Agriculture Reference
In-Depth Information
population growth is hypothesized to negatively
impact life histories of natural enemies. Plant
morphology also may impact natural enemy effec-
tiveness against aphids, with natural enemies
favored generally by morphological traits that
enhance their access to cereal aphids, for example,
reducing the number of leaf hairs to facilitate for-
aging activity by natural enemies. Most studies
on host plant-natural enemy interactions have
focused on other cereal aphids, so future research
on this topic with BCOA is needed.
during the fall causes irreversible damage to
plants. Population densities of 30 aphids per
culm, with a feeding period of 7 days, caused
reductions of at least 40% in grain weight (Kieck-
hefer and Kantack 1988). In addition to leaf
damage, greenbug feeding reduces root length
and dry weight (Burton 1986; Riedell and Kieck-
hefer 1995). Thus, greenbug infestations of winter
wheat at the seedling stage may adversely impact
the plant's ability to withstand environmental
stresses such as drought and heat during later
developmental stages, which is particularly
important during grain fi lling.
In addition to direct plant damage caused by
feeding, the greenbug is also a vector of viruses,
notably
Barley yellow dwarf virus
. The fi rst con-
fi rmed widespread barley yellow dwarf epidemic
transmitted by greenbug into winter wheat
occurred in southern Idaho during 1977 to 1978
(Forster 1990).
Although the greenbug has been known to exist
since the 1880s, it was not until the 1950s, when
greenbug-resistant wheat began to be developed,
that greenbug populations or biotypes were iden-
tifi ed that differed in their ability to damage resis-
tant plants. Biotypes are genetically distinct
populations, and each biotype is a phenotypic
expression of an indefi nite number of genotypes
(Puterka and Peters 1990). Greenbugs are classi-
fi ed to a particular biotype primarily based on the
plant virulence gene(s) they have in common, as
manifested by their ability to damage plants with
known resistance genes. Eleven greenbug bio-
types (named A through K) have been identifi ed
using multiple wheat and sorghum (
Sorghum
bicolor
L.) genotypes containing greenbug resis-
tance genes to differentiate among these greenbug
populations.
The most common method of controlling
greenbug in wheat has been the use of chemical
insecticides. Treatment thresholds vary by plant
growth stage (e.g., 2 to 4 aphids per tiller at the
seedling stage, 2 to 8 aphids per tiller when plants
are 7 to 15 cm in height, and 8 to 20 aphids per
tiller when the plants are 15 to 40 cm in height)
and by crop condition (Royer et al., 1997b).
Chemicals listed for use against greenbug in wheat
include chlorpyrifos, dimethoate, disulfoton, imi-
GREENBUG
Economic impact and distribution
The greenbug (
Schizaphis graminum
Rondani) is
a serious, perennial aphid pest of wheat with a
worldwide distribution. A 1993 survey revealed
that over 3 million hectares (41%) of dryland and
500,000 ha (93%) of irrigated wheat in the
western US were infested with greenbug (Webster
and Amosson 1995). Damage from greenbug
during such periodic outbreaks can be substan-
tial, such as 1976, when losses exceeded $80
million in Oklahoma alone (Starks and Burton
1977). In central Oklahoma, greenbug outbreaks
generally occurred following a year of above-
normal precipitation during the spring and
summer; above-normal temperature during
the winter, spring, and fall; and below-normal
temperature during the summer (Rogers et al.,
1972).
Biology, plant damage, and control
methods
The greenbug is a small (approximately the size
of a sesame seed) light green sap-sucking arthro-
pod pest of wheat (Color Plate 22). It feeds on
sieve tube sap through a proboscis, shaped like a
soda straw and called a stylet. This feeding activ-
ity results in distinguishable leaf damage symp-
toms (yellow-red necrotic lesions at the feeding
site surrounded by a larger area of chlorosis) that
eventually lead to general necrosis of the leaf.
Greenbug feeding on seedling winter wheat
