Plants & Human Affairs

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Plants & Human Affairs (BIOL106)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe

Case Study:  Monarchs & Transgenic Corn

 
Background

    Bt corn refers to corn varieties that have been genetically engineered with a Bt gene (called Cry). A Bt corn plant will produce Bt toxin within its cells. Insects that eat the tissues of the corn plant, for example a European corn borer feasting on leaves or a corn rootworm munching on roots, will eat the toxin and die. Different varieties of Bt corn have different properties because they contain different proteins. Some varieties have been engineered to manufacture the toxic Cry protein in all cells of the plant all season long. At least one variety does not make the Cry protein in the corn kernels - in this variety the amount that was made in other cells of the plant gradually decreased toward the end of the summer.   

Case Study. (adapted in large part from the Transgenic Crops web site hosted by Colorado State University.  This is an excellent site.)   
    The following headline appeared in the scientific journal Nature on May 20, 1999, �Transgenic pollen harms monarch larvae.�  In a brief article, the authors described their results from experiments in which larvae of Monarch butterflies were fed milkweed leaves dusted with pollen from Bt corn plants. Over the course of four days, 44% of the caterpillars that ate Bt corn pollen died. The scientists fed another set of caterpillars milkweed leaves dusted with pollen from conventional corn, and fed a third set of caterpillars no pollen, only milkweed leaves, which are the normal diet for Monarch caterpillars. There were no deaths among the caterpillars that were fed conventional pollen or no pollen.  Other data related to this experiment appear in Tables 1-5.  Based on your knowledge of Bt and the data that are provided, answer the following questions about this experiment.

Questions

  1. Do you expect all Bt corn pollen to kill Monarch butterfly larvae? (i.e., is there just one kind of Bt corn, or different kinds?)

  2. Do you expect Bt corn pollen to kill other insects in addition to Monarch butterfly larvae?

  3. Do you expect parts of a Bt corn plant other than the pollen to kill insects?

  4. Is Bt likely to be deposited on the food sources of Monarch butterfly larvae in areas where corn is grown?

  5. Do Monarch butterflies live in areas where corn is grown?

  6. Does the larval stage of the Monarch butterfly live in areas where corn is grown?

  7. Are Monarch larvae likely to be harmed by the Bt pollen that falls on their food  source?

  8. Are there any laboratory tests of toxicity of Bt corn pollen?

  9. Are there any field tests of toxicity of Bt corn pollen?

  10. Would Monarch�s be safe if a new variety of Bt corn is introduced that produces 7 micrograms of Bt toxin per gram of pollen?

  11. What do you expect if an existing variety of Bt corn is planted in a very windy area?

 

Table 1:  Amount & Distribution of Bt toxin in maize

Bt gene

Promoter element(s)

amount of Bt protein in pollen (micrograms per gram of pollen)

Bt 176

1) A promoter derived from a pollen-specific protein kinase in maize, linked to the Bt CryIA(b) gene. This promoter causes the Bt protein to be produced in the pollen.
2) A promoter derived from phosphoenolpyruvate carboxylase in maize, linked to the Bt CryIA(b) gene. This promoter causes the Bt protein to be produced in green tissues such as stems and leaves. There is little or no Bt protein produced in the kernels.

1.1 to 7.1

Bt 11

The promoter from the cauliflower mosaic virus 35S gene, linked to the Bt CryIA(b) gene. This promoter causes the Bt protein to be produced in all tissues of the plant.

0.09

MON810

The promoter from the cauliflower mosaic virus 35S gene, linked to the Bt CryIA(b) gene. This promoter causes the Bt protein to be produced in all tissues of the plant.

0.09

   

Table 2.  Toxicity of Bt pollen

Bt transgene

butterfly species

LD50

(grains /cm2)

reduced feeding

(grains /cm2)

no effect*

Bt 176

Monarch

389

 

133

 

 

Bt 176

Monarch

67

 

23

 

 

Bt 176

Monarch

 

5 to 10

 

 

Bt 176

Black swallowtail

613

 

 

 

Bt 11

Monarch

 

 

> 1,600

 

MON810

Monarch

 

 

> 1,600

 

DBT418 (no longer available)

Monarch

 

 

> 1,600

 

CBH351 (StarLink, no longer available)

Monarch

 

 

1,001 to 1,600

 

   

Table 3. Amounts of pollen deposited within a corn field and at various distances away from the field.

distance

pollen (grains/cm2)

pollen

(grains/inch2)

source

in the field

0 to 506

0 to 3,264

Hansen-Jesse and Obrycki, 2000

0 to 1,600

0 to 10,320

Pleasants et al., 2001

65 to 425

419 to 2,741

Sears et al., 2001

at the edge of the field

0 to 1,100

0 to 7,095

Pleasants et al., 2001

158 to 266

1,019 to 1,716

Sears and Stanley-Horn, 2000

0.2 meters away

0 to 427

0 to 2,754

Hansen-Jesse and Obrycki, 2000

0.5 meters away

260

1,677

Zangerl et al., 2001

1 meter away

0 to 222

0 to 1,432

Hansen-Jesse and Obrycki, 2000

0 to 1,300

0 to 8,385

Pleasants et al., 2001

170

1,097

Zangerl et al., 2001

2 meters away

0 to 400

0 to 361

Pleasants et al., 2001

192

1,238

Zangerl et al., 2001

3 meters away

0 to 56

0 to 361

Hansen-Jesse and Obrycki, 2000

5 meters away

0 to 11

0 to 71

Hansen-Jesse and Obrycki, 2000

0 to 200

0 to 1,290

Pleasants et al., 2001

34 to 175

219 to 1,129

Sears and Stanley-Horn, 2000

10 meters away

0 to 4

0 to 25

Hansen-Jesse and Obrycki, 2000

  

Table 4. Eggs laid on milkweeds in corn fields versus nonagricultural areas and contribution to Monarch population of corn fields versus nonagricultural areas. (Oberhauser et al., 2001)

habitats

eggs/milkweed plant

proportion of breeding habitat

contribution of each habitat to the total Monarch population

Minnesota/Wisconsin
    corn
    nonagricultural

 

0.0612

0.124

73

0.0134

0.0011

1

Ontario
    corn
    nonagricultural

 

0.0125

0.302

0.055

0.0133

0.045

1

Iowa
    corn
    nonagricultural

 

0.190

0.436

45

0.114

0.0023

1

  

Table 5. Period of pollen shed in corn fields. (Oberhauser et al., 2001)

 

Site name

Dates of pollen release

Minnesota/Wisconsin

Farm

July 31-August 7

Amery

July 27-August 3

AG

July 25-August 1

Rosemount

July 21-28

Cedar Creek

July 10-26

Iowa

595/280

July 7-13

Shipley S

July 7-13

LincolnWay

July 11-17

Coopers

July 5-19

Ankeny

July 6-12

Maryland

Wye Farm

July 19-31

Wye Island

July 6-17

Gannon

July 7-19

Airport

July 27-August 9

Beaverdam

July 6-18

Ontario, Canada

Fergus

August 7-14

New Hamburg

August 1-15

Drayton

August 10-17

Linwood

August 9-16

Palmerston

August 4-18

 

 Reference:
    
This case study is adapted in large part from �Agricultural Biotechnology and  the Monarch Butterfly:  Are Monarchs Threatened by Bt Corn?� which is available from the Transgenic Crops: An Introduction and Resource Guide web site at Colorado State University

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Last updated:  03/18/2005   � Copyright  by SG Saupe