Be
on the Look-out for Alfalfa Weevil Feeding in 2000
Mike Rankin
Crops and Soils Agent - Fond du Lac County
University of Wisconsin - Extension
John Wedberg, UW Extension
Entomologist, anticipates a potential problem with alfalfa weevil this
year. Alfalfa weevil, an
insect that we essentially had no problems with during most of the
1990’s, has caused light to severe damage in some fields during the past
two years. Weather conditions
over this past winter and so far this spring favor another alfalfa weevil
problem in 2000. We had
sufficiently warm weather in March for egg-laying to occur and we could
see feeding activity earlier than the normal range of 300 to 400 growing
degree units (Base 48).
It ‘s generally suggested to take action when 40% of the plants
exhibit tip-feeding injury. Once
you start to notice feeding activity (via spot-checking), the recommended
scouting technique is to randomly pick 30 stems while walking through the
field in a M-shaped pattern. In
those cases where the field is in the bud-stage when the treatment
threshold is reached, many farmers prefer to cut the alfalfa rather than
spend the money on chemical control.
Of course, this strategy requires careful monitoring of regrowth to
make sure the weevil problem is over.
The following are some alfalfa weevil lessons learned from past
years:
-
Fields
seeded in 1999 could have earlier and more extensive damage than older
stands. In past years,
first-year fields have been used as indicators of things-to-come.
Check these fields first.
-
It is
possible to harvest fields before egg-laying is completed and end-up
with economic injury showing-up in the second crop when it is 6 to 10
inches tall.
-
Southern
slopes and light-colored knolls within fields will often be the areas
where activity is first noted. This
micro-climate effect has previously been noted by growers who farm
hilly terrain.
-
When
scouting contour strips, sample each strip as an individual field,
even if strips are only separated by 30 to 40 rows of corn.
There can be tremendous variation in weevil populations from
the top to the bottom of a hill.
- Dry
weather is not conducive to outbreaks of the fungal pathogen that can
decimate alfalfa weevil larvae populations.
Thus, it appears we won’t really be able to count on natural
control for 2000.
 For
more information contact Mike Rankin
 
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