CONSERVATION TILLAGE HANDBOOK SERIES
Chap. 8, Crops and Varieties -No. 15, September 1993
Fly Management in Conservation Tillage Systems for the Inland Pacific
Pike, Roger Veseth, Baird Miller, Roland Schirman, Larry Smith and Hugh
is as Variable as the Weather!
Hessian fly's life cycle is highly weather dependent. Rainfall patterns
largely determine the level of fly infestation each year in dryland
cropping regions in the Inland Northwest that receive less than 25
inches annual precipitation. Consequently, it does not reach an economic
threshold every year in this production region. Extended periods of
wet weather in the spring, late summer or fall can result in an increased
number of flies emerging and/or generations produced. Another point
to remember is that volunteer wheat and some native plants and weeds
are alternate hosts of the Hessian fly. Managing for complete elimination
of Hessian fly is impossible in favorable years since it can move
in from surrounding fields and non-crop areas. Some significant infestations
of Hessian fly have occurred in southeast Washington and north central
Idaho over the past five years. The most recent infestation was in
1993, which was attributed to the wet spring weather and delayed planting.
Practices Compatible with Farm Conservation Plans?
- Hessian fly is
an exception to the general rule that insect pests of cereals in the
Northwest are not influenced by choice of tillage and residue management.
Conservation tillage systems leave more infested stubble and volunteer
plants on or near the soil surface than under more intensive tillage
practices. Because few fly larvae or puparia are buried to an adequate
depth to prevent emergence, there can be a greater survival of the pest
under conservation tillage. Fortunately, the potential for Hessian
fly damage in the subsequent wheat crop depends more on management options
for the following crops than it depends on tillage and residue management
practices after an infested crop. Using an integrated approach,
Hessian fly can be effectively managed under conservation tillage systems.
Purpose and Other References
- This PNW publication
will present management considerations for minimizing crop losses from
Hessian fly under conservation tillage systems. Other publications provide
more details on the history, identification, and life cycles of Hessian
fly. One in-depth, color publication which serves as an excellent reference
is "Hessian Fly in Washington," Washington State University
Bulletin XB0909. It is available through county offices of WSU Cooperative
Extension or it can be ordered through the WSU Cooperative Extension
Bulletins Office in Pullman, (509) 335-2857.
Cycle and Crop Injury
- Under normal weather
conditions, there are three or more major fly emergences each year in
the Inland Northwest region, two in the spring and one in the fall.
If late summer rains occur, there may be two emergences in the fall,
instead of the usual one in this region. Two fall emergences occur more
frequently in some areas, such as in Asotin County, WA, and Nez Perce
County, ID, where climatic conditions favor the pest. The first spring
emergence begins as temperatures reach a mean of 45-50F, commonly after
April 1 (depending on the area and yearly variations). The second generation
typically emerges in late May and June. Fall emergence normally takes
place between mid-August and mid-October. Larval activity generally
ceases about mid-October with the onset of cold weather.
- Adults are fragile,
mosquito-like flies that are weak fliers and live only 1-2 days. They
prefer to lay eggs on the leaves of newly emerged and very young wheat
plants in preference to older wheat plants and alternate hosts. Larvae
hatch from the eggs within 3-10 days and migrate to the base of the
leaf at the crown of the seedling or to the node in jointed wheat. There,
the larvae feed for about 2 weeks and develop to the pupal stage, often
referred to as the "flaxseed" because of its similarity in
size, shape and color to the seed of flax. Adults emerge a short time
later, depending on the weather and time of year. With favorable weather,
the adults can emerge in as few as 15 days after the eggs are laid.
- No single generation
of Hessian fly ever completes its development uniformly. The emergence
of at least some of each generation can be delayed 6 months to a year
or more. This delayed emergence is a survival mechanism of the insect
to maintain itself through unfavorable environmental conditions.
- Hosts -
Wheat is the preferred host of the Hessian fly. Winter wheat rarely
sustains economic losses in the Inland Northwest. Since there are usually
two spring generations of the fly, spring wheat (particularly that seeded
late) has a greater exposure to infestation and typically sustains greater
damage than winter wheat. Spring and winter barley, triticale and rye
are generally considered resistant. Oats are not infested. Wild grasses
such as quackgrass, western wheatgrass, ryegrasses, jointed goatgrass
and timothy have been observed as alternate hosts for Hessian fly. Damage
is relatively minor on these alternate host plants, but they help sustain
the pest when wheat is not immediately available. An exception is in
the Asotin Co., WA/Nez Perce Co. ID area where some significant yield
losses have been reported in winter wheat and spring barley in years
favorable to the fly.
- Crop Injury
- Hessian fly causes plant injury when the larvae feed on the juices
in the stem tissue at the crown of young plants or just above the nodes
on jointed wheat. Infested seedlings and tillers become stunted and
the leaves become broader and darker green. Injury is more severe on
newly emerged and young seedlings compared to older plants. Infestations
during the seedling stages may lead to reduced stands that are open
to greater weed problems.
- Infested tillers,
particularly in the younger plants, usually wither and die. If the tillers
survive, their growth and yield will be reduced. Economic grain losses
can be expected when 15 to 20% of the tillers become infested. Infestation
as high as 70-80% have been reported in spring wheat under irrigation
and some dryland cropping areas in the Inland Northwest.
- Larval feeding
on jointed wheat plants weakens the stem at the point of feeding and
can result in lodging or stem breakage. Feeding can also interfere with
the grain filling process, resulting in losses of grain yield and quality.
Options Under Conservation Tillage
- For Hessian fly,
as with most crop pests, there is no one management choice that will
provide complete control. The most effective control will be through
the use of an integrated management approach which takes into account
all applicable management options. Growers need to balance control strategies
with other yield limitations and management considerations. Management
impacts on water conservation and erosion protection are just two examples.
- Water Conservation
- Management effects on water storage and resultant yield potential
need to be considered. In this dryland production region with predominantly
winter season precipitation, the amount of soil water stored overwinter
is a primary yield-limiting factor. Consequently, the influence of the
tillage and residue management practices on water storage must be considered
when developing strategies for control of Hessian fly and other pests.
Reduced water storage potential with intensive tillage and residue removal
could limit yield more than fly damage.
- Erosion Protection
- Most farm conservation plans spell out the amounts of surface
residue required to control wind and water erosion. Growers need to
contact their local conservation district to make sure that the Hessian
fly management practices they select will allow them to meet the goals
of their farm conservation plans.
- The following
management options can help to minimize crop losses from Hessian fly
in conservation tillage systems.
- Crop Choice
Winter Wheat Instead of Spring Wheat - In the Northwest,
winter wheat generally does not sustain economic yield losses
from Hessian fly, but can serve as a host to sustain or build
fly populations to attack spring wheat the following spring.
During the vulnerable seedling stage, winter wheat is usually
exposed to only one flight of flies. Although crop losses in
winter wheat are generally low, damage may occur when it is
planted early after an infested spring wheat crop and late summer
rains have facilitated early fly emergence and growth of volunteer
- Local exception
-- Asotin County, WA and Nez Perce County, ID have climatic conditions
which favor Hessian fly infestations of winter wheat. Therefore, growers
in this area should consider other management options discussed later
to minimize fly damage in winter wheat, as well as alternative crops.
Less Susceptible Winter Cereals - Plant winter barley or triticale
where the choice is practical. Winter barley and triticale are seldom
infested and do not sustain noticeable yield losses.
Crops in the Rotations - Crop rotation is a key management tool
for Hessian fly throughout the rotation. Winter rapeseed or Canola,
winter pea and other non-host crops could be included in the fall
where possible. Including non-cereal crops in the rotation can also
effectively reduce the incidence of a number of other insect and disease
pests in spring and winter wheat.
"Green Bridge" of Volunteer and Host Weeds - Control
volunteer wheat and grass weeds in fallow, after harvest and through
planting of the next crop. Volunteer wheat and other host grasses
which are allowed to grow for 2 to 3 weeks during fly emergence periods
can become infested with Hessian fly and increase populations. Non-selective
herbicides often replace some or all tillage operations for controlling
volunteer plants and weeds under conservation tillage. It is important
to spray early, before the Hessian fly emerges and "flaxseed"
develop on these plants. Early control of volunteer, beginning in
August if necessary, will reduce the number of host plants for Hessian
fly production. Early control of volunteer grain and other host weeds
can also reduce potential build-up of aphids and other insect and
disease pests which can attack fall or spring cereal crops.
Seeding - Delayed fall seeding dates to avoid Hessian fly emergence
(fly-free dates) is one of the most common management tools worldwide.
If Hessian fly is a concern, delay seeding of winter wheat until after
Hessian fly flights are reduced by cool fall temperatures, usually
early to mid October. However, remember that delaying the winter wheat
seeding date past the optimum seeding time also reduces yield potential
and erosion protection benefits. Potential yield loss from a delayed
seeding date must be weighed against the potential yield loss from
Hessian fly and other pests if a "normal" or earlier seeding
date is used.
a Healthy Productive Crop - Use optimal fertilizer rates, cultural
practices and crop protection in order to insure a healthy vigorous
crop which can tolerate a higher degree of infestation without noticeable
at Planting - The first choices in managing Hessian fly should
include the integration of crop rotation, seeding date, early control
of volunteer wheat and weeds and other the management options listed
above. Where important control options are not feasible, use of labeled
insecticides at planting could be considered where Hessian fly potential
is high. Granular and liquid insecticides are currently registered
for in-furrow application at fall planting for control of Hessian
fly. This insecticide application can also help to reduce the impacts
of other crop pests, such as Russian wheat aphid and aphids carrying
the Barley Yellow Dwarf virus. Contact your county extension office
for more information. New seed-applied systemic insecticide treatments
have been successful experimentally and should be available within
- When is Hessian
fly potential high for spring wheat? The highest level of infestation
in spring wheat usually occurs when a susceptible variety is planted
after an infested spring or winter wheat crop, and wet, mild weather
has delayed planting by 3 to 6 weeks. The delayed seeding date increases
the potential for fly emergence when spring wheat is in the most susceptible
seedling stage. Late summer rains can also permit increased fly production
for attacking spring wheat the following spring.
- A number of important
spring management options are listed below.
- Crop Choice
Resistant Varieties - If spring wheat must be planted in
spite of the possibility of Hessian fly damage, plant an adapted,
Hessian fly-resistant variety. Wakanz is one soft white spring
wheat variety that is resistant to Hessian fly. Other are being
developed. Some hard red spring wheats have good resistance
to the fly. Tables 1 and 2 show the results of some recent field
evaluations of varietal resistance and tolerance to the fly.
When considering resistance or tolerance in the choice of variety,
a large number of other factors, such as yield potential, disease
resistance, straw strength, date of maturity, and volunteer
carryover must be considered in variety selection. Each factor
must be weighed against others in making the final decision.
Contact your county extension agent for additional information
on performance of varieties in your area.
- If a susceptible
variety must be grown when Hessian fly is a concern, the spring wheat
should ideally be planted after non-host crops, or after cereals which
were not significantly infested with Hessian fly. Also consider other
management tools described below.
Susceptible Spring Cereals - Plant spring barley or triticale
instead of spring wheat where feasible. These crops generally do not
sustain significant yield losses from Hessian fly and have less than
1 percent infestation when infestations of susceptible spring wheats
are up to 45 percent (Table 1). However, significantly higher levels
of spring barley infestations have been reported around Nez Perce
Co., ID and Asotin Co., WA because climatic conditions are
- Table 1.
Summary of Hessian fly resistance in spring cereal varieties based
on a 1993 replicated, single field trial at Dayton, WA (K. Pike,
B. Miller and R. Schirman,WSU).
- * 14 spring
barley varieties included in the Dayton study did not have more
than 0.3 percent infested tillers
- more favorable
for Hessian fly infestations. In these areas favorable to fly infestations,
growers may find these options of less benefit.
Non-host Crops in the Rotation - A change in crop rotation,
particularly with the inclusion of non- host crops, should be a
management consideration where possible since Hessian fly is primarily
a potential problem for spring wheat under irrigation and in dryland
cropping areas with about 18 to 25 inches or more annual precipitation.
Rapeseed/Canola, peas, lentils, oats, triticale and other non-host
crops could be grown as an alternative to or preceding spring wheat.
Rotations with many of these crops also effectively reduce the incidence
of a number of soilborne diseases of wheat.
the "Green Bridge" of Volunteer and Host Weeds - As
in the fall, continue early control of volunteer wheat and grass
weeds in fields to be planted in the spring, as well as those to
be fallowed. Early control of volunteer and weeds with a non-selective
herbicide before the fly emerges and/or flaxseed develop will reduce
fly populations. Early control of volunteer cereals can also reduce
the potential for Russian wheat aphids, aphids carrying Barley Yellow
Dwarf, other insect pests and soilborne root diseases.
2. Summary of Hessian fly resistance in spring wheat varieties
based on a 1991 replicated, single field trial south of Lewiston,
ID (L. Smith, UI)
11 experimental lines had infestations of 1% or less
Westbred 906R also had no infested tillers in a 1990 study
in the Spring - If a susceptible spring wheat variety must be
planted in spite of potential Hessian fly damage, plant as early as
possible to reduce infestation potential during the early seedling
stage, when the crop would suffer the greatest yield loss. However,
adjustment of the seeding date to avoid fly infestation is difficult
because the times of spring fly emergence change annually with weather
conditions. Optimal seeding dates to reduce economic losses are in
March or earlier in most areas.
a Healthy Productive Crop - Use optimal fertilizer rates, cultural
practices and crop protection in order to insure a healthy, vigorous
spring crop which can tolerate a higher degree of infestation without
noticeable yield losses.
at Planting - The priority choices in managing Hessian fly in
the spring should include the integration of crop rotation, fly resistant
varieties, volunteer control, seeding date and other the management
options listed above. When important control options are not feasible,
use of labeled insecticides at planting could be considered where
Hessian fly potential is high. Granular and liquid insecticides currently
are registered for in-furrow application at spring planting. The insecticides
could also help control Russian wheat aphids, aphids carrying Barley
Yellow Dwarf, and other crop pests. Contact your county extension
office for more information. New seed-applied systemic insecticides
should be available within 1-2 years.
- After a wheat
crop damaged by Hessian fly, a common question is whether the infested
straw should be handled differently than normal to minimize carryover
of the fly problem to future wheat crops. There are practices which
can reduce the survival of the fly in infested residue and volunteer.
However, Hessian fly can still be a problem for the subsequent winter
or spring wheat crop because fly populations and generations are so
highly dependent on rainfall patterns, and the pest is mobile, moving
in from surrounding fields and alternate host plants. Consequently,
crop rotation, varietal selection, early control of volunteer and host
weeds, adjusting seeding dates and other practices mentioned in the
above management options for establishing the next wheat crop are much
more important to minimize Hessian fly damage than are tillage and residue
management practices after an infected crop. Following is a brief summary
of the effects of different tillage and residue management approaches
after harvest of infested crops, and where they might apply.
- Intensive Tillage
on Land Which is Not Highly Erodible - Moldboard plowing and rolling/packing
of fields with infested crop residue after harvest will prevent fly
emergence if puparia are buried to a depth of 3-4 inches in firm soil.
However, this level of intensive tillage in the fall is not possible
on highly erodible land, which includes much of the Inland Northwest
cropland. Furthermore, subsequent fall or spring tillage, which returns
some of the infested residue to or near the surface, permits emergence
of Hessian fly from remaining viable puparia and can partially negate
the control from the primary tillage.
- Stubble Burning
not Recommended - Although stubble burning can kill the flaxseed
stage in above-ground portions of the plant (especially stems infested
after jointing began), burning has little effect on survival of those
in the crown at or below the soil line, where a high percentage of the
flaxseed usually occur. Stubble burning is not recommended because it
provides inadequate Hessian fly control and can leave the land more
vulnerable to erosion.
- Practices on
Highly Erodible Land - On highly erodible land, growers need to
utilize conservation tillage practices which at least result in the
amount of crop residue required in their conservation plans. Use of
as many control strategies listed in the preceding Hessian fly management
options as feasible will provide effective control under conservation
- After harvest
of a wheat crop infested with Hessian fly, growers should not significantly
change tillage and residue management practices. Severe tillage to deeply
bury infested residue should only be considered on cropland which is
not highly erodible. Stubble burning is not recommended because of minimal
reduction in fly survival and increased potential for soil erosion.
Hessian fly can be managed within the tillage and residue management
practices included in farm conservation plans. To control Hessian fly
in conservation systems, growers should consider an integrated approach
utilizing all feasible control options including: resistant spring wheat
varieties, crop rotations with less susceptible or non-host crops, early
control of volunteer wheat and host weeds, adjusted seeding dates and
management for a healthy crop. In-furrow insecticides could be considered
for reducing infestation potential during seedling establishment when
other control options are not feasible.
- Pacific Northwest
Conservation Tillage Handbook Series publications are produced jointly
by University of Idaho Cooperative Extension System, Oregon State University
Extension Service and Washington State University Cooperative Extension.
Pike, WSU Entomologist, IAREC, Prosser, WA;
WSU/UI Extension Conservation Tillage Specialist, Moscow,
WSU Extension Agronomist, Pullman, WA;
Schirman, WSU Extension Agent, Dayton, WA;
UI Extension Agricultural Agent, Lewiston, ID;
UI Extension Entomologist, Moscow, ID