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Alfalfa Weevil, Wheat Leaf and Stripe Rust On Rise
5/14/2007
Assefa
Gebre-Amlak
Regional Pest Management Specialist
Colorado State University
Extension
North Region |
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Wheat
This week the Colorado State University diagnostic
lab received several wheat samples from southeastern Colorado with rust. Wheat
leaf rust was found in Prowers County on variety Ripper.
Wheat stripe
rust was found in Bent, Baca and Prowers Counties. In Baca County, the variety
was Ripper and in the other two counties the variety was not indicated.
Apparently the wheat is in the boot to heading stage. Dr. Ned Tisserat CSU
Plant Pathologist says that the rust levels are still pretty low, so that is
the good news.
Alfalfa
We are seeing increase in alfalfa
weevil infestations in many fields in northeastern Colorado and Weld County.
Larvae feeding in the folded leaves can heavily damage stem terminals,
but initial damage is not always clearly visible. The closed, overlapping
foliage of the stem terminals should be unfolded to detect feeding damage.
Heavily infested stands have a grayish or frostlike appearance due to the dried
defoliated leaves.
Damage also may reduce hay quality due to loss of
leaf tissue, leaving only the lower quality stems. Damage to regrowth buds may
occur when the plants break dormancy and after first cutting.
Monitoring
alfalfa weevil: Two sampling techniques may be used.
Sweep sampling
using a standard sized 38 cm diameter net is the most efficient method for
estimating larval population. Sampling should begin when 148 degree days have
been accumulated, when the larvae are expected to be primarily second instars
and when alfalfa hay has reached at least 10 inches in height. Ten, 180 degree
sweeps are taken while the sampler is walking through the field. Count the
number of larvae per sweep and repeat this sampling procedure, taking a minimum
of three samples for fields up to 20 Acres, four samples for fields up to 30
Acres and five samples for larger fields.
Weevil infestation may be
patchy or uniform depending on terrain, weather and soil. Inspection for
weevils in every distinctive section of the field will aid in determining the
pattern of the infestation.
Bucket method or stem count method is used
to determine the number of weevil larvae per stem. Take three six-stem samples
in fields one to 19 acres, four samples in fields 20 to 29 acres, and five
samples in fields 30 acres.
Decision making: the economic
threshold for a sweep sample is 20 larvae per sweep. The economic threshold for
the stem sampling method is 1 ½ - 2 larvae per stem.
A
non-insecticide control measure for alfalfa weevils is an early first harvest
if an economic infestation is not detected until late in the growth of the
first cutting. Harvesting alfalfa in an immature stage provides good control of
larvae for the first crop. Rapid removal of hay will accelerate larval
mortality due to desiccation by direct sunlight.
An early first cutting
tends to cure more rapidly because lighter windrows dry quickly, and forage
quality is enhanced by higher crude protein and lower fiber content. Additional
steps should be taken to ensure that surviving larvae do not cause economic
damage to the regrowth. If larval survival under the windrows is high and
baling is delayed (e.g., due to rainfall), damage to regrowth may be
exacerbated. Regrowth should be inspected at a height of one to two inches to
determine larval density.
Corn
Two insects that may cause
damage to your corn seeds and seedlings are seedcorn maggots and pale western
cutworm. Pay attention to lack emergence and seedling damage. We will provide
details on their damage and management next week.
For more detailed
management information on wheat diseases and alfalfa weevil check our IPM guide
at http://highplainsipm.org. |
Page Created and Maintained by: Perry D. Brewer, Area
Extension Agent (Technology Education/Youth)
5/16/2007 |
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