Biology Reference
In-Depth Information
lation exhibited survivorship more than 40% greater than mites from the
bean population. Mites from the bean and cucumber population, how-
ever, lost a small but significant degree of fitness on their original bean
host. In similar experiments, Agrawal (2000) obtained similar results.
Fry (1989) extended the studies of Gould (1979) by examining the
ability of the two-spotted spider mite to adapt to two other hosts that
were initially only marginally acceptable: tomato (
Lycopersicon esculentum
)
and broccoli (
Brassica oleracea
). In laboratory populations, mites showed
reduced mortality and increased acceptance of these plant hosts in less
than ten generations. Thus, rapid evolutionary adaptation by this mite is
possible to plant hosts differing considerably in defensive chemistry.
Many of the major insect pests of agricultural crops have made eco-
logical and evolutionary shifts from wild plants to crop species since their
domestication. In some cases, these insects show high phenotypic and
genotypic variability and are able to adapt to many host plants.The green
peach aphid (
Myzus persicae
), for example, is known to feed on more than
400 species of plants belonging to more than 50 plant families (Weber
1985). Although they reproduce parthenogenetically, populations of this
aphid exhibit numerous genotypes. Within crop fields, selection for cer-
tain genotypes may occur during an individual growing season, as Weber
(1986) found in crop fields in Germany. This pattern of rapid, fine-scale
adaptation has been observed in a number of native and alien insects
(Mopper 1996).
A more restricted pattern of feeding on multiple hosts is shown by the
southern cowpea weevil (
Callosobruchus maculatus
), native to sub-Saharan
West Africa.This weevil is also able to use many host species, but within
only one plant family, the Fabaceae. In addition, the southern cowpea
weevil shows somewhat closer evolutionary adaptation to particular host
species. This weevil has become a cosmopolitan pest of legume seeds in
field and warehouse situations.Although its original host was probably the
wild relative of the cowpea or black-eyed pea (
Vigna unguiculata
) in Africa,
it now shows biotypes adapted to a wide variety of crop legumes (Wasser-
man 1986). The weevil now attacks crop legumes belonging to at least
nine genera and more than a dozen species of legumes. Specific host races
of the cowpea weevil show genetically based oviposition preferences pre-
sumably related to differences in host plant chemistry. The pea aphid
(
Acyrthosiphon pisum
) shows a similar pattern of biotype formation on
plants in the legume family (Auclair 1978).
A still more restricted pattern of host use is shown by the boll weevil
(
Anthonomus grandis
), one of five closely related weevils whose ancestral
