Contact Information August 28, 2000 | vol.1 CBS News Covers Iowa Researcher's Study on Bt Corn Pollen and Monarch

By Butterfly Larvae and Cindy Lynn Richard
August 21, 2000 Council for Agricultural Science and Technology

News in perspective: Field trial results from 1999 and preliminary results from year 2000 do not support the laboratory studies that monarch or swallowtail butterfly larvae are threatened under natural field conditions, when exposed to Bt-containing corn pollen.

As the result of a letter by Cornell University researcher John Losey to the scientific journal Nature in Spring 1999, there was a good deal of media coverage of the potential effect of Bt (Bacillus thuringiensis) corn pollen on the beautiful Monarch butterfly. Despite nearly 20 years of prior consideration by the Environmental Protection Agency (EPA), the Agency called for information generated in the fields under real world conditions. Many field studies have been conducted by more than 8 separate groups of researchers in the USA and Canada. The studies, carried during the summer 1999 and 2000 planting seasons, evaluate the potential for exposure of the Monarch and Swallowtail Butterfly larvae to corn pollen containing Bt.

The most recent report by John Obrycki, professor of entomology at Iowa State University in Ames, and one of his graduate students published the results of a laboratory feeding study similar to the original laboratory feeding study by Losey. The Obrycki study is designed as a set of compounded, worst-case conditions bearing little resemblance to real world conditions. The unrealistic conditions include

Highest dose - Obrycki sat milkweed plants on the edge of corn fields during pollination, then brought the plants into the laboratory where larvae were placed on the leaves to feed and then given no alternative food source; and Highest potency strain - Obrycki used a strain of Bt corn pollen for which the monarch larvae are extremely sensitive (40 to 50 times more sensitive). This strain makes up only 2% of the total Bt-corn acres planted.

There is some positive news for the Monarch butterfly from the Obrycki study. Those larvae that survive even high doses of Bt-containing pollen go on to develop to maturity and reproduce normally. Additionally, current field studies are looking to see if butterflies are laying their eggs at the same time that corn plants are naturally pollinating. Even in areas where there is an overlap in time when migrating butterflies lay their eggs and the normal two-week pollination period in corn fields, field studies show that the level of pollen outside the corn fields drops off rapidly from the edge of the fields. Given a choice, butterfly larvae will not eat pollen with or without Bt.

Background information: Bt corn is designed to fend off the European corn borer. The European corn borer is far more sensitive to Bt from transgenic corn than the monarch or swallowtail butterflies. Bt corn is a strain of corn, which is protected from infestation by the European corn borer through the insertion of a gene that allows the plant to make Bt (Bacillus thuringiensis). Bt naturally occurs in soil and is sprayed frequently by organic and conventional farmers to control pests such as the European corn borer.

The Environmental Protection Agency had evaluated the possibility that butterflies and other non-target organisms could come into contact with Bt-containing pollen. EPA completed this evaluation prior to allowing the Bt-containing plants to be grown on a large scale and before the Bt corn could be marketed. In a July 2000 ruling by the Washington D.C. Court of Appeals, the EPA was found to be prudent in its evaluation of Bt-containing crops.

As predicted, laboratory studies such as those conducted by Losey and Obrycki, demonstrate that the monarch butterfly larvae prefer leaves with no pollen, but when given no other food the larvae will eat the pollen. Using laboratory studies, Obrycki and Sears have confirmed that as the dose or amount of Bt pollen eaten by the larvae increase, more of the larvae die. Those larvae that survive even high doses of Bt-containing pollen go on to develop to maturity and reproduce normally. Preliminary results indicate that the No Observed Adverse Effect Level (NOAEL) for monarch larvae exposure to Event 176 pollen is on the order 100 grains per sq cm. Note that the monarch larvae are 40 to 50 times more sensitive to this particular stain of Bt pollen then other strains and that this strain represents only 2% of the Bt corn acres planted. The Lowest Observed Adverse Effect Level (LOAEL) for strains of Bt other than Event 176 is 135 grains/sq cm.

Researchers at several universities in the USA and Canada are completing a second summer of field research to assess the impact of Bt corn and Bt corn pollen on butterflies and other non-target organisms (butterflies and moths other than the corn borer). Actual field studies show the following: corn pollen concentration drops from 80-100 grains/sq cm at fields edge down to about 1.4 grains per sq cm between 1 and 5 meters from the edge of the field; in general, 90% of the pollen collected in traps was found within 5 meters of the edge of the field; therefore, levels of corn pollen in the field at pollination are in the range of the lowest effect levels. Levels of pollen outside the field are well below those likely to effect the monarch or swallowtail butterflies or their offspring.

Environmental Protection Agency to Petition for rulemaking and collateral relief concerning the registration and use of genetically engineered plants expressing Bacillus thuringiensis endotoxins," is available on the EPA website at [A short summary of the EPA document is posted on the University of California-Davis website. This document is available at]

Sears, M.K., H.R. Mattila and D.E. Stanley-Horn. 2000. "Preliminary report on the ecological impact of B.t. corn pollen on populations of non-target lepidoptera, including the monarch butterfly, in Ontario. Can." Food Inspect. Agency, Plant Biotech. Office [News story on Sears research]

Hansen, LC and Obrycki, JJ. 2000. "Field deposition of Bt transgenic corn pollen: lethal effects on the monarch butterfly" Department of Entomology, Iowa State University, Ames IA 50011.USA

Hellmich, R. August 18, 2000. Personal communication.

Losey, J.E., Rayor, L.S., and Carter, M.E. 1999. Transgenic pollen harms monarch larvae. Nature 399: 214.

For more information, contact Cindy Lynn Richard, CAST Biotechnology Communications Coordinator, 202-408-5383,

Updated August 22, 2000 2:00 p.m. CDT

This document is available on the CAST web site at

Return to the home page of Better Foods