Biology Reference
In-Depth Information
sensory surface than those of eastern aphids. This modification may be
related to the expanded host plant use by southwestern aphids.
All apparent host plant shifts may not be adaptive, however. Zangerl et
al. (2002) examined the shift of the parsnip webworm (
Depressaria pasti-
nacella
) from its principal European host, wild parsnip (
Pastinaca sativa
), to
cow parsnip (
Heracleum lanatum
) in North America. In this case, not only
is survival of webworm larvae poorer on cow parsnip, a North American
native species, than on wild parsnip, but predation by birds appears to be
much higher. Cow parsnip thus may represent a population sink that is
maintained only by high reproductive success on wild parsnip. Why the
webworm continues to use cow parsnip as a host is unclear but may result
from loss of genetic factors for host discrimination as a founder effect
when the webworm was introduced to North America.
The wild parsnip and parsnip webworm relationship appears to fit
Thompson's (1993) geographic mosaic pattern. Each species can act as a
selective agent for the other (Berenbaum et al. 1986; Zangerl and Beren-
baum 1993). Wild parsnip contains a set of five defensive chemicals
known as furanocoumarins for which the webworm has specific detoxi-
fying metabolic systems. In different populations of wild parsnip in Illi-
nois and Minnesota, Berenbaum and Zangerl (1998) found that the pat-
terns of abundance of different furanocoumarins were matched by the
activity of detoxifying enzyme systems in the webworm, suggesting that
these populations were coevolutionary hot spots. In later studies in Illi-
nois (Zangerl and Berenbaum 2003), this matching pattern was also
observed in many populations. In other populations, particularly those
near populations of cow parsnip, the degree of matching was weak. The
relative abundance pattern of furanocoumarins in cow parsnip differs
from that in wild parsnip, so the presence of both plants alters the coevo-
lutionary pattern for both wild parsnip and the parsnip webworm.Thus,
the presence of cow parsnip tended to create a coevolutionary cold spot.
Ecological Host Shifts by Introduced Herbivores
Introduced vertebrate and invertebrate herbivores use many species of
plants native to their new homes. In general, vertebrate herbivores use the
same range of plant life forms as they used in their native region. Most
invertebrate herbivores, especially terrestrial forms, tend to be more selec-
tive. For example, about 2,000 species of alien insects have become estab-
lished in North America, about 400 of which are herbivores that feed on
