Cabbage White Butterfly

Common Name: European Cabbage Butterfly, Cabbage White – Indigenous to Europe, its larvae subsist on plants from the Mustard family. Cabbage is the most important food crop in that family and a major dietary staple in many parts of the world.

Scientific Name: Pieris rapae – Pieria is part of the Greek region of Central Macedonia and was the home of the nine muses of Greek Mythology. Companions of Apollo, the god of music, they are usually depicted as maidens dancing in diaphanous robes, like white butterflies. Rapa is Latin for turnip, one of the plants of the Mustard family.

Potpourri: The undulations of flitting white butterflies across a meadow’s expanse evinces the harmony and poetry of nature. It is no wonder that the cabbage whites were named for the nine muses who deified those qualities in Greek mythology. Beauty belies the beast that can be an alter ego. The cabbageworm larvae of the butterfly are voracious predators of a whole family of plants that are at the foundation of the human food pyramid. They have circumnavigated the globe to become a universal invasive species. Their namesake goddesses could also be cruel. The nine daughters of King Pierus of Macedonia challenged the muses for their poesy and were turned into magpies for their effrontery. [1]

Pieris is the type genus for the butterfly family Pieridae that is prosaically called the whites and sulphurs for their nearly monochromatic wing coloration. The single color wings are adorned only with an occasional darker bar or dot to provide some degree of distinction of speciation for sexuality. For example, a cabbage white with one spot is a male (depicted) whereas the female has two spots. The other salient characteristic of the family is having six fully developed legs in both sexes. While one would think that this would be standard fare for insects – hexapod by definition – most butterflies have repurposed the two front legs to do something other than walk. [2] The only other pierid frequently encountered during a walk in the woods is the Cloudless Sulphur (Phoebis sennae) which is yellow, or perhaps butter-colored?

The erratic acrobatics of flying insects with delicate wings of rainbow colors limned with black bars could not go unnoted by the earliest of the primordial talking apes who named them. These eventually became papillon in French, paroparu in Filipino, kapalak in Uzbek, and balanbaalis in Somali among many others, mostly distinctive. The origin of the word butterfly has stumped lexical certainty resulting in several competing etymologies. One is that a yellow butterfly, like that depicted, is a butter-colored fly, concatenated to butterfly. Doubtful.  The butter analogy has been extended to imply “smooth as butter” and even to the observation that butterfly frass was like butter in color and consistency. The scatological theory is drawn from the Middle Dutch boterschijte which translates to “butter shitter.”  A pervasive Norse superstition contends that witches take the form of flying insects and steal milk and butter from peasant farms. The most mundane theory is that the Saxon word for larva is buda and the fly is floege – taken together as buda-er-floege the butterfly was the “fly from the larva.” [3] Take your pick.

The global success of the cabbage butterfly is due to a combination of fortuitous mutation and human intervention all involving the Mustard family of plants. Known more formally as the Brassicaceae (brassica is Latin for cabbage) or Cruciferae (“cross-bearing” for the cross shape of the four petals of its flowers) family, it contains about four thousand species including cabbage, broccoli, radish, turnip, and, of course, mustard. As with most plants and animals, there is a continuing evolutionary pas-de-deux between the former not wanting to be eaten and the latter needed plant food to survive. The brassicaceous plants rose to prominence about 90 million years ago by producing a sulfur containing compound called glucosinolate that releases a pungent oil called isothiocyanate when the plant is damaged, as would be the case when hungry insects bite; the repellent keeps most of them away. That is, except the Pieridae family of butterflies that evolved to tolerate the tangy mustard oils and followed the cabbages wherever they went.  The various sulfurous compounds are what gives this family of plants, notably horseradish, wasabi, and hot mustard, the acrid bite for which they are known and for which they have been cultivated and transported by man. [4,5]

The ménage-a-trois between cabbage whites, cabbage, and cabbage eaters set the stage for the global reach of all three. Neolithic agricultural innovators slowly but surely crossbred and selectively planted wild mustards to produce a cornucopia of cultivars from European petit choux (little cabbages in French – Brussels sprouts in the US) to Chinese bok choy. Domestication of wild mustard (Brassica rapa) and black mustard (Brassica nigra) began about 4,000 BCE in the Levant area of the eastern Mediterranean, followed about two thousand years later by wild cabbage (Brassica oleracea). The circuitous path of Mesopotamian cabbage patches across Eurasia followed the Silk Road, the first notable trade route across the continent. The name Silk Road is misleading in suggesting a settled path, product, and protocol. In reality it is a generic term for the exchange of goods, culture, and religion that gradually and inexorably extended across the vast Eurasian steppes from the Black Sea to the Pacific Ocean. As the cabbages moved eastward, their namesake butterfly moved with them, P. rapae following B. rapa. A citizen science effort called the Pieris Project collected and evaluated the DNA from cabbage white butterflies around the globe to date species migration according to the time of their mutation. The Asian subspecies of P. rapae diverged from its European ancestors in about 800 CE, coinciding with the period of peak traffic on the Silk Road from 600 to 900 CE.[6]

Moving across the steppes by caravan is one thing. Crossing the ocean to North America is quite another. The European cabbage white butterfly was first sighted in Quebec, Canada in 1860 but no one knows exactly how it got there. The leading American entomologist Samuel Scudder reported to the Boston Society of Natural History that “Mr. Wm. Couper, a taxidermist and general collector, addicted especially to Lepidoptera and a good observer, living in Quebec, first captured a few specimens in 1860 in the immediate vicinity of that city; he looked upon it as a great rarity and indigenous to Canada.” Scudder verified in 1863 that the “rarity” was the European Cabbage White. [7] And that was just the beginning of a diaspora. The Commonwealth Agricultural Bureau International (CABI) tracks invasive species that seriously threaten the global food supply. According to their assessment: “Pieris rapae is a very serious pest of crucifers in Europe, North America, Japan, China, Australia and New Zealand. Unless controlled, damage from P. rapae larvae can result in total crop loss.” [8] Efforts to contain the cabbageworm contagion have resulted in some successes. This was only possible subsequent to a scientific full court press to understand the vulnerabilities of the butterfly. The cabbage white may well be the most studied lepidopteran on Earth.

The wily European cabbage white butterfly took advantage of its tolerance of glucosinolate produced by brassicaceous plants cultivated and spread by equally wily humans to master survival as a species. The adult females emerge from the chrysalis stage and set out with a singular goal to eat, mate, and lay as many eggs as possible. Butterflies feed primarily on the nectar that most flowers produce to entice insects to promote plant cross-pollination. While cabbage whites flit from blossom to blossom, they are specifically attracted to the flowers of Brassica rapa, which provides UV markings pointing to the central part of the corolla as a guide. Insects have a wider range of color vision (and non-color UV above the visible range) than primates with four different cones as compared to our trichromatic three. [9] Some butterflies, notably the closely related swallowtails, have eight cones. As they have limited neural networks, they don’t process the pixilated input as we do, but they do have a more directed responsiveness to a particular color (or UV pattern). It is with this superior targeted vision that a male cabbage butterfly (with one spot) finds a female (with two spots) to consummate the sexual union that underlies all animal diversity and evolution. The female then shifts from a random flower to flower pattern to a linear pattern with the express goal of oviposition. [10] Most animals that follow the evolutionary path of probability in producing huge numbers of eggs so at least one will survive to adulthood and regeneration put them all in one place. Not so the cabbage white. With instinctual cunning, each conical golden egg is laid singly on remote plants to avoid dense clusters of feeding larvae more likely to attract predators. It is survival of the fittest after all. [11]

A female cabbage white butterfly lays over 300 eggs. As there are usually four generations in a single season, a single butterfly could produce more than thirty million great-great grandchildren every summer absent predation. Emerging from winter refuge as chrysalides in early May, the first generation adults seek out vernal mustard family plants like common winter cress (Barbarea vulgaris) as repository for the first round of eggs.  Five days later, they are nursery and bodega for the first generation of caterpillar larvae that do nothing but eat for two to three weeks before the metamorphosis of pupation. In July, the second generation emerges just in time to lay eggs on the cole or crucifer crops like cabbage and cauliflower that predominate in truck farming enterprise. The emergent larvae chew away at the leaf edges, leaving only the tough, stringy veins and yellow-brown globules of frass to mark their passage. Without some means of combatting the scourge, all would be lost. [12]

How do you kill the white butterfly that lays the golden eggs? Pesticides are proven but problematic. Enter Bacillus thuringiensis (frequently called Bt), a soil bacteria discovered in Japan in 1901 for its role in decimating the silk-producing larvae of sericulture. The different strains of the bacterium produce different proteins that are activated by enzymes in the larval gut that create toxins that consume the intestines, killing the host. There are some twenty different strains of Bt that have been developed for use on a very narrow range of insect pests to minimize the unintended consequence of indiscriminate poisons like DDT. Cabbageworms can be effectively controlled by spraying Bt in early summer at the start of the growing season. One way to reduce the widespread application of aerosol pesticides is to genetically modify the affected plants to make it themselves, now known infamously as GMOs. The genes that produced Bt toxin were isolated by scientists at the University of Washington in 1982. By the mid-1990s, GMO or Bt corn that killed European corn borers was developed, and by 1998, 15.6 million acres were growing, one-fourth of the entire US corn crop. [13] Twenty years later, it is above ninety percent – GMOs are a fait accompli.

The preferred method to reduce the impact of insect pests is biological control. Other than the predators at the top of the food chain, like eagles, anacondas, orcas and us, becoming an entrée is a fact of life in the animal kingdom. Insect larvae offer an enticing dollop of protein without the drama and danger of a fight to the finish. That is except for those larvae that either make their own toxins or eat plants that are toxic to almost everything else. Cabbageworms do both. They eat glucosinolate-laden mustards and produce a mixture of fatty acids similar to the insect repellent linolenic acid. In one trial, ants offered a cabbageworm snack retreated en masse to clean their nether reaches of the offending chemical. [14] Clever, but not quite good enough. Only about ten percent of the larvae ever make it to the pupa stage. The rest are knocked off one by one in the sub rosa battle zone of the cabbage patch. Smaller larvae are eaten mostly by ground beetles and harvestmen (daddy-long-legs that look like spiders but aren’t). Birds account for about twenty percent of the older and bigger but not wiser cabbageworms. The balance are subject to the most gruesome fate in nature’s quiver of atrocities, the parasitoid wasps. [15]

Wasps are the consummate predators of the arthropod Phylum. Some sting to kill, carting off their prey to be eaten at leisure or fed to their progeny in the nest that is their palladium. Most are the more insidious parasitoids that do not kill their prey outright but lay their eggs on it so that it becomes a mobile canteen for the wasp’s larval offspring when they hatch. In the most extreme cases, the wasp larva consume the caterpillar from the inside out, saving the vital organs for last in a gruesome death of a thousand cuts. There are some fifty thousand species of parasitoid wasp globally and about four thousand in the US, where the annual savings are estimated at twenty billion dollars in the prevention of crop damage. The use of wasps to control the cabbageworm has a storied past. The chief entomologist of the USDA arranged for the first importation of a parasitoid wasp from England in 1881, but it failed as did a second introduction from the Balkan region in 1970. Finally, the Asian species Cotesia glomerata was successfully introduced in the 1980s to largely eliminate the cabbageworm as a serious pest. It lays its eggs on the larva as it feeds. When they hatch, the wasp larvae polish off the caterpillar as their first meal. [16]

There are still plenty of cabbage white butterflies in spite of the global scientific effort to offset their exploitive adaptations with chemical and biological weapons. The mustard family thrives in the wild, oblivious of human manipulation to improve its succulence according to our tastes. Butterflies are free, eating what they can find, and adapting to what they can eat. Both the mustards and the whites are doing quite well in the harmony of the ecological web.

 

References:

1. Larousse Encyclopedia of Mythology, Hamlyn Publishing Group, London. Translated from the original French, New Edition, 1968. Pp. 119-120.
2. Marshal. S. Insects, Their Natural History and Diversity Firefly Books, Ontario, 2006 p.166.
3. Adams, Ernst, Notes and Queries, 1876 concerning the etymology of butterfly https://english.stackexchange.com/questions/331851/etymology-of-butterfly
4. Ehrlich, P. and Raven, P. “Butterflies and Plants” Scientific American, 1967. pp 105-113.
5. DeMarco, E, “Evolutionary arms race created flavors like wasabi” Science 30 June 2015.
6. Ryan, S. “Global invasion history of the agricultural pest butterfly Pieris rapae revealed with genomics and citizen science” Proceedings of the National Academy of Sciences, 1 October 2019 116(40) pp. 20015–20024. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6778179/
7. Scudder S. H., “The Introduction and Spread of Pieris rapae in North America, 1860-1885” Memoirs of the Boston Society of Natural History, Boston, Volume 4 Number 3. 1887. https://www.biodiversitylibrary.org/bibliography/38374.
8. https://www.cabi.org/isc/datasheet/41167
9. Osorio D, Vorobyev, M “A review of the evolution of animal colour vision and visual communication signals”. Vision Research. June 2008 48 (20): pp. 2042–2051. https://www.sciencedirect.com/science/article/pii/S0042698908003222?via%3Dihub#fig2
10. Omura, H. et al. “Chemical and chromatic bases for preferential visiting by the cabbage butterfly, Pieris rapae, to rape flowers”. Journal of Chemical Ecology. 6 April 1999 Volume 25 Number 8, pp. 1895–1905.
11. Root, R. and Kareiva, P. “The search for resources by cabbage butterflies (Pieris rapae): ecological consequences and adaptive significance of Markovian movements in a patchy environment”. Ecology. February 1984 volume 65 (1) pp. 147–165.
12. https://vegento.russell.wisc.edu/pests/caterpillar-pests-of-cole-crops/   
13. Charles, D. Lords of the Harvest, Basic Books: New York, 2001, pp 1–23, 41–50, 60–63.
14. Hill, D. “Caterpillar Pests Ooze Insecticide” Science, 6 May 2002.
15. Dempster, J. “The Control of Pieris rapae with DDT. I. The Natural Mortality of the Young Stages of Pieris”. Journal of Applied Ecology. 1967 Volume 4 (2), pp. 485–500.
16. Pennisi, E. “The Little Wasp That Could” Science 15 Jan 2010, Volume 327, Issue 5963, pp. 260-262.