1North Central RegionalExtension PublicationNCR # 553

Biotechnology Information Series

Insect-resistant Crops Through Genetic Engineering

insects. Strains of Bt are effectiveagainst European corn borers andcotton bollworms (Lepidoptera),Colorado potato beetles (Coleoptera),and certain flies and mosquitos(Diptera). Bt is not harmful tohumans, other mammals, birds, fish,or beneficial insects.

Bt was first identified in 1911when it was discovered that it killedthe larvae of flour moths. Bt wasregistered as a biopesticide in the U.S.in 1961. Today it is used in insecticidesprays sold to home gardeners andothers worldwide.

In 1983, the World HealthOrganization used Bt in West Africa tocontrol disease-carrying blackflies. Inthe U.S., various strains of Bt are usedto control spruce budworms and gypsymoths in forests, cabbage worms inbroccoli and cauliflower, loopers orbudworms in cotton and tobacco, andleaf rollers in fruits.

However, less than one percentof all pesticides used in the U.S. eachyear contain Bt (Monsanto). As aningredient of commercial sprays, Bt isrelatively expensive and has somedrawbacks. Although some pesticideskill on contact, Bt must be eaten byinsects to be effective. Sunlight breaksdown Bt, and rain washes it fromthe plants. Therefore, Bt must beapplied exactly where and whenthe target insects are feeding andthey must consume it quicklybefore it disappears.

Bt and Biotechnology

Today, plants can be geneti-cally engineered to produce theirown Bt. Genetic (recombinantDNA) engineering is the modifica-tion of DNA molecules to producechanges in plants, animals, or otherorganisms. DNA (deoxyribonucleicacid) is a double-stranded moleculethat is present in every cell of anorganism and contains the heredi-tary information that passes fromparents to offspring. This heredi-tary information is contained inindividual units or sections of DNAcalled genes. The genes that arepassed from parent to offspringdetermine the traits that theoffspring will have.

In the last twenty years,scientists made a surprising discov-ery DNA is interchangeableamong animals, plants,bacteria...any organism! In additionto using traditional breedingmethods of improving plants andanimals through years of cross-breeding and selection, scientistscan now isolate the gene or genesfor the traits they want in oneanimal or plant and move them intoanother. Of course, when a trait iscontrolled by several genes, thetransfer process is more difficult.The plants or animals modified inthis way are called transgenic.

Where the Story Began

For centuries, humans havesearched for crop plants that cansurvive and produce in spite of insectpests. Knowingly or unknowingly,ancient farmers selected for pestresistance genes in their crops,sometimes by actions as simple ascollecting seed from only the highest-yielding plants in their fields.

With the advent of geneticengineering, genes for insect resis-tance now can be moved into plantsmore quickly and deliberately. Bttechnology is only one example of waysgenetic engineering may be used todevelop insect resistant crops now andin the future.

The Bt Chapter

Bacillus thuringiensis,commonly known as Bt, is abacterium that occurs naturally inthe soil. For years, bacteriologistshave known that some strains ofBt produce proteins that killcertain insects with alkalinedigestive tracts. When theseinsects ingest the protein pro-duced by Bt, the function of theirdigestive systems is disrupted,producing slow growth and,ultimately, death.

Bt is very selective differ-ent strains of the bacterium killdifferent insects and only those

2Out of Bt. . . Into the Plants

DNA technology makes itpossible to locate the gene thatproduces Bt proteins lethal toinsects and transfer the gene intocrop plants. The process is de-picted in Figure 1.

First, scientists identify astrain of Bt that kills the targetedinsect. Then they isolate the genethat produces the lethal protein.That gene is removed from the Btbacterium and a gene conferringresistance to a chemical (usuallyantibiotic or herbicide) is attachedthat will prove useful in a later step.

The Bt gene with the resis-tance gene attached is insertedinto plant cells. At this point,scientists must determine whichplant cells have successfullyreceived the Bt gene and are nowtransformed. Any plant cell thathas the Bt gene must also have theresistance gene that was attachedto it. Researchers grow the plantcells in the presence of the antibi-otic or herbicide and select theplant cells that are unaffected byit. These genetically transformedplant cells are then grown intowhole plants by a process calledtissue culture. The modifiedplants produce the same lethal Btprotein produced by Bt bacteriabecause the plants now have thesame gene.

Research to transfer insectresistance genes from Bt to cropplants is well under way. Corn,cotton, and potatoes are three ofthe many commercial crops tar-geted for Bt insect resistance.

Bt in Corn ConfersResistance to EuropeanCorn Borers

Modern corn hybrids havesome resistance to European cornborers (ECB), but they still sustaindamage from moderate and highlevels of infestation. Geneticallyengineered insect-resistant corn isproviding a new defense againstan old enemy.

The EnemyThe European corn borer is a

major pest of corn in NorthAmerica and Europe. Researchersestimate that yield loss due toECB averages five percent, butany one year or location can havemuch higher damage (Ciba Seeds).The number of larvae in theprevious season, the type of winterweather, natural parasites anddisease, and weather conditionsduring the growing season canaffect ECB levels. For example,during the 1990 and 1991 growingseasons, ECB levels were veryhigh across much of the U.S. CornBelt and losses were heavy. But

the cool, rainy weather and dis-eases during critical parts of the1992 growing season reduced ECBpopulations and their damage inmany areas.

ECB larvae attack corn twotimes during the growing seasonacross most of the U.S. Corn Belt.First-brood larvae feed on plantleaves before flowering, injure leaftissue, and eventually bore intothe stalk. Second-brood larvaebegin feeding in the leaf sheathand collar area of corn plants afterflowering. Eventually, they boreinto the stalk and ear shank wherethey can cause broken stalks anddropped ears.

Figure 1

3The greatest source of ECBdamage may not be visible. ECBdisrupts the physiological pro-cesses of corn and can reduceyields even when stalk lodging(breaking) problems are notevident. Physiological damage cancause low test weight of harvestedcorn, small ears, or early death ofthe plant.

Chemical sprays must beapplied before the borer tunnelsinto the stalk to be effective.Multiple applications may berequired, and, even then, completecontrol is not possible.

The Bt SolutionScientists know that a family

of insecticidal proteins from Btkills ECB. The bacteria producethe protein as an insoluble crystal.When a susceptible caterpillar,like the corn borer, eats thecrystal, part of it binds to, pen-etrates, and collapses the cellslining its gut, causing death.

Although Bt genes have beenintroduced into tobacco, tomatoes,cotton, and other broadleaf plants,gene transfer technology for cornis a recent achievement. Thedevelopment of corn plants ex-pressing Bt proteins requiressubstantial changes in the Bt

genes, including the creation ofsynthetic versions of the genes,rather than the microbial Bt geneitself.

For example, one companyhas developed three versions of asynthetic Bt gene that switcheson in pollen, in green tissue, or inother parts of the corn plant (CibaSeeds). Along with a herbicideresistance gene, the three Btversions were shot by a biolisticgene gun into immature cornembryos taken from developingseeds about 15 days after pollina-tion. The gene gun bombarded theembryos with tiny gold particlescoated with the genes.

Some plants received geneswith a non-specific promoter(switch) that resulted in produc-tion of the Bt protein in manyparts of the corn plant. Otherplants received a mixture of geneswith switches that resulted in Btprotein production only in thepollen and green tissue. Theherbicide resistance gene allowedscientists to determine whichplant cells had received a goldparticle carrying DNA and suc-cessfully incorporated the gene.Subsequent screening for Btproduction showed which cellsreceived both herbicide resistance

and the Bt gene(s).When the cells were grown

into plants and field tested forresistance, it was found that theBt plants fared better than any ofthe control plants. In general,plants with both the pollen andgreen tissue Bt genes appearedmore resistant to second-broodcorn borer damage than plantswith Bt genes that expressedthemselves in many parts of theplant. Another advantage of theBt genes with specific switches(pollen and green tissue) was thatthey produced insecticidal proteinin those parts of the plant at-tacked by both first-and second-brood ECBs, while minimizingproduction in seed and other partsof the plant where protection isnot critical.

Using Bt technology tocontrol corn borers is an interna-tional effort. One U.S. company isusing a biolistic gene gun tobombard Indonesian corn with Btgenes that produce proteins lethalto the Asian corn borer (Wilson).This pest is responsible for signifi-cant yield losses, up to 40 percent,in much of the corn-growing areasof Indonesia.

The warm Indonesian cli-mate and the life cycle of the

Asian corn borer allow 9to 12 overlapping genera-tions annually, making ita threat to corn virtuallyyear-round. IntroducingBt insect resistance genesinto commercial Indone-sian corn seems to be themost feasible and effec-tive way available tocontrol the pest.

Comprehensivetesting must be done,approval by relevantauthorities obtained, andpatent questions an-swered before any com-pany can commercializecorn hybrids containingthe Bt gene.

The corn plant on the left is resistant to European corn borers, thanks to Bttechnology. The corn plant on the right is not.

4Bt Resistance for theColorado Potato Beetle

The potato is the mostpopular vegetable among NorthAmerican consumers. It is esti-mated that potatoes, in some form,are consumed during one of everythree meals (Monsanto). Despitetheir popularity, potatoes are notthat easy to grow. Each year,potato growers lose a significantamount of their crops to an insectcalled the Colorado potato beetle.The beetle feeds on the growingplant leaves and stems during thegrowing season, stunting the plantand cutting yields. Repeatedapplications of chemical sprays areusually needed to control the pest.

The Economic PictureAccording to the USDA,

potato producers in the U.S. spend$20 to $40 million each year forcontrol of the Colorado potatobeetle (Monsanto). In an averageyear, growers spray their fieldswith insecticides as many as fivetimes at a cost of up to $150 per acre.

If insect-resistant plants wereavailable, researchers estimatethat the use of chemical insecti-cides in potatoes could be reducedas much as 30 to 40 percent.Translated into dollars, growerscould save $6 to $16 millionannually, based on USDA esti-mates of current insecticide use.

Similar Science, Different TargetThe science being used to

develop potatoes resistant to theColorado potato beetle is similar tothe European corn borer technol-ogy. One of the advantages of Bt isthe diversity of strains that can beused to target different insect pests.

Like the corn researchers,potato scientists are using aspecific type of the naturallyoccurring soil bacterium Bacillusthuringiensis. A single gene fromthe Bt is spliced into a chromosomeof a potato cell. When the cellcarrying the Bt gene is grown intoa plant, the plant produces aprotein that is toxic to the Colorado

potato beetle. When the beetlefeeds on the genetically improvedplant, the toxic protein interfereswith its digestive system and it dies.

The protein is highly selec-tive, affecting only the Coloradopotato beetle. It does not harmhumans, animals, or beneficial insectsthat help control other crop pests.

The OutlookOne company conducting

insect-resistant potato researchexpects farmers to be plantingtheir beetle-resistant potatoes bythe mid-1990s (Monsanto newsrelease). This company began itsdevelopment of insect-resistantpotatoes with the Russet Burbankvariety, since it comprises approxi-mately 40 percent of U.S. potatoacreage. However, companyresearchers are also working toimprove other popular potatovarieties such as Atlantic, Supe-rior, Norchip, and Shepody.

Like all genetically improvedfood crops, the exact timetable forcommercialization of insect-resistant potatoes depends notonly on the scientific tests thatmust be conducted, but also onapprovals by the appropriateregulatory agencies. (See Regu-lating Bt Plants.)

Bt Resistance for Cotton

Cotton is another cash cropbenefiting from Bt insect-resis-tance research. With a marketvalue of $4.5 billion, cotton is thenations fifth largest crop(Monsanto). Yet, annually cottongrowers expect to lose up to 15percent of each field due to insectdamage.

Caterpillar insects, like thetobacco budworm, cotton boll-worm, and pink bollworm, feed oncotton plants. Experts say thatthese and similar pests are re-sponsible for millions of dollars indamage to the American cottoncrop each year.

Caterpillar EconomicsCaterpillar insects are respon-

sible for 60 to 70 percent of allinsect damage to cotton plants(Monsanto). Efforts to control themchemically account for about 60 to70 percent of a cotton growerspesticide costs. In the MississippiDelta growing region, a cottongrowers pesticide costs can average$40 to $45 per acre each season. InArizona, pest control costs areusually higher, perhaps double thatof the Mississippi Delta.

The potato plant on the left did not produce the Bt protein toprotect it from the Colorado potato beetle. The plant on theright had the Bt gene.

5Chemical sprays containingBt are seldom practical in cotton,since Bt breaks down in sunlight,washes away in rain, and is nearlyimpossible to apply to the plantparts where insects feed. How-ever, other chemical insecticidesare used that may need to beapplied as many as 10 times inone season. By planting insect-resistant cotton, researchers hopefarmers can reduce the number ofinsecticide applications they mustpay for.

Same Song, Third VerseCreating insect resistance in

cotton by using Bt technology is aprocedure similar to that used incorn or potatoes. Researchers findone or more strains of Bt thatproduce a protein fatal to some ofthe most damaging cotton insectpests. The gene that regulates theproteins production is insertedinto the genetic structure of cellsfrom a cotton plant. The cells thatsuccessfully receive the Bt genesare grown into whole plantsthrough tissue culture and enter along period of laboratory and fieldtesting. When certain caterpillarpests feed on the new cottonplants, the protein damages theirdigestive systems, and they die.

Commercialization TimetableCurrent estimates are that it

will be several years before insect-resistant cotton plants are com-mercially marketed. They mustmeet the regulatory requirementsadministered by the USDA and theEnvironmental Protection Agency(EPA). (See Regulating Bt Plants.)

The Insect ResistanceProblem

Transgenic Bt plants likecorn, potatoes, and cotton have apossible problem what if insectsbuild up a resistance to the lethalproteins?

The potential of pests todevelop resistance against thedefense mechanisms of crops iswell-known and is not unique togenetically engineered plants.Insects may develop resistance to acrop defense no matter how it wasdeveloped. The crop defense mightbe a chemical or biological agent, agene already in the crop speciesand transferred to commercialplants by conventional plantbreeding methods, or a geneintroduced by recombinant DNAtechnology. Because more than500 insects and mites already have

acquired resistance to a number ofinsecticides, there is concern thatsimilar resistance to Bt toxins coulddevelop (McGaughey and Whalon).

Several major pests, includingthe tobacco budworm, Colorado potatobeetle, Indianmeal moth, and dia-mondback moth, have demonstratedthe ability to adapt to Bt in thelaboratory. It has been reported thatthe diamondback moth evolved highlevels of resistance in the field as aresult of repeated use of Bt(McGaughey and Whalon). As Bt useincreases on more acres, some scien-tists predict that insect resistance toBt will be a major problem. Consider-able controversy exists about how Btshould be managed to prolong itsusefulness.

Bt Durability in Non-Transgenic Crops

The crystal proteins of Bt thatare lethal to insects have been widelyused as microbial insecticides intraditional corn and vegetable cropsfor years. In recent years, fieldpopulations of insects tolerant to theseBt proteins have been reported. Todate, these cases are limited and havebeen associated with frequent andprolonged use of Bt on geographicallyisolated insect populations (CibaSeeds). Scientists have been able toinduce higher levels of resistance inthese field populations by taking theminto the laboratory and exposing themto specific Bt insecticidal crystalproteins.

A laboratory population oftobacco budworm that is resistant tomore than one class of crystal proteinshas been reported. However, theresistance of this population is nothigh and its ability to survive issignificantly reduced when theproteins are absent.

In the case of ECB-resistantcorn, it is thought that the develop-ment of insect resistance may bedelayed by the availability of non-Btcorn fields (refuges) that are stillsusceptible to the borers. Borers thatcould not survive ECB-resistant fieldsmay do well in the refuges. There theycould breed with resistant borers,diluting the resistance trait in thegene pool.

The large boll of cotton on the left was produced by a plantdesigned with a Bt gene to fend off insects. The smaller boll onthe right was grown in the same field on an unprotected plant.

6Bt Durability in TransgenicCrops

About 13 U.S. and Europeancompanies that fund academic Btresearch have formed a Bt Manage-ment Working Group to supportresearch on the interaction oftransgenic plants with target pests.

The plant industry is employ-ing several approaches to dealwith insect resistance totransgenic insecticidal plants,including the following:

Very high levels of insecti-cidal crystal proteins (ICPs) arebeing produced in new geneticallymodified plants. The only insectsthat can survive feeding on suchplants will be those that alreadypossess a high-level resistancegene as part of their genetic code.Such genes are expected to beextremely rare in the insectpopulation. Insects with morecommon genes for partial, or low-level, resistance will not survivethe very high levels of ICPs.

Additional insecticidal genesare being sought. The idea is todevelop transgenic plants thatexpress several insecticidal genestargeting different sites within theinsect. In order to survive plantswith such multiple defenses, aninsect would probably have topossess multiple resistance genes.Some scientists believe it is highlyunlikely that insects can acquiresuch genes. In any case, thesescientists anticipate that plantsexpressing additional insecticidalgenes will be available beforeinsect resistance to a single, highlyexpressed ICP from Bt develops.

Regulating Bt Plants

Insect-resistant crops devel-oped through Bt technology aresubject to the same regulations asother genetically engineeredplants. It should be noted thatU.S. regulatory policy for geneti-cally engineered plants hasevolved over time and additionalchanges in the requirementsdiscussed below may occur. Three

movement and field testing ofcertain genetically engineeredcrops such as corn, soybeans,cotton, potatoes, tomatoes, andtobacco. The streamlining wasbased on an extensive history ofsafe use in field trials in theUnited States. In the streamlinedprocess, an applicant can field testspecific crops if the crop andgenetic material meet certainUSDA performance standards.For a field test that does not meetthese performance standards, thecurrent 120-day field test permitprocess would be followed.

Environmental ProtectionAgency (EPA)

The EPA recently proposednew policies for review of applica-tions for field trials of geneticallyengineered plants. It proposedthat substances introduced intocrops by genetic engineering toprovide disease or insect resistance(Bt crops are in this category) fallunder the Federal Insecticide,Fungicide, and Rodenticide Actand the Federal Food, Drug, andCosmetic Act.

To date, the USDA hasconsulted with EPA when review-ing applications to test such cropson a small scale. The EPA willmost likely formally review aproduct when a company decidesto move into large-scale testing. Aswith traditional pesticides, anExperimental Use Permit (EUP)will be required for tests thatexceed 10 acres total in the U.S.

After receiving the EUP, acompany follows a process thatinvolves extensive testing of thecrop to ensure food and environ-mental safety before the crop iscommercialized. In addition, it islikely that some applicants alsowill consult the Food and DrugAdministration (FDA) before apest-resistant food crop is commer-cialized, even though a recent FDApolicy proposal emphasized thatsuch products fall under the EPAsjurisdiction and are not subject toFDA pre-market approval.

government agencies share respon-sibility for administering plantbiotechnology regulations in theUnited States.

U.S. Department of Agriculture(USDA)

The USDA administers theFederal Plant Pest Act (FPPA),which regulates interstate move-ment, importation, and fieldtesting of genetically engineeredplants. A special permit from theUSDA and approval by the indi-vidual state departments ofagriculture are required to moveany genetically engineered organ-ism into the U.S. or betweenstates. Applicants for a permitmust provide details about thenature of the organism, its origin,and its intended use. If granted,the permit certifies that theapplicants facility and proceduresmeet certain operating standards,such as appropriate levels ofcontainment to prevent accidentalescape of the organism. Guide-lines set by the National Institutesof Health for laboratory researchinvolving recombinant DNAmolecules are included in thesestandards and must be followed bythe applicant.

The USDA oversees fieldtesting of genetically engineeredcrops. The applicant must providecomplete information about theplant, including all new genes andnew gene products, their origin,the purpose of the test, the experi-mental design (how the test will beconducted), and precautions to betaken to prevent the escape ofpollen, plants, or plant parts fromthe field test site. The approvalprocess can take a maximum offour months.

Before a genetically engi-neered crop can be sold commer-cially, a petition must be filed forUSDA exemption. This petitionrequires more information than afield test permit, including environ-mental product safety information.

Recently, the USDA stream-lined the permit process for

7Food and Drug Administration(FDA)

The FDA also has very broadauthority under the Federal Food,Drug, and Cosmetic Act to regulatethe introduction of new food crops,whether they were conventionallyproduced or created by geneticengineering. FDA policy statesthat any new food developedthrough genetic modification mayrequire testing and/or labeling ifthe genetic material introducedinto the host plant comes fromoutside the traditional food supply,is a known or suspected cause ofallergic reactions, or significantlyalters the nutritional compositionof the food (Federal Register).Every company or individual thatproduces food or food productsthrough recombinant DNA tech-nology is legally required to assureits safety and quality before itenters the food supply the sameassurance required of traditionalfood products.

The Bt OutlookCrops that have been geneti-

cally engineered for Bt resistancecould dramatically lower produc-tion costs and provide farmerswith new insect control optionswithin the next few years. Thesuccess of their commercializationdepends on several factors, includ-ing the regulatory climate, patentissues, and the ability of scientiststo deal with targeted insects thatdevelop resistance to the lethalproteins. Several companiescurrently working on Bt insectresistance in crops foresee market-ing their products in the mid-1990s.

References

Ciba Seeds. Ciba Seeds Intro-duces Gene; New AgreementBetween INRA, CIBA for Studieson Insect-Tolerant Corn; PlantBreeding Technology and DNAMarkers; Questions & Answers Benefits of Insect-resistantCorn for Farmers; Strategy forManaging Insect Resistance toTransgenic Bt Corn; U.S. Gov-ernment Regulation of PlantBiotechnology News releases.Cibas Bt Maize Work Sum-marized article. Ciba Seeds, P.O.Box 18300, Greensboro, NorthCarolina.

Federal Register, Statement ofPolicy: Foods Derived from NewPlant Varieties; Notice. Part IX,Department of Health and HumanServices, Food and Drug Adminis-tration. Federal Register, Vol. 57,No. 104, Friday, May 29, 1992.p. 22984-23005.

Gould, Fred. EvolutionaryBiology and Genetically Engi-neered Crops. Bioscience. Vol.38, No. 1, January 1988. p. 26-33.

Gould, Fred; Martinez-Ramirez,Amparo; Anderson, Arne; Ferre,Juan; Silva, Francisco J.; andMoar, William J. Broad-spectrumResistance to Bacillusthuringiensis Toxins in Heliothisvirescens. Proceedings of theNational Academy of Science,USA. Vol. 89, September 1992.p. 7986-7990.

Marois, James J.; Grieshop, JamesI.; and Butler, L. J. (Bees). Envi-ronmental Risks and Benefits ofAgricultural Biotechnology. InAgricultural Biotechnology, Issuesand Choices, p. 67-79. Edited byBill R. Baumgardt and Marshall A.Martin. West Lafayette, Indiana:Purdue University AgriculturalExperiment Station, 1991.

McGaughey, William H. andWhalon, Mark E. ManagingInsect Resistance to Bacillusthuringiensis Toxins. Science.Vol. 258, November 27, 1992.p. 1451-1455.

Monsanto. B.T. The InsideStory; Insect-Resistant CottonReplaces Insecticides; SelfDefense for Potatoes Newsreleases. The Agricultural Group,Public Affairs Department, 700Chesterfield Parkway North, St.Louis, Missouri.

Wilson, H. M. Personal communi-cation. ICI Seeds, Slater, Iowa,March 30, 1993.

Wrage, Karol. Bts: Flagship for21st Century BioControls: Indus-try Execs, University ResearchersSay Future Promising and Profit-able. Biotech Reporter. Vol 10,No. 5, May 1993. p. 8-9.

Written by Glenda D. Webber,Office of Biotechnology, IowaState University.

Photo Credits

Page 3 Courtesy ofCiba Seeds.

Page 4 Courtesy ofMonsanto.

Page 5 Courtesy ofMonsanto.

8North Central Regional Extension Publications are subject to peer review andprepared as a part of the Cooperative Extension activities of the 13 land-grantuniversities of the 12 North Central States, in cooperation with the ExtensionService - U.S. Department of Agriculture, Washington, D.C. The following statescooperated in making this publication available.

University of Illinois Michigan State University University of Nebraska69 Mumford Hall 10B. Ag. Hall Dept. of Ag. Communications1301 W. Gregory Drive East Lansing, MI 48824-1039 Lincoln, NE 68583-0918Urbana, IL 61801 517-355-0240 402-472-3023217-333-2007

* Iowa State University University of Minnesota North Dakota State University119 Printing & Publ. Bldg. 3 Coffey Hall Ag. Comm, Box 5655, Morrill HallAmes, IA 50011-1050 St. Paul, MN 55108 Fargo, ND 58105515-294-5247 612-625-8173 701-2137-7881

Kansas State University University of Missouri University of WisconsinUmberger Hall 115 S. Fifth St. Ag. Bulletin, Rm. 245Manhattan, KS 66506 Columbia, MO 65211 30 N. Murray St.913-532-5830 314-681-5557 Madison, WI 53715

608-262-3346

* Publishing state

For copies of this and other North Central Regional Extension Publications,write to: Publications Office, Cooperative Extension Service, in care of theUniversity listed above for your state. If they do not have copies or your state isnot listed above, contact the publishing state as marked with an asterisk.

Programs and activities of the Cooperative Extension Service are available to allpotential clientele without regard to race, color, national origin, age, sex, reli-gion, or disability.

In cooperation with NCR Educational Materials Project

Issued in furtherance of Cooperative Extension work, Acts of Congress of May8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture andCooperative Extension Services of Illinois, Indiana, Iowa, Kansas, Michigan,Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wiscon-sin. Robert M. Anderson, Jr., Director, Cooperative Extension Service, IowaState University, Ames, Iowa 50011.

Printed and distributed in cooperation with Extension Services, U.S. Depart-ment of Agriculture, Washington D.C., and the Cooperative Extension Servicesof Arkansas, Colorado, Kentucky, Louisiana, Maine, Montana, Pennsylvania,and Tennessee.

January, 1995