Biology Reference
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pared to those known for 35 other species of
Phyllonorycter
native to Eng-
land and associated with 32 different host plants.
Each of the two newly colonizing species of
Phyllonorycter
was found
to support 16 species of parasitoids, a diversity that was essentially equal
to that shown by native moth species. Furthermore, the parasitoids asso-
ciated with these colonist moths did not show a greater proportion of
generalists. One of the parasitoids associated with the species mining
leaves of
Platanus
, in fact, was a specialist on this host moth. It was discov-
ered during this study and very likely immigrated to England at the same
time as its host.The suite of parasitoids associated with the new hosts was
not predictable from the pattern of parasitism of native moths and their
plant hosts.Thus, a normal number of parasitoids became associated with
the two colonist moths in only a few years.The rapidity with which these
associations developed was certainly aided by the proximity of England to
the continental source area for the two moth species, and by the fact that
many moth congeners and their associated parasitoids already existed in
England.
For alien insect herbivores, the
Phyllonorycter
example is probably
somewhat extreme, and the accumulation of parasitoids is usually not as
fast nor as full. For a set of 87 herbivorous insects, for example, Cornell
and Hawkins (1993) compared the number of insect parasitoids that had
been recorded in areas where they were native and in areas to which they
had been introduced. For most of these species, the time since introduc-
tion was known and varied from 1 to 155 yr. Analysis of these data
showed, first, that in areas of introduction, the herbivore species had accu-
mulated fewer parasitoids, about 4.0, than they possessed in their native
regions, about 7.7.The number of parasitoids per herbivore in the native
region, however, was positively related to that in the region of introduc-
tion. That is, herbivores with many parasitoids in their native region
tended to have many in the region to which they were introduced, and
vice versa.The intensity of parasitism was also higher in most cases in the
native region of herbivores.
Many herbivorous arthropods apparently present a substantial chal-
lenge to exploitation by native parasitoids, however, and the natives may
require evolutionary adjustments in seasonal activity pattern, behavior, or
ecology to attack the new hosts. Evidence of this is the fact that the num-
ber of parasitoids accumulated by an herbivore in its region of introduc-
tion showed a slightly positive relationship with time since introduction
(Cornell and Hawkins 1993). Variability in this relationship was great,
