Research
Wheat Stem Sawfly (Hymenoptera: Cephidae) Revisited
MICHAEL
J. WEISS AND
ABSTRACT The wheat stem sawfly, Cephus c;nctus Norton, is
a major pest of spring wheat in the northern Great Plains of North
America. Cropping systems developed during the early settlement
years, summer fallowing and strip cropping, and drastic increases
in wheat production led to wheat stem sawfly becoming a pest. The
development and release of a solid-stemmed resistant cultivar,
'Rescue', in the 1940s and the subsequent release of improved
cultivars have reduced losses significantly in regions heavily infested
by the sawfly. The difference in yield potential of resistant cultivars
compared with susceptible cultivars is less in the western range of
the pest, and explanations are provided. Although resistantcultivars
have reduced losses, the undesirable agronomic characteristics and
lower yield potential have suppressed grower acceptance of the
solid-stemmed cultivars. The lack of an accurate damage-prediction
program forces growers to gamble between planting a susceptible
cultivar and suffering yield loss because of the wheat stem sawfly
or planting a resistant cultivar and suffering yield or quality
WENDELL L. MORRILL
250-500 mm, as well as from Manitoba, Saskatchewan, Alberta,
and British Columbia (Ries 1933; Davis 1953, 1955).
Adult emergence begins in late Mayor early June and lasts 3-4
wk. The wheat stem sawfly is a relatively weak flier. Although
females have been observed to migrate atleast 2.2 km (Anonymous
1955), the female will usually deposit eggs in stems near the
emergence site (Criddle 1911, Ainslie 1920, Holmes 1975). Adults
are active during sunny conditions, when temperatures range from
17 to 32°C and wind speeds are minimal (Seamans 1945). The
female inserts a single egg in the stem per visit. Holmes & Peterson
(1960) demonstrated that females prefer to oviposit in the elongating (uppermost) internode. Succulent stems with a diameter of2.83.4 mm from which the head has not yet emerged are preferred
(Holmes & Peterson 1960).
Eggs hatch in ",7 d (Ainslie 1920) and larvae feed on parenchyma
and vascular tissue within the stem (Holmes 1954). There are four
to five instars depending on the host (Farstad 1940). If more than
reduction. Continued economic losses in spring wheat and recent
one egg is deposited in the stem, only one larva survives because of
infestations of winter wheat mandate continued research on this
pest.
cannibalism. Completion of larval development usually coincides
with plant senescence. Early researchers hypothesized that larval
movement down the stem was a response to decreasing stem
moisture (e.g., Davis 1955); however, Holmes (1975) provided
evidence that light (visible and infrared) transmitted through the
stem was the main stimulus for downward migration. Upon
com pletion of larval development, the larva will girdle the inside of
a stem with a V-shaped notch; the height above the soil surface
depends on soil and stem moisture (Holmes 1975). Immediately
below the notch, the larva plugs the stem with frass. The stem
usually breaks at the notch, forming a "stub" that serves as an
overwintering chamber. The stub is hollow and allows the larva to
overwinter below the soil surface, thus protecting it against the
severe winter climate. The following Ma y, the larva pu pates and the
adult chews an emergence exit through the plug or side of the stub.
Crop Damage. The prediction of Riley & Marlatt (1891, 178)
that "it may be expected at any time to abandon its natural food
plant in favor of the small grains" came true when the wheat stem
sawfly was first reported attacking wheat in 1896 near Indian
Head, Saskatchewan, and Souris, Manitoba. Serious damage was
recorded in Manitoba during 1912 (Criddle 1913) and 1921
(Criddle 1922) and in Saskatchewan during 1921 (Criddle 1922),
1922, and 1926 (King 1929). In Alberta and Saskatchewan, the
wheat stem sawfly caused an estimated yield loss of 50 million
bushels during 1941 and 15-30 million bushels during 1944 and
1946 (Bierne 1972). The annuallosses in Saskatchewan from 1926
to 1958 ranged from 1.4 to 10.3% of potential yield (Bierne 1972).
In North Dakota, Munro (1948) estimated an annual loss of 3-5
million bushels during 1940-1947. In North Dakota and Montana
T
HE WHEAT STEM SAWFLY, Cephus cinctus Norton, is a
native North American insect that is a major insect pest of
wheat (Triticum spp.). Within the northern Great Plains of
North America (western Minnesota, North Dakota, northern
South Dakota, eastern and central Montana, western Manitoba,
Saskatchewan, and eastern Alberta), this insect has caused devastating losses to both spring bread wheat (T. aestivum L.) and durum
wheat (T. durum Desf.). During 1989, extensive damage was
observed in winter wheat (T. aestivum L.) in central Montana, for
which losses were estimated at 80% (W.L.M., unpublished data).
Numerous control strategies provided only marginal success until
the development and subsequent release of resistant (solid-stemmed)
hard red spring bread wheatcultivars. Growers' reluctance to select
resistant cultivars is based on perceptions that the solid-stemmed
cultivars have an inherently lower yield potential, will undergo a
reduction in the solidness of the stem because of environmental
conditions, will experience inherently higher stem lodging because
of the solid stem, and possess inferior disease resistance and grain
quality, or both. Additionally, recommendations for areas and
years in which a solid-stemmed cultivar should be selected have not
been identified. The objectives of this article are to review wheat
stem sawfly literature, evaluate current cultural practices, and
present the current status of the wheat stem sawfly.
Sawfly Biology. The wheat stem sawfly has been collected from
areas of every state west of the Mississippi in the northern and
central plains of North America where the annual precipitation is
Winter 1992
241
Fig. 1. Typical wheat stem sawfly-induced
cutting of the wheat stem
with resulting stub.
during 1952, Davis (1953) estimated losses to be 8 million bushels.
Although the insect was found throughout western North America,
it only achieved severe pest status in the Spring Wheat Belt of North
America (northwestern North Dakota and eastern and central
Montana in the United States and western Manitoba, Saskatchewan,
and eastern Alberta).
The wheat stem sawfly affects yield both physiologically and
physically. It affects the yield of the infested stem physiologically by
cutting vascular bundles, reducing the flow of nutrients and water
to the developing kernels. Several early researchers failed to detect
a yield loss caused by sawfly feeding. The female selects larger
diameter stems, which have an inherently higher yielding ability;
the researchers erroneously compared the yield of the larger infested stems with the yield of the smaller, noninfested stems.
Holmes (1977), comparing similar diameter stems, found a range
in yield reduction of 10.8-22.3 % attributable to reductions in the
number and size of kernels. The loss in protein content ranged from
0.6 to 1.2%. According to Holmes (1977), rainfall in July and
August, date of infestation, and time of cutting in relation to plant
development contributed to the variation in losses.
Physical damage is the most visible and can be extreme. The
larva weakens the stem by cutting a V-shaped notch at the base,
which results in stem lodging and reduction in the amount of grain
that can be harvested (Fig. 1). Farstad &Jacobson (1945) estimated
the amount of grain lost because of stem cutting to be 800-1,600
kglha. More recently, M.J.W. (unpublished data) found only a 58
kg/ha loss. Several factors influence the amount of lodging of the
cut stems; for example, high winds with precipitation will result in
extreme lodging rates, thus increasing the harvest loss.
Effect of Cultural Practices. Turnock (1971) developed a scenario to explain the evolution of the wheat stem sawfly as pest. We
agree with the explanations; however, we believe the three major
reasons were: 1) rapid expansion of wheat acreage; 2) producer
utilization of the summer-fallow concept; and 3) producer utilization of the strip cropndashfallow rotation cropping system.
Wheat acreage in the northern Great Plains expanded rapidly in
the early 1900s. For example, in Montana wheat was produced on
=101,170 ha during 1909; by 1919 production had increased to
=1,214,040 ha (Howard 1959). During 1885, a Scottish farmer in
Alberta "accidentally" discovered that if a field was fallowed for a
16-mo period, the following growing season the wheat yield was
greater than in a field that was not fallowed. Because of the
242
increased yield potential, this cropping system spread quickly
throughout the northern Great Plains (Howard 1959). The use of
the "saved" soil moisture ensured that wheat would not senesce
early, thus allowing the sawfly larva to complete development.
Favorable moisture also results in a wheat stem of sufficient
diameter to ensure preferential oviposition in wheat rather then in
native grass hosts. As the use of the summer-fallow system increased, wind-caused soil erosion became a severe problem. In
1918, two farmers in Alberta found that decreasing the field width
perpendicular to the prevailing winds decreased the wind erosion.
By using alternating strips of crop and fallow, the wind erosion was
decreased (Howard 1959). However, use of narrow strips instead
of large (wide) fields increased the severity of wheat stem sawfly
damage. Females emerging from adjacent strips of the previous
year's crop infested the wheat from two sides. In many cases they
were able to infest the entire stand, which resulted in severe sawflyinduced stem lodging. In larger fields, the damage was confined
mostly to edges of the fields, and the resulting damage was not as
severe (Holmes 1982).
Control Strategy Attempts. Norman Criddle, a farmer hired by
the provincial government of Manitoba, was the first to explore,
develop, and recommend various control strategies. Criddle's
(1911,1913,1915,1922)
recommendations included deep moldboard plowing, early mowing of rye grasses for hay, refraining from
mowing brome grass to permit parasite emergence, planting of trap
crops of nonhost crops and grasses (females oviposit within the
stem, but larvae do not complete development), planting nonhost
crops, and early harvesting. Later, others (e.g., Jacobson & Farstad
1952) recommended a delayed planting date to reduce damage.
These control strategies were applied with moderate success but fell
into disuse when they no longer could be used economically in a
mechanized agriculture system. Today, the only method that is still
used to some extent is the rotation to nonhostcrops. Unfortunately,
the economics associated with continuous spring wheat production
often make crop rotation to a nonhost, such as spring barley,
unattractive to producers.
Attempts at biological control have been unsuccessful. During
1930, 15,000 wheat stems infested with the European wheat stem
sawfly, Cephus pygmaeus (L.), and associated parasitoids were
imported from England. From the stems, =6,000 adult Collyria
calcitrator (Gravenhorst) were recovered and released in Saskatchewan, but they did not become established. Further releases of C.
calcitrator over a 9-yr period and releases of Heterosphilus cephi
Rohwer and Pediobius nigritarsis (Thompson) were unsuccessful
(Clausen 1978).
A second major importation of parasites of C. pygmaeus larvae
from Russia and Sweden was made during 1952-1954. Releases of
17,000 C. calcitrator and 3,000 Bracon terebella Wasmael were
made in North Dakota and Montana, but none became established
(Smith 1961). Smith (1961) postulated that the importation failed
because of the parasitoid'shost specificity for the European species.
Development of Resistant Cultivars. In the early 1920s, Schegolev
(1926) reported that solid-stemmed cultivars were less subject to
damage by the European wheat stem sawfly. Stem solidness is
caused by the development of pith inside the stem. During 1933, the
Canadian government initiated a research program to find an
agronomically suitable wheat cultivar that was resistant to the
wheat stem sawfly. Farstad (1940) was the first to relate stem
solidness to resistance to the wheat stem sawfly. Solid-stemmed
wheat germ plasm was collected and a cultivar from Portugal, $-615
(Platt & Farstad 1953), was crossed with the cultivar 'Apex' to
produce a solid-stemmed cultivar, 'Rescue' ($toa 1947). 'Rescue'
performed as hoped, because losses for 'Rescue' did not exceed 5%,
whereas losses in susceptible varieties reached 95% in 1947 in
AMERICAN
ENTOMOLOGIST
Teton County, Mont. (Plattet aI.1948). An estimated $3.8 million
was saved in 1948 by using 'Rescue' in sawfly infested areas
(Anonymous 1946).
Unfortunately,
the agronomic characteristics
of 'Rescue' were
poor. Yields were 8-15% less than other varieties, and it lacked
desirable baking and milling qualities (Stoa 1947). The solid stem
characteristic
was not always well expressed, especially when
overcast days coincided with stem elongation (Platt 1941, Holmes
1984). Resistance was also affected by synchronization
of sawfly
flight and development of wheat plants (Holmes & Peterson 1962).
Two thousand lines of domestic and foreign wheats were screened
for sawfly resistance in northern Montana,
but no additional
characteristics
were found to be related to resistance (Eckroth &
McNeal 1953).
To date, 12 resistant cultivars have been released from Canadian and U.S. breeding programs (Table 1). In Montana, resistant
cultivars account for a larger share of the spring wheat acreage than
in North Dakota. The resistant cultivar 'Fortuna' accounted for
60% of the spring wheat acreage in Montana in 1970 (Fig. 2).
'Fortuna'
provided yield equal to or better than many of the
susceptible cultivars in eastern Montana
and extreme western
North Dakota; however, because of melanism (false black chaff),
a genetic disorder that discolors the seed and causes yield losses
during hot and humid conditions, its acceptance by producers has
decreased.
Yield Potential of Resistant Cultivars. The reasons for low
grower acceptance of wheat stem sawfly-resistant cultivars include
lower yield potential, inconsistency
of the solid-stemmed
trait
caused by environmental conditions, increased lodging rate (M.] .W.,
personal observation), reduced disease resistance, and lower grain
quality. We be]ieve that the major reason for grower reluctance is
the lower yield potential of the resistant cultivars.
Wecompared the yields of the susceptible and resistant cultivars
in the Advanced Hard Red Spring Wheat Yield Trials conducted at
the North Central Research Center (Minot, N.Dak.) and the
Williston Research Center (Williston, N.Dak.) during 1970-1989
and from Northern Agricu]tural Research Station (Havre, Mont.)
during 1970-1990
and Central Agricu]tural
Research Station
(Conrad, Mont.) during 1978-1990.
We used the average of the
three highest yielding cultivars (based on the previous years'
results) that most likely would be planted by growers to estimate a
probable yield for the susceptible cultivars in the absence of the
wheat stem sawfly; cu]tivars did change over the years used. The
80
"0
I
~
~
~
<Ii-
---0-
North Dakota
-tr-
Montana
60
40
20
o
1940
1950
1960
1970
1980
1990
Year
Fig. 2. Percentage of spring wheat acreage planted to a wheat stem
sawfly-resistant cultivar.
cultivars were not always the same at each location. All of the
resistant cultivars in the trial were averaged to estimate a yield for
the resistant cultivars. The yield advantage (in the absence of the
sawfly) for the susceptible cultivars ranged from 0 to 0.73 tlha at
Williston, and from 0.37 to 1.4 tlha at Minot. In Montana, the yield
advantage (in the absence ofthe sawfly) for the susceptiblecultivars
ranged from 0 to 0.62 tlha at Conrad and 0-0.65 tlha at Havre.
Two sources of yield loss occur because of sawfly infestation.
The first is reduced yield of infested plants, which has been
estimated to range from 10.8 to 22.3% (Holmes 1977). Based on
a conservative estimate of loss from Holmes (1977) we used 10%
as our estimate of loss in this article. The second is loss of cut stems
that cannot be recovered during harvest; this loss is a function of
percentage of cut stems and the percentage of recovery (harvest) of
the lodged stems. For this article, we assumed the recovery to be
85% of the cut stems. For a major infestation we assumed an 80%
infestation rate and 50% of the stems cut for a total yield loss of
15.5%. For a minor infestation we assumed a 10% infestation with
20% of the stems cut for a 2.7% total yield loss. In addition, we
assumed no yield loss caused by the sawfly for the resistant
cultivars. If a major infestation occurred at Williston every year, the
use of resistant cultivars would have provided a yield advantage in
8 of 19 yr and 5 of 19 yr at Minot. If a major infestation occurred
every year at Conrad, the use of resistant cultivars would have
Table 1. Agronomic characteristics of wheat stem sawfly-resistant cultivars
Relative
Cultivar
Releasing agencY'
Yr
released
Rescue
Chinook
Cypress
Sawtana
Fortuna
Tioga
Canuck
Lew
Leader
Glenman
Lancer
Curless
AgCan
AgCan
AgCan
MAES, ARS
MAES, NDAES, ARS
NDAES, ARS
AgCan
MAES, NDAES, ARS
AgCan
MAES
AgCan
NDAES
1946
1952
1962
1962
1966
1974
1975
1976
1982
1984
1985
1986
Yieldh
Height"
Maturityh
Kernel web
Protein 'Yah
Baking
qualityb
Disease
susceptiblilityd
Low
Low
S
S
I
S
S
S
S
S
S
SD
S
SD
Low
Med
Med--early
Low
Early-med
Med
Med-Iow
Med
Med
Med
Med
Early-med
Med
Med
High
Med
Med
Med
High
High
High
Med
High
High
Low
High
High
Med
Low
High
High
Med
High
Low
High
High
Med
Good
Good
Good
Med
Good
Good
Med
Good
Low
Good
Good
SR, LR, B, LS
SR, LR, B, LS
Unkown
SS
SR
BC
CRR, LS
SR, STR, LS
SR, LR, LS, B
SR
SR
SR, LR
Low
Low
Low
Med
High
Med
High
High
Med
"AgCan, Agriculture Canada; MAES, Montana Agricultural Experiment Station; NDAES, North Dakota Agricultural Experiment Station; ARS,
Agricultural Service.
hMcd, intermediate.
-S, Standard height; SD, semi-dwarf.
dSusceptiblc to SR, stem rust; SS, stinking smut; STR, stripe rust; LR, lear rust; B, bunt; BC, black chaff; CRR, common root rot; LS, loose smut.
Winter 1992
243
A
~
~
~
OJ
"C
OJ
';;'
...
<:>
I!J
low infestatlon
0
actual resistant
~
B
high Infestation
~
~
OJ
:EOJ Q.
OJ
';;' "
high infestation
i:!I
low infestation
0
actual resistant
.,
Q.
... ~
OJ
~
~
:l
<:>
.~ C
.~ c
';
';
~ 'ij;~
~
~
'ij;
OJ
,!:.
OJ
,!:.
70 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89
70 71 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89
Year
Year
c
D
7071 72 73 74 75 76 77 78 79 80 82 83 84 85 86 87 88 8990
Year
Year
Fig. 3. The yield ratio of wheat stem sawfly resistant to susceptible cultivars for yield in the absence of wheat stem sawfly damage and assuming a low
(2.7% yield loss) and high (15.5% yield loss) infestation. Values >1 indicate that the yield of the resistant cultivars was higher than the yield of the
susceptible cultivars. (A) Williston, N.Dak., (B) Minot, N.Dak., (C) Conrad, Mont., and (D) Havre, Mont.
provided a yield advantage in 9 of 11 yr and 18 of 20 yr at Havre.
If a minor infestation occurred at Williston, the use of resistant
cultivars would have provided a yield advantage in 1 of 19 yr;
resistant cultivars at Minot would not have provided a yield
increase in any of the years examined. During a minor infestation
at Conrad, resistant cultivars would have provided a higher return
in 6 of 11 yr and 6 of 20 yr at Havre (Fig. 3A-D). We offer two
explanations for the differences between locations: 1) yield performance of solid-stemmed cultivars is enhanced or hollow-stem
cultivars yield performance is reduced in areas of reduced rainfall;
and 2) bcttcr progrcss has been made increasing the yield potential
of hollow-stem cultivars than for solid-stemmed cultivars. In the
northern Great Plains, precipitation levels from east to west decline, with the long-term annual average for precipitation being
50
40
OJ
C
III
a.
5
u
a't
c
30
6.
6.
20
III
Ol
:!
10
6.
Y
=
1.9 + 53.9X
RA2
= 0.75
0
0.0
0.2
0.4
0.6
0.8
1.0
Mean females/sweep
Fig. 4. Relationship between adult females per sweep and stems cut by
the wheat stem sawfly, Culbertson, Mont., during 1984.
244
436.4 mm at Minot, 267.4 mm at Williston, 244 mm at Conrad,
and 210 mm at Havre.
Undoubtedly, solid-stemmed cultivars were a significant development for the management of the wheat stem sawfly. However,
the stagnant yield potential of solid-stemmed cultivars relative to
the increases achieved in the yield potential of the hollow-stem
cultivars and the increased efficiency of harvesting (reducing harvest losses) have caused a decrease in the utilization of the solidstemmed cultivars as a management tool. Use of solid-stemmed
cultivars is an advantage only during some years of severe infestation. Therefore, prediction of infcstation levels is important. Accurate prediction of sawfly populations based on current levels has
not been successful (Holmes 1982). Local populations can increase
dramatically, with moderate infestations ranging from 7 to 9% and
increasing to 70-80% during 1 yr (Holmes 1982).
Growers should select resistant cultivars in areas with lower
rainfall and histories of high consistent sawfly infestations. In areas
where infestations are inconsistent, resistant cultivars should be
selected if >5% of the stems were infested during the previous
season. Fields could be swathed or special effort could be made to
harvest as early as possible to reduce losses from lodging if high
numbers of adult sawflies are observed or if a high percentage of
stems are found to be infested when heads are filling. Adult sawflies
are readily visible and can be sampled easily with a sweep net and
correlated with stem cutting (Fig. 4).
Alternative cultural practices include blending susceptible and
resistant cultivars (Weiss et al. 1990) or adjusting the planting date.
Solid-stemmed cultivars should be planted as early as possible to
maximize use of spring moisture (Black 1983), or the planting of a
susceptible cultivar should be delayed until 15 May to 1 June to
avoid attack (Weiss et al. 1987).
In summary, the wheat stem sawfly quickly adapted from native
grasses to spring grains. Cultural practices including strip farming
AMERICAN ENTOMOLOGIIT
and alternate-year summer fallow encouraged survival in wheat.
Concern over heavy losses led to development of resistant solidstemmed 'Rescue' wheat and subsequent improved cultivars. Despite the previous research contributions, C. cinctus continues to be
a major yield constraint in wheat in the northern Great Plains.
Acknowledgment
This review was conducted as part of the North Dakota Agricultural Experiment Station Project 1537 and Montana Agricultural
Experiment Station Project 101153. Published with the approval of
the director of the North Dakota Experiment Station as publication
number 2011.
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Received for publication]
May 1992.
December
1991, accepted 27
0
Michael]. Weiss is an associate professor in the Department of
Entomology at North Dakota State University, Fargo, ND 58105.
He has studied the wheat stem sawfly in western North Dakota and
eastern Montana since 198]. Wendell 1. Morrill is an associate
professor in the Entomology Research Laboratory at Montana
State Univeristy, Bozeman, MT 59717. He has studied economic
thresholds for small grain insects since 1979 in the northern great
plains.
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