Biology Reference
In-Depth Information
an increased fitness of individual checkerspots selecting this plant. Clearly,
an evolutionary shift from a native plant to an alien had occurred.
Host Shifts by Generalist and Specialist Herbivores
The massive introduction of plants to new continents has made a vast
number of new hosts available to native herbivores. In California, for
example, more than a third of the native butterfly species have been
reported to oviposit or feed on alien plants (Graves and Shapiro 2003).We
shall examine the accumulation of native herbivores by alien plants in
detail in chapter 15.
Herbivores have a wide range of potential tactics that can be used and
refined by evolution to permit the use of new host plants (Karban and
Agrawal 2002).These include changes in oviposition behavior, morphol-
ogy, feeding behavior, and physiology by the herbivores themselves. Her-
bivores can also manipulate the physiology of host plants, as in the induc-
tion of gall tissues. Many adjustments to new plant hosts can occur
without genetic change, but others can be partly or completely genetic in
basis. In fact, genetic variability for host acceptance probably exists in
most herbivorous insects (Jaenike 1990).
Generalist herbivores have accepted many alien plants without show-
ing evolutionary shifts. For example, larvae of several of the large Sat-
urniid moths, which use 40 or more genera of food plants, have begun to
feed on the alien purple loosestrife (
Lythrum salicaria
) in New York (Bar-
bour and Kiviat 1997). In Arizona, the bruchid seed beetle,
Stator limbatus
,
has begun to feed on Texas ebony (
Chloroleucon ebano
), a shrub now
widely planted in urban areas, despite the fact that it does not feed on this
plant in Texas (Fox and Savalli 2000). Since both the plant and the beetle
occur together in Texas, a genetic difference in host plant acceptance
between Texas and Arizona beetles seems likely.The valley pocket gopher
(
Thomomys bottae
), a generalist feeder on native plants, also uses a wide
range of alien grasses and forbs in California coastal grasslands (Hunt
1992) and throughout its range in western North America. Nevertheless,
as we have noted, the lag time in utilization of alien plants by native her-
bivores may be a major factor contributing to the initial success of many
invasive alien plants (see chapter 7). For some alien plants, on the other
hand, barriers to utilization by native herbivores are very strong. Prickly
pear cacti (
Opuntia
spp.), naturalized in parts of Australia and South Africa
for 150-250 yr or longer, are still not used by any native tissue-feeding
insects (Moran 1980).
