9-14-06  TABASHNIK ANSWERS TO MATTEN MODELING QUESTIONS

Date: Thu, 14 Sep 2006 13:45:11 -0400

From: Matten.Sharlene@epamail.epa.gov

Bruce -- I have some modeling analysis questions (Vol. 5 of the
submission). Please send answers to me ASAP.  I am trying to finish
everything up today, if possible, but at the latest tomorrow
afternoon. Thank you for your help.

 Sincerely, Sharlene 

 

1.  Resistance allele frequency, genetics of resistance, likely
inheritance pattern is based on Cry1Ac resistance only? Yes or No?  

Yes.     Do you have the data on the acreage in Arizona of Cry1Ac cotton
vs. Cry1Ac + Cry2Ab cotton?

  

How are these parameters addressed for Cry2Ab resistance?

We are not considering Cry2Ab resistance in the modeling.

There could be two different resistance mechanisms and two different
inheritance patters?  

Yes.

How should I use the model to address resistance to both toxins?

If some or all of Arizona’s Bt cotton had two toxins, Cry1Ac + Cry2Ab,
evolution of resistance would be much less likely than it is with only
Cry1Ac.  If you want, I can summarize the reasoning for that conclusion.
 By modeling resistance to cotton that produces only Cry1Ac, we are
choosing a more pessimistic scenario than one that includes cotton with
Cry1Ac + Cry2Ab.  So, in this aspect, all of the modeling predictions
are conservative, i.e., they tend to overestimate resistance risk.

 

2.  What values/parameters would be changed based on the 2006 field
data?

We will use data from the field to test the model’s predictions about
the ratios of sterile:native moths and resistance allele frequency.  If
the model’s predictions are wrong,  incorrect assumptions will be
changed to improve the accuracy of the model.

  

Are additional simulations planned to used these data?

Yes.  We are also conducting additional simulations now.  We have
modified some of the assumptions and run another set of scenarios using
sensitivity analysis varying emigration (10, 55, or 75%), dominance
(recessive or dominant), number of fields in the region (400 or 4096),
percentage of Bt fields (80, 95, 100), carrying capacity per field
(2,885,  28,885, 288,850), and numbers of steriles per ha per release in
Bt and non-Bt cotton fields (0,0; 2,10; 100, 500; and 200, 1000). 
Again, with one exception, all scenarios yielded regional loss in 2
years.  The only case in which loss did not occur was with no steriles
released in Bt or non-Bt cotton (0, 0).  The lowest non-zero release
rate tested was 2 per ha per release in Bt cotton and 10 per ha per
release in non-Bt cotton (2,10).  This low release rate scenario yielded
regional loss in 2 years.     

 

3.  The modeling output is represented in a qualitative way.  Do you
have another table that summarizes each of the twelve scenarios?  I
think this would be clearer.

If you let me know more specifically what additional information you
seek, we can probably provide it.  Regional loss means zero PBW in the
region.  So, we could put zeroes in the table instead of “regional
loss.”  R allele frequency is not relevant when regional loss occurs.

 

4. Do I understand that for 11/12 scenarios, resistance did not evolve
and no PBW were present after the 4 year eradication program.  

Correct.  In 11/12 scenarios, no PBW were present after 4 years.

One scenario had a 98% population suppression with a r frequency of 0.02
(below resistance threshold of 0.50).  The r frequency in question is
Cry1Ac only?

Yes.

 

5.  Sterile release rates in the model.  The sampling/trapping protocol
description in vol. 1 indicates that steriles were released at a rate of
20 steriles/acre/day (3X per week) in Bt fields and 100 steriles (or
more)/acre/day (3X per week) in non-Bt fields.  I converted these to
hectacres and so the sterile rate in Bt fields = 8 steriles/ha/day and
in non-Bt fields = 40 steriles/ha/day.  However, in the model, I see
that the default values are much higher than what was actually done in
2006, i.e., 75 steriles/ha/day in Bt cotton fields and 500
steriles/ha/per release in non-Bt cotton fields.  How do you explain
this difference?  How does this difference affect the model outcome?

Your conversions are off by ca. 6.25X (too low) because the conversion
requires multiplying X 2.47 (not dividing by 2.5).  Also, the model
assumptions are stated in steriles per release (not per day).  The
sensitivity analysis results submitted to EPA at the beginning of August
included 1, 2, 3, 4, 5, 10, and 15 steriles per ha per release in Bt
cotton.  These lower levels also yielded regional loss (i.e., no pink
bollworm) in 2 years.  More recent simulations show regional loss with 2
per ha per release in Bt cotton and 10 per ha per release in non-Bt
cotton. 

