greatest chance of survival. We suggest that methoxypyrazines act as context setting signals for warning colors and as attention alerting or “go-slow” signals for distasteful toxins, thereby mediating the relationship between warning signal and toxicity. Furthermore, we found that moths that were heterozygous for hindwing coloration had more effective defense fluids compared to other genotypes in terms of delaying approach and reducing the latency to drop the moth, suggesting a genetic link between coloration and defense that could help to explain the color polymorphism. Conclusively, these results indicate that color, smell, and taste constitute a multimodal warning signal that impedes predator attack and improves prey survival. This work highlights the importance of understanding the separate roles of color, smell and taste through the predation sequence and also within-species variation in chemical defenses.

Performance of Danaini larvae is affected by both exotic host plants and abiotic conditions

The consequences of the introduction of invasive plants for the diet of herbivorous insects have been little explored in nature where, potentially, abiotic and biotic factors operate. In this study, we examined the larval performance of two Neotropical Danaini butterflies when using either a native or an exotic Apocynaceae species as host plant in both field and laboratory experiments. Hosts greatly differ in their amount of latex exudation and other physicochemical traits, as well as in the amount of evolutionary time they have interacted with herbivores. First, herbivore performance on the hosts was investigated under laboratory conditions. Larvae of both Danaini species took more time to develop on the exotic host; larval survivorship did not vary between hosts. Second, first instar survivorship on both hosts was evaluated in two field sites, one site per host. To do so, in both sites half of the larvae were bagged (protected against both abiotic and biotic factors) while the remainder were nonbagged (exposed). The interaction between larval exposure with the use of the exotic host reduced larval survival. We concluded that the combined effects of host plant traits and abiotic factors reduced survival of herbivores in field conditions. Therefore, the performance of herbivores when using hosts of different origins should be considered together with the multiple ecological factors found in natural environments, as these factors can modify the result of plant-herbivore interactions.

Mimicry in viceroy butterflies is dependent on abundance of the model queen butterfly

Mimics should not exist without their models, yet often they do. In the system involving queen and viceroy butterflies, the viceroy is both mimic and co-model depending on the local abundance of the model, the queen. Here, we integrate population surveys, chemical analyses, and predator behavior assays to demonstrate how mimics may persist in locations with low-model abundance. As the queen becomes less locally abundant, the viceroy becomes more chemically defended and unpalatable to predators. However, the observed changes in viceroy chemical defense and palatability are not attributable to differing host plant chemical defense profiles. Our results suggest that mimetic viceroy populations are maintained at localities of low-model abundance through an increase in their toxicity. Sharing the burden of predator education in some places but not others may also lower the fitness cost of warning signals thereby supporting the origin and maintenance of aposematism.

Kathleen Prudic et al. examine the persistence of mimicry in viceroy butterflies in locations with low model abundance. They show that when queen butterflies are less abundant, viceroy butterflies become more abundant, but also increase their chemical defenses to gain protection from predation.

Mediation of cardiac glycoside insensitivity in the monarch butterfly (Danaus plexippus): Role of an amino acid substitution in the ouabain binding site of Na(+),K (+)-ATPase

The Monarch butterfly (Danaus plexippus) sequesters cardiac glycosides (CG) for its chemical defense against predators. Larvae and adults of this butterfly are insensitive towards dietary cardiac glycosides, whereas other Lepidoptera are sensitive and intoxicated by ouabain. Ouabain inhibits Na(+),K(+)-ATPase by binding to its α-subunit. We have amplified and cloned the DNA-sequence encoding the respective ouabain binding site. Instead of the amino acid asparagine at position 122 in ouabain-sensitive insects, the Monarch has a histidine in the putative ouabain binding site, which consists of 12 amino acids. Starting with the CG-sensitive Na(+),K(+)-ATPase gene fromDrosophila, we converted pos. 122 to a histidine residue as inDanaus plexippus by site-directed mutagenesis. Human embryonic kidney cells (HEK) (which are sensitive to ouabain) were transfected with the mutated Na(+),K(+)-ATPase gene in a pSVDF-expression vector and showed a transient expression of the mutatedDrosophila Na(+),K(+)-ATPase. When treated with ouabain, the transfected cells tolerated ouabain at a concentration of 50 mM, whereas untransformed controls or controls transfected with the unmutatedDrosophila gene, showed a substantial mortality. This result implies that the asparagine to histidine exchange contributes to ouabain insensitivity in the Monarch. In two other CG-sequestering insects, e.g.,Danaus gilippus andSyntomeida epilais, the pattern of amino acid substitution differed, indicating that the Monarch has acquired this mutation independently during evolution.

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.

Comparative unpalatability of mimetic viceroy butterflies (Limenitis archippus) from four south-eastern United States populations

Viceroy butterflies (Limenitis archippus), long considered palatable mimics of distasteful danaine butterflies, have been shown in studies involving laboratoryreared specimens to be moderately unpalatable to avian predators. This implies that some viceroys are Müllerian co-mimics, rather than defenseless Batesian mimics, of danaines. Here, I further test this hypothesis by assessing the palatability of wild-caught viceroys from four genetically and ecologically diverse populations in the southeastern United States. Bioassays revealed that viceroys sampled from three sites in Florida and one in South Carolina were all moderately unpalatable to captive redwinged blackbird predators, which ate fewer than half of the viceroy abdomens presented. Red-wings commonly exhibited long manipulation times and considerable distress behavior when attempting to eat a viceroy abdomen, and they taste-rejected over one-third of viceroys after a single peck. These findings, the first based on wild-caught butterflies, support the hypothesis that the viceroy-danaine relationship in some areas represents Müllerian mimicry, prompting a reassessment of selective forces shaping the interaction. Moreover, considerable variation in palatability of individual viceroys, and in behavior of individual birds, contributes to the complexity of chemical defense and mimicry in this system.

Unpalatability of viceroy butterflies (Limenitis archippus) and their purported mimicry models, Florida queens (Danaus gilippus)

Understanding the dynamics of defensive mimicry requires accurately characterizing the comparative palatability of putative models and mimics. The Florida viceroy butterfly (Limenitis archippus floridensis) is traditionally considered a palatable Batesian mimic of the purportedly distasteful Florida queen (Danaus gilippus berenice). I re-evaluated this established hypothesis by directly assessing palatability of viceroys and queens to red-winged blackbirds in a laboratory experiment. Representative Florida viceroys were surprisingly unpalatable to red-wings; only 40% of viceroy abdomens were entirely eaten (compared to 98% of control butterfly abdomens), and nearly one-third were immediately tasterejected after a single peck. In fact, the viceroys were significantly more unpalatable than representative Florida queens, of which 65% were eaten and 14% taste-rejected. Thus, viceroys and queens from the sampled populations exemplify Müllerian rather than Batesian mimicry, and the viceroy appears to be the stronger model. These findings prompt a reassessment of the ecological and evolutionary dynamics of this classic mimicry relationship.