Biology Reference
In-Depth Information
plant, a toxic, alien rangeland weed, was controlled in the western United
States by a leaf beetle,
Chrysolina quadrigemina
. After this system had been
in operation for about 54 yr, Maron and Luke (2000) examined the effec-
tiveness of the beetle in suppressing St. John's wort. Field populations of
St. John's wort in the western United States were more tolerant of her-
bivory than St. John's wort plants in either the midwestern United States,
where the beetle is absent, or in Europe, where St. John's wort is native.
Genetic analyses suggested that selection had favored increased tolerance
by St. John's wort to this biocontrol agent.
A similar case in Australia involves common St. John's wort and a mite
(
Aculus hyperici
) introduced in 1991 for biological control.This mite is one
of six biocontrol species that have been established in the hope of achiev-
ing control of this rangeland weed (Mayo and Roush 1997). Introduc-
tions of the mite succeeded in some locations and failed in others (Jupp
et al. 1997). Four morphologically distinct forms of St. John's wort are
known in Australia, and it appears that these forms are differentially sus-
ceptible to the mite.
Another Australian case involves rush skeletonweed (
Chondrilla juncea
)
and a rust (
Puccinia chondrillina
) introduced as a biocontrol (Burdon et al.
1981).The rust was effective in controlling one of the three forms of rush
skeletonweed, but the remaining two resistant forms quickly invaded areas
where the susceptible form had been eliminated.
Several cases of evolutionary resistance of target animals of biocontrol
agents have also been reported. The parasitoid wasp
Aphidius ervi
was
introduced to the northeastern United States in 1959 as a biological con-
trol for the pea aphid (
Acyrthosiphon pisum
), an alien pest of alfalfa (
Med-
icago sativa
), red clover (
Tr ifolium pratense
), and other legumes. The intro-
duced wasps, originating from alfalfa fields in France, have spread and
become a successful control agent for pea aphids throughout much of
North America.
The genetic structure of populations of pea aphids and parasitoid
wasps has been the focus of ongoing studies by several investigators (see,
e.g., Henter and Via 1995;Via 1999; Hufbauer and Via 1999).These stud-
ies revealed that aphids in alfalfa fields suffer less parasitism than those in
red clover, with the difference being a greater physiological resistance to
parasitism in alfalfa aphids.This difference cannot be explained by nutri-
tional or secondary chemical differences related to the two crops and thus
appears to be genetic. Genetic variability in resistance to parasitoids is
abundant in the alfalfa populations but undetectable in those in red
