Agriculture Reference
In-Depth Information
have difficulty predicting a priori which single agent will establish and func-
tion most successfully throughout the target weed's range, and (ii) synergistic
effects of control agents that attack different host plant tissues and life stages
are desired. As more potential control agents are screened and released, there
is a better chance of having at least one agent establish successfully. Andow,
Ragsdale & Nyvall (1997) reviewed data for 74 insect species released for
control of 35 target weeds in cool temperate regions.For those target weeds on
which no insects established, the mean number of species released was 1.2. In
contrast, for target weeds on which at least one insect species became estab-
lished, the mean number of species released was 4.2. For the weed species for
which complete biological control was achieved, the mean number of species
released was 5.4.
Euphorbia esula
and
Centaurea maculosa
are two rangeland weeds introduced
into the western USA and Canada for which multiple herbivore species have
been imported and released since the 1960s. Eleven of the 18 insect species
released to control
E. esula
have been established successfully at one or more
locations (Table 8.1); 10 of the 13 insect species released against
C. maculosa
have been established successfully (Table 8.2). Some of the established species
inflict substantial damage to the targeted weeds under field conditions.
Insects that are particularly effective for suppression of
E. esula
include flea
beetles (
Aphthona cyparissiae
,
A. czwalinai
,
A. flava
,
A. lacertosa
, and
A. nigriscutis
)
that mine the plant's roots as larvae and eat its leaves as adults; another beetle
(
Oberea erythrocephala
) that bores into stems and root crowns; midges (
Spurgia
capitigena
and
S.esulae
) that form galls in shoot tips and prevent flowering; and
moths (
Hyles euphorbiae
and
Lobesia euphorbiana
) whose larvae defoliate the
plant and kill stems (Julien & Griffiths, 1998; Swiadon, Drlik & Woo, 1998).
Successfully established species that are particularly effective at suppressing
C.maculosa
include a root-mining moth (
Agapeta zoegana
); a beetle (
Cyphocleonus
achates
) that kills small plants and shoots arising from stools of larger plants;
and a moth (
Metzneria paucipunctella
) and two gallflies (
Urophora affinis
and
U.
quadrifasciata
) that attack seed heads (Grossman, 1989; Julien & Griffiths,
1998).
Although insect herbivores have substantially reduced
Euphorbia esula
and
Centaurea maculosa
densities at some locations, they have failed to completely
control the weeds throughout their full geographical ranges (Julien &
Griffiths, 1998).However, control may become more widespread in the future
as introduced control agents disperse, increase in number, and deplete weed
seed banks in the soil (Harris, 1997). Control is also likely to improve as more
is learned about the ecological characteristics and requirements of the
herbivores.
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