Biology Reference
In-Depth Information
Specialist herbivores, primarily insects and other arthropods, have,
however, also shown host shifts to many alien plants. Many of the host
shifts that were first identified involve the use by native herbivores of
introduced crop plants. Recently, several cases have been noted in which
native insects have begun to concentrate their feeding on other types
of invasive alien plants. A genetic basis for most of these shifts is not
yet evident, but for many, the possibility of a genetic basis is being
recognized.
Bowers et al. (1992), for example, examined the shift of the Baltimore
checkerspot butterfly (
Euphydryas phaeton
), a close relative of Edith's
checkerspot, to ribwort plantain in NewYork State.The native host of the
Baltimore checkerspot is turtlehead or balmony (
Chelone glabra
), a mem-
ber of the family Scrophulariaceae. Ribwort plantain, introduced from
Europe in the late 1700s or early 1800s, has become a common weed of
disturbed, open habitats in New York. Recently, however, the Baltimore
checkerspot has begun to feed on ribwort plantain extensively as a host.
Ribwort plantain, belonging to a different plant family than turtlehead,
the Plantaginaceae, contains chemicals known as iridoid glycosides that
are similar to those present in turtlehead. These chemicals are stored in
tissues of the butterfly, making the butterfly unpalatable to vertebrate
predators, although butterflies reared on plantain appear to have much
lower levels of iridoid glycosides. Some populations of Baltimore check-
erspots now reproduce exclusively on ribwort plantain. Laboratory stud-
ies of adult and larval preferences (Bowers et al. 1992), however, showed
that turtlehead is still preferred and that growth is faster on this native
host. Some benefits may exist for butterflies that use plantain, the most
likely being escape from parasitoid insects. In addition, abundance of the
native host plant, turtlehead, is declining, whereas plantain is flourishing.
Thus, although no evidence yet exists for genetic specialization of Balti-
more checkerspots for plantain, the potential for such adaptation exists.
Several butterflies have also expanded their host use from native legumes
to introduced legumes (Barbour and Kiviat 1997). Continued investiga-
tion will certainly reveal many more cases of this sort.
In western Europe, a native budworm,
Choristoneura murinana
, attacks
silver fir (
Abies alba
) and several other native conifers (Du Merle et al.
1992). With the widespread introduction of exotic conifers from North
America, North Africa, and eastern Europe, this lepidopteran species
has broadened both its host range and its geographical range. In some
cases, these new hosts are more favorable to larval development than sil-
ver fir, their original primary host. Thus, the expansion of host and geo-
