With over 142,000 described species worldwide, moths are a smashing evolutionary success, second among animals only to beetles in number of species. Over 12,000 species, grouped into 65 families, are found in North America alone. The moth fauna of the Southwest is particularly rich, as it includes the northern limit of distribution for many primarily Neotropical species. Within the order Lepidoptera, moth species outnumber butterflies and skippers nearly 15 to 1, with many species left to be described, especially among the numerous “microlepidopteran” families.
Why are moths so successful? All moths undergo complete metamorphosis; that is, their life cycles progress through egg, larval, pupal, and adult stages. Thus, the typical moth lives 2 ostensibly distinct lives, filling 2 distinct ecological niches; it is born as a terrestrial, vegetarian eating machine and is “reborn” as a winged creature of the night, hell-bent on completing its reproductive cycle. Yet this is not unusual for insects. Moths share a common body plan, including a head with large compound eyes and sensitive olfactory appendages (antennae). Also, in all but the “primitive” families (Micropterigidae, etc.), moths have long, tubular mouthparts fused into a proboscis; a powerfully muscled midsection (thorax) with two pairs of scale-covered wings and three pairs of legs; and a long, segmented abdomen that includes digestive, circulatory, respiratory, and reproductive structures. Male moths can often be distinguished from females by their broader, comb-like antennae, valve-like abdominal claspers, and smaller, more slender bodies. As in beetles, moths from different families vary widely in wing venation, shape and coloration, larval and adult feeding habits and behaviors, mating systems, population structures, thermal biology, and sizes, ranging from the minute clothes moth (Tineidae) with its ¼ to 3/8 inch (7-10 mm) wingspread, to the bat-sized hawkmoths (Sphingidae) and giant silkmoths (Saturniidae). Unlike beetles, the overwhelming majority of moth species are herbivorous as larvae and adults; there are far fewer examples of carnivores, fungivores, and detritivores among moth lineages. The complex relationships between moths and their host plants may hold keys to understanding why there are so many moths.
Given the stupendous diversity of moths, and our incomplete knowledge of moth distribution and abundance, especially in the Southwest borderlands, our purpose in this section is to outline a few of the salient characteristics of moth biology and suggest a few activities by which the reader might gain an appreciation for moths as dynamic, complex organisms.
Arctiidae (Tiger Moths), Geometridae (Inchworms), Lasiocampidae (tent caterpillars), Lymantriidae (tussock moths), Micropterigidae (mandibulate moths), Noctuidae (owlet moths), Prodoxidae (yucca moths), Saturniidae (giant silkmoths), Sphingidae (hawkmoths), Tineidae (clothes moths)
|Spanish names: palomilla, mariposa de noche, polilla|
Five Features of Moth Biology in the Southwest
1. Plants, Caterpillars, and the Arms Race
The caterpillars of most moths are highly specialized and eat only one or a few plant species. Unfortunately, moth caterpillars are infamous for the exceptional cases; the decimation of crop plants by extreme generalists such as the cabbage looper (Trichoplusia ni; Noctuidae), and the destruction of wool clothing and stored grains by moths in the family Tineidae. The repeated association of certain moth and butterfly lineages with specific families of host plants worldwide suggests that these relationships are ancient. A closer examination reveals complex suites of plant defenses, both chemical (terpenoids, alkaloids, phenolics, cyanide-generating compounds) and physical (hairs, spines, tough leaves, oozing resins, and latex), designed to keep caterpillars at bay. Humans owe a debt of gratitude to moths and other insects for such biochemical plant wealth, which, quite coincidentally, provides us with a pharmacopoeia of natural drugs, insecticides, flavors, and fragrances.
Caterpillars, in turn, have evolved numerous physiological and behavioral strategies to counteract these defenses, from detoxification or rapid excretion of plant toxins to avoidance of older, better defended leaves. Tobacco hornworm (Manduca sexta) larvae will avoid snipping the veins of tobacco leaves, thus reducing the amounts of nicotine marshalled by the plant in its defense. Some specialized caterpillars co-opt the toxins from their host plants for their own defenses, and advertise their acquired distastefulness with bright, vivid colors.
There are additional, more subtle levels to the wars between caterpillars and their host plants. When caterpillars remain undaunted by chemical or physical deterrents, plants may use extrafloral nectaries or other foodstuffs to purchase the services of ants and wasps as caterpillar exterminators. These security guards can be bribed, however, and certain caterpillars do so with glandular secretions and resume eating plant tissues with impunity. And so on. Caterpillars on plants are vulnerable to many other hazards. The scents of wounded leaves and grass, the by- products of caterpillar foraging, are attractive to the parasitic wasps and flies that appropriate caterpillar tissues for the nutriment of their own young. In addition, caterpillars are preyed upon by birds, wasps, and other visually foraging predators. In order to survive, they defend themselves by being distasteful or covering themselves with stinging spines, or through bluff and deceit: they mimic leaves, twigs, galls, flower buds, bird droppings, and even snakes.
2. Moths as Pollinators
Few people realize that the voracious hornworm, looper and armyworm caterpillars that defoliate desert wildflowers, crop plants and garden vegetables, eventually become nectar-feeding adult moths that render important pollination services to many of the same plants. Moth pollination is more prevalent in the Southwest than in other regions of North America, largely due to warm evenings, favorable climate, and proximity to the moth-rich canyons and thorn- scrub of northern Mexico. Moths visit flowers in search of nutritious rewards, usually nectar, and transfer pollen as a consequence of their contact with floral structures and forging movements between flowers. Many night-blooming plant species, especially in grasslands and dune areas, appear to be specialized for moth pollination, but since most moths feed opportunistically from a variety of flowers, disperse widely during their lifetimes, and neither defend territories (like hummingbirds) nor provision young with floral rewards in local nests (as do many bees), most moth-pollinated plants employ alternative reproductive strategies. These include self-pollination, recruiting other (diurnal, or day-active) pollinators, or simply waiting for the next flowering season. Thus, moth pollination is a risky proposition, and moth-flower mutualisms are not very exclusive.
One noteworthy exception to this pattern is the relationship between yucca flowers and the small, white moths (of the genera Tegiticula and Parategiticula in the family Prodoxidae) that spend most of their lives associated with yucca plants. Yucca moths are among the few examples of “active” pollinators, animals that intentionally collect pollen from anthers and apply it to stigmatic surfaces. A female yucca moth uses her unique mouthparts (tentacles) to gather a pollen ball from yucca anthers, then walks or flies to another flower, deposits a number of eggs within the flower’s ovaries, and slam-dunks the pollen ball into its stigmatic cavity. Like wasps that bury the bodies of paralyzed spiders with their eggs, the mother moth’s pollination services ensure that her young will have food (developing seeds) when they emerge as hungry caterpillars.
The yucca plant and moth are absolutely dependent upon one another for reproductive success, yet the terms of their contract are usually complex. First, the yucca plant must sacrifice a significant percentage of its seeds as food for the moth larvae, although limited feeding damage enhances seed germination. Second, if yucca moth females deposit too many eggs within a single flower, the plant can selectively abort that flower, effectively killing all larvae within it. Finally the yucca-moth mutualism (living together in such a way as to increase each other’s reproductive success) is vulnerable to exploitation by cheaters: other moth species lay eggs within fertilized flowers but do not pollinate the flower.
3. Migration and Dispersal
The Southwest is an unusually good place to witness impressive directional movements of insects, especially of conspicuous desert butterflies like the snout butterflies and painted ladies. Such movements are noteworthy for the animals’ prolonged flight in the same direction, at a fixed speed, just a few feet above the ground. True round-trip migration, such as that performed annually by monarch butterflies and many birds, is a relativity rare phenomenon. Most cases of mass movements by moths probably are examples of one-way dispersal, such as the northward flush of black witches (Ascalapha odorata) into our region from northern Mexico toward the end of the summer. Dispersal need not be limited to adults; the movements of thousands of green- and black-striped hornworms (larvae of the white-lined sphinx moth Hyles lineata), across the desert floor provides one of the most memorable images of the summer monsoon. There are many potential causes of mass dispersal, such as periodic population explosions, seasonal changes in day length or humidity, and the availability of food or hostplant resources, but the actual causes are unknown in many cases. The movement of adult moths over great distances, often across political boundaries, has important implications for moths as biotic resources for plants (through pollination), predators, and parasites. With increasing fragmentation and conversion of wild habitats to agricultural lands and subdivisions, these movements also affect populations of moths and their biological interactions with plants and other animals.
4. Moths and Bats: Eaves- dropping That Really Pays Off
While the age of the earliest fossil moths suggests that they shared the world with dinosaurs and flying reptiles, we probably can never know if or when moths or their ancestors abandoned daylight for a relatively predator-free night. However, with the fall of the dinosaurs and the rise of the mammals, new and deadly predators of the night skies arrived: the bats. Fast, maneuverable fliers equipped with sensitive sonar guidance systems, bats are the number one threat for night-flying moths. But moths have developed an array of sensory and behavioral strategies that enable them to avoid becoming evening snacks for a bat.
Many night-flying moths have pairs of ears positioned on both sides of their abdomens that are tuned to exactly the sound frequencies emitted by hunting bats. These sensitive ears allow the moths to eavesdrop on the hunting cries of bats and to attempt to avoid them. Moths have two levels of escape behavior at their disposal when they hear a bat using sonar to search for food. If their bat-detecting ears inform them that a bat is on the way, but still distant, the moth turns away from the direction that the cries are coming from and leaves the area. However, if the bat gets very close before it is detected, the moth suddenly executes a series of high-speed acrobatic maneuvers, usually ending in a dive for the ground or the shelter of nearby bushes. Some moths confuse bats by emitting sounds similar to those emitted by a bat closing in on prey. Sometimes the moths can evade the hunting bats; sometimes they become dinner.
Very small moths and very large moths usually do not have bat-detecting ears. The smaller moths are too small a morsel for the bats to chase after, and many of the larger moths, such as hawkmoths and giant silkmoths, may be too large for bats to catch and eat. There is another small group of species in the tiger moth family (Arctiidae) that actually advertise their poisonous nature to hunting bats. Moths in this group are poisonous due to toxins in the plant species that their larvae eat. Diurnally-active, poisonous insects typically bear bright and conspicuous color pattern so that visually hunting predators learn to associate their horrible flavor or poison-induced sickness with the bright colors. But what do you do if you are poisonous, active at night, and your major predator uses sound and not vision to identify its prey? The small group of tiger moths, which share these characteristics, make unique sounds that their potential bat predators can detect and associate with their poisonous nature. When the sensitive ears of one of these moths detect a hunting bat in the area, at first it attempts to avoid the bat. If the bat gets too close, just as the moth initiates its evasive maneuvers, it also emits a burst of high-frequency clicks that the bat detects with its very sensitive ears. Bats that have learned to associate these clicks with an unpleasant meal initiate their own evasive maneuvers and leave the moth alone.
5. Mating Systems: The Scent of a Female
It is easy to understand why the colorful and conspicuous day-active butterflies typically have mating systems that rely heavily on visual communication of species and sexual identity. In contrast, even though moths possess visual systems especially adapted for their active night life, most species identification and sexual information in moths is communicated via air-borne chemical signals known as pheromones. In some insects, such as the highly social bees, ants, and termites, a complex chemical “language” exists that coordinates activities in the colony and allows for group defense and the dominance of the queen. Moths and many other insects appear to have only a very imited chemical vocabulary, usually amounting to “Hey baby, I’m a fantastic guy,” and “OK, I’m ready to mate.”
In a large majority of the moth species so far studied, the female moth determines when mating will occur by releasing her sex-attractant pheromones. These pheromones typically are a blend of closely-related chemical compounds, which she synthesizes in a special gland near the tip of her abdomen. The sex-attractant is unique to each particular species, and males are rarely confused into following the scent trail of the wrong species. Male moths often trace a side-to-side zigzagging flight track as they follow the wind-borne pheromone trail to its source. In most moth species studied, once the male arrives at the female’s location and physical contact is made, mating proceeds almost immediately. However, in some moths, upon his arrival a male releases his own unique courtship pheromone and fans it over the female with his wings. It has been demonstrated in a few species, and suspected for others, that the female moth uses the quantity or quality of the male’s pheromone to assess his “quality” as a potential mate. It is interesting to note that many of the chemical compounds identified from male pheromones are also common components of the scents of flowers. At least some male moths remember to bring a bouquet!
Five Active Ways to Learn About Moths
1. Light Traps
The inexorable attraction of moths to light is axiomatic in Western culture, yet no truly satisfactory explanations for this behavior have come to light, so to speak. Nevertheless, there is no better way to gain an initial appreciation for the diversity of moths in your area than by setting out a strong ultraviolet or mercury vapor light, hung over a white sheet in an otherwise dark, wild place. Insect lights that run off automobile cigarette lighters or portable generators are available from biological supply houses; alternatively, you could run an extension cord from your porch outlet. Pull up a chair and watch the guests arrive; take notes on which species arrive at different hours of the night and at different times of year. Examine the degree of wear and tear on the wings: are the moths freshly emerged and naive, or are they time-worn and battle weary? Look at the moths’ antennae and genitalia: are they all of one sex, or are equal sex ratios attracted to the lights? What do you observe when you take your light to a different habitat or elevation? Who shows up to eat the moths that are attracted to your light? Which moths never seem to get eaten? If you are interested in collecting moths, this is a good way to start.
Relatively little is known about the adult feeding behaviors of most moth species. There are entire families of moths, such as the giant silkmoths (Saturniidae), tussock moths (Lymantriidae) and the familiar tent caterpillars (Lasiocampidae), that don’t feed at all as adults. Others, including many hawkmoths (Sphingidae) and owlet moths (Noctuidae) are avid nectar feeders and can be important pollinators of night-blooming plants. Some of these nectar-drinkers also tipple at flowing sap or rotting fruit, resources that are exploited to a much greater extent in tropical than in temperate regions. Setting out fermented bait is an exciting way to observe these moths in action; you may attract a different fauna than you will find at light traps, and you’ll have a creative way to dispose of stale beer and overripe bananas! Simply mix old beer, rotting fruit and sugar or molasses and let it brew in a warm, dark place. When the concoction becomes pungent, paint or splash it onto tree trunks along a trail at dusk, then return after dark with a lantern and a field guide. You may see large underwing moths (Catocala spp.)—whose camouflaged, bark-like forewings conceal colorfully banded hindwings—as well as other owlet moths, inchworm moths (Geometridae), ants, crickets, and millipedes. When you learn the species by sight, keep notes on which ones are attracted to bait at different times of year. Experiment with the kinds of bait you use, adding the proverbial “eye of newt” to the cauldron; remember that most insects are less squeamish about road kill and excrement than we are—the funkier it smells, the better!
3. Night-bloomers and Moth Gardens
One of the greatest thrills for a moth enthusiast is watching a large hawkmoth unfurl its 4 inch (10 cm) long proboscis to drink from a trumpet-shaped flower while hovering in place at its threshold. Hawkmoths are effective pollinators of a guild of specialized night-blooming plants throughout the Southwest, including sacred datura (Datura wrightii), sweet four o’clocks (Mirabilis longiflora), and tufted evening primrose (Oenothera caespitosa), which produce pale, fragrant flowers with nectar tubes as long as the moths’ extended tongues. In exchange for reproductive services, these plants provide copious sucrose-rich nectar, the high-octane fuel required by hawkmoths to maintain hovering flight. The strong floral perfumes are thought to attract moths from a distance, after which the moths appear to be guided to these pale trumpets by visual cues. Plants with bunches of small tubular flowers, such as scarlet gaura (Gaura coccinea) and fairy duster (Calliandra eriophylla), attract many owlet and inchworm moths, which may spend up to 20 minutes perched on a single inflorescence, drinking leisurely from each flower. As mentioned above, yuccas are pollinated exclusively by small, satin-white yucca moths, whose frenetic mating, pollinating, and egg-laying activities are endlessly entertaining. Find populations of these and other night-blooming plants and wait until dusk, then watch as their flowers open and the moths arrive. Do flowers and their moth visitors segregate by size? Do moths stick to one type of flower, or do they sample from a buffet? Dab a few flowers of one plant with day-glo paint powder, wait until a few moths pass through, then scan all the nearby flowers with a portable ultraviolet lamp (the kind used to illuminate scorpions) to follow the moths’ trails. Better still, plant your own moth garden with a variety of night- blooming, fragrant plants, and provide a nectar filling station for wayfaring moths.
4. Raising Caterpillars
In the Southwest, as in most places, chances are good that if you find an unfamiliar caterpillar, it will grow up to be a moth. This is especially true for caterpillars that mine within leaves, bear stinging spines, and spin plush silken cocoons. Modern insect science has benefited greatly from the contributions of dedicated amateur entomologists who have reared adult moths from caterpillars, and described their larval anatomies and behaviors, food plant preferences, and the timing of their life cycles. Despite the efforts of amateurs and scientists, little is known about the immature stages of most adult moths, and many await discovery.
Caterpillars can be raised in jars, terraria, or even plastic bags, as long as old leaves and grass are removed regularly and growth of mildew is prevented. When you find a new caterpillar, provide it with plenty of freshly cut leaves from the plant it was eating or walking on, along with twigs, dry leaves and soil on which to pupate. If it refuses to eat, you may have to offer it a living plant, or a buffet of different plant species from the area in which you found it. As the caterpillar grows, make drawings or photographic records of its different larval stages (instars), the pupa (cocoon) and the adult that emerges from it. Once you have reared a given species successfully, you can experiment with alternative food plants. Will your caterpillars accept leaves of another plant related to its host plant? Would they rather die (some do)? Never let caterpillars run out of food, as even temporary starvation enhances the probablility of death due to viral infection. Be persistent, as many moth life cycles involve dormancy (diapause) during one or more of the stages of development, and some caterpillars are very difficult to rear in captivity. Also, don’t be discouraged if a parasitic wasp or fly emerges from your pupa; instead, photograph it or collect the specimen. Records of this kind are extremely important, as we know even less about moths’ parasites then we do about moths!
5. Captive Moth Sirens
Many moth species, especially the giant silk moths and hawkmoths, feature mating systems in which newly emerged females emit volatile sex pheromones that attract males from a considerable distance. Rearing moths from caterpillars may provide you with many virgin females, and the means to conduct an interesting census of male moths in your area. (You can also obtain female moths at light traps and bait, but these may already have mated.) Place female moths into small mesh or wire cages; position the cages along a trail, near a forest clearing, or on your porch; then observe. If males of this species are about, they should find their way to the cages in short order. At what time do they arrive? How abundant are they in suburban areas, as compared with rural habitats? Do males from more than one species arrive at the cages? If you are interested in moth breeding, this is an excellent way to attract wild mates and begin the life cycle anew.