Lecture 9: GM and insect resistance: Bacillus thuringiensis (Bt)

Question 1

B.t. =

Answer 1

Bacillus thuringiensis

Question 2

Bacillus thuringiensis is an

Answer 2

aerobic gram-positive spore forming bacterium

Question 3

Bacillus thuringiensis produces:

Answer 3

a range of insect toxins known as delta-endotoxins during sporulation

Question 4

Bacillus thuringiensis affects insects by

Answer 4

the crystals produces within the spore dissolves in the insect gut, they are then broken down by the insect digestive system to form the active toxin, and then disrupt the membranes of the gut causing death

Question 5

how is bacillus thuringiensis used in the field:

Answer 5

toxins are extracted fro bacteria & applied to fields as a spray

Question 6

Natural Bt crystals contain

Answer 6

over 21 different toxins

Question 7

delta-endotoxins how do they work:

Answer 7

enzyme in insect gut interacts and causes inactive toxin to be cleaved –> forming active toxin

Question 8

benefits of Bt toxins over chemical pesticides:

Answer 8

• Specificity
• -highly specific for insect pests
• very limited range of activities
• effective against Lepidoptera & Coleoptera (and some Diptera)
• Readily degraded in the environment
• well understood
• many conventional pesticides are highly toxic

Question 9

Conventional Bt toxin usage (non-GM):

Answer 9

The protein in free form has been applied to fields in massive amounts

Question 10

Bt targets:

Answer 10

highly effective against major pests including diamondback moth (Plutella xylostella), tobacco hornworm & Colorado beetle

Question 11

advantages to Bt toxin:

Answer 11

• No toxic effects against mammals
• Public assurances that it was ‘impossible’ to develop since banteria & host coexisted for millions of years (NONSENSE)

Question 12

disadvantages of Bt toxin:

Answer 12

• Nonsense regarding impossible chance of resistance as were providing a very sturgeon selective pressure (high exposure)
• rather too readily broken down (requires repeated spraying)
• toxin must come into contact with target organism

Question 13

Genetic modification of crops to express Bt toxin- ADVANTAGES

Answer 13

• no need to spray crops repeatedly so reduces insecticide application
• all parts of the plant can contain toxin (depends on promoter) - useful for protection against root pests
• highly effective

Question 14

Genetic modification of crops to express Bt toxin - DISADVANTAGES

Answer 14

• selection pressure for insect resistance is higher
• -toxin present all the time
• -originally only single Bt toxins used
• Bt gene may be transferred to other plants (may creat super-weeds)
• impact on non-target organisms potentially higher

Question 15

does it work? example of plant where GM plants expressing Bt toxin are protected against target insect pests

Answer 15

Chrysanthemum expressing Bt toxin is protected against beet armyworm larvae

Question 16

Bollgard in cotton ???

Answer 16

• Bt cotton commercialised by Monsanto as Bollgard
• Cotton is one of the most heavily sprayed crops grown in the US & insect protection is the highest variable cost associated with cotton growing

Question 17

Bollgard in cotton protects against

Answer 17

• Tobacco budworm (Heliothis virescens)

- Pink bollworm (Pectinophora gossypiella)

Question 18

how many toxins does Bollgard 1 contain?

Answer 18

a single Bt toxin (cry1A)

Question 19

how many toxins does Bollgard || contain?

Answer 19

2 Bt toxins (cry1A + cry2Ab) (STACKED)

Question 20

how many toxins does Bollgard || contain?

Answer 20

2 Bt toxins (cry1A + cry2Ab) (STACKED - to avoid resistance)

Question 21

New studies suggest use of Bt _____ yield

Answer 21

STABILISES

• Monsanto study
• Strip trials - triple stacked maize vs isogenic control
• field trials 2005-2009
• if the conventional variety ‘struggle’d the GM variety tended to do better
• GM Bt stabilises yield

Question 22

Does GM Bt reduce spray insecticide usage?

Answer 22

• 80 commercial fields growing in Arizona growing non-transgenic, Bt and Bt+herbicide resistant rice were studied
• there was no significant difference in yield between the crop types
• Significantly more insecticide was applied to non-transgenic crops in comparison with transgenic crops.
• CATTANEO ET AL (2006)

Question 23

Does the use of GM Bt plants alter biodiversity?

Answer 23

Comparison of ant and beetle density & Species richness between uncultivated fields and cultivated fields
-no add on effect altering biodiversity

Question 24

effect of planting Bt crops on non-Bt crop farmers?

Answer 24

• Planting of Bt crops can help non-Bt farmers, but can also not.
• example of not: Mirid Bug Outbreak Bt Cotton in China to neighbouring fields and thrived
• LU ET AL (2010)

Question 25

Problems with Bt: RESISTANCE

Answer 25

• commercial over-application of Bt sprays (NOT GM) has lead to field resistance in Diamondback moth (attacks brassicas)
• Resistance to Bt has been observed in lab & field
• Cross-resistance develops (resistance to one toxin makes organism resistant to other toxins –> Zhao et al (2005)

Question 26

Problems with Bt: EFFECTIVENESS

Answer 26

• Bollgard protection is not complete (not a ‘magic bullet’
• initial highly-publicised failure as selected cotton variety was susceptible to drought
• resistance increasing

Question 27

Problems with Bt: IMPACT ON NON-TARGET ORGANISMS

Answer 27

• highly-publicised impact impact of pollen from Bt maize on Monarch butterfly (now largely discounted)
• toxin persists in the environment in plant material

Question 28

Resistance in field isolates of Helicoverpa sea (corn ear worm) to Bt cotton expressing Cry1A

Answer 28

-No difference in the Lethal Concentration (50%) (LC50) between lab and field strains in 1992-1993.
-Significant differences in 2002- 2004.
BUT
• cross-resistance to cry2A limited
• controlled by insecticide use
• not responsible for any crop
failures– Tabashnik et al (2008)

Question 29

Does wind dispersed pollen (60m+) from Bt maize have a negative impact on non-target butterfly larvae conditions?

Answer 29

• yes & no
• part of the problem is that pollen from any plant (GM or not) has an effect on the larvae
• the effect varies between plant varieties

Question 30

plant material containing Bt can ___ in the environment

Answer 30

PERSIST

-get into water ways

Question 31

Why do so many studies not compare like-with-like regarding GM and Bt crops

Answer 31

• access to GM plants is restricted by contracts
• GM licensing takes a long time therefore non-GM varieties (planted on a large scale) are always newer than the GM varieties
• The debate is heated!

Question 32

solutions to problems of resistance?

Answer 32

• use as part of Integrated Pest Management Scheme
• Use multiple toxins (stacking) (e.g. Bollard 2)
• rotate crops
• plant mosaics
• Use refuges to maintain sensitive population

Question 33

problems with refuges to maintain sensitive population

Answer 33

• unpopular with farmers
• Biology uncertain (although seems to be working)
• compliance poor (but improving); in USA 29% of US farmers broke rules in 2000, in 2001 only 13%
• seed companies are meant to be the primary inspectors (clear conflict of interest)
• not effective in many countries particularly if farmers retain seed