RESEARCH NEWS
Updates on Bt Corn Research
Ken Hough, Director, Research and Market Development
Bt Corn and Insecticide Use
Results from a recent Iowa State University study (conducted by Dr. Marlin Rice and graduate student Clinton Pilcher)
show that farmers have significantly reduced their use of insecticide for control of European corn borer through
their use of Bt corn.
Of the 2,000 farmers surveyed from Iowa, Nebraska, Minnesota, Kansas, Illinois and Pennsylvania who grew Bt corn,
26 per cent decreased their insecticide use in field corn in 1999. Of the others, 13 per cent said their insecticide
use stayed the same two per cent reported increased insecticide use (54 per cent indicated they do not use insecticide).
By comparison, similar surveys conducted in 1996 and 1997 showed that farmers growing Bt corn reported decreasing
their pesticide use in corn by 13% and 19% respectively. Over this time period, acreage of Bt corn in the mid-west
has grown to 22 million acres of the total 75 million acres planted to corn.
In the Iowa State survey, 82 per cent of the growers cited prevention of yield loss to corn borer as their primary
reason for planting Bt corn; 27 per cent said they chose Bt corn to eliminate their insecticide use for this pest.
The survey results also indicate 41 per cent of the growers said corn borer caused more yield loss than they had
previously thought, compared to 29 per cent and 12 per cent, respectively, who said yield loss from corn borer
was about as expected or less than expected. Comparing yields of Bt corn with similar maturity non-Bt hybrids,
45 per cent of the respondents observed higher yields with Bt hybrids, 41 per cent had similar yields and seven
per cent reported lower yields for the Bt corn.
Almost 85 per cent of the Iowa State survey participants responded “yes” when asked if they would follow a recommended
resistance management plan to prevent corn borer from becoming resistant to the Bt gene in Bt corn (which is the
same ‘active ingredient’ as found in the foliar Bt insecticide, used widely by organic and conventional farmers).
Results from a similar Ontario survey conducted in the summer of 1999, showed that 86.5 per cent of Ontario growers
were complying with the recommended practice of planting no more than four-fifths of their corn acreage to Bt hybrids.
(The Ontario results also showed that 75 per cent of the farmers not complying with this refuge strategy had less
than 75 acres of corn.)
Bt Corn and Monarch Butterflies
Last spring, Dr. John Losey, a Cornell researcher, reported on a laboratory study in which about 40 per cent of
Monarch butterfly caterpillars fed on milkweed leaves dusted with Bt pollen died. He also reported that caterpillars
fed on milkweed dusted with pollen from non-Bt corn did not gain weight or develop as well as caterpillars feeding
on milkweed leaves with no corn pollen, but the mortality rate with non-Bt pollen was zero. Losey suggested the
results might indicate there could be some risk to Monarch populations, but that much more research was needed
to confirm the real effects of Bt corn in the rural environment. Several field and laboratory studies were undertaken
in 1999 by a number of researchers at USDA and Universities (Cornell, Guelph, Maryland, Kansas, Michigan State,
Nebraska, Penn State, Illinois, Iowa, Iowa State, Stanford) to clarify the effects of Bt corn on Monarch butterflies
in ‘real world’ situations. The preliminary results, reported at a November 2 conference held in Chicago, are briefly
summarized below.
- Results from laboratory studies at the University of Guelph (Dr. Mark Sears)
showed that concentrations of 500 to 700 grains of Bt pollen (Bt event 176) per square centimeter (cm2) were required
before the youngest, most sensitive Monarch caterpillars started to die. Sears believes that 100 grains/cm2 of
Bt 176 pollen would be the ‘no effect’ level, based on these preliminary observations. Sears estimates pollen from
Bt event 176 is about 10 to 50 times more toxic than the most widely used Bt event, Mon810 (YieldGard). Similar
laboratory studies at other institutions corroborate these results.
- The concentration of corn pollen drops by about 90 per cent within a few (three-
to five) meters from the edge of corn fields. Corn pollen is relatively heavy, so most is deposited very near to
the corn plant (levels of 78- to 229 grains/cm2 observed within corn fields). Levels drop substantially within
three metres from the field edge (17- to 60 grains/cm2),and beyond (1.4- to 2.4 grains/cm2 at five- to 10 metres
from the field edge). Although winds can move corn pollen away from the corn field, it is dispersed over a wide
area so the concentration of pollen on leaf surfaces is low (well below the threshold level which could harm Monarch
or other beneficial caterpillars).
- Several researchers, including Cornell’s Losey, found that Monarch caterpillars
preferentially fed on pollen-free milkweed leaves, compared with milkweed leaves dusted with corn pollen (Bt or
non-Bt), where both were available to the larvae. Caterpillars feeding on milkweed foliage treated with corn pollen
(either Bt or non-Bt) began eating later and ate less than larvae on pollen-free leaves.
- Surveys of milkweed showed that patches along roadsides were more dense than
within corn fields where farmers use weed management to control milkweed and other weeds. Milkweeds within the
tall corn canopy may be less accessible or attractive to Monarchs than milkweed populations in more open areas
widely dispersed along roadsides.
- Field studies indicate that pollen does not adhere easily to milkweed leaves,
and that wind and rainfall tend to dislodge up to 90 per cent of the pollen from the leaves. It has been suggested
that the Bt in pollen is degraded by exposure to sunlight. This will be assessed further in ongoing studies.
These preliminary results of field observations and the additional laboratory
studies provide assurance that wild populations of Monarch butterflies are at little risk from the widespread use
of Bt corn. An assessment of many other moth and butterfly species showed minimal overlap of the insect habitats
with Bt corn production.
These studies will continue. Researchers need to confirm the validity of the issues noted in this article, and
look into several more. These include more thorough investigations of such aspects as:
- Monarch egg laying behavior in corn and non-corn environments.
- Duration of pollen viability and decay of Bt activity in the external environment.
- Relative proximity and geographic distribution of butterfly (milkweed) and corn
habitats.
- Impact of reduced insecticide use in Bt corn (see related item elsewhere in this
article) on populations of Monarch and other butterfly/moth species.
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