From the previous post we learned that wild parsnip, Pastinaca sativa, the weedy variant of the domesticated root crop of the same species, contains toxic chemical compounds called furanocoumarins which act as chemical defenses against herbivores, but which also cause photodermatitis ( a chemical skin burn exacerbated by exposure to sunlight) in humans. The wild parsnip of the U.S. in the midwestern and northeastern states is descended from plants intentionally introduced by European colonists nearly 400 years ago. Associated with the wild parsnip is the parsnip webworm, a moth whose larval stage feeds on developing flowers of Pastinaca. The webworm is able to tolerate the toxins by virtue of its ability to produce enzymes that can detoxify the furanocoumarins. This Eurasian native pest did not arrive in North America until about 250 years after its host plant.
Because the wild parsnip and its caterpillar enemy reached the U.S. more than two centuries apart scientists, using herbarium specimens, could determine toxins levels of parsnip plants that lived both before and after the appearance of the webworm in North America. Results show that toxicity of Pastinaca plants in some areas increased rapidly, within 20 years of the moth’s introduction, an apparent adaptive response to the presence of the webworm (Berenbaum and Zangerl, see citations below). Also, the toxin content of parsnips collected from 1850 to 1889, that is, prior to and just after the introduction of the parsnip webworm, was lower than in recently collected plants in both the U.S. and in Europe. In related, current-day field work conducted across sample areas varying in webworm population density, the types and quantities of furanocoumarins produced by the wild parsnips correlated positively with the detoxifying enzyme levels of local webworms.
Although concentrations of the toxin and the detoxifying enzymes may vary depending on several factors, an important one appears to be coevolutionary forces which favor higher concentrations of both the parsnip toxins and the detoxifying enzymes activity of the herbivore. Toxin levels vary within wild parsnip populations such that more toxic plants are better able to limit herbivore damage and therefore reproduce more. Similarly, webworms capable of higher enzyme activity are better able than their cohorts to detoxify the chemical defenses of the parsnip and therefore preferentially reproduce. The fact that the webworm continues to successfully consume Pastinaca plants in the modern period in the U.S. where toxins concentrations are elevated suggests that it, in turn, has adapted to the higher toxin levels of its food plant, a response supported by chemical analyses of webworm enzymes. In contrast, in both the United States and Europe where webworms are rare, P. sativa produces less toxin.
In Europe where, of course, the wild parsnip and the webworm have long been in contact and likely coevolved, there are a host of additional food plants- seven other species of Pastinaca and four species of Heracleum), giving the webworm choices. In the Netherlands, for example, the webworm prefers hogweed or European cow-parsnip (Heracleum sphondylium), even where P. sativa is available- reflecting the lower furanocoumarin content of H. sphondylium. Similarly, the presence in the U.S. of native populations of H. lanatum provides an alternate host for the webworm that reduces selection intensity because of the plant’s relatively lower toxin content. Complicating matters in Europe but not in North America, there’s a high probability of a parasitic wasp attacking the webworm particularly when the latter feeds on hogweed, but this does not apparently offset the preference of the moth for H. sphondylium. It appears that the herbivore is able to utilize the parsnips as a food choice but not without considerable cost.
Pastinaca sativa has been grown in New Zealand for over a century while the webworm has until recently been absent. Possibly the parsnips that were initially imported were relatively untoxic, but whether or not that is true, in the absence of its specialist herbivore, the parsnips produced only low levels of chemical defenses given that there was no selection pressure favoring more potent toxins. Now that the webworm has escaped in New Zealand, it has become a serious predator of the parsnip. Whether future parsnip populations will be able to respond by increasing mean toxicity levels remains to be seen.
Berenbaum M. and A. Zangerl 2006. Parsnip webworms and host plants at home and abroad: trophic complexity in a geographic mosaic, Ecology 87: 3070-3080.
Zangerl A. and M. Berenbaum 2005. Increase in toxicity of an invasive weed after reassociation with its coevolved herbivore. PNAS 102: 15529-15532.