Biology Reference
In-Depth Information
sian fly now occurs throughout wheat-growing areas of North America.
Wheat breeders are engaged in an ongoing battle to create varieties that
are resistant, with 60 such varieties released between 1950 and 1983.The
mechanism of resistance is the production by the wheat plant of sub-
stances that kill the larvae hatching from eggs laid in plant tissues. Even
on resistant varieties, however, some flies survive and reproduce. A grad-
ual selection of a new virulent fly biotype thus occurs over a period of
years. At least 16 Hessian fly biotypes are now known, and two or more
are likely to occur in any area where wheat is grown (Ratcliffe et al.
2000).
Host plant shifts from native plants to introduced crop plants and then
to other native or alien plants in other regions have also been docu-
mented.The Colorado potato beetle (
Leptinotarsa decimlineata
), for exam-
ple, expanded its host range from native plants to cultivated potato
(
Solanum tuberosum
) when potatoes were introduced to the western
United States (see chapter 13). This beetle then spread through eastern
North America where potatoes are grown. In the eastern United States,
the potato beetle has adopted horse nettle (
Solanum carolinense
), a wide-
spread native plant, and bittersweet (
S. dulcamara
), a European introduc-
tion, as hosts (Hare 1983; Hare and Kennedy 1986). In North Carolina,
where the beetle has been present only since the late 1800s, it has evi-
dently made an evolutionary adjustment to horse nettle, so its survival on
this species is equal to or greater than on potato (Table 9.1). In Connecti-
cut, on the other hand, survival is considerably lower on horse nettle than
on potato. This difference may be related to the seasonal difference in
potato growing in North Carolina and Connecticut relative to the phe-
nology of horse nettle. Potatoes are grown earlier in the season in North
Carolina than in Connecticut, so after harvest, a long period exists when
horse nettle is an available host plant. During this period, two generations
of potato beetles can develop on horse nettle, so selection for adaptation
to this host may be strong.
Ongoing Evolution
These examples of rapid evolution by introduced herbivores appear to be
the tip of an adaptive iceberg. Laboratory studies show that the potential
for rapid evolution is widespread, and the examination of populations in
agricultural ecosystems has now revealed clear examples of rapid evolu-
tion.Very likely, similar patterns will be detected in alien forest herbivores.
As we shall see in the next chapter, alien predators and parasites are also
