Charge questions

8. Does the Panel have any

recommendations for distributing nonmultiplicative interactions in models to

evaluate multi-gene pyramided

products?

9. Please comment on the reliability of the

estimates of resistance evolution by

each of the three models in light of the

biological and parameter uncertainties

identified by BPPD.
Modeling

Seed blends vs. Structured refuge

Differences

Larval movement (blends)

Adult dispersal (refuges)
Models

Monsanto - implicit everything

Dow - explicitly spatial

EPA/ORD - compartment, implicitly

spatial, frequency-only

Ives et al. (2010) - spatially implicit and

explicit, density-dependent or

frequency-only
Goal: Understand processes that affect rate of

resistance evolution

Make comparisons among scenarios:

“All else equal,” what is the best strategy?

Given different models and target insects, need to

understand differing results

Estimates of time to resistance failure are inherently

suspect (even if results from 3 models are similar)

It is easier to assess whether one scenario leads to

faster or slower resistance evolution than

predicting when failure will occur
SS SR RR

SS SR RR

S

u

r

viv

al 1

0

S

u

r

viv

al 1

0

Faster evolution

Survival on Bt plants

(not realistic values)

Higher survival

Lower survival

Heuristic explanation of high-dose strategy
SS SR RR

S

u

r

viv

al 1

0

SS SR RR

S

u

r

viv

al 1

0

Slower evolution

(due to relative

survival of SR to SS)

Add refuge

Higher survival

Lower survival

Survival at

population level

(not realistic values)

Information provided on SS survival

Contrast between data and what models use/need
Results preview

1. If adults have high dispersal among fields and larvae do not move

among plants, then blends and refuges are equivalent.

2. Structured refuges: Limited adult dispersal can speed but mainly

slows resistance evolution (depending on male vs. female dispersal

rates, rotation patterns, etc.).

3. Blends: For high-dose case, larval movement among plants will

mostly speed resistance evolution, with an important role of increased

survival of heterozygotes.

4. Expression of resistance in heterozygotes is key to the durability of

seed blends, yet key information is not known.

5. Monsanto, Dow, and EPA/ORD models for the durability of 5% seed

blends are NOT conservative, especially for SWCB

I will present examples highlighting processes that are not adequately

addressed in any of the models.
General Model

Ives, A. R., P. R. Glaum, N. L. Ziebarth, and D. A. Andow. 2010.

The evolution of resistance to two-toxin pyramid transgenic

crops. Ecological Applications in press

Shares characteristics with other models (not independent)

2 Bt toxins (CRW) that act independently

2 partially recessive resistance alleles

Random mating within fields

Pre-mating male dispersal, post-mating female dispersal

Keep track of both density and allele frequencies

Either density-dependent survival of larvae on plants, or not

(frequency-only model = extinction model)

Form of density dependence irrelevant
RR 0.01 0.06 1

SR 0.0006 0.0035 0.06

SS 0.0001 0.0006 0.01

SS SR RR

Survivals on Bt plants

Relative survival of heterozygotes, h = 0.05

sXXYY = sXXsYY

Different from EPA/ORD, but will return to this
1. If adults have high dispersal among fields

and larvae do not move among plants, then

blends and refuges are equivalent.
2. Structured refuges: Limited adult dispersal

can speed or slow resistance evolution,

depending on male vs. female dispersal

rates, rotation patterns, etc.

Two opposing forces:

Limited dispersal means insects remain in

refuges (provided incomplete rotation)

slows resistance

Limited dispersal can cause resistance

hotspots

speeds resistance

I will give a brief description only to allay fears

about structured refuges.
Moderate adult dispersal

white = areas

of higher allele

frequency

generation 91 generation 92
Low adult dispersal

white = areas

of higher allele

frequency

generation 32 generation 33
Assumptions

5% refuge fields

exponential dispersal kernel (including some

adults remaining in natal fields)

pre-mating male dispersal, post-mating

female dispersal

refuge rotation either every 3 generations, or

never
Refuge rotation every 3 generations

Includes contrasting effects of (i) the probability of

leaving natal fields and (ii) the distance dispersed

once natal fields are left (Andow, Charge Question 1)
No refuge rotation
2. Structured refuges: Limited adult dispersal

can speed or slow resistance evolution,

depending on male vs. female dispersal

rates, rotation patterns, etc.

Limited dispersal will generally slow rather

than speed resistance evolution.

But extremely limited dispersal can greatly

speed resistance evolution.
3. Blends: For high-dose case, larval movement

among plants will mainly speed resistance evolution,

with an important role of increased survival of

heterozygotes.

Unknowns:

Behavior: movement rates, response to density,

stage at movement, response to Bt vs. non-Bt

plants, mortality during moves

Susceptibility: pre-movement selection, postmovement selection, survival of heterozygotes to

multiple toxins

Studies need to target unknown information

Onstad et al. comment expands on unknowns
3. Blends: For high-dose case, larval movement

among plants will mainly speed resistance evolution,

with an important role of increased survival of

heterozygotes.

Unknowns:

Behavior: movement rates, response to density,

stage at movement, response to Bt vs. non-Bt

plants, mortality during moves

Susceptibility: pre-movement selection, postmovement selection, survival of heterozygotes to

multiple toxins

Studies need to target unknown information

Onstad et al. comment expands on unknowns
RR 0.1 0.24 1

SR 0.024 0.06 0.24

SS 0.01 0.024 0.1

SS SR RR

Seed blends: survival of larval stages

Pre- and post-movement stages

Assume fixed probability of moving between

stages (same for Bt and non-Bt plants;

independent of density)

Move randomly to other plants

sXXYY = sXXYY

1 sXXYY2

 (like EPA/ORD, but not on a daily time scale)

 Stage-specific survivals
Larval movement =

probability of leaving

initial plant

SSSS survival includes

survival on non-Bt plants

Relative RS survival

includes survival on nonBt plants
RR 0.058 0.1051 1

SR 0.02 0.0233 0.0861

SS 0.018 0.019 0.038

SS SR RR

EPA/ORD Survivals (low-dose, CRW)

RR 0.01 0.06 1

SR 0.0006 0.0035 0.06

SS 0.0001 0.0006 0.01

SS SR RR

Survivals used here (high dose, 2-toxin ECB)
Larval movement has

almost no effect for lowdose crops

Relative RS survival

unaffected with low-dose

crops
Does the timing of larval movement (stage of

strongest selection) affect resistance evolution?

RR 0.1 0.24 1

SR 0.024 0.06 0.24

SS 0.01 0.024 0.1

SS SR RR

RR 0.01 0.06 1

SR 0.0006 0.0035 0.06

SS 0.0001 0.0006 0.01

SS SR RR

RR 0.01 0.06 1

SR 0.0006 0.0035 0.06

SS 0.0001 0.0006 0.01

SS SR RR

RR 1 1 1

SR 1 1 1

SS 1 1 1

SS SR RR

RR 0.1 0.24 1

SR 0.024 0.06 0.24

SS 0.01 0.024 0.1

SS SR RR

RR 1 1 1

SR 1 1 1

SS 1 1 1

SS SR RR

Stage 1 Stage 2

Case 1 Case 2 Case 3
Does the timing of larval movement (stage of

strongest selection) affect resistance evolution?

When there is pre-movement selection (case

2), movement slows resistance evolution

Removes density-dependent mortality of

stage 1 susceptibles on non-Bt plants

Case

1=split

2=stage 1 selection

3=stage 2 selection
Does the timing of larval movement (stage of

strongest selection) affect resistance evolution?

Case

1=split

2=stage 1 selection

3=stage 2 selection
Does the timing of larval movement (stage of

strongest selection) affect resistance evolution?

RR 0.1 0.24 1

SR 0.024 0.06 0.24

SS 0.01 0.024 0.1

SS SR RR

RR 0.31 0.5 1

SR 0.16 0.24 0.5

SS 0.1 0.16 0.31

SS SR RR

RR 0.31 0.5 1

SR 0.16 0.24 0.5

SS 0.1 0.16 0.31

SS SR RR

RR 0.032 0.12 1

SR 0.004 0.015 0.12

SS 0.001 0.004 0.032

SS SR RR

RR 0.1 0.24 1

SR 0.024 0.06 0.24

SS 0.01 0.024 0.1

SS SR RR

RR 0.032 0.12 1

SR 0.004 0.015 0.12

SS 0.001 0.004 0.032

SS SR RR

Stage 1 Stage 2

Case 1 Case 4 Case 5
Does the timing of larval movement (stage of

strongest selection) affect resistance evolution?
Summary

In moderate/high dose cases, larval movement

speeds resistance evolution, but not in low-dose

cases

SAP for Pioneer’s Optimum AcreMax 1

recommended 20% seed mix

To my knowledge, there has been no systematic

study of the consequences of selection preversus post-movement

Most importantly, we need to know the strength of

selection on heterozygotes, both pre- and postmovement
Charge question

8. Does the Panel have any

recommendations for distributing nonmultiplicative interactions in models to

evaluate multi-gene pyramided products?
EPA/ORD (with Cry1F=SS)

RR 0.00140 0.00176 0.00502

SR 0.00140 0.00144 0.00176

SS 0.00140 0.00140 0.00140

SS SR RR

Cry2Ab2 0.00190 0.09976

Cry1A.105 0.00275 0.11500

Survivorship of SS Dominance
RR 0.00265 0.00277 0.00502

SR 0.00146 0.00153 0.00277

SS 0.00140 0.00146 0.00265

SS SR RR

EPA/ORD (with Cry1F=SS)

RR 0.00140 0.00176 0.00502

SR 0.00140 0.00144 0.00176

SS 0.00140 0.00140 0.00140

SS SR RR

With h1 = h2 = 0.05
Increase survivals by 5x

RR 0.01 0.06 1

SR 0.0006 0.0035 0.06

SS 0.0001 0.0006 0.01

SS SR RR

Same survivals pre- and postmovement

RSRS = cross resistance
5x survival of SRSS or SRSR halves the time to

resistance failure
RR 0.01 0.06 1

SR 0.0006 0.0035 0.06

SS 0.0001 0.0006 0.01

SS SR RR

Increase survivals by 5x

RR 0.01 0.06 1

SR 0.0006 0.0035 0.06

SS 0.0001 0.0006 0.01

SS SR RR

Asymmetric vs. Symmetric
Asymmetric

Symmetric
Differences between loci in rate of

evolution

Rate of resistance is driven largely by the locus

that evolves most rapidly

compare three cases:

sA,SS = sB,SS = 0.01

sA,SS = 0.01, sB,SS = 0.001

sA,SS = sB,SS = 0.001

Summary

Estimates (guesses) of relative survivals of

SRSS and SRSR genotypes are poor

They have a large impact on the rate of

resistance evolution

Asymmetric changes in survivals of

heterozygotes appear to have effects almost as

large as symmetric changes

Overall rate of resistance evolution seems to be

driven by the fastest evolving locus
Charge question

8. Does the Panel have any

recommendations for distributing nonmultiplicative interactions in models to

evaluate multi-gene pyramided products?

We need to use models to examine what

gene interactions will most likely give the

most rapid resistance evolution

(sensitivity analyses are limited)

Ultimately, how genes in each target

species interact is an empirical question
Charge questions

9. Please comment on the reliability of the

estimates of resistance evolution by

each of the three models in light of the

biological and parameter uncertainties

identified by BPPD.
Monsanto model

When larvae move, a key effect is on the

relative survival of heterozygotes

For ECB and SWCB, Monsanto models

movement of larvae implicitly as a decrease in

refuge size to account for increased larval

mortality

This largely misses the point
Monsanto model

underestimates the

importance of larval

movement and

overestimates the time to

resistance failure.
Dow model

Models CRW with high-dose parameters which

generally cause slower resistance evolution

than low-dose parameters

Nonetheless, under low-dose conditions, larval

movement likely makes little difference, so

seed blends are less of an issue for CRW

Only investigated change in allele frequencies

over 20 years, so results are hard to assess
EPA/ORD model

Does not include density - “extinction model”

Density-dependence is important:

Larval mortality decreases survival of susceptibles

It thereby increases the relative survival of moreresistant heterozygotes, and speeds resistance

evolution

(ii) For ECB and SWCB, apparently little

increased survival of heterozygotes

(iii) issues involving sensitivity analysis
Frequency-only and

density-dependent

models coincide when

populations are going

extinct (and resistance

evolution is rapid)
5. Monsanto, Dow, and EPA/ORD models for

the durability of 5% seed blends are NOT

conservative, especially for SWCB

Monsanto - larval movement had no effect on

relative survival of heterozygotes

Dow - used high-dose survivals for CRW (as

pointed out by BPPD)

EPA/ORD - (i) no density-dependent larval

survival; (ii) for ECB and SWCB, apparently

little increased survival of heterozygotes; (iii)

issues involving sensitivity analysis
What are the relative survivals of SRSS, SSSR,

and SRSR heterozygotes?

What are the relative strengths of selection on

pre- vs. post-movement larvae?

These questions cannot be answered with theory

RR 0.1 0.24 1

SR 0.024 0.06 0.24

SS 0.01 0.024 0.1

SS SR RR

RR 0.31 0.5 1

SR 0.16 0.24 0.5

SS 0.1 0.16 0.31

SS SR RR

RR 0.31 0.5 1

SR 0.16 0.24 0.5

SS 0.1 0.16 0.31

SS SR RR

RR 0.032 0.12 1

SR 0.004 0.015 0.12

SS 0.001 0.004 0.032

SS SR RR

RR 0.1 0.24 1

SR 0.024 0.06 0.24

SS 0.01 0.024 0.1

SS SR RR

RR 0.032 0.12 1

SR 0.004 0.015 0.12

SS 0.001 0.004 0.032

SS SR RR

Stage 1 Stage 2

Case 1 Case 4 Case 5
Charge questions

9. Please comment on the reliability of the

estimates of resistance evolution by

each of the three models in light of the

biological and parameter uncertainties

identified by BPPD.

Hard to say

Hate to guess

I wouldn’t place money on them
Conclusions

1. Structured refuges are better than seed

blends for ECB and SWCB

Issues of compliance and variation in blended

seed clustering not addressed

2. Monsanto, Dow, and EPA/ORD models

for the durability of 5% seed blends are

NOT conservative, especially for SWCB
Information that is lacking

1. CEW needs to be addressed with data

and inter-regional models

2. Survival of heterozygotes, epitasis, and

how they interact with larval movement

for ECB and especially SWCB (need for

a “model system”)

3. Quantitative genetic models for lowdose products

4. Monitoring/remediation strategies for

stacked products
How reliable are the estimates of time to

failure for the Monsanto, Dow, and

EPA/ORD models?

Nick would say NOT

Graham said that the absence of cases of

resistance means that the models have

either been right or too conservative

But what about Helicoverpa punctigera in

Australia? This is among the last pests

you would expect to evolve resistance.
Helicoverpa punctigera

Resistance developing to cry2Ab in Bollgard II

Downes, S., T. Parker, and R. Mahon. 2010. Incipient

resistance of Helicoverpa punctigera to the Cry2Ab Bt toxin

in Bollgard II (R) cotton. PloS ONE 5(9).

but F1 screen

was significantly

lower in 2009/10

2009/10
Helicoverpa punctigera

Resistance developing to cry2Ab in Bollgard II

Downes, S., T. Parker, and R. Mahon. 2010. Incipient

resistance of Helicoverpa punctigera to the Cry2Ab Bt toxin

in Bollgard II (R) cotton. PloS ONE 5(9).

but F1 screen

was significantly

lower in 2009/10

2009/10

Do we have enough information to model resistance accurately?
Recommendation

EPA should consider in their risk assessment

a broader range of IRM tactics

For example:

1. Introduce 20% SmartStax seed blend

2. Monitor insect population

Monsanto/Dow data demonstrate survival of susceptibles

at the field scale, but we need information at the regional

scale

3. In 5 years, if insect populations are not too low,

consider a 10% seed blend

4. In 10 years, consider a 5% seed blend