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Phenotype matching between wild parsnip and parsnip populations

Wild parsnip plants can be characterized by their furanocoumarin production, and parsnip webworms can be characterized by their ability to metabolize those furanocoumarins.  Each plant and each insect can then be assigned a phenotype.   We collected both seeds and pupae from four Midwestern populations.  The seeds were analyzed for concentrations of four furanocoumarins--xanthotoxin, bergapten, isopimpinellin, and sphondin.  The larval offspring of the adults collected as pupae were analyzed for their capacity to metabolize those same four furanocoumarins.  By adjusting each value (furanocoumarin production or furanocoumarin metabolism) by the average across populations, we able to combine both plants and insects in a cluster analysis.

cluster.jpg (20776 bytes) Cluster analysis was used to group individual plants and insects into four clusters that can be considered multivariate phenotypes.  Each cluster contains insects and plants from each of the four populations.  This means that all of the variation found in these populations is represented in each population.   However, the frequencies of each phenotype differs between populations.
Matching frequencies
The frequencies of the four phenotypes in each population of plants was compared to the frequency of insect phenotypes.  An extraordinary degree of matching was discovered in three of the populations (Winona, Peotone, and Charleston).  The only population in which the frequency of plant phenotypes differed significantly from that of the insects was the one in Urbana.  We suspect that the failure of the Urbana populations to match may be due to a difference in sampling methodology.  In each of the three matching populations, we collected both plants and insects within well-defined boundaries.    In Urbana, the plants were collected from a well-defined area, but the insects were collected from a much larger area but still within the same continuous parsnip population. These results suggest that coevolution between this plant and its herbivore is very tight.

 

 matching.jpg (70304 bytes)
In order to determine what factors might influence matching, a large study investigating 20 populations occuring in Illinois and Wisconsin was undertaken.   Among the notable findings were that matching can occur within a single growing season.  We know this because populations of wild parsnip from which webworms were eradicated the previous year by insecticide were recolonized the following year and matched their plant populations.  For this to have happened so quickly, differential mortality of maladapted insect phenotypes must have occurred to produce the matching.   The alternative explanations involving differential mortality of plant phenotypes and selective oviposition of female webworms are highly doubtful.  In the case of the plants, webworms rarely kill parsnips outright, although they can severely reduce the plant's seed output, and the female webworms lay eggs before the plant parts that their offspring feed on even exist, so it is unlikely they can predict what their offspring will encounter.  A transplant experiment of larvae that managed to make it to fifth instar on parsnips in the field showed that indeed larvae are more likely to die if moved to another plant than if moved to a different location on their natal plant. 

However, not all populations in the study exhibited matching and the factors found to contribute to mismatching were the presence of an alternative hostplant, cow parsnip (Heracleum lanatum), or the presence of particularly high concentration furanocoumarin plants (the plant and insect data sets are now available as excel files).  

 

Does this kind of matching make any difference? Don't larvae adjust metabolism to meet demands anyway?

last updated 4/14/1999