Bt Crops

Question 1

Insect Resistance Development to Bt Crops.

Answer 1

High degree of selection pressure accelerates the development of resistance.

Question 2

IRM for Bt crops in U.S. and Canada.

Answer 2

“high dose/refuge” strategy.
High dose Bt plants that can kill RS insects.
Refuge: An area of corn or cotton (or a portion of seeds in RIB) planted without the Bt trait where SS pests on non-Bt plants can survive and produce a local population capable of inter-mating with any possible RR from Bt plants.

Question 3

Describe the High Dose/ Refuge Strategy

Answer 3

The large SS populations from refuge plants should mate with rare RR from Bt plants.
Thus, most offspring carrying R alleles will be RS. Since RS should be killed by high dose Bt plants, R frequencies should be maintained at low levels for a long period of time.

Question 4

Key Assumptions of High/Dose Refuge Strategy.

Answer 4

• High dose: > 95% mortality against RS
  OR resistance is functionally recessive or at least partially recessive
• Initial resistance allele frequency is very low(< 0.001)
• Random mating between susceptible and resistant insects.

Question 5

Factors Contributing to the Long-Term Success of Bt Crops in North America.

Answer 5

fundamental requirements of the ‘high-dose/refuge’ IRM strategy have applied to all of the four pests that continued to be susceptible (ECB, SWCB, TBW, PBW)

1) low initial R allele frequency
2) high dose/recessive resistance
3) random mating

Question 6

Gene-stacking and Gene-pyramiding

Answer 6

Gene-stacking: >1 Bt genes for controlling different target species.
– YieldGard Plus: Cry1Ab+Cry3Bb1 for corn borers and rootworms
Gene-pyramiding: >1 Bt genes for same target species
– Bollgard II: Cry1Ac and Cry2Ab2 for moth pests
requires lack of cross-resistance