Entomologists, nature lovers and the agricultural biotech industry have recently focused their attention on the monarch butterfly (Danaus plexippus), one of the most recognized butterflies in North America.

Heightened interest followed a highly publicized study in the May 20, 1999, issue of Nature that reported on the possible effects of Bt corn pollen on monarch butterfly larvae. In laboratory feeding experiments, Cornell University researchers found that monarch butterfly larvae fed milkweed leaves artificially coated with pollen from Bt corn ate less, grew slower and suffered a higher death rate than larvae that consumed milkweed leaves free of corn pollen.

By design, the researchers did not mimic natural conditions. In the laboratory, the caterpillars were given no choice but to feed on leaves artificially covered with corn pollen. The Cornell study raises interesting questions, but further research is needed to determine what impact, if any, Bt corn pollen has on monarchs in their natural, free environment.

The decline of monarch butterfly populations has been a concern for decades. Monarchs are threatened by a variety of factors, including loss of habitat in overwintering sites and summer breeding grounds, as well as unfortunate encounters with moving vehicles on our nation's roadways. The main causes of premature death and population loss, according to Dr. Orley "Chip" Taylor, director of Monarch Watch and chair of the Department of Entomology at the University of Kansas, fall into these categories:

• Loss of Habitat
  - Overwintering sites. Logging of forest trees that provide a winter home for monarchs.
  - Summer migration grounds.
  Urbanization and weed management to control milkweeds and other weeds that compete with crop production and are noxious to grazing livestock.
• Use of broad-spectrum insecticides.
  Monarch larvae developing on milkweeds within agricultural areas may be killed by application of conventional insecticides targeting crop pests.
• Predation. Examples are birds, rodents, spiders and other invertebrates that prey on the butterfly. In addition, many parasites and pathogens are known to attack monarchs.
• Physiological death. Possible causes are depleted fat bodies, infections and worn, broken wings.
• Catastrophic mortality. Includes episodes such as drought, snowstorms and sub-freezing temperatures.

The oyamel trees, on which monarchs cluster, are valuable lumber sources for the local residents, the ejidatarios. Deforestation is a serious concern to monarch enthusiasts because of the high butterfly concentrations in just a few sites. Only 11 to 14 wintering sites have been identified in the transvolcanic mountains of Mexico, and each is just a few hectares in size and contains millions of monarchs. With logging, not only are trees lost for roosting, but their removal also opens up the forest canopy, exposing the butterflies to rain, snow and freezing temperatures.

Five sanctuaries were established in 1985 to protect the monarch habitat, yet pressures on all sites continue. Ecotourism has not been as lucrative as lumbering, and logging continues today even in protected areas. Conservationists remain committed to protect and hopefully increase land so critical to monarch survival. They are working to raise funds to lease the oyamel trees and provide the ejidatarios with financial compensation. Ideally, they would like the Mexican government to buy the land outright from the landowners and set it aside as permanent monarch sanctuary.

The monarch's magnificent migration

Orange and black monarch butterflies are divided into two populations in the United States - those living east of the Rocky Mountains and those west of the Rockies. Monarchs east of the Rockies overwinter from November to March in the fir forests of the Sierra Madre mountains near Mexico City. In the summer, three to five generations successively migrate up to 3,000 miles to the northern United States and southern Ontario. Amazingly, the butterflies that return to Mexico in the fall are typically the great-great-grandchildren of those that left in the spring.

It is the eastern group that interests scientists studying the possible effects of Bt pollen on monarch larvae. On their migratory route, the butterflies breed in the Corn Belt, which includes states such as Nebraska, Iowa, Illinois and Minnesota, where the majority of Bt corn is grown.

Monarch life cycle

Monarchs lay their eggs exclusively on plants in the milkweed family (Asclepiadaceae), usually on the underside of a leaf. The larvae eat the plants not only for nutrition, but also for toxins in the sap - cardenolides - which provide a chemical defense against many predators. There are more than 106 milkweed species in North America, but the common milkweed (Asclepias syriaca) is the predominant host for monarchs in the Midwest.

As they grow, monarchs undergo complete metamorphosis with four distinct stages: egg, larva (caterpillar), pupa (chrysalis) and adult. Monarchs pass through these stages in about a month. The adults can continue to live another two to six weeks in summer and those that migrate may live all winter, or a total of seven to nine months.

Bt Basics

Bt corn has built-in insect protection that comes from a naturally occurring microorganism called Bacillus thuringiensis or "Bt." For four decades, organic farmers have sprayed the bacterium on plants as an alternative to conventional insecticides.

Bt corn produces a protein that binds to specific receptors in the midgut of sensitive insects, but does not affect other insects, birds or animals in that way because they lack those receptors. The corn has been proven to have no effect on agriculturally beneficial insects including honeybees, ladybird beetles and green lacewings, as well as a variety of other predatory insects and spiders.

Bt corn helps protect the environment because its built-in insect protection eliminates the need to apply conventional insecticides for corn borer protection. According to a recent study led by Marlin Rice, professor of entomology at Iowa State University, Midwest growers who planted Bt corn have reduced insecticide use each year from 1996 to 1998 (See Figure 4). In this way, Bt corn helps enhance beneficial insect populations that otherwise would be threatened by the use of pesticidal sprays.

Given the many threats already encountered by monarchs, the Cornell study has focused the attention of environmental groups on the regulatory approval process for Bt corn. They wonder whether the EPA fully considered the potential harm of Bt corn pollen on non-target species like the monarch butterfly. In June 1999, the Environmental Defense Fund petitioned the EPA to limit the sale of Bt corn and require that growers plant borders around Bt cornfields to limit potential exposure of monarchs to Bt corn pollen. The EPA is carefully reviewing the petition and will take appropriate steps, if indicated, to avoid adverse environmental impact. In the meantime, the EPA is explaining the process they used to establish the safety of insect-protected crops in the environment.

EPA evaluates toxicity and exposure

The EPA regulates environmental impact of insect-protected crops to make sure use won't cause unreasonable adverse effects on the environment, beneficial insects and other life. Before approving Bt corn in 1995, the EPA concluded that the corn does not present any "unreasonable adverse effects" to butterflies.

Dr. Janet Andersen, director of the EPA's Biopesticides and Pollution Prevention Division (BPPD), explained that to reach this conclusion, the EPA evaluated both toxicity and exposure, which together determine risk. Toxicity is the quality of being hazardous; exposure is the condition of being subjected to a hazard.

The EPA, Andersen said, assumed that Bt corn is toxic to non-target Lepidoptera, a group of insects that includes butterflies and moths. However, the EPA determined that exposure of the monarch larvae to Bt pollen would be limited. This conclusion was based on the fact that the majority of pollen moves only a short distance away from cornfields and that exposure of monarchs would be limited only to larvae developing on milkweeds within the cornfield or very near to cornfields during pollen shed. Since only a portion of the milkweed population is likely to be exposed to Bt pollen and only a portion of those plants would be expected to harbor monarch larvae, the EPA scientists concluded that Bt corn does not present any "unreasonable adverse effects" to butterflies.

The Cornell laboratory study demonstrated that under conditions of high exposure, consumption of large amounts of Bt pollen is hazardous to small monarch larvae. But to what extent does this scenario occur in the natural environment, under typical field conditions? In response to the Nature article, leading researchers, entomologists and weed scientists headed to cornfields in the summer of 1999 in an effort to answer the following question: Does Bt corn pollen move away from Bt cornfields in sufficient quantities to adversely affect the monarch butterfly in nature?

The researchers presented their findings on Nov. 2, 1999, at the Monarch Butterfly Research Symposium in Chicago. The symposium was sponsored by the Agricultural Biotechnology Stewardship (ABS) Working Group, which counts among its members biotech seed companies and associations. Officials from the U.S. Department of Agriculture (USDA) and EPA also participated.

The research presented at the symposium was conducted by multiple researchers in various geographic locations and was done independently of one another. It is significant that multiple researchers achieved comparable results and reached similar conclusions on important questions. The studies show that the concentration of Bt pollen adhering to milkweeds within just a few meters of cornfields is typically too low to cause mortality of even small monarch caterpillars that might be present during pollen shed.

Pollen movement - How is pollen deposited around cornfields? How much pollen reaches milkweed leaves?

Necessary pollen levels to affect larvae - How much Bt pollen must be eaten to have an effect on monarch larvae?

Milkweed distribution - Where are milkweed plants located in and around cornfields?

Weather impact - Does exposure of Bt corn pollen to sun, wind and rain reduce the biological activity of Bt pollen grains? Does exposure reduce retention on milkweed leaf surfaces?

Some 20 scientists in the United States and Canada researched these questions this summer and presented their preliminary findings at the monarch butterfly symposium. Here’s what they found.

Monarch biology

Monarch migration and Bt pollen shed may not coincide

Scientists acknowledge that exposure to Bt pollen varies from region to region. Dr. Galen Dively, pest management specialist and professor of entomology at the University of Maryland, monitored Maryland cornfields and monarch development in 1999. He found that corn plants had completed pollen shed before monarch caterpillars were first observed feeding on milkweed.

Monarchs prefer milkweed away from corn

Tests by Dr. John Losey, Cornell University assistant professor of entomology, show that monarchs avoid laying eggs on milkweed surrounded by corn. Losey placed potted milkweeds by themselves or surrounded by corn plants and found that monarch adults preferred to lay eggs on milkweeds in the open. If this is typical behavior for monarchs in natural environments, then larvae would be less common on milkweeds very close to cornfields.

When given a choice between feeding on milkweed leaves with no pollen or high levels of Bt pollen, the majority of monarch larvae chose no pollen, reported researchers from USDA-Agricultural Research Service (ARS) and Iowa State University.

Pollen movement

Corn pollen doesn’t travel far

Due to its large particle size (90 to 100 microns), most corn pollen deposits stay within the cornfield, said Dr. Stuart Weiss of Stanford University. He noted that at wind speeds of 5 meters per second (11 miles per hour) or less, most pollen grains fall close to the field edge.

The volume of pollen falls off sharply just a few feet away from cornfields, reported Dr. Mark Sears, chair of the University of Guelph's Department of Environmental Biology. Using pollen traps set on poles about the height of milkweed, Sears and his colleagues determined that 90 percent of pollen grains traveled less than 5 meters (16.5 feet) from the field edge. Because pollen density dropped off rapidly a short distance from the field edge, "The risk to monarch larvae is very minimal," he said.

Likewise, Dively also set out to measure how much pollen lands on milkweed leaves in and around Maryland cornfields. He, too, found the pollen drops off rapidly beyond the field edge. He reported that pollen levels on milkweed within the field at the end of full pollination run between 200 and 250 pollen grains per square centimeter. Going outside the field, within the first 3 meters (10 feet), the pollen level drops 90 percent – to 20 to 30 grains per square centimeter.

Milkweed leaves capture only 30 percent of available pollen, reported Dr. John Pleasants, professor in the Department of Zoology and Genetics at Iowa State University. Wind speed and direction also affect pollen deposition, as illustrated in Figure 6, which shows an Iowa field that experienced prevailing winds from the south. Like his colleagues, Pleasants found the pollen levels decrease rapidly going away from the field edge and that uncommonly "high" pollen densities (150 grains per square centimeter) were not found on milkweeds just 2 meters (6.6 feet) from a Bt cornfield (See Figure 7).

Typical pollen levels had little to no effect

Different types of Bt corn pollen seem to have varying degrees of toxicity, Iowa State University and USDA researchers reported. Feeding trials with monarch larvae and two types of Bt pollen - YieldGard (MON810) and StarLink (CBH351) - indicated that leaves with high pollen densities of 150 grains per square centimeter had no effect on larval weight or survivorship. Pleasants and Dr. Richard Hellmich showed that pollen densities dropped to harmless levels beyond 1 meter (3 feet) from the edge of the field. Thus, the potential effects on monarch caterpillars that may be present very near to pollinating corn would be limited. However, KnockOut and NatureGard varieties (Event 176), which represents just 2.5 percent of Bt corn, was more toxic, with higher mortality rates and lower weight gains.

These findings are no surprise since the original Nature article already demonstrated toxicity in lab conditions when caterpillars consumed large amounts of the most toxic Bt protein. Again, the question at hand is how toxicity and exposure work together to determine risk in a real-world situation.

Milkweed distribution

Milkweed density highest in roadsides

Dr. Doug Buhler, research agronomist at the USDA-ARS, and Dr. Robert Hartzler, associate professor and Extension weed scientist in the Agronomy Department at Iowa State University, found that Iowa roadsides had 48 common milkweed patches per hectare, while corn and soybean fields had only seven patches per hectare. They concluded that common milkweed in roadsides may be especially important to monarchs due to the frequent occurrence of milkweed in roadsides and the uniform distribution of roadsides across the landscape. In another survey, Hartzler and Buhler also found that the majority of milkweeds along roadsides were distant from field edges. Milkweed density was approximately twice as high adjacent to the road (186 patches per hectare) and in the center of the roadside (165 patches per hectare) as it was next to the field edge (89 patches per hectare).

Few milkweeds were found very close to cornfields

Researchers from several universities and industry surveyed the occurrence of milkweeds along roadsides in Iowa, Nebraska, Illinois, Indiana and Maryland. Dr. David Isenhour, entomologist for Monsanto Company, reported that survey results showed from 15 to 41 percent of roadside milkweeds were adjacent to cornfields. In addition, the Iowa surveyors found only 19 percent of roadside milkweeds occurred within 1 to 3 meters of the field edge. Considering both proximity and location, less than 8 percent of roadside milkweeds were found to be exposed to meaningful amounts of pollen.

Future Research
Allowing science to lead decisions

These latest findings provide reassurance that the beloved monarch butterfly is not likely to be imperiled by Bt corn. While the data do not eliminate the possibility that some monarch caterpillars could be impacted if they consume milkweeds with extremely high levels of Bt pollen, there is general agreement among researchers that the likelihood of exposure is small, and the impact on monarchs generally will be relatively low.

Those most concerned about potential impacts of Bt corn pollen on monarch populations are focusing attention to potential in-field effects rather than effects outside of the cornfield. They are concerned that any effect on monarch survival inside the cornfield during the short pollen shed interval could have a lasting impact on the monarch population. However, there is no data to support this position, which ignores the comparative in-field effect of broad-spectrum insecticide sprays. On the contrary, it would seem from early monarch census information that monarch numbers may have increased by about 40 percent in 1999 over 1998 (See Figure 2), while Bt corn plantings increased by about 30 percent. "If Bt corn pollen affects monarchs, these data indicate that any impacts are likely to be small and not threatening to monarch survival," said Dr. Val Giddings of BIO.

While some scientists continue to question the environmental safety of Bt corn, others are eager to gather more data. "More data will likely help to allay the concerns of some Bt corn critics, and help to establish the environmental safety and benefits of Bt crops in general," said Giddings. Researchers already are looking ahead to the summer of 2000, when monarchs will leave the overwintering grounds in Mexico and return to America's Corn Belt. Although much of the needed data has been gathered, scientists and butterfly enthusiasts are eager to explore other research interests (See "The Legacy of Science" below) to answer additional questions:

• Why do monarch populations differ from year to year?
• Where are the milkweeds that are most important for monarch development?
• Where in the milkweed habitat is monarch density highest and lowest? What is the potential for exposure of Bt pollen to other species?
• Could Bt corn help reduce butterfly mortality in cornfields by reducing use of insecticides?

For scientists, the answer to one question always leads to more questions. Important data were collected and reported at the Chicago Monarch Butterfly Research Symposium. While these data help to alleviate concerns that monarchs are threatened by Bt corn, scientists are interested in exploring other avenues of interest. A summary of these interests follow:

1 Monarch butterfly biology

• Estimate the size of the annual migrating population.
• Measure mortality rates during migration.
• Determine reproductive values throughout the year.
• Quantify butterfly production in corn, soybeans and other habitats.
• Determine the number of eggs laid and larvae present within and nearby cornfields and other habitats.
• Determine the impact of milkweed patch density on attractiveness to adult butterflies.
• Examine adult egg laying behavior in field habitats.

• Measure sensitivity of various larval stages to varying Bt pollen concentrations.
• Examine fitness through the entire life cycle following exposure to Bt pollen at different life stages.
• Determine the impact on fitness following partial exposure to sublethal amounts of Bt pollen.
• Determine the potential for recovery of monarch larvae following sublethal exposure.

• Examine the relationship between pollen deposition and leaf position, plant height and surrounding habitat.
• Determine the duration of pollen viability and decay of Bt biological activity.
• Examine influence of the extent of leaf pilosity (hairiness) on pollen retention.
• Determine the impact of latex release from milkweed on pollen retention.
• Measure pollen retention by young milkweed leaves and flowers and potential to impact developing larvae.

• Examine the impact of corn and soybean rotations on milkweed growth and survival.
• Examine the impact of weed management systems in the corn and soybean rotation.
• Determine the relative importance of bluevine milkweed (honeyvine milkweed) for supporting monarch development.
• Evaluate the potential use of Integrated Roadside Vegetation Management to help support butterfly populations.

• Determine the spatial overlap of corn plantings and habitats for different butterfly species.
• Examine exposure potential of other moth and butterfly species that may occur in areas of corn production.
• Determine the potential for Bt corn to reduce insecticide applications and resulting impact on monarch survival.
• Evaluate sensitivity of other species to various Bt proteins.
• Evaluate sensitivity of monarch larvae relative to other species.
• Determine potential impacts of Bt pollen on the milkweed tusset moth.

Photography

Monarch Watch, www.MonarchWatch.org.

Marlin Rice, professor of entomology, Iowa State University.

Figure 1

Monarch Watch 1997 Season Summary, page 35.

Figure 2

Taylor, O. R. (1999, November 2). Monarch butterflies: Population dynamics and the potential impact of add-on mortality. Monarch Butterfly Research Symposium, Chicago. James, C. (1999, 1998 and 1997). Global Review of Commercialized Transgenic Crops. International Service for the Acquisition of Agri-biotech Applications.

Figure 3

Taylor, O. R. (1999, November 2). Monarch butterflies: Population dynamics and the potential impact of add-on mortality. Monarch Butterfly Research Symposium, Chicago.

Figure 4

“Farmers reduce insecticide use with Bt corn.” (1999, November 22) Associated Press Newswires. James, C. (1999, 1998 and 1997). Global Review of Commercialized Transgenic Crops. International Service for the Acquisition of Agri-biotech Applications.

Figure 5

National Corn Growers Assn. Web site, www.ncga.com/02profits/ know_where/know_grow_approved.htm

Figure 6

Pleasants, J. M., Hellmich, R. L., and Lewis, L. C. (1999, November 2). Pollen deposition on milkweed leaves under natural conditions. Monarch Butterfly Research Symposium, Chicago.

Figure 7

Adapted from Pleasants, J. M., Hellmich, R. L., and Lewis, L. C. (1999, November 2). Pollen deposition on milkweed leaves under natural conditions. Monarch Butterfly Research Symposium, Chicago.