About Butterflies and Moths – Part 3
By Mary Free, Extension Master Gardener
If a butterfly or moth is going to successfully perpetuate its line, then it has to persevere in a competitive and perilous world. To increase their chances of survival, species have inherited certain traits or developed certain behaviors to avoid or to deal with predators.
Approximately 96% of North American birds feed insects to their young, and slow-moving, nutrient-rich caterpillars with their soft and flexible bodies are prime targets. Besides birds, parasitoids, insect predators, spiders, and reptiles (including Eastern box turtles, snakes, and lizards) attack caterpillars. Adult lepidopterans also are vulnerable to predators, especially when eclosing (emerging), basking, perching, or puddling (sipping mineral-rich liquids from wet ground).
To improve their chances of survival, some lepidopterans are masters of disguise, hiding from their enemies in plain sight. Green caterpillars or those with longitudinal markings on their thorax and abdomen often blend in with the plants they are eating and those with stripes and disruptive coloration are difficult to spot in dappled light. Other caterpillars (e.g., viceroy and early instars of eastern black and tiger swallowtails) sport colors and patterns that resemble old bird droppings. Spanworms deceive not only by the colors, bumps, and rings on their bodies that look like twigs with leaf scars and lenticels but by their resting, twig-like posture–straight, tilted, and taut. The adult blinded sphinx moth (Paonias excaecata) can be mistaken for a dead leaf.
Left to right: Speckled green fruitworm moth caterpillar, eastern black swallowtail butterfly early and 5th instars, filament bearer (horned spanworm moth), and blinded sphinx moth caterpillar.
Other caterpillars are armed with defense mechanisms. Some have stinging or itch-inducing (urticating) hairs. Io moths (Automeris io) have stout bristles that pierce the skin and release a poison when touched. Hickory tussock moths (Lophocampa caryae) have long, flexible hairs that puncture the skin and inject venom or break off and cause mechanical injury. Although wooly bear caterpillars, like yellow bear (Spilosoma virginica), have similarly long hairs, they are not poisonous. However when handled, the hairs can pierce the skin and break off causing irritation. Those tempted to pick up these fuzzy creatures should be careful not to rub their eyes afterward in case hairs have imbedded in their fingers.
Left to right: Io and hickory tussock moth caterpillars, yellow bear (Virginia tiger moth), tobacco hornworm (Carolina sphinx moth), eastern black swallowtail butterfly and eastern tent caterpillars.
Some caterpillars exude chemicals. When a tobacco hornworm (Manduca sexta) feeds on host coyote tobacco plants, it ingests toxic nicotine. Most of this chemical is excreted, but the caterpillar retains a small amount to “be repurposed for defensive functions through respiration as a form of defensive halitosis” (Kumar et al. 2014). This toxic “breath” is effective in warding off a specific nemesis, the wolf spider. A swallowtail caterpillar also can repel other insects with an odor those predators find offensive. It is released when the horn-like organ (osmeterium), held behind its head, is everted. The forked, snake-tongue shape might also surprise birds or reptiles. The eastern tent caterpillar (Malacosoma americanum) repels attacking ants by vomiting an intestinal liquid that contains hydrogen cyanide and benzaldehyde derived from the black cherry leaves that its eats (Peterson et al. 1987). Other lepidopterans, like the silver-spotted skipper, also can vomit defensive chemicals when disrupted. It is unclear if they also produce clicking or chirping sounds similar to the silk and peacock moths mentioned below.
When camouflage is not an option, some lepidopterans flaunt what they have, and nothing is more conspicuous than an eyespot–“a roughly circular pattern on the wing, with at least two concentric rings or with a single color disc and a central pupil” (Monteiro 2008). The common buckeye (Junonia coenia) butterfly is named after its bold, target-shaped eyespots found on both the upper forewing and hindwing. Butterfly experiments have shown that it is the size, prominence, and number of eyespots that intimidate predators rather than the arrangement of eyespots to mimic vertebrate eyes as originally thought. However, the effectiveness of eyespots varies with the environment and with the predator. Predation actually increased when lepidopteran color blended into the background because it made the eyespots more conspicuous revealing the prey. And, if the eyespots did not scare away the predator in the first place, then they served to make the prey easier to find (Kodandaramaiah 2011).
Instead of using eyespots to intimidate predators from attacking, some lepidopterans use eyespots among other elements to deflect predator attacks. “False-head” patterns appear on the closed wings of some butterflies, like hairstreaks and eastern tailed-blues (Everes comyntas), who often eat with their head down and hindwings up. Their posterior hindwings are contoured like a head with eyespots in the margins and slender tails, which move to imitate antennae. This adaptation fools a predator into attacking the hindwings rather than the head and body, so the butterfly loses a piece of wing rather than its life. In a behavioral study of a jumping spider (Phidippus pulcherrimus) and twelve different lepidopteran species, the false-head pattern of the red-banded hairstreak (Calycopis cecrops) butterfly deceived the spider and, unlike the other lepidopterans, the hairstreak escaped attack 100 percent of the time (Torrent 2013). A study of 20,709 museum specimens found that the more false-head elements on a wing, the higher proportion of damage to the false-head area confirming that this adaption was successful in deflecting predator attacks (Galicia et al. 2019).
The silver-spotted skipper (Epargyreus clarus) caterpillar uses a different method of deflection. When it is not eating, it shelters in a folded leaf. To prevent an accumulation of waste (frass), which has chemicals that could attract predators like wasps to its hiding place, it deliberately distances itself from its frass. Martha Weiss of Georgetown University discovered that it did so by “ballistically” ejecting the frass up to five feet away. For a human, that would be about the width of a soccer field.
Aposematism is the use of visual signals, like colors or patterns, to warn predators that the animal is poisonous or distasteful. Predators that recognize (through experience or evolution) their aversion to such species tend to avoid them. A well-known example is the monarch (Danaus plexippus) butterfly whose bright coloration and striking pattern signal enemies that it is poisonous (due to the cardiac glycosides the caterpillar sequesters from eating milkweed leaves). A 2004 study showed that the unique yellow, white, and black bands on the monarch caterpillar provided protection “in the field, both on and off of milkweed hosts” (Hitchcock 2004).
Left to right: Monarch butterfly on aster and monarch caterpillar on host swamp milkweed.
Although aposematism commonly is regarded as a visual warning, some hypothesize that some caterpillars employ acoustic aposematic signals to warn predators of chemical defenses. North American silk moths (Antheraea polyphemus) “produce airborne sounds, resembling “clicks,” with their mandibles,” which often precede or accompany the defensive chemicals it regurgitates (Brown, et al. 2007). Studies of the common European great peacock moth (Saturnia pyri) show that it can modulate the high-pitched chirping produced by its mandibles to the frequency range of both birds and bats to warn these “predators of defensive chemical secretions” (Bura et al. 2009). To alert predators to their toxicity, some tiger moths use multimodal warnings–both coloration and sound–that “vary according to the seasonal and daily activity patterns” of birds that are visually-oriented and bats that are acoustically-oriented (Ratcliffe & Nydam 2008). In spring and summer, mostly diurnal tiger moths ward off birds with their aposematic colors while the colors of nocturnal tiger moths are more subtle. Some late emerging species, though, can also produce ultrasonic clicks audible to bats, which corresponds with an increase in bat activity.
“Imitation is the sincerest form of flattery,” but it also is a survival technique employed by some lepidopterans. The coloring and pattern of the eastern tiger swallowtail, dark form female (Papilio glaucus), eastern black swallowtail (Papilio polyxenes), spicebush swallowtail (Papilio troilus), and red-spotted purple (Limenitis arthemis) appear similar to that of the poisonous pipevine swallowtail (Battus philenor). It has been suggested that this Batesian mimicry may afford protection to harmless and palatable species if predators mistake them for the poisonous species they avoid. However, a 2008 study showed that it did “not work efficiently enough over evolutionary time to permit consequences such as the evolution of longer physiological life spans” (Beck & Fielder 2009).
Left to right, top/bottom: Batesian mimics eastern tiger swallowtail dorsal/ventral, eastern black swallowtail dorsal/ventral, spicebush swallowtail dorsal/ventral, red-spotted purple dorsal/ventral, and pipevine swallowtail dorsal/ventral.
On the other hand, because aposematism is effective in providing protection, it is hypothesized that Müllerian mimicry is more successful in warding off predation. This occurs when different poisonous species share the same warning color pattern (like monarch, viceroy, and queen butterflies), so that they share the burden and the benefit as predators learn to avoid them. Lastly, there is the Texas wasp moth (Horama panthalon) that mimics a species in an entirely different order–paper wasps in the Hymenoptera.
Left to right: Müllerian mimics monarch, viceroy, and queen butterflies and wasp mimic Texas wasp moth.
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