Plants & Human Affairs

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:

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?)
Do you expect Bt corn pollen to kill other insects in addition to Monarch butterfly larvae?
Do you expect parts of a Bt corn plant other than the pollen to kill insects?
Is Bt likely to be deposited on the food sources of Monarch butterfly larvae in areas where corn is grown?
Do Monarch butterflies live in areas where corn is grown?
Does the larval stage of the Monarch butterfly live in areas where corn is grown?
Are Monarch larvae likely to be harmed by the Bt pollen that falls on their food source?
Are there any laboratory tests of toxicity of Bt corn pollen?
Are there any field tests of toxicity of Bt corn pollen?
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?
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
at the edge of the field	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
2 meters away	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.