Transcript Decision Making • Have discussed the “Maximum Tolerable Level” but have not defined it. • Several Points to Make: – More than 1 “Level”

Decision Making
• Have discussed the “Maximum Tolerable Level”
but have not defined it.
• Several Points to Make:
– More than 1 “Level” is usually needed.
– There are many kinds of such levels (cf. p. 200 – 201
in text for one list).
– “Action Levels” or “Thresholds” are one general
method of decision making. We will discuss the other
one (Optimization) later.
• The leader in this field has been L. Pedigo. Be
sure & read his article in the “Reading
Assignments”
Pest Population Density
The General Problem
Maximum Tolerable Level
Time (Weeks)
Pest Population Density
We actually see this:
Maximum Tolerable Level
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Time (Weeks)
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8
One problem is that we need to allow for management
response time – The time between when a control decision
is made and when it takes effect
Pest Population Density
Assume it takes 1 week to
decide a control is needed,
apply it, and for it to work
Maximum Tolerable Level
Decision must
be made here
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Time (Weeks)
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Pest Population Density
The other problem is uncertainty
Maximum Tolerable Level
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Time (Weeks)
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Solution to both problems (mgmt response time &
uncertainty) is to create two levels
Pest Population Density
The maximum pest level that
one is willing to tolerate.
Maximum
EconomicTolerable
Injury Level
Level
Economic Threshold
The pest level at which action must be
taken in order to avoid exceeding the EIL.
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Time (Weeks)
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Quick Notes on EILs & ETs
• ET is always < EIL
• Units of ET & EIL are the same
– Often pest density (absolute or relative)
– Can also be injury (e.g. % defoliation)
– Can also be implicit factors (e.g. leaf wetness)
• EIL & ET are hard numbers calculated
from equations developed through field
research.
The Basic EIL Model
The basic concept is that the EIL is the point at
which the cost of a control = the value of
damage that will be avoided by the control.
Value of damage avoided is a product of:
Crop market value (V)
Pest population density (P)
Injury caused by each pest individual (I)
Damage resulting from that injury (D)
Proportion of total damage that cannot be avoided by
the control (K)
The Basic EIL Model
V  P'I  D  K  C
C
EIL  P' 
V  I  D K
Example
• Assume:
– It costs $50/A to apply a given control (C)
– A crop is worth $40/bushel (V)
– Leaf area equal to two leaves/row foot are
eaten by each pest individual/plant (I)
– The loss of two leaves/row foot results in the
loss of one bushel/A (D)
– Even if you apply the control, you will still lose
10 % of the crop (K = 0.1, no units)
Example, Continued
C
EIL  P' 
V  I  D K
50
EIL 
 6.25
40  2 1 0.10
Understanding the Units is Key
C
EIL  P' 
V  I  D K
50
EIL 
 6.25
40 1 2  0.10
$/A
EIL 
$ lv/rowft
bu/A


bu pest/plant lv/rowft
Here’s how the units balance
$/A
EIL 
$ lv/rowft
bu/A


bu pest/plant lv/rowft
EIL  1/1/(pest/plant)
Result:
 pest/plant
EIL = 6.25 pests/plant
One of the principal advantages of EILs
is their objectivity and scientific basis
C
EIL  P' 
V  I  D K
I, D, and K are determined empirically through field
& laboratory experimentation.
C is, for the most part, easily determined.
For most agricultural crops, V is commonly
available.
The principal source of subjectivity is in
“Value”: Ex: Tree Crops & Gypsy Moth
Pest Population Density
Forest Ranger
C
EIL  P ' 
V  I  D K
Municipality
Lumber Company
Resort Owner
Note that in all of these cases: C, I, D,
& K are all the same. Only V changes.
Time (Weeks)
Some examples of EILs & their
derivation.
• EIL for Mexican Bean Beetle in Soybean –
Details the development of an EIL.
• EILs for sorghum midge on sorghum –
See Table 1 in the middle of the article.
• Common stalk borer in Nebraska corn
• Sweet potato whitefly on cantaloupe
How are ETs calculated?
•
Most common method is heuristic. Most
common rule of thumb is 1/3 EIL.
• Two examples of more formal methods
are:
(1) ET = EIL/r
(2) ET = EIL/(expected rate of change in
pest population)
General notes on ETs
• ETs are the predictive part of an EIL/ET
pair – one acts on an ET in order to
prevent the EIL from being exceeded.
• ETs are one type of “Action Threshold”.
Other types were in Pedigo & your text
(pp. 201 – 202).
• Note your text’s discussion of limitations of
thresholds.
Advantages of Thresholds
• Conceptually easy to understand makes them easy to
implement/adopt. Can also be represented in many
formats: single numbers, tables, charts.
• Scientific basis to threshold criteria
• Flexibility gives broad applicability
– Can be applied to a variety of pests in many situations
– Can utilize many variables as the action variable. Climatic
variables often used for pathogens.
– Have been extended to take into account many other issues.
Examples include
•
•
•
•
Age distribution
Multiple controls (e.g. biocontrol)
Environmental Impacts (i.e. macroeconomic “C” values)
Risk
Closely read the remainder of this
chapter
• This is the only place where the following
topics are discussed:
– Use of field history
– Field location & size
– Monitoring climate
– Use of computer/mathematical models
– Aesthetic effects
– Risk Assessment
– Economics