Dates for initiating alfalfa weevil scouting are provided, based on 30 years of climatic data used to estimate appropriate degree day accumulations for northern and southern Nebraska. Sampling activities for alfalfa weevil need to be timed properly because it is inefficient to sample when the pest is not active or present. Conversely, delayed sampling is financially risky because economic damage can occur before a management practice is implemented. Integrated pest management programs often use degree day accumulations to initiate activities, while producers often use calendar dates. Calendar scheduling is traditionally based on subjective experience rather than research

Meyer, Steven J.

Peterson, Robert K.D.

English

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Historical Materials from University of Nebraska-Lincoln Extension Extension 1995 G95-1263 When to Sample for Alfalfa Weevil Steven J. Meyer University of Nebraska - Lincoln Robert K.D. Peterson Dow-Elanco, bpeterson@montana.edu Follow this and additional works at: https://digitalcommons.unl.edu/extensionhist  Part of the Agriculture Commons, and the Curriculum and Instruction Commons Meyer, Steven J. and Peterson, Robert K.D., "G95-1263 When to Sample for Alfalfa Weevil" (1995). Historical Materials from University of Nebraska-Lincoln Extension. 1111. https://digitalcommons.unl.edu/extensionhist/1111 This Article is brought to you for free and open access by the Extension at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Historical Materials from University of Nebraska-Lincoln Extension by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. G95-1263-A  When to Sample for Alfalfa Weevil Dates for initiating alfalfa weevil scouting are provided, based on 30 years of climatic data used to estimate appropriate degree day accumulations for northern and southern Nebraska. Steven J. Meyer, Extension Specialist, Agricultural Climatology Robert K. D. Peterson, Research Biologist, DowElanco  Sampling activities for alfalfa weevil need to be timed properly because it is inefficient to sample when the pest is not active or present. Conversely, delayed sampling is financially risky because economic damage can occur before a management practice is implemented.  Integrated pest management programs often use degree day accumulations to initiate activities, while producers often use calendar dates. Calendar scheduling is traditionally based on subjective experience rather than research. Consequently, calendar dates are not as precise as degree day accumulations. This is not to imply that calendar dates have no value; indeed, they can be a valuable technique for determining when to initiate sampling activities particularly when real-time degree day information is not available. This approach can be improved, however, by using climatic "normal" temperature data (1961-1990) to determine the probable date when a specified number of degree days will have accumulated.  Typically, two degree day (DD) accumulations are used to initiate alfalfa weevil sampling in Nebraska -- 250 and 300 DD. Differences in autumn egg-laying patterns in Nebraska lead to differences in the timing of egg hatch and larval populations the next spring. In Nebraska, autumn egg-laying varies depending on climatic conditions. Research at Iowa State University suggests that alfalfa weevil sampling should begin at 250 DD in the southern half of Iowa and at 300 DD in the northern half. It's likely that these DD accumulation requirements are similar for Nebraska. Thus, the accumulation of 250 DD should be used south of a line from Omaha to Ogallala and 300 DD north of that line.  Figures 1-3 identify dates for the accumulation of 250 DD at the 10%, 25%, and 50% probability levels, based on the climatic "normal" temperature data of 1961-1990. Dates for the accumulation of 300 DD at the 10%, 25%, and 50% probability levels are given in Figures 4-6. Growers in southern Nebraska should use the information in Figures 1-3, while growers in northern Nebraska should use information in Figures 4-6. Those close to the dividing line may want to use a combination when making their decision.  The dates in the six figures provide alfalfa growers with a sense of the risk assumed if sampling is initiated on these dates. In Figure 1, for example, if sampling is initiated by March 26 in southwestern and south central Nebraska, the alfalfa grower will have begun sampling at or before the beginning of alfalfa weevil egg hatch in nine of ten years. If sampling is initiated by March 31, the grower will have begun sampling at or before the beginning of egg hatch in three of four years (Figure 2). And if the grower waits until April 10, they will have begun sampling at or before the beginning of egg hatch in one of two years (Figure 3).  The dates in Figure 4 indicate that if sampling is initiated by April 10 in north central and northeastern Nebraska, the alfalfa grower will have begun sampling at or before the beginning of alfalfa weevil egg hatch in nine of ten years. If sampling is initiated by April 15, the grower will have begun sampling at or before the beginning of egg hatch in three of four years (Figure 5). And if the grower waits until April 20 (April 25 in parts of north central and northeastern Nebraska), they will have begun sampling at or before the beginning of egg hatch in one of two years (Figure 6).  Although there is some probability that alfalfa weevil egg hatch will be missed when any date is used, there is usually still time for management (if needed) because economic damage typically will not occur until at least one week after egg hatch. Therefore, the 25% probability level is an effective level to use. Using the 10% probability level would require the decision maker to sample too early most years.Real-time degree day accumulations are the most accurate predictors of alfalfa weevil egg hatch and should be used whenever possible. However, when real-time degree day accumulations are not available, the information presented here can be used to initiate alfalfa weevil sampling activities.    Figure 1. Date on which an alfalfa grower using the 250 DD base will initiate alfalfa weevil sampling at or before egg hatch in 9 of 10 years. The 250 DD base is typically used for southern Nebraska.    Figure 2. Date on which an alfalfa grower using the 250 DD base will initiate alfalfa weevil sampling at or before egg hatch in three of four years. The 250 DD base is typically used for southern Nebraska.    Figure 3. Date on which an alfalfa grower using the 250 DD base will initiate alfalfa weevil sampling at or before egg hatch in one of two years. The 250 DD base is typically used for southern Nebraska.    Figure 4. Date on which an alfalfa grower using the 300 DD base will initiate alfalfa weevil sampling at or before egg hatch in 9 of 10 years. The 300 DD base is typically used for northern Nebraska.    Figure 5. Date on which an alfalfa grower using the 300 DD base will initiate alfalfa weevil sampling at or before egg hatch in three of four years. The 300 DD base is typically used for northern Nebraska.    Figure 6. Date on which an alfalfa grower using the 300 DD base will initiate alfalfa weevil sampling at or before egg hatch in one of two years. The 300 DD base is typically used for northern Nebraska.   File G1263 under: Insects AND Pests C-39; Field Crops Paper version issued September 1995; 5,000 printed.   Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Elbert C. Dickey, Director of Cooperative Extension, University of Nebraska, Institute of Agriculture and Natural Resources. University of Nebraska Cooperative Extension educational programs abide with the non-discrimination policies of the University of Nebraska-Lincoln and the United States Department of Agriculture.  

G95-1263 When to Sample for Alfalfa Weevil

UNL | Libraries

University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
Historical Materials from University of 
Nebraska-Lincoln Extension Extension 
1995 
G95-1263 When to Sample for Alfalfa Weevil 
Steven J. Meyer 
University of Nebraska - Lincoln 
Robert K.D. Peterson 
Dow-Elanco, bpeterson@montana.edu 
Follow this and additional works at: https://digitalcommons.unl.edu/extensionhist 
 Part of the Agriculture Commons, and the Curriculum and Instruction Commons 
Meyer, Steven J. and Peterson, Robert K.D., "G95-1263 When to Sample for Alfalfa Weevil" (1995). 
Historical Materials from University of Nebraska-Lincoln Extension. 1111. 
https://digitalcommons.unl.edu/extensionhist/1111 
This Article is brought to you for free and open access by the Extension at DigitalCommons@University of Nebraska - 
Lincoln. It has been accepted for inclusion in Historical Materials from University of Nebraska-Lincoln Extension by an 
authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
G95-1263-A 
 
When to Sample for Alfalfa Weevil 
Dates for initiating alfalfa weevil scouting are provided, based on 30 years of climatic data used to 
estimate appropriate degree day accumulations for northern and southern Nebraska. 
Steven J. Meyer, Extension Specialist, Agricultural Climatology 
Robert K. D. Peterson, Research Biologist, DowElanco  
Sampling activities for alfalfa weevil need to be timed properly because it is inefficient to sample when 
the pest is not active or present. Conversely, delayed sampling is financially risky because economic 
damage can occur before a management practice is implemented.  
Integrated pest management programs often use degree day accumulations to initiate activities, while 
producers often use calendar dates. Calendar scheduling is traditionally based on subjective experience 
rather than research. Consequently, calendar dates are not as precise as degree day accumulations. This 
is not to imply that calendar dates have no value; indeed, they can be a valuable technique for 
determining when to initiate sampling activities particularly when real-time degree day information is 
not available. This approach can be improved, however, by using climatic "normal" temperature data 
(1961-1990) to determine the probable date when a specified number of degree days will have 
accumulated.  
Typically, two degree day (DD) accumulations are used to initiate alfalfa weevil sampling in Nebraska -
- 250 and 300 DD. Differences in autumn egg-laying patterns in Nebraska lead to differences in the 
timing of egg hatch and larval populations the next spring. In Nebraska, autumn egg-laying varies 
depending on climatic conditions. Research at Iowa State University suggests that alfalfa weevil 
sampling should begin at 250 DD in the southern half of Iowa and at 300 DD in the northern half. It's 
likely that these DD accumulation requirements are similar for Nebraska. Thus, the accumulation of 250 
DD should be used south of a line from Omaha to Ogallala and 300 DD north of that line.  
Figures 1-3 identify dates for the accumulation of 250 DD at the 10%, 25%, and 50% probability levels, 
based on the climatic "normal" temperature data of 1961-1990. Dates for the accumulation of 300 DD at 
the 10%, 25%, and 50% probability levels are given in Figures 4-6. Growers in southern Nebraska 
should use the information in Figures 1-3, while growers in northern Nebraska should use information 
in Figures 4-6. Those close to the dividing line may want to use a combination when making their 
decision.  
The dates in the six figures provide alfalfa growers with a sense of the risk assumed if sampling is 
initiated on these dates. In Figure 1, for example, if sampling is initiated by March 26 in southwestern 
and south central Nebraska, the alfalfa grower will have begun sampling at or before the beginning of 
alfalfa weevil egg hatch in nine of ten years. If sampling is initiated by March 31, the grower will have 
begun sampling at or before the beginning of egg hatch in three of four years (Figure 2). And if the 
grower waits until April 10, they will have begun sampling at or before the beginning of egg hatch in 
one of two years (Figure 3).  
The dates in Figure 4 indicate that if sampling is initiated by April 10 in north central and northeastern 
Nebraska, the alfalfa grower will have begun sampling at or before the beginning of alfalfa weevil egg 
hatch in nine of ten years. If sampling is initiated by April 15, the grower will have begun sampling at or 
before the beginning of egg hatch in three of four years (Figure 5). And if the grower waits until April 
20 (April 25 in parts of north central and northeastern Nebraska), they will have begun sampling at or 
before the beginning of egg hatch in one of two years (Figure 6).  
Although there is some probability that alfalfa weevil egg hatch will be missed when any date is used, 
there is usually still time for management (if needed) because economic damage typically will not occur 
until at least one week after egg hatch. Therefore, the 25% probability level is an effective level to use. 
Using the 10% probability level would require the decision maker to sample too early most years.Real-
time degree day accumulations are the most accurate predictors of alfalfa weevil egg hatch and should 
be used whenever possible. However, when real-time degree day accumulations are not available, the 
information presented here can be used to initiate alfalfa weevil sampling activities.  
  
Figure 1. Date on which an alfalfa grower using the 250 DD base will initiate alfalfa weevil 
sampling at or before egg hatch in 9 of 10 years. The 250 DD base is typically used for southern 
Nebraska.  
  
Figure 2. Date on which an alfalfa grower using the 250 DD base will initiate alfalfa weevil 
sampling at or before egg hatch in three of four years. The 250 DD base is typically used for 
southern Nebraska.  
  
Figure 3. Date on which an alfalfa grower using the 250 DD base will initiate alfalfa weevil 
sampling at or before egg hatch in one of two years. The 250 DD base is typically used for 
southern Nebraska.  
  
Figure 4. Date on which an alfalfa grower using the 300 DD base will initiate alfalfa weevil 
sampling at or before egg hatch in 9 of 10 years. The 300 DD base is typically used for northern 
Nebraska.  
  
Figure 5. Date on which an alfalfa grower using the 300 DD base will initiate alfalfa weevil 
sampling at or before egg hatch in three of four years. The 300 DD base is typically used for 
northern Nebraska.  
  
Figure 6. Date on which an alfalfa grower using the 300 DD base will initiate alfalfa weevil 
sampling at or before egg hatch in one of two years. The 300 DD base is typically used for 
northern Nebraska.  
 
File G1263 under: Insects AND Pests 
C-39; Field Crops 
Paper version issued September 1995; 5,000 printed. 
  
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation 
with the U.S. Department of Agriculture. Elbert C. Dickey, Director of Cooperative Extension, 
University of Nebraska, Institute of Agriculture and Natural Resources. 
University of Nebraska Cooperative Extension educational programs abide with the non-discrimination 
policies of the University of Nebraska-Lincoln and the United States Department of Agriculture.  


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G95-1263 When to Sample for Alfalfa Weevil

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