Agriculture Reference
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rare among other plant-insect interactions (Harris
et al., 2003).
Despite the development of virulence in
Hessian fl y populations, the strategy of deploying
a single H gene, followed by deployment of a
different H gene when the fi rst is overcome by
virulent biotypes, is considered cost-effective,
especially for cereal crops such as wheat (Cox and
Hatchett 1986; Berzonsky et al., 2003; Harris et
al., 2003). Indeed, single H genes transferred to
elite wheat genotypes and deployed in agriculture
have eliminated the need for other control tactics
in North America and North Africa, where the
Hessian fl y is a serious pest (Pedigo 2002). It has
been suggested that H genes overcome by viru-
lent biotypes can be reintroduced at a later time
(Foster et al., 1991), because the modifi ed Avr
gene that conferred Hessian fl y virulence to the
H gene will be eliminated from the Hessian fl y
population when that H gene is removed as a
source of selection pressure.
since 1980, several studies have shown direct
impact to wheat by BCOA without confounding
effects of BYD. Seedling infestation has produced
maximum grain yield loss (24%-65%), whereas
yield loss from boot-stage infestations was lower
and less frequent, and no yield loss resulted from
infestation at later stages (Kieckhefer et al., 1995;
Voss et al., 1997). Despite the lack of visible symp-
toms associated with its feeding, BCOA causes
equal or greater yield loss as do cereal aphids that
cause visible symptoms (i.e., greenbug and Russian
wheat aphid) (Pike and Schaffner 1985; Kieckhe-
fer et al., 1995). Yield loss from seedling infection
by Barley yellow dwarf virus (BYDV) is greater
than from seedling infestation by BCOA, and
combined effects of BCOA and BYDV are greater
than either treatment alone (Riedell et al., 1999,
2007). The BCOA also reduces baking quality of
wheat (Basky et al., 2006).
Insecticides have been widely used to manage
BCOA and reduce incidence of BYD (Burnett
and Plumb 1998). However, problems are associ-
ated with insecticide use. It may not always
provide economic return (Royer et al., 2005),
BCOA has developed insecticide resistance
(Cheng et al., 2004), and environmental costs are
associated with insecticide use. Alternative use of
natural enemies and host-plant resistance may be
advantageous.
Natural enemies to BCOA occur as fungal and
viral pathogens, polyphagous predators, aphi-
dophagous predators, and hymenopterous para-
sitoids (Carter et al., 1980). However, the different
kinds of natural enemies may vary in their
effi cacy for managing BCOA and limiting BYD
(Sanderson et al., 1992).
Fungal epizootics affect cereal aphid popula-
tions, but viral epizootics have not been reported
(Latgé and Papierok 1988). However, despite
reaching epizootic levels at times, fungal entomo-
pathogens depend on relatively high humidity
and high aphid densities (Basky and Hopper
2000). Consequently, mycoses are typically low in
BCOA populations, with incidence greatest at or
near times of peak aphid abundance. Typical
levels of natural infection therefore may not limit
BCOA populations and BYD incidence below
economic levels (Sanderson et al., 1992).
BIRD CHERRY-OAT APHID
The bird cherry-oat aphid (BCOA), Rhopalo-
siphum padi L. (Color Plate 21), is a common
aphid pest of wheat worldwide (Vickerman and
Wratten 1979). Bird cherry ( Prunus padus L.) and
closely related tree species are primary hosts at
higher latitudes, while oat ( Avena sativa L.),
wheat, other cereals, and many grasses are sec-
ondary hosts from spring through fall.
Biology, plant damage, and
control methods
The BCOA has multiple, asexual generations on
secondary hosts (e.g., wheat). Migrants are pro-
duced in autumn and fl y to primary hosts where
mating occurs and overwintering eggs are depos-
ited. In the northern hemisphere, parthenogenic
BCOA routinely overwinter on cereals and grasses
at latitudes below 40ºN.
The BCOA was fi rst identifi ed as a wheat pest
because it vectors viruses that cause barley yellow
dwarf (BYD), one of the most important viral dis-
eases of cereals (Lister and Ranieri 1995). However,
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