KEY POINTS:
The parsnip is under siege but its battle is teaching scientists important lessons about biological control of weeds.
Researchers say that the vegetable and its weed relative, cow parsnip, are being attacked by the parsnip webworm, a Northern Hemisphere pest that was discovered less than four years ago
at Port Chalmers in Otago.
The study has serious implications for scientists using biological controls to manage invasive weeds by importing the insects that attack them in their native land, said University of Illinois entomology professor May Berenbaum.
While such strategies may appear to be effective initially, the plants may be able to adjust to the insect threats over time by upping their chemical defences.
Professor Berenbaum said the belated incursion of the webworm into NZ was providing an open-air laboratory for relationships between the parsnip plants.
This "unique window" on the complex interaction of plant and insect enemies, offered the best view yet of how these species influence one another, she said in an international journal, the Proceedings of the National Academy of Sciences.
The researchers, who included Dr Margaret Stanley, an Auckland University lecturer with expertise in "invasion biology", have found New Zealand parsnips have significantly lower levels of certain chemical defences than parsnips growing in Europe and North America, where webworms have been a constant threat.
Now the parsnip webworms are dramatically affecting the ability of the NZ plants to reproduce: the webworm caterpillars eat the parsnip flowers and burrow into their stalks.
"In certain populations affected by webworms, 75 per cent of the plants were completely devoid of any reproductive parts," said Art Zangerl, a senior research scientist in the Illinois University's department of entomology.
"The affected plants were contributing zero fitness, which is really dramatic," he said. "We don't often see that."
Fitness is a measure of a species' ability to successfully reproduce. Environmental factors that reduce the fitness of an organism - for example, by destroying all of its offspring - can influence the course of its evolutionary trajectory. Survivors less susceptible to that attack enjoy a reproductive advantage, contributing more offspring, and more of their genetic attributes, to subsequent generations.
In New Zealand, the newly arrived parsnip webworms are a major selective agent, Dr Zangerl said, wiping out a majority of the flowering parsnips.
The altered chemical defences of New Zealand parsnips are probably allowing the webworms to feast on most of the plants in any given locale, Professor Berenbaum said.
The parsnip's chemical defences normally include a good dose of furanocoumarins, a class of organic compounds that can be toxic to insects that eat the plant.
While the parsnip webworm has evolved to tolerate large doses of furanocoumarins in its diet (it can eat up to 5 per cent of its body weight of these toxins) the chemicals do limit its capacity to inflict damage.
Professor Berenbaum said what isn't clear is whether the absence of parsnip webworms in New Zealand for more than 100 years allowed the parsnips to let down their guard.
She said that with no aggressive enemy, it could be simply that the parsnip "had 100 years to relax".
- NZPA
