variegated cutworm - mint injury and threshold model

Help on MINTSIM Variegated cutworm mint sampling, injury, and threshold model

The variegated cutworm simulation model, called MINTSIM, is used for determining the need for treatment and for most efficient use of sampling of variegated cutworm in peppermint. Peppermint growth, cutworm development, feeding behavior, and injury are simulated. The beneficial effects of biological control by parasites are also simulated.

Formerly, with IPMP 2.0, the programs for developing sequential sampling plans for sweep net and ground search sampling methods had been separate from MINTSIM. MINTSIM provides comprehensive estimates for a wider variety of field conditions and will generally provide more accurate estimates of thresholds.

The model allows extension workers, consultants and field workers to input conditions of a given peppermint field to predict economic loss and benefits of treatment. It makes use of near-real time calculation of degree-days to predict cutworm development and feeding behavior. The model is conservative: factors such as cutworm mortality due to birds is not included, and cutworm injury rates may be slightly overestimated. This conservative nature of the model is in accord with the tendency to avoid risk of unforeseen pest outbreaks that most decision makers adopt. Refer to disclaimer below.

The following inputs describe the current field conditions that determine model results. Of these inputs, the most important are the cost of control, current oil price, and cutworm sampling data (if available). The weather station selected may also be important, especially under relatively extreme temperature conditions. Try running the model using default values or a range of values to see if the model recommendations vary little or a lot. Usually the model is relatively insensitive.

Control cost
The total direct and indirect costs of treatment for the pest. This value varies with the product used, the cost of application, cost of clean-up, cost in having to prevent re-entry of workers into the field, if any, and costs in terms of indirect effects on the environment (which are usually not easy to estimate). The control cost is very important for determination of economic thresholds and should be estimated as accurately and completely as possible. The higher the control cost, the higher the damage tolerance and economic threshold.

Control % kill or Efficacy
The total proportion of cutworm pests that are expected to be killed by the treatment, given as a percentage (0 to 100%). Most effective treatments are 80% effective or better. The % kill is very important for determination of economic thresholds and should be estimated accurately. A thorough post-treatment sample can be used to determine % kill for future applications in most cases. As the expected % kill decreases, the tolerance for crop damage and associated economic thresholds will increase.

Oil price
The price of peppermint oil ($/lb). The price expected for the commodity. It is important that this value is estimated accurately. The higher the commodity price is, the lower the damage tolerance and economic threshold will be.

Expected yield
The expected yield of peppermint oil in lbs/acre. This value is not important for calculation of the economic threshold and it is not required to have an accurate estimate of this value.

Field class and harvest date
Three classes of fields are reflected in the model: those with early, average, and late harvest times. Fields which are managed for early harvests (early August) usually have early fertilizer treatments and have relatively tall mint plants early in the season, which will attract cutworms. Late-harvest fields, in contrast, may flame late or even mow in June, and are less attractive to cutworms and are harvested in late August. Select the stategy that best reflects the current field conditions. Also select a predicted harvest date (a precision of +/- 7 days is expected). If you are unsure, run the model twice using a range of harvest dates.

Weather region and location
This section accesses an extensive database of online weather stations throughout the Pacific Northwest from a related website at To use this section, click on the region where the most appropriate weather station for your conditions exists. In the same column as the selected region, select the actual station, 7-day forecast, and long term historical average forecast locations.

Sampling data
This program will commonly be run under 2 situations: during the season when sampling cutworms to determine the need for treatment, and at other times to become familiar with the program and the concepts it is based on. Either way, an approximate economic threshold is estimated and sampling plans can be generated.

If no sample data are available, select the "No" option and (normally) leave the total density set at 5.0 cutworms per sq. ft. The next section (3 rows) can then be skipped. This mode allows the indicated density to hatch and complete development, thus simulating a cutworm population and its effects on peppermint throughout the season.

If the "Yes" option is selected, then the input sample densities will begin development and feeding on the input sampling date. Thus, "past" injury is not calculated in this mode, which is consistent with computing the benefit of treatment, since it is assumed that the decision to treat will be made soon after the sampling date.

Sampling cutworms
Two methods of cutworm sampling are commonly used: sweep net and ground search. The sweep net method is most commonly used first, while cutworms are in the early stages (1-4). As cutworms develop, they tend to feed lower on the plant, spending more time on the ground surface where they tend to spend the night. This is why the ground search method is most commonly used to sample the 4-6 instars. If the population is spread out in all stages, then it is sometimes helpful to sample using both methods at the same time. This program tries to adjust for the biases of each sampling method to estimate "absolute" densities to simulate the population more accurately in the model.

Ground search methodology
Ground searches assume that you shake the peppermint foliage above an area of approximately 1 sq ft (12 x 12 inches), and count the number of cutworms of each instar that fall to the ground (or where on the ground already). This method recovers large instars mre easily than small instars. The longer the time used to inspect for cutworms, the more are recovered. Therefore, the time spent conducting the ground searches is used to help determine absolute densities. We recommend an average search time of 1 minute per sample. At least 10 samples should be taken throughout the field, although 20-30 will provide much better precision, especially later in the season when sweep sampling is no longer very helpful.

Sweep net methodology
Using the sweep net assumes that you use a 15-inch diameter net and sweep in an arc of 180 degrees. This option assumes that you have taken a sample of cutworms and loopers using the sweep net method. The program will combine estimates from both methods to help determine absolute densities. At least 10 samples of 10 sweeps per sample should be taken throughout the field, although 20 or more samples will provide much better precision, especially early in the season when the ground search method is not yet very helpful.

Estimating cutworm larval stages
While the model expects you to enter the average number of each substage ("instar") for both sampling methods, this may seem a difficult and even daunting task.You could use the following basic guidelines:

1st instar - very tiny, almost microscopic (less than 1 mm length)
2nd instar - tiny, the smallest you can readily see (ca. 2 mm length)
3rd instar - small, unlikely you will notice in ground search samples (ca. 3 mm length)
4th instar - small, but possible to see in ground search samples (ca. 0.6" length)
5th instar - medium size (ca. 1" length)
6th instar - large size (ca. 1.5" length)
It may be easiest to collect all larvae sampled, take them back to the laboratory, and sort them out into groups that match these instar categories. Alternatively, record the sample numbers in the field into broader categories such as: tiny (instars 1-2), small (instars 3-4), medium (instars 4-5), and large (instars 5-6). The degree of precision in estimating larval stages is considerably less important that the amount of time spent collecting samples. In other words, worry more about sampling the entire field than in trying to guess the instars accurately.

Output options
Use this section to select the degree of detail you would like to see in the model output. The options include:

Currently there is no option to graph the daily output or harvest summary, a feature that was available in earlier versions of this program.

Run the model - explanation of the model itself
The MINTSIM model combines research on variegated cutworm egg hatch, development, feeding behavior, survival, and sampling, along with research on peppermint growth and oil yield on a per-leaf stratum basis. As a result, the model tracks the feeding of cutworms up and down the peppermint plant, and where that feeding injury ends up at harvest. Once harvested, any feeding which occurred on leaves that had abscissed (decayed and fallen off) is not included in the tally of damage. Also, different mainstem and lateral leaves contain different quantities (and quality) of peppermint oil, and this information is also used to compute final cutworm losses at harvest. The program was developed as part of a PhD thesis in 1987, from data collected in peppermint fields in the mid-Willamette Valley of Oregon. The program was first written in Turbo Pascal to run in PC-DOS in 1987. It was modified slightly for use with IPMP in 1989, and for IPMP 2.0 in 1994. The program has been converted to Free Pascal, an open-source Turbo Pascal 32-bit compiler, to run on the web and to incorporate sequential sampling plans for IPMP 3.0 in 2002.

Detailed output
The MINTSIM simulation model tracks the development of mint and the variegated cutworm. The daily detailed results provides a summary of important model variables during the course of model execution. The output table includes these variables and sample data extracts:

              |----------Mint plant status---------|                       
          Cum |----No.---|Injury (g)|-Cumul injury-|
      Deg Deg Nod Lat Absc     Lat-       Lat- Absc|--Cutworm densities #/ft²--|
  Day Day Day es eral iss Main eral  Main eral iss  L1&2 L3   L4   L5   L6  Pupa
  --- --- --- -- ---- --- ---- ----  ---- ---- ---  ---- ---  ---  ---  --- ----
   1  20   39  5   0   0    0   0     0    0     0  0.0  0.0  0.0  0.0  0.0  0.0
   2  25   63  5   0   0    0   0     0    0     0  0.0  0.0  0.0  0.0  0.0  0.0
   3  27   90  5   0   0    0   0     0    0     0  0.0  0.0  0.0  0.0  0.0  0.0
  36  24  783  6   5   4   47   3   156    7     3  1.1  1.5  1.1  0.7  0.1  0.0
  37  23  806  7   6   4   59   3   215   10     3  0.7  1.3  1.2  0.9  0.1  0.0
  38  24  830  7   6   4   86   4   301   15     3  0.5  1.0  1.2  1.2  0.3  0.0
Of these, the [Day], [Deg Day], and [Cum Deg Day] signify the counting day, daily calculated degree-days (from the selected weather site), and the cumulative degree-days. All developmental processes are driven by daily degree-days.

The Mint plant status variables include leaf numbers, daily cutworm injury tally, and cumulative injury tally. The leaf number variables include the number of mainstem leaf nodes [Nodes], the number of lateral leaf nodes [Lateral], and the number of leaf pairs abscissed or decayed [Absciss]. Leaf abscission occurs for the older, lower leaves which naturally age and decay in part as a function of shading by the younger leaves. The plant status variables show the growth of the plant, which is affected only by the user-selected field type and the degree-days. The tracking of the relative position of injury on mainstem and lateral leaves and of leaf abscission is central to how MINTSIM calculates crop losses and economic threshold values.

The mint injury variables include the leaf dry wt. (g) of the mainstem leaves [Injury - Main] and the lateral leaves [Injury - Lateral]. These injury values represent the total amount of feeding for the day on all leaves by the simulated cutworm population. There is no daily injury to abscissed leaves because cutworms prefer to feed only on healthy green peppermint leaves.

The cumulative injury variables simply accumulate the daily injury up until the date of harvest. At harvest, the cumulative injury for all non-abscissed leaves is converted into oil lost according to research on the average oil amount known from each mainstem leaf node and from the average of all lateral leaves.

The cutworm density variables indicate the daily status of the population. The densities in # per sq. ft. of the first and second instars [L1&2] is combined, and the densities of all other instars [L3], [L4], [L5], [L6], and of pupae [pupa] are shown individually. Tracking of individual stages (instars) of the population is critical because: 1) Larger stages feed at much higher rates and are much more damaging 2) The timing of injury is critical, so we track the development of stages and their feeding, 3) The location of feeding is somewhat dependent upon the stage, with larger stages feeding lower on the plant than the younger stages, and 4) the action of parasites contributing to the partial biological control of the variegated cutworm occurs at specific stages, specifically attack is usually to the 2nd through 4th instars, while the death of the cutworms usually occurs by the 4th or 5th instar, before the majority of the potential injury is inflicted.

Harvest detail output
After the last day of the simulation, as specified by the user inputs, the collected peppermint leaves are converted into oil, and the leaf injury is converted into oil consumed. A gas chromatograph was used to determine mainstem leaf oil concentrations on a per leaf basis, and for lateral leaves on average (not shown). In this table (Table 2), it is interesting to compare the leaf dry wt. consumed per leafnode for abscissed vs. non-abscissed leaves.

In Table 2 below, the amount of dry wt. consumed was nearly the same for the non-abscissed (Node # 1-8) vs. abscissed leaves (Node # 9-17). This means that close to 1/2 of cutworm injury was to leaves that ended up NOT contributing to the yield. Delaying the harvest date would lessen the actual loss of oil even further. So the timing of cutworm feeding is critical to determining whether it will affect actual yield loss.

Table 2. Detail results for individual leaf pairs at harvest:
         Dry wt.    Oil   Poten- % Oil                       
   Node    cons-   cons-   tial   cons-  Absc-  Date  Date  
    #      umed    umed     Oil   umed   issed  added absc. 
   ----  ------   -----   -----  -----   -----  ----- -----
   FLR                     4.78    
    0                      1.57    
    1         0    0.00    3.43   0.00    no     69    
    2         0    0.00    4.45   0.00    no     62    
    3         0    0.00    4.31   0.00    no     56    
    4         0    0.00    4.84   0.01    no     50    
    5        10    0.02    4.89   0.35    no     43    
    6        80    0.13    4.56   2.77    no     37    
    7       269    0.41    4.56   9.04    no     31    
    8       583    1.10    4.50  24.51    no     25    
    9       438    0.77    2.36  32.88    part   18     0
   10       772    1.37    4.45  30.70    yes    12    68
   11       461    0.82    4.45  18.36    yes     6    60
   12       166    0.29    4.45   6.59    yes     1    52
   13        31    0.06    4.45   1.24    yes     0    44
   14         3    0.00    4.45   0.10    yes     0    35
   15         0    0.00    4.45   0.01    yes     0    27
   16         0    0.00    4.45   0.00    yes     0    19
   17         0    0.00    4.45   0.00    yes     0    10
Daily weather data and degree-days
If the third output option is selected, you will see the daily details of the temperature and degree day calculations, as output from a version of the online weather data and degree-day tool,, which is available for several phenology models for mint pests.Because MINTSIM now uses this utility and real-time weather data (and 10-day forecasts), it is recommended that you show these details to verify that the weather data are not in error - otherwise thresholds and model outputs could also be incorrect. See disclaimer below.

Mintsim outputs
The % damage tolerance is a simple calculation of the cost of control divided by the value of the crop times the proportion killed:

% damage tolerance = cost of control/(oil price x estimated yield x % kill)
All of these values are input by the user and povide a starting point for economic (damage) thresholds for all pests.

This value provides a convenient reference point for determining the need for control. As the cost of control decreases or the oil price increases, the damage tolerance decreases. Likewise, as the % kill decreases, the damage tolerance increases.

The % damage inficted is determined by the simulation

Sequential sampling table
The table for sequential sampling is based on the absolute density of cutworm larvae per sq ft, and is of limited use for sweep net samples, unless you use this program to estimate absolute densities from sweep net (and ground search) samples. The ground search method is described
above, and is the best way to sample larger cutworm larvae. A separate sequential sampling scheme for using sweep net samples only is not available at this time.

Sequential sampling can save time and money by allowing early decisions when the pest density is well above or below the economic threshold. The table takes into consideration factors including: cost of control, current calculated threshold, control efficacy, current oil price, and risk of sampling error. It requires that sampling procedures are followed as described.

To interpret the table (see example below), note whether your sample total (no. cutworm larvae of all instars for a given sample size) is below the 'No Treat' level, above the 'Treat' level, or in between. If the population is in between these levels, then 10 or more additional samples are recommended before making a decision to treat.

In the example table below, the economic threshold is 3.3 cutworms /ft², which is the "breakeven" density: below this density damage is not estimated to exceed the control cost, whereas above this density damage IS expected to exceed the control cost. In the table, notice that the "Continue Sampling" range is much wider for few samples than for more samples. Thus, with more samples, more precise estimates are available, and more correct decisions to treat are made. But IF the early sampling results are well below or above the threshold (No Treat or Treat columns), then an early decision can be made which will save time and sampling costs.

Estimated Economic Threshold:     3.35 cutworms / ft²
                          Total No. Cutworms / ft² for all samples   
 ------Samples--------No Treat-------Continue Sampling--------Treat-----
          10           < 14.5           14.5 - 48.9           > 48.9   
          15           < 29.0           29.0 - 63.3           > 63.3   
          20           < 43.5           43.5 - 77.8           > 77.8   
          25           < 58.0           58.0 - 92.3           > 92.3   
          30           < 72.4           72.4 -  107           >  107   
          35           < 86.9           86.9 -  121           >  121   
          40           <  101            101 -  136           >  136   

MINTSIM Disclaimer
This program is a complex simulation model which depends upon all conditions input and set correctly, use at your own risk. Be carefull of all options selected, weather data validity and that you understand the models correct usage and interpretation. Failure to read all instructions and to follow procedures correctly will result in incorrect estimates. No particular products or control actions are implied by this program. No liability is assumed for errors whether caused by user, input data, or programmer.

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This project funded in part by grants from the Mint Industry Research Council and the Oregon Peppermint Commission
This program last updated July 1, 2002
Contact Len Coop at if you have any questions about this information.