Economic Thresholds
of Alfalfa Weevils

Economic injury level (EIL) is the number of an insect pest on the field  that
represents the breakeven point between the cost of control, and the value
of the damage on alfalfa that could be prevented if a control action is taken.

Economic threshold (ET) is the number of an insect pest on the field that
represents the actual time at which control actions must be taken to prevent
the EIL from being reached and exceeded.

If control measures can be taken without delay, then EIL and ET are one
and the same.  ET recognizes the fact that a lead time is usually needed
from the time damaging insect numbers are detected on the field to the
actual application of a control tactic.

For most situations, It is recommended that ET be 80% of the EIL.

Dr. Bob Peterson, a former classmate at the University of Nebraska,
developed a procedure that could be used to calculate EIL based on
current market value of alfalfa and chemical-plus-application costs.

    (click on the thumbnail for a larger image)

Example 1:  An alfalfa grower from Lucas, SD took 30 alfalfa stems
at random from his 40-acre field then gently dislodged the alfalfa weevil
larvae inside a plastic bucket.  He determined the average count of the
weevils to be about 3 larvae per stem.  The crop sprayer is charging
$10.00 per acre to spray the field with an insecticide (chemical plus
application).  The alfalfa is on the early bud stage of development and
the predicted market value is $65 per ton.  Is it economically justifiable
to spray the field?

From the table above, the EIL at the intersection between a control cost
of $10.00 and $65 per ton market value is 4 larvae per stem.

ET = 80% of EIL = 0.8  x  4 = 3.2 larvae per stem.  Thus, treating the field
immediately will be economically justifiable.

There is no regrowth! How long can I wait? (Relax, you have time!)

Alfalfa weevils can also delay the regrowth of the second alfalfa crop.
A separate guideline was developed by Dr. Steve Danielson (University
of Nebraska, as usual) to address this concern. The procedures indicated
on the tables below should be self explanatory.  Notice that the number
of days of delayed re-growth that could be tolerated is shorter if the alfalfa
grower is under a strict 28-day cutting schedule.

            (click on the thumbnails for larger images)
      Table 1                     Table 2

Example 2:  An alfalfa grower just cut his first cutting alfalfa near the ND
border.  If the alfalfa weevils delay the regrowth of the second crop, how
long can he wait before considering treating the field?  He is under a cutting
schedule based on bloom, and the estimated market value of alfalfa is $40
per ton.  The crop sprayer in the area is charging $12 per acre to spray the
field with an insecticide (chemical plus application).

From Table 1 above, he should consider treating the field if there are no signs
of regrowth 15 days after harvest, and if (and only if) the larvae are still actively
feeding on the tender bud and leaf regrowths.

Example 3:  An alfalfa grower from Armour, SD just cut her first cutting alfalfa.
If the alfalfa weevils delay the regrowth of the second crop, how long can she
wait before considering treating the field?  She is under a strict 28-day cutting
schedule and sells high quality alfalfa for $80 per ton.  The crop sprayer in the
area is charging $11 per acre to spray the field with an insecticide (chemical
plus application).

From Table 2 above, she should consider treating the field if there are no signs
of re-growth  4 days after harvest, and if (and only if) the larvae are still actively
feeding on the tender bud and leaf regrowths.

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                                                                   Prepared and posted  by Mike Catangui on May 8, 2000.