Columbia, Missouri
May 8, 2008
Alfalfa weevils have increased to
threatening levels in central and northern Missouri, said a
University of Missouri
Extension entomologist. Farmers should scout fields for signs of
damage so weevil populations can be managed.
"Alfalfa weevil larvae are two to three times the economic
threshold in many fields," said Wayne Bailey. "On Wednesday, 100
percent of plants were showing signs of foliar damage in central
Missouri."
The economic threshold is one larva per alfalfa stem, plus 30
percent of plants showing signs of feeding damage.
Bailey said that central and northern Missouri will likely see
damage similar to recent problems in southern Missouri. Alfalfa
farmers in all parts of the state, however, should scout fields
to determine if weevils are present.
"Problems can quickly develop and result in substantial loss of
forage yield and quality," he said. "Warm temperatures allow for
rapid larval growth and increased consumption of leaf tissues."
Most larvae in fields surveyed by MU Extension specialists were
in early stages of growth. This means damage is limited to the
upper whorl of leaflets, Bailey said. Damage increases, however,
as the weevils mature.
"These larger larvae rapidly move about the plant, and they may
consume significant amounts of leaf tissue," Bailey said.
Beyond yield losses, heavy defoliation weakens alfalfa plants
and can lead to increased weeds in fields.
Farmers have several options. First, scouting fields to
determine weevil numbers is essential, as this can influence
management strategy.
To scout fields, farmers should pick five random locations and
sample 10 alfalfa stems at each spot, for a total of 50 stems
per field.
"This is best accomplished using a five-gallon bucket and a
sharp knife," Bailey said. "Carefully cup the terminal of each
alfalfa stem with your hand. Then cut the stem off near the soil
surface and carefully place it inside the bucket. Tap it
vigorously to dislodge any larvae."
Cupping the stem is necessary to ensure larvae don't drop off
outside the bucket and cause a miscount. Weevils should be
controlled if numbers have reached the economic threshold,
Bailey said.
Applying a foliar rescue insecticide is the most common control
strategy, he said. Farmers should read insecticide labels and
use only those recommended for alfalfa weevil.
Other control options are early harvesting, grazing and
biological control. The success of these options depends on
number of larvae, stage of alfalfa growth and field conditions.
Early harvest can be used instead of an insecticide if alfalfa
is within seven to 10 days of the normal harvest stage, Bailey
said. "This season, early harvest may be a viable option as
alfalfa plants have grown rapidly with the cool, wet conditions
this spring."
Most larvae die during haying due to mechanical crushing or sun
scorching after the alfalfa canopy is gone, he said. Fields
still need to be monitored after harvesting to ensure larvae
haven't returned.
Management-intensive grazing can reduce weevil larvae by 90
percent, Bailey said. This method requires a large number of
cattle grazing on a small number of acres so that alfalfa growth
is quickly eaten.
"This can effectively eliminate the risk from alfalfa weevil as
long as most spring-laid eggs have hatched," Bailey said.
Grazing on wet fields, however, can be risky due to the
potential for hoof damage. Cattle bloat is another risk. Farmers
who opt for this strategy should wait until alfalfa plants are 6
to 8 inches tall and weevil numbers are at the economic
threshold, Bailey said. Only the upper two-thirds of the alfalfa
should be grazed so it can recover for the next cutting.
Biological control is a long-term strategy that uses a fungus or
parasites to kill weevil larvae. Farmers can promote weevil
parasites on their farms by using pesticides only when
necessary, Bailey said.
However, farmers should not rely on biological control of
alfalfa weevils this year, he said. "The wet conditions this
spring seem to be having little effect on weevil numbers, and
the cold weather isn't going to slow them down." |
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