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A Rough Day in the Life of...
A female butterfly is ovipositing this morning. The sun is warm, the winds are calm, and the bouquet of phlox and other blossoms fills the air. The hint of so much nectar is good, for, after "laying" a number of eggs, this female will often pause to nectar and regain her energy before ovipositing again. And before the day is out, she may have deposited a hundred eggs or more on her preferred host plants. And in the course of her lifetime, she'll likely lay even more. But let's imagine that we have the ability to monitor those hundred eggs over the next three or four weeks. Their lives are highly eventful and provide just a glimpse of the complex interaction of butterflies and other living creatures.
On this same day or in the next few days those host plants will likely be visited by one or more critters. Various spiders, ants, and other insects along with small lizards may happen upon some of the eggs, recognize a tasty food source and have dinner. It's easy to assume that some small predators learn to associate these particular host plants with victuals. Clearly wasps do. Research has shown that as few as ten or as many as sixty percent of the hundred eggs we're watching will be devoured. However, for those that survive predation to hatch out, the challenge of becoming a butterfly is just beginning.
The cliché remains: the caterpillar that hatches from the egg is primarily an eating machine, destined, if it survives through all four or five of its instars, to increase its weight over a thousand times. Each instar is the result of having stretched its particular skin at that time to the maximum and having shed that skin and replaced it with a baggy, new one, which it proceeds to grow into. And as it grows the larva or caterpillar becomes ever so much more attractive to hungry predators. It is in the larval stage that a butterfly faces its greatest danger, for the caterpillar, focused almost exclusively on eating and lacking the ability to move quickly, remains highly vulnerable. And the many partially eaten leaves of the host plant form a telltale sign that attracts the attention of sharp-eyed birds. The list of predators is long: birds and lizards would seem to pose the greatest threat, but insects do considerable damage. Insect predators include stinkbugs, lacewings, wasps, ladybugs, earwigs, spiders, and fire ants, to name a few. However, several insects inflict harm in a most insidious manner.
Wasps and flies (not common house flies) are major threats to larvae. I'll never forget that in less than a week's time, of the thirty-six Monarch caterpillars on my Mexican milkweed (A. curassavica), only twelve survived. I could see the partially devoured remains of many lying over a branch or on the ground. While I'm not sure of this, I've read that some wasps take parts of the larvae home to feed baby wasps in the nest, bird-fashion. And some insects don't eat the caterpillar but use it simultaneously as a nursery and food source. Most notable is the tachinid fly, one of which is Lespesia archippivora. These and other tachinids are parasitoids, which lay their eggs on or within other insects such as butterfly caterpillars. As Chip Taylor of Monarch Watch notes, "The parasitoid larvae develop within the host usually feeding on "non-essential tissues" until they near maturity, when their increased feeding and growth kills the host." Chip says that late in the season tachinids have killed as many as ninety per cent of Monarch larvae on milkweed near the University of Kansas.
And in some cases the connection between parasitoid and caterpillar has a fascinating twist. Joann Karges was telling me recently about having read Gilbert Waldbauer's interesting book, What good are bugs? In it he writes about a strange phenomenon in the relationship of Cabbage Whites and the ichneumon wasp. Evidently the parasitic wasp larvae, having fed on the insides of the caterpillar, then emerge and pupate on the outside of its body. "The caterpillar though doomed may live for another six days and in this period actually makes a protective covering over the wasp cocoons and 'stands guard' to protect them from other predators until they emerge - and the caterpillar dies."
Depending on temperature, as many as three to four weeks have passed in our story since the female oviposited. At this point those caterpillars remaining - and their numbers are small in comparison with the one hundred eggs we began watching -- leave their host plants and seek out a remote, secluded place to pupate, usually on a stem deep within another plant and often at a considerable distance. Sometimes, however, larvae stay put. Black Swallowtails in particular occasionally seem to flaunt themselves, pupating on the very plant that they used as a host, albeit in a color that matches the plant or blends in with the background. The particular form each species chooses for its pupa has an overriding purpose, to imitate the shape and color of a natural object such as a leaf or twig. Other than camouflage, a lack of movement by a chrysalis or pupa perhaps acts as a deceptive sign of lifelessness. If disturbed, its only defense is a sudden flexing or jerking movement that may deter very small predators. The best defense of some other butterflies, such as skippers, is to pupate by spinning a nest among folded leaves or, like the Coral Hairstreak, to pupate beneath dead leaves.
Regardless of the pupa's strategy for defense, Phil Schappert (see Book List for his excellent new work) estimates that from forty-five to ninety percent of all pupae succumb to predation and environmental causes. I've noticed a large number of pupae that have clearly been killed by parasitoids. You'll see a dark hole on the side of the chrysalis, a sure sign that an insect such as the ichneumon wasp has emerged. But perhaps some of the most underestimated threats to butterflies are environmental or weather-related ones like pathogens and fungi, strong winds and heavy rains. In 2002 many people lamented how few butterflies they saw. But if you recall, we had a remarkably wet spring and early summer with record rainfall. Butterflies were highly vulnerable to the fungal diseases and pathogens that resulted, particularly in the egg, larval, and pupa forms. However, even in a year of fairly normal environmental factors, few survive. Of the original one hundred eggs that we began monitoring, only one or perhaps two adult butterflies emerge from the pupae to mate and pass on their genes.
Naturally predation continues even with the adult butterfly's ability to fly and dodge, to be or to mimic a highly distasteful species, to camouflage itself as a leaf when still, or like the Hairstreak or Swallowtail, if spotted, to misdirect predators to its hind wings or tails. Birds remain major predators and anoles and other lizards lurk on plants. But new threats appear in the form of vast spider webs (research has shown that most butterflies caught in webs tend to be males) and numerous creatures that dine by virtue of their patience. Praying mantids excel in hanging out near blossoms, whereas crab spiders calmly wait within larger flowers -- both ready to pounce on a nectaring butterfly. Dragonflies may be seen eating a butterfly twice their size and robber flies clearly know no fear when seizing one. Finally, when roosting at night, butterflies are particularly vulnerable to predators such as mice, possums, and raccoons. In fact, mice are some of the most successful predators of Monarchs roosting in large numbers, notably so in Mexico.
And so butterflies, from egg to adult, serve as an important source of protein and nourishment near the bottom of nature's food pyramid. Given what we've learned about the mathematics, the sheer volume of food necessary to support successively larger predators as we go up the food chain, it makes sense that so high a level of mortality is inherent in the life cycle of a butterfly. Yet when I encounter this winged wonder out at the Nature Center or happen to look out my window to see one flying in the garden, I'm still amazed that they exist in such numbers, much less by the beauty they bring into our lives.
Here are the most useful guides I know of to help you plan and develop your own butterfly garden or meadow or to learn more about our butterflies. Remember that both the Tarrant County Butterfly Society (see "More Info") and the Dallas County Lepidopterists' Society (see link below) meet monthly and often have field trips in which you are quite welcome to participate.
Butterfly Gardening for the South by Geyata Ajilvsgi. 1990. Taylor Publishing Company.
Butterflies of Houston and Southeast Texas by John and Gloria Tveten. 1996. University of Texas Press.
Butterflies through Binoculars: The West by Jeffrey Glassberg. 2001. Oxford University Press.
Four Wings and a Prayer by Sue Halpern. 2001. Random House.
Native Texas Plants by Sally Wasowski and Andy Wasowski. 1997. Gulf Publishing Company.
Texas Wildscapes: Gardening for Wildlife by Noreen Dalmude and Kelly Conrad Bender. 1999. Texas Parks and Wildlife Press.
http://www.dallasbutterflies.com/ -- excellent site of Dale Clark, local lepidopterist.
http://www.monarchwatch.org/ -- have a question about monarchs?
http://www.learner.org/jnorth/ -- answers to a trillion questions about monarchs. Most important, each spring they compile an updated map based on current sightings so that you can follow the migration of the monarchs north. They also map the journey south.
http://www.naba.org/ -- the North American Butterfly Association; much info.
http://forums.gardenweb.com/forums/butterfly/ -- some practical help on butterflies and gardening to attract them.
http://www.txcn.com/sharedcontent/features/texasgardener/BI_BUTTERFLY.6a12d05e.html -- garden guidelines by Charlene Rowell of Heard Museum and Tina Dombrowski of Texas Discovery Gardens.
http://www.texasdiscoverygardens.org/TopTenButterflyPlantlist.htm -- list of some useful host and nectar plants.
For more information, you may contact:
Tarrant County Butterfly Society (817) 923-8474
Joann Karges or (817) 923-8474
Henry Turner or (817) 922-9484