Quick Facts...
- Several kinds of aphids infest small grains.
- The Russian wheat aphid (RWA) and the greenbug are the most destructive species in the state.
- Chemical control of other aphid species rarely is necessary.
- Use the key (Figure 5) to determine that aphids are present in the crop.
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| Figure 1. The Russian wheat aphid. |
Russian Wheat Aphid
The Russian wheat aphid (RWA) was first reported in Colorado in March 1986.
Good RWA management practices have emphasized resistant varieties, cost-effective
use of insecticides and certain cultural practices. Resistant varieties
and biological control agents have been effective enough to replace the
need for insecticides in many cases, however, recent occurence of new
biotypes have made resistant varieties much less useful.
The RWA damages small grains by injecting saliva into and
sucking sap from plants. Yield losses of 50 percent or more to
this pest can be expected if economic infestations are left
untreated. U.S. research has shown that RWA is not an important
vector of barley yellow dwarf virus or other cereal diseases.
Life Cycle
Two forms of RWA are found in Colorado during the year: a
wingless female and a winged female. It is difficult to
determine if an individual aphid will be winged or wingless until
it is near maturity.
In Colorado, most severe spring infestations of winter grains
are caused by wingless aphids that overwintered in the crop.
Winged aphids begin to appear in April and May and
flights peak during July in most wheat-producing areas
of the state. At this time, winged aphids include both local
aphids and immigrants from the south.
Winged aphids infest late-maturing winter wheat and spring
grains but not corn, millet or sorghum. They
also infest a number of cool-season grasses, particularly
wheatgrasses. Damage to newly seeded grasses can be
significant.
These grasses serve as alternate hosts for RWA during the
period between grain harvest and the appearance of new wheat
in the fall. Volunteer wheat and barley also may become infested.
Volunteer wheat and barley are important sources of
RWA for the new fall crop as soon as it emerges. Weather
conditions that favor cool season grasses and volunteers will
increase the number of aphids infesting the new wheat crop in the
fall.
Movement to the new crop occurs in October and early November.
RWA can survive the winter in most Colorado grain
growing areas, except the San Luis Valley. Winter weather
conditions that are detrimental to RWA in Colorado include
several cycles of wet snow followed by a rapid melt and a quick
freeze, prolonged exposure to temperatures below 15
degrees F, and extended periods of snow cover.
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| Figure 2. Discoloration caused by the Russian wheat aphid. |
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| Figure 3. Wheat plants damaged by the Russian wheat aphid. |
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| Figure 4. "Fish hook" distortion caused by trapped awns. Each damaged head is compared to an undamaged head.
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Signs of Infestation
RWA can be found in winter wheat, usually on the younger leaves, from emergence
in the fall to grain ripening. Aphid feeding prevents young leaves from
unrolling. RWA colonies are found within the tubes formed by these tightly
curled leaves. This not only makes it difficult to achieve good insecticide
coverage but also interferes with the ability of predaceous and parasitic
Insects to reach and attack aphids. New beneficial insect species better
able to attack RWA were identified in areas where the aphid is native.
Promising species are being imported and released by the Colorado Department
of Agriculture.
Leaves infested by RWA have long
white, purple or yellowish streaks. Under some conditions,
infested wheat tillers have a purplish color. Heavily infested
plants are stunted and some may appear prostrate or
flattened.
After flowering, some heads are twisted or distorted and have
a bleached appearance. Heads often have a "fish hook" shape
caused by awns trapped by tightly curled flag leaves. At this
time, most RWA are found feeding on the stem within the flag
leaf sheath or on developing kernels. There may be poorly formed
or blank grains. The entire head sometimes is killed.
Cultural Controls
Although insecticides provide the most effective RWA control,
several other practices can help minimize the need for
chemical applications.
- Control volunteer wheat and barley.
Although many grass species help RWA survive the
summer, volunteers are the
most important source of infestation for the new crop in the
fall. Try to have a three week volunteer-free period prior to
emergence of fall seedings.
- Adjust planting dates. Plant
winter wheat as late as possible in northeastern, southwestern
and western Colorado.
Recommended planting dates have not been determined for southeast
Colorado. Plant spring grains as early as possible.
- Produce a healthy, stress-free crop.RWAs often get their start in stressed fields or
stressed portions of fields and cause
relatively more damage to stressed plants. Test the soil and
fertilize accordingly. Plant certified, treated seed. Select a
variety that is well adapted to local growing conditions.
Resistant Varieties
These resistant varieties are still the most economical and effective
managment option for Biotype 1. Also, they are well adapted to Colorado
production conditions and should be good variety choices regardless of
their resistance to Russian wheat aphid. Biotypes other that Biotype 1
must be managed with the methods developed before resistant varieties
were available. These include biological control, cultural controls and
judicious insecticide treatments based on approproate scouting and economic
threshold information.
There are also important differences among the small grain crop species.
Oats are resistant to RWA. Although heavy infestations have been observed,
little economic damage has been detected. Resistant barleys have been
developed including Stoneham and Sidney.
Insecticides
Insecticides recommended for RWA control are in the High Plains Integrated
Pest Management Guide, which is available at www.highplainsipm.org.
Be sure to read and follow all label directions. Insecticides
may be applied at one of several times during the growing season.
Planting-Time Soil and Seed Treatments
Treatments with soil insecticides and certain seed treatments can control
RWA for a substantial period of time if adequate soil moisture is available.
Since the fall RWA flight is expected six to eight weeks after planting,
such treatments may not last long enough.
The risk of significant fall infestations is generally
considered to be low in Colorado. Consider using planting-time
treatments only when the risk of fall infestation is high, for
example: when planting near uncontrolled volunteers, when
planting early, when planting near other common alternate hosts
such as one of the wheatgrasses, when planting near
rangeland or CRP, or if the area has a history of fall RWA
infestations.
Fall Aerial or Ground Applications of Foliar insecticides
These should be considered if more than 10 to 20
percent of the plants in a field show RWA damage and weather
conditions are expected to remain favorable for an extended
period. To determine the level of infestation, walk a diagonal or
zigzag pattern across the field and stop at least 10 times to
examine 20 consecutive plants. It also is best to examine some
damaged plants for RWA, but they may be difficult to find on
cooler days or during bad weather. Scout fields every two
weeks in the fall. Visit a field more often if a RWA
infestation is detected.
Fall control generally has been more effective than spring
control. The risk of increased winter injury and some loss of
yield potential if treatment is delayed until spring needs to be
balanced against the possibility that RWA overwintering
success will be low.
Spring insecticide Treatments
These are recommended according to the guidelines below.
Plants with even a single infested or damaged tiller should be
considered infested.
| Regrowth to early boot stage |
5 to 10 percent damaged and infested
plants. |
| Early boot to flowering |
10 to 20 percent damaged and infested
plants. |
| After flowering |
More than 20 percent damaged and infested
tillers. |
A more accurate method to determine the need for treatment is
to walk a diagonal or zigzag pattern across the field, stop 10
times and collect 10 tillers at random at each
stop. (Avoid bias in selecting tillers at each stop. For example,
take the 10 tillers closest to your foot or every fifth tiller starting with
the one closest to your foot). More efficient sampling procedures
are described in Sampling Russian Wheat Aphid in the Western
Great Plains, Great Plains Agricultural Council Publication
138.
Examine the tillers and count the number that contain RWA.
This number is the percent infested tillers and can be
compared to the economic threshold calculated with the following
formula:
ET = (CC * 200) / (EY * MV)
where:
ET =Economic threshold or the percent infested tillers above
which an insecticide application will be cost effective.
C =Control cost per acre (insecticide plus application).
EY =Expected yield per acre.
MV =Market value per bushel.
After flowering, substitute 500 for 200 in the numerator of the
formula. If the calculated ET is lower than the percent
infested tillers observed, a treatment should be cost effective.
There probably is no benefit from insecticide applications
made after the crop has reached the soft dough stage.
Scout fields at least weekly in the spring. Spring foliar treatments
have been the most common type of treatment in Colorado over the past
10 years. Chorpyrifos is available in several restriced use products and
has been the most consistent products commercially. Both cool temperatures
and drought stress can interfere with the plant's ability to absorb systemic
insecticides. If the crop is stressed, consider using a contact insecticide
alone or a contact/systemic tank mix. Avoid herbicide/insecticide
tank mixes if the crop is stressed.
Make ground applications in at least 10 gallons of spray
volume per acre. Aerial applications can be made in one gallon of
spray volume per acre prior to jointing and after flowering. Otherwise, use 2 gallons per acre.
If greenbugs are present in the field see the recommendations
below.
insecticide applications generally are cost effective in
Colorado, but do not completely prevent yield losses.
Control of Greenbug and Other Aphids
Like the RWA, greenbugs damage small grains by injecting toxic
saliva into and sucking sap from the plant. It also is an
important vector of barley yellow dwarf virus.
For economical control of the greenbug combine biological control, cultural
practices and, if necessary, insecticide treatments. Lady beetles and
parasitic wasps often keep the greenbug below economically important levels.
Certain barley varieties are resistant to greenbugs and can help reduce
the need for insecticide treatments. insecticides for control of small
grain aphids are found in the High Plains Integrated Pest Management
Guide (www.highplainsipm.org).
Some greenbugs are resistant to some insecticides. Resistant
greenbugs are not widespread. If a control failure is
experienced, it is unlikely that control will be achieved by
retreatment with the same product or a different insecticide from
the same chemical family. Most insecticides currently registered
on small grains are from one of two closely related
families. The best way to avoid such problems is to treat
greenbug infestations only when it is absolutely necessary, based
on greenbug or RWA action thresholds. Ask your local Colorado State University Extension county office or other reliable
source if these or other products have failed in the area.
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| Figure 5. Key to common wingless aphids on Colorado small grains. |
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