Biology Reference
In-Depth Information
graphic range increases the potential of
Choristoneura
to become a serious
forest pest.
Theory relating to patterns of specialization of herbivorous insects has
received considerable attention from insect ecologists because of its rele-
vance to pest evolution, and therefore, pest management. Many pest
insects exhibit biotypes, which are ecologically or genetically distinct
forms adapted to particular hosts (Futuyma and Peterson 1985).To ecol-
ogists, biotypes that are genetically distinct are simply ecotypes for which
the habitat and food resource are specific plants. Different biotypes may
vary,however, in degree of reproductive isolation. Some may be capable
of interbreeding freely when they come in contact; others may be repro-
ductively isolated, so they do not come in contact or do not interbreed if
they do. Biotypes that show reproductive isolation are effectively distinct
species, whether or not they are so classified.
Given that an herbivorous species uses two or more host plants,
selection can favor the evolution of distinct biotypes under certain con-
ditions. First, some advantage of specialization for specific plant hosts
must exist. That is, adaptation to plant features that influence growth,
survivorship, and reproduction must be possible. Second, there must be
genetic variability on which selection can act. For many native herbi-
vores, genetic variability may be greater than that for alien herbivores
that may have experienced a genetic bottleneck during the introduction
process (see chapter 3). Third, the dispersion of plant hosts in space or
time must be such that gene flow between populations of the herbivore
on different plant hosts does not overwhelm the influence of selection.
Plant hosts may differ in the habitats occupied, resulting in reduced
movement of individual herbivores between areas with the different
hosts. Alternatively, the phenology of flowering or fruiting of the differ-
ent hosts may be such that individuals of the herbivore become isolated
in time. When spatial or temporal isolation is only moderate, a degree
of specialization may result, but interbreeding may prevent full special-
ization of populations for different plant hosts.When isolation is strong,
selection may eventually lead to strong reproductive isolation and spe-
ciation.
If a new potential host plant becomes intermixed in a plant commu-
nity with host plants already used by an herbivore, selection may favor
expansion of the feeding niche to include the new host plant. Accessory
benefits of a host plant shift may involve reduced predation or parasitism
by specialist enemies associated with the native host plant.
