New Iowa State Study on Effects of Bt Corn Pollen on Monarch Larvae

[Paul Cherubini]

The following abstract of an Iowa State Study was copied from the
website of the North Central Branch of the Entomological Society of
America. It appears to be a better study than the Cornell one as it
measured pollen deposited on milkweed growing within a corn field and up
to 3 meters away from the edge of the corn field under real world
conditions. Exactly how the researchers experimentally manipulated the
larvae and arrived at their 19% mortality figure (including 3% mortality
in a no pollen control group) is unclear at this point.

May 25, 1999 - Data from Iowa State University Department of Entomology
shows a relationship between the mortality of first instars (larvae) of
the monarch butterfly (Lepidoptera: Danaidae) under field conditions
with Bt corn.

Non-target effects of Bt corn pollen on the Monarch butterfly
(Lepidoptera: Danaidae) 

L. Hansen, Iowa State University, Ames , IA 50011 and J. Obrycki, Iowa
State University, Ames, IA 50011 Contact e-mail: lrahnsen at iastate.edu 

The expression and dispersal of Bt toxin in the pollen from transgenic
crop plants may pose a risk to non-target Lepidopterans. When Bt corn
pollen lands on the leaves of plants in and around corn fields, it
exposes non-target Lepidopteran larvae feeding on these plants to Bt
toxins.

In Iowa, where the landscape is dominated by row-crop agriculture, areas
effected could include a significant portion of non-cultivated areas,
including remnant prairies, roadside ditches and wetlands. The monarch,
Danaus plexippus (Lepidoptera: Danaidae) was chosen as the non-target
Lepidopteran species for this study because it is a common butterfly
species that feeds on several species of milkweed, Asclepias. Asclepias
syriaca (Asclepiadaceae) commonly grows in road ditches, grassy areas
between fields, and within corn fields, so it is likely to have Bt corn
pollen deposited upon it.

The first step of this study was to determine the amount of corn pollen
deposited on A. syriaca leaves within and adjacent to a Bt corn field at
0 m, 1m, and 3m.

The highest levels of pollen deposition was found on plants within the
cornfield, and lowest levels found at three meters from the edge of the
corn field. 

Leaf samples taken from within and at the edge of the corn field were
used to assess mortality of first instar monarch, D. plexippus exposed
Bt and non-Bt corn pollen. 

Within 48 hours, there was 19% mortality in the Bt corn pollen treatment
compared to 0% on non-Bt corn pollen exposed plants and 3% in the no
pollen controls. 

MONSANTO May 20 Press Release: 

The exposure of milkweed to corn pollen is very low because only a very
small portion of milkweed grows in close enough proximity to corn fields
for exposure to corn pollen. This information supports the conclusion of
a very low likelihood of effect of Bt pollen on non-target insects like
the Monarch butterfly in their natural habitats.

PIONEER HI-BRED Seed CompanyMay 24 Web Site Statement:

We have no proof that pollen containing Bt has a negative effect on
monarch populations in a natural environment. A major question is
whether or not monarch larvae eat pollen on milkweed plants in the
natural environment.  Contact between Bt pollen and milkweed depends on
where the milkweed is in relation to the corn field. Wind and rain can
obviously impact pollen on milkweed.