Biology Reference
In-Depth Information
abundance of the three host plants. Where all three were common, per-
formance was best on hoe nightshade.Where only buffalo bur and potato
were common, beetles reared on hoe nightshade performed poorly, with
mortality of almost 70%.Thus, some genetic differentiation of potato bee-
tle populations for their most common hosts seemed to exist. As we
noted in chapter 9, as a result of its expanded geographical distribution,
this beetle has adapted genetically to another native solanaceous plant in
the eastern United States.
A slightly different situation is illustrated by the shift of the common
sulfur butterfly (
Colias philodice
) from native legume plants to alfalfa (
Med-
icago sativa
). When alfalfa was introduced to Colorado in the late 1800s,
the common sulfur began to use it as a host in areas of alfalfa cultivation
(Tabashnik 1983a). In localities without alfalfa cultivation, common sul-
fur butterflies have maintained their traditional host relationships. Studies
of preference, larval growth, and survival on native host plants and alfalfa
revealed that some divergence of populations in alfalfa-growing and non-
alfalfa-growing areas has occurred (Tabashnik 1983b). All populations are
able to use both native and crop host plants. Sulfur butterfly larvae from
alfalfa areas showed greater survival and faster growth to pupation on
alfalfa than on native legume hosts. Larvae from areas without alfalfa
showed greater survivorship and heavier pupal weights on native hosts
than on alfalfa. Nevertheless, no preference of the alfalfa butterflies for
oviposition on alfalfa rather than native hosts was evident (Tabashnik
1983b). Thus, a degree of differentiation in genetic adaptation was evi-
dent, but all populations were able to use both ancestral and recently
adopted crop host plants.
Perhaps the evolutionary shift that has progressed furthest is that of the
apple maggot fly (
Rhagoletis pomonella
) from native hawthorns (
Crataegus
spp.) to introduced apple (
Malus sylvestris
) (Bush 1969; McPheron et al.
1988). This fly may also use dogwood (
Cornus florida
) as a native host
(Smith 1988), although these populations may be a distinct species (Feder
et al. 1998). The apple maggot fly first began to attack domestic apple
trees in the mid-1800s. Studies in the 1980s showed that populations of
the fly on hawthorns and apple were genetically differentiated but not
completely reproductively isolated (Feder et al. 1988; McPheron et al.
1988). Behavioral differences exist between hawthorn and apple races
such that flies from hawthorn populations strongly prefer to lay eggs on
hawthorns, whereas flies from apple populations show a slight preference
for laying on apple (Prokopy et al. 1988). Extensive sets of field experi-
ments with marked flies from hawthorn and apple populations have sup-
