Agriculture Reference
In-Depth Information
Polyphagous predators (e.g., spiders, ground
beetles) often colonize cereal fi elds before
aphids and have been associated with low aphid
densities (Sanderson et al., 1992; Schmidt et al.,
2004). By stemming initial BCOA populations,
polyphagous predators have potential to prevent
damage by BCOA and limit BYD (Östman
et al., 2003).
Aphidophagous predators, for example, lady
beetles [
Coleomegilla maculata
(DeGeer) and
Hip-
podamia convergens
Guérin-Méneville] and their
larvae, hover fl y [
Allograpta obliqua
(Say)] larvae,
and lacewing [
Chrysoperla plorabunda
(Fitch)]
larvae, consume large numbers of BCOA and
contribute to biological control of cereal aphids.
However, these predators typically peak during
surges in cereal aphid populations and generally
do not preempt sizeable aphid infestations (Sand-
erson et al., 1992; Elliott and Kieckhefer 2000).
Moreover, predator activity causes BCOA to
move among cereal plants and may contribute to
increased BYD transmission and incidence (Bailey
et al., 1995; Smyrnioudis et al., 2001).
Parasitoids of BCOA typically occur at low
levels in cereal fi elds, and parasitism tends to peak
at or around the time of maximum aphid abun-
dance (Sanderson et al., 1992; Basky and Hopper
2000). Nonetheless, parasitism levels may be ade-
quate to preclude some treatments for aphids and
thus have been included in aphid treatment
thresholds (Giles et al., 2003). Moreover, parasit-
oid attack may make BCOA more sedentary and
help reduce spread of BYDV (Smyrnioudis et al.,
2001).
mechanisms that produce high nymphal mortal-
ity, prolong early growth stages, and lower birth
rate near the time of head emergence (Wiktelius
and Pettersson 1985). The BCOA is less sensitive
to mechanisms that increase the proportion of
winged forms and decrease landing rates on
plants. Plant resistance traits that limit winged
forms may suppress BYD by reducing BCOA
dispersal. Also, traits that inhibit BCOA from
reaching the phloem may prevent inoculation and
acquisition of BYDV (Gibson and Plumb 1977).
Tolerance is a form of host-plant resistance
that allows plants to recover or limit the effects of
infestation. It may limit direct yield loss from
BCOA (Dunn et al., 2007) but could allow BCOA
population buildup and promote secondary spread
of BYDV (Gibson and Plumb 1977). Thus, toler-
ance may need to be coupled with traits confer-
ring BYD resistance.
Although promising sources of resistance to
BCOA have been identifi ed in wheat, resistance
genes have not been characterized or routinely
utilized to develop new resistant wheat cultivars.
While some of these sources of resistance to
BCOA may indirectly limit BYD, conversely
some forms of BYDV resistance may indirectly
suppress BCOA population growth. Transgenic
wheat plants with coat-protein-mediated resis-
tance to BYDV maintained a lower virus titer
than susceptible, conventional wheat and conven-
tional, BYD-tolerant wheat lines. They also
lowered fecundity, shortened the reproductive
period, and reduced the number of progeny of
BCOA compared with noninfected transgenic
plants (Jiménez-Martínez et al., 2004) The
authors suggested that BYDV-resistant trans-
genic plants could further limit BYD under
fi eld conditions by reducing BCOA population
growth.
Finally, host-plant characteristics such as resis-
tance traits and plant morphology can impact the
effectiveness of natural enemies. Theoretically,
plant resistance based on tolerance should not
affect aphid fi tness, and thus no indirect negative
effects on natural enemies are predicted (Brewer
and Elliott 2004). Empirical studies have gener-
ally supported this prediction. In contrast, a resis-
tance trait that causes greater mortality or reduced
Utilization of host-plant resistance
Host-plant resistance to BCOA has been identi-
fi ed in wheat and other monocotyledonous plants,
and it has been derived from dicotyledonous
transgenes (Migui and Lamb 2003; Liang et al.,
2004; Hesler et al., 2005; Dunn et al., 2007).
Although several promising sources of resistance
to BCOA are available, none have been incorpo-
rated into contemporary wheat cultivars.
Host-plant resistance may differ in its applica-
bility to managing BCOA and limit BYD. Models
predict that BCOA is most sensitive to resistance
