|
Background and Objective:
The hessian fly is a 3-5 mm
long, black midge that farmers and researchers rarely see in their
fields because of their small size and short life. The pest was
introduced from Europe into the United States, possibly by
mercenaries from the German state of Hesse at the time of the War of
Independence. Since then this small insect has been one of the most
important insect pests of winter wheat in the United States. The
larva of the hessian fly feeds on the stem of young winter wheat
plants, between the leaf sheath and the stem, and so is rarely
visible. When the larva pupates for the winter, it remains hidden
under the leaf sheath. If an infested tiller is examined closely,
by pulling back the leaf sheath, the pupa is visible and looks very
much like a flax seed. Feeding by this larvae can cause death in
young winter wheat plants in the fall, and contributes to what is
generally seen as winter kill the following spring. Much research
has been conducted in the United States, primarily in Indiana and
Kansas, on identifying resistance genes and incorporating resistance
in U.S. winter wheats. This program has been very successful, and
continues today as new virulent strains of the hessian fly develop.
Hessian fly has also infested
spring wheat across Canada for decades but has rarely been considered an
important pest, partly because spring wheat shows different symptoms
than winter wheat. The most noticeable symptom is a break in the
infested stem just prior to harvest that is often confused with typical
lodging. The economic impact of the hessian fly has not been well
quantified. Manitoba extension entomologists report fields with 10-20%
stem breakage some years. Our own survey of hessian fly indicates an
infestation level of 14% of all tillers across Manitoba, and the
resulting yield loss probably is in the order of 5%. Damage is most
severe in late seeded fields, which have been common in the wet springs
of the past few years. No control methods other than crop resistance
are available.
Another midge, the wheat midge
which attacks developing seeds, has been a more important pest than
hessian fly in recent years. A research program to develop wheat with
resistance to wheat midge has been underway at the Cereal Research
Centre of Agriculture and Agri-Food Canada for about a decade. This
research has been funded in part by ARDI. We have discovered a very
effective resistance gene that essentially eliminates damage by the
wheat midge. The gene has been characterized and is called Sm1.
It is a naturally occurring wheat gene that can be transferred by
classical wheat breeding techniques into spring wheat adapted to
Manitoba conditions. In February 2007 resistant wheat cultivars were
approved for registration and will be available to producers as soon as
seed stocks are increased. We found the wheat midge resistance gene
Sm1 in U.S. winter wheats that are also resistant to hessian fly.
Different genes confer resistance against wheat midge and hessian fly,
but the fact that they occur together and in winter wheats that we are
already crossing with Canadian spring wheats provided the opportunity to
combine resistance to both insects in a spring wheat adapted to Manitoba
conditions.
The proposed research provides a
rare opportunity to incorporate resistance to a secondary but
significant pest, hessian fly, simultaneously with crop resistance for
the key pest of wheat, wheat midge. This research will increase the
competitiveness of all wheat production in Manitoba. Not having to
worry about wheat midge and protecting the wheat from the currently
uncontrollable hessian fly damage will increase the profitability and
sustainability of wheat production in Manitoba. The economic benefits
to producers would begin to accrue when wheats are developed with dual
resistance. Benefits of resistance would consist of an average yield
increase of 3-5% ($15-25 million) per year, for hessian fly damage. An
added benefit would be the elimination of downgrading as a result of
weathering of seed from lodged heads caused by hessian fly. A small
investment in developing control for hessian fly would pay for itself
many times over. Furthermore, the integrated nature of this research
assures that funds will be used efficiently because the resistance
research and screening for the one pest can be conducted simultaneously
with that of the key pest, wheat midge.
Procedure and Project Activities:
The following research was
conducted to exploit resistance to hessian fly simultaneously with
our program for resistance to wheat midge:
-
Establish a laboratory colony
of Manitoba hessian fly so that we can screen our wheat midge
resistant lines and identify the ones also resistant to hessian fly.
-
Develop field screening
methods for hessian fly resistance so that we can efficiently confirm
that our hessian fly resistance holds up under field conditions.
-
Integrate hessian fly
screening into our breeding program on resistance for wheat midge.
First we tried to establish a
laboratory culture of hessian fly to begin identifying wheat midge
resistant lines which are also hessian fly resistant. Unexpected
difficulties were encountered because nearly all the hessian flies we
collected from broken wheat stems were parasitized. We could not
initially understand how the wheat could be so heavily infested by
hessian fly if more than 90% of all fly larvae were killed by parasites.
To continue improving our screening methods and identifying resistant
lines, we arranged to have our wheats tested for resistance at the
London Laboratory of Agriculture and Agri-Food Canada, in Ontario, which
worked very well. In the meantime we continued with field trials at
Glenlea, Manitoba, and tried to figure out how the hessian fly survived
with such high parasitism.
This initial field work led to
the discovery that in spring wheat there is a second generation of the
hessian fly that causes the stem breakage in spring, and this generation
is heavily parasitized by a number of species of parasitic wasps.
Unfortunately, although the parasitism is high, hessian fly larvae are
not killed in time to prevent stem breakage. We also discovered an
earlier generation of hessian fly that produces the stem breaking
generation. The early generation attacks much younger plants and kills
individual tillers that are infested. This generation went unnoticed
because the rapidly growing young plant replaces the killed tillers, but
at a cost to wheat production that we have not yet had time to fully
estimate. This early generation is not heavily parasitized and sustains
the hessian fly on spring wheat. We were then able to establish a
laboratory culture of hessian fly from this early generation, and
subsequently develop the laboratory methods to screen spring wheat for
resistance to hessian fly using a Manitoba strain of the pest. Now
screening of lines in the breeding program is routinely conducted
throughout the year with this Manitoba strain of hessian fly. Using
these methods, large numbers of breeding lines and spring wheat
cultivars currently grown in Manitoba were tested for resistance to
hessian fly. Promising lines and cultivars were then re-tested in the
field at Glenlea to confirm that the resistance was effective in the
field.
Throughout the process of
screening and field testing we worked closely with the spring wheat
breeders to assure that the most promising breeding lines, especially
those with wheat midge resistance, were included in the testing program.
Results and Discussion:
Screening Methodologies
that Enable Detection and Selection of Hessian Fly Resistant
Spring Wheats
Methodologies were successfully
developed for identifying resistant plants in the laboratory and in the
field. In the laboratory, a culture of the Manitoba strain of hessian
fly was established to assure that the test insects express the
virulence characteristics of the Manitoba population, and that tests
reveal the suitable reactions for wheats grown in Manitoba. This aspect
of the work proved more difficult than originally expected because of
high parasitism (>90%) of the hessian fly in the targeted growth stage
of wheat. As part of this aspect of the study, however, we identified a
previously unknown early generation of hessian fly that is much less
parasitized, and provided large numbers of healthy insects. Laboratory
rearing methods were adapted from the literature and contacts with other
hessian fly researchers across North America. The methodology now
produces large numbers of diapausing hessian fly which can be brought
out of cold storage and used when needed in tests. A testing arena and
suitable exposure methodology was developed for spring wheat, which
allows routine screening for hessian fly resistance using the same
bioassay facility, growth cabinets, and greenhouses as we routinely use
for wheat midge screening. Hundreds of wheat lines can now be screened
at the seedling stage for hessian fly resistance at any time of year.
In the field at Glenlea a
nursery was established to provide reliable field densities of hessian
fly for large scale confirmation of resistance reactions. Agronomic
practices, particularly seeding dates, were defined that allow
successful tests. The nursery now permits reliable screening of advanced
material and cultivars for resistance to hessian fly under normal
Manitoba agricultural conditions.
Assessment of the Types and
Extent of Hessian Fly Damage in Spring Wheat
Preliminary research and reports
by plant breeders and Manitoba extension entomologists identified
pre-harvest stem breakage as the major type of damage caused by hessian
fly in spring wheat. The stems break at lower nodes, but remain
attached, so that heads rest on the soil. The heads from these stems
may not be picked up at harvest, and if picked up are probably weathered
and reduce crop quality. We documented that this type of damage is
indeed important, with some cultivars more or less susceptible to stem
breakage, not because they are more or less resistant to hessian fly,
but because some lines are less susceptible to stem breakage when
attacked by hessian flies than other lines. We also determined that
this damage greatly reduces seed size and yield of the affected tillers.
Of particular interest, however,
was the discovery of an early generation of hessian fly, and the insight
that stem breakage is caused by the later generation of hessian fly.
The early generation kills young individual infested tillers of spring
wheat, a previously unnoticed and unreported type of damage. This early
damage is more similar to that traditionally seen in winter wheat in the
autumn which results eventually in apparent winter kill; obviously the
consequences of this damage are different in spring wheat that continues
growing and producing tillers rather than going dormant. This early
damage is widespread at Glenlea and certainly contributes to reductions
in wheat yield, but a more widespread survey of the extent of seedling
damage to spring wheat in Manitoba was beyond the scope of this project.
Documentation of the extent of the problem could be important in
helping wheat producers assess the need to plant spring wheats resistant
to hessian fly.
Determination of Relative
Susceptibility of Manitoba Spring Wheat Cultivars to Hessian Fly Damage
All but one currently grown
Manitoba spring wheat cultivars are susceptible to infestation by
hessian fly. All of these wheats are also prone to stem breakage just
before harvest when tillers are infested with hessian fly.
Nevertheless, some Manitoba cultivars tend to break less than others,
and these more tolerant wheats could be recommended for production where
farmers have experienced hessian fly losses.
One recently registered spring
wheat, Superb, suitable for production in Manitoba proved to be the only
spring wheat cultivar with a substantial level of resistance to hessian
fly. Superb is infested by relatively few hessian flies, because fewer
larvae are able to develop on this wheat and because its straw resists
breaking even if a larva is able to survive on Superb. Superb has the
U.S. wheat Grandin as a parent, and this wheat shows a similar
resistance to hessian fly. We suspect that the resistance was
accidentally incorporated as a result of the large hessian fly
resistance program in the United States on winter wheat. The cultivar
Superb now is available, or soon will be available, to Manitoba
producers concerned about hessian fly damage in their wheat. Superb is
not, however, resistant to wheat midge, a more serious problem than
hessian fly in most years.
Advanced Breeding Lines that
Express Both Wheat Midge and Hessian Fly Resistance
The development of the methods
described above led to an intensive effort to screen breeding lines for
hessian fly resistance as well as wheat midge resistance, which is an
ongoing program. As a result, many lines with resistance to both
insects have been advanced in the breeding program. The combined
resistance was anticipated because the wheat midge resistance gene
Sm1 was originally identified in winter wheats known to have
resistance genes for hessian fly. Although these genes segregate
independently, an effective screening program can identify those lines
that retain both types of resistance. In addition we have initiated a
continuous backcrossing program to incorporate two different resistance
genes for hessian fly along with the wheat midge resistance gene in a
germplasm suitable for production in Manitoba. This backcrossing
program is reaching the point where the germplasm can be turned over to
wheat breeders, and contribute the appropriate resistance genes to their
advanced lines. As a result of both aspects of this work, many spring
wheat lines with resistance to both insects, and adapted to Manitoba
conditions, are available to the spring wheat breeding program.
The hessian fly is a
significant pest of spring wheat in Manitoba, with an early generation
causing tiller death in seedlings and the later generation causing
stem breakage just prior to harvest. This damage can be prevented by
growing a resistant spring wheat. The only resistant cultivar of
spring wheat available to Manitoba farmers is Superb - as identified
in this project - but Superb is not resistant to wheat midge. A
number of genes for hessian fly resistance are in the parentage of
breeding lines used to develop wheat midge resistance. As a result of
this project, lines in the breeding program are now routinely screened
in the laboratory and in the field for resistance to hessian fly as
well as wheat midge. Spring wheats with resistance to both pests are
being advanced through the breeding program with the aim of
registering them as cultivars for production in Manitoba.
|
HomeContact
UsAbout
ManitobaDepartmentsLinksPrivacy 
