Agriculture Reference
In-Depth Information
42 days (Aalbersberg et al., 1987), and females can
produce 13 to 46 nymphs per generation. Repro-
ductive rates increase and generation time is
reduced as air temperature increases. The RWA
is holocyclic in its native range. Parthenogenic
females give rise to males and females in the fall
to produce eggs that overwinter and hatch in the
spring. However, RWA is only known to over-
winter in the US as parthenogenic females.
The RWA preferentially feeds on new plant
growth and damages the leaves by causing char-
acteristic longitudinal white streaking, purple dis-
coloration, and rolling of the leaves. Prolonged
infestations lead to prostrate plant growth, stunt-
ing, and heads being trapped by the rolled leaves.
Ultimately, the damage leads to reduced seed
weight, end-use quality, and yield (Walters,
1984). Infestations that occur from the boot
growth stage onward are the most damaging to
yield (Kriel et al., 1986). Rolling of the plant
leaves plays a critical role in the aphid's survival
by providing a tubular refuge from predators,
parasites, and insecticides (Webster et al., 1987).
Management strategies for the control of RWA
include cultural, biological, chemical, and host-
plant resistance. Host-plant resistance has been a
cornerstone in managing RWA in the US and
South Africa. Cultural control options in the US
consist of delaying the planting of winter wheat
or barley until November to avoid early fall infes-
tations, and planting spring grains in early March
to establish strong seedlings prior to spring RWA
infestations (Hammon et al., 1996). The control
of volunteer wheat and barley after harvest has
eliminated sources for fall infestations (Walters
1984). These methods are very useful and are
typically employed whenever they fi t a grower's
production system.
Biological control agents that have been evalu-
ated for RWA management in the US include
predators, parasitoids, and pathogens. Life-table
studies on RWA determined that key predators,
Hippodamia
spp. and
Chrysoperla
spp., had no
effect on aphid densities or plant biomass
(Randolph et al., 2002). The only endemic para-
sitoid,
Diaeretiella rapae
(McIntosh), parasitized
RWA at very low levels (<5%). Similarly, three
species of common aphid fungal pathogens
were ineffective control agents (<2.5% aphid
control) for RWA in dryland wheat (Wriaght et
al., 1993). Biological control agents are, however,
important components of IPM programs for
cereals.
Chemical control by insecticides has been the
most widely used method for RWA management
in the absence of resistant cereal cultivars. Sys-
temic or mixtures of contact and systemic insec-
ticides (e.g., chlorpyrifos, dimethoate, disulfoton,
imidacloprid, malathion, methyl parathion,
parathion) are used when aphid levels reach
an economic infestation level of 5% to 20%
infested tillers per plant, depending on time
of season and plant growth stage. Plants can
withstand higher levels of infestation as they
mature.
Utilization of host-plant resistance
Host-plant resistance has been the most effective
and economical means of managing RWA in the
US and South Africa. The fi rst breeding effort to
develop RWA-resistant wheat was initiated in
South Africa shortly after the introduction of the
RWA in 1978. Genes conferring resistance to
RWA were identifi ed in
T. aestivum
L. and des-
ignated as
Dn1
,
Dn2
, and
Dn5
(Dutoit 1989),
which led to the fi rst RWA-resistant cultivar
(TugelaDn). By 2006, 27 resistant cultivars had
been released in South Africa (Tolmay et al.,
2007). South African and US researchers have
now characterized nine resistance genes for wheat.
Dominant genes
Dn1
,
Dn2
,
Dn4
,
Dn5
,
Dn6
,
Dn8
,
and
Dn9
originated from
T. aestivum
. The reces-
sive
dn3
gene originated from
T. tauschii
. Gene
Dn7
is a dominant gene resulting from the inter-
generic transfer from rye to common wheat
(Marais et al., 1994). The
Dn4
gene (Quick et al.,
1996) was incorporated into seven RWA-resistant
cultivars, which successfully managed RWA in
the US from 1995 to 2002.
A RWA population can contain biotypes that
have the ability to damage previously resistant
cultivars. Their occurrence can have a serious
impact on managing RWA with host-plant resis-
tance. Biotypes were fi rst reported in RWA popu-
lations from the former Soviet Union, Europe,
