Chris DiFonzo, Field Crops Entomologist
RESEARCH JUST PUBLISHED IN "NATURE"
GOAL OF EXPERIMENT
Reports of this research have since been picked up by news outlets around the country and stories have been printed in many papers, for example, "Corn strain is deadly to monarch butterflies", Detroit Free Press, May 20, 1999; "Butterfly-killing corn", Associated Press, May 19, 1999.
USEFUL BACKGROUND INFO
BACKGROUND - the Monarch butterfly:
BACKGROUND - Bacillus thuringiensis (Bt):
Bt is an important insecticide in production of organic crops, and it is used as a safer alternative in gypsy moth spray programs in many states, including Michigan. Bt bacteria is highly specific to insects and is listed as practically "non-toxic" to mammals. Bt has much less impact on beneficial insects compared to conventional broad spectrum insecticides. However, as with any pesticides, Bt applications are not completely benign. Non-target impacts of Bt insecticide sprays on insects related to the intended target pest (for example, other caterpillars in the forest besides gypsy moth) have been noted in the literature for years. The negative impact of Dipel, a Bt product made up of several caterpillar-killing strains of Bt, on monarch larvae feeding on plants in vegetable fields was reported in 1986.
BACKGROUND - Bt corn
Genetic engineering technology allowed scientists to find the specific genes in Bt bacteria which code for the protein crystal. These genes have been inserted into plants, allowing crops like potato and corn to produce Bt toxin directly in their own tissues. In most (not all) cases, the toxin is found in all tissues of the plant, including the pollen. These so-called "Bt-crops" circumvent the problems of UV breakdown, coverage, and timing by making Bt toxin available continuously throughout the season in all tissues. The cost of Bt crops is decreasing, making Bt seed competitive with conventional insecticides.
Bt corn genetically engineered to control the most important pest of corn, the European corn borer (ECB), is becoming increasingly popular with growers in the United States. The advantages of Bt corn are the following:
Limitations of Bt corn are the following:
Q AND A*s ABOUT THIS ISSUE
Is this new news?
No - the Cornell findings have been discussed at scientific meetings last year. Other research groups have also been investigating this phenomena. Iowa State entomologists did a similar feeding study using milkweed leaves actually taken from Bt corn fields (L. Hansen and J. Obrycki, poster presentation, NCB-ESA meetings, Des Moines, Iowa, March 1999). They found that after 2 days, 19% of first-instar monarchs fed on milkweed leaves from Bt corn fields died, versus 0% fed on milkweed not exposed to Bt pollen and 3% fed on leaves without pollen. In addition to the monarch work, many labs are conducting experiments to determine the impacts of Bt corn on beneficial insects like ladybugs and lacewings. In general, most studies have shown Bt corn has little impact on non-targets, although much of this work is not complete. Subtle, long-term impacts still could turn up.
How far does corn pollen travel?
The Nature article sites a distance of at least 60 meters (~ 200 ft). Production of inbred corn seed requires separating blocks by 600 feet to prevent cross-pollination. These distances are the outside range of corn pollen travel. Corn pollen is fairly heavy, and most of it seems to settle directly in the field.
How much Bt corn is in Michigan?
According to the most recent statistics, 2.6 million acres of field corn and 12,500 acres of fresh market sweet corn were planted in Michigan in 1997; only a fraction of this acreage is Bt transgenic. Compared to other states in the mid-west, Michigan and Ohio have fewer acres of Bt field corn. Depending on the growing region, the amount of Bt ranges from less than 5% of the acreage up to about 20% in some parts of southeast Michigan. Corn fields in Michigan tend to be smaller, and the cropping pattern more diverse, than in the rest of the corn belt. Thus much of Michigan is not within 600 feet of a corn field, much less a Bt corn field.
What is the status of milkweed (Asclepias spp.) in Michigan?
Milkweed plants are found throughout Michigan along roads, in fields, pastures, swamps, and around homes. Some types are available commercially to plant in butterfly gardens. In agricultural fields, milkweed (mostly common milkweed) is considered a weed species and thus is a target for cultivation or herbicide applications. However, Jim Kells, MSU weed specialist, reports that milkweed is a "second tier" weed species in Michigan - i.e. found as single plants or groups of plants in a corn field or on a field margin, but generally not common enough to be the target of a weed control program. Thus in Michigan, milkweed is more often found along roadsides, in ditches, and field margins rather than in corn fields themselves. Milkweed species in Michigan eaten by monarchs are listed below. Of the species, common milkweed is the most likely to be located near corn fields, and thus dusted with corn pollen.
Common (A. syriaca) - cultivated fields, pastures, open woods, roadsides.
Eastern whorled (A. verticillata) - meadows, pasture, waste areas. Seldom
in cultivated fields.
Why is the monarch population declining in the United States?
How serious is the Bt pollen threat compared to the big picture? The number of monarchs has been decreasing over the years, but for reasons not related to transgenic crops. The two most important factors associated with the decline of this butterfly are:
Another factor that can negatively impact monarchs is the use or drift of conventional insecticides in agricultural and suburban areas. Most conventional (OP, carbamate, pyrethroid) insecticides used in corn production are broad-spectrum, in other words, they kill most of the insects in the field, pest or not. Pyrethroids in particular are very active against beneficial and non-target species. Ironically, one of the advantages of Bt corn for corn borer control is that it replaces the application of broad-spectrum insecticides. Use of Bt sprays, such as Dipel, targeted at caterpillar pests have also been shown to affect monarchs (L. Brower, 1986, Atala. Vol.).
In the whole scheme of things - big picture - is this something for people to worry about?
Our personal opinion is "no" short-term, "maybe" long-term. The Cornell findings are interesting. They suggest there could be non-target impacts of Bt pollen on monarch butterflies and, by extrapolation, non-target impacts on other non-pest caterpillars that live in and around corn fields. These results should prompt further research to, for example, find out if monarchs are actually being harmed in the field, test different levels of Bt pollen and different sizes of monarch larvae, examine the timing of corn pollen shed versus the timing of the monarch life cycle, measure actual accumulation of pollen on plants in and around corn fields, and look at other species of caterpillars. Also, this work shows once again that no pest management technique is a "magic bullet"; there are benefits and limitations of transgenic technology.
However, our sense is that with the current level of Bt corn in Michigan, most of our milkweed is not within range of Bt pollen. This is especially true of milkweed in suburban areas (for example, residents in Detroit identified in a Detroit Free Press article as planting butterfly gardens). Monarchs in Michigan are currently at more risk from problems at overwintering sites, reduction in milkweed, and the use of insecticides rather than Bt pollen shed. Again, in the big picture, Bt corn is a much safer method of pest management, and has less detrimental impact on all aspects of the environment (monarchs included) than the use of broad-spectrum insecticides.
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Last Updated on 6/3/99
By Karen Lutz