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Equally important are the agricultural crop lossesdue to weed infestation. Weeds deprive the crops ofmoisture and nutritive substances from the soil. Insome instances they even shade crops and hindertheir growth. The use of herbicides in the control ofweeds is now a standard practice in the UnitedStates, and it has been estimated that we would have10 to 12 percent of our population working onfarms, manually dissecting weeds from the crops,were it not for herbicides. Microorganisms also contribute heavily toagricultural crop destruction. Most plant diseasescan be controlled to some extent with modernsynthetic fungicides. In general, fungal plantdiseases are basically more difficult to control withchemicals than are insects because the fungus is aplant living in close quarters with its host. Thismakes it especially arduous to find chemicals whichwill selectively destroy the fungus without harmingthe plant. Other organisms that cause plant diseasesare viruses, rickettsias, algae, nematodes, andmycoplasma.

INTRODUCTIONThe term pest derives from the Latin pestis forplague and is used to describe plants (mostly weeds),vertebrates, insects, mites, pathogens and otherorganisms which occur where we do not want them.Large agricultural endeavors have intensified theproblem by concentrating a large population of pestsin discrete areas. With the availability of vastquantities of succulent food and often the absence ofnatural enemies, pests can reproduce quickly andbecome a serious agricultural problem. Insects, inparticular, often exhibit a dramatic and rapid in-crease in population size under these conditions.The average soil density of insects is about 9 millionper acre with about 10 thousand “in flight” above it.The world’s main source of food is plants and theanimals that rely on plants for their diet. Among the800,000 species of insects, about 10,000 are plant-eating and add to the devastating loss of cropsthroughout the world.

Michael W. Davidson

M.J. Page

M.F. French

d r u g s f o r b u g s

Figure 1. (above left) DICHLORANFiqure 2. (above right) MONURON

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Approximately one-third of the world’s agricul-tural food output is destroyed by pests duringgrowth, harvesting, and storage. In the United Statesalone this amounts to about $20 billion annuallydespite the heavy use of pesticides and other currentcontrol methods. One is tempted to wonder what thelosses would be without pesticide use. Pesticides are a big business with a lot of moneyat stake. Annually, United States manufacturersproduce almost a billion kilograms of syntheticorganic pesticides, valued at over $5 billion, with aneven higher retail value. More than 50 U.S. firmsmanufacture pesticides of one type or another,although 14 firms account for about 85 percent oftotal overall pesticide sales. Although our domesticconsumption of pesticides has grown substantiallyover the past few years, the rest of the world is alsousing the chemicals at an escalating rate as morecountries develop their agricultural and industrialeconomies. The effects of pesticides on nontarget organismsand the environment have been a source of bittercontroversy. The most easily identified pesticidenontarget consequences were those of the persistentorganochlorane insecticides (such as DDT) and theirmetabolites. These deadly chemicals have devas-tated populations of certain species of fish and birds.Less readily apparent are the consequences ofpesticide residues in food and the environment andeffects on humans and domestic animals. The accompanying selection of photomicrographsrepresents a small portion of the author’s collection

entitled Drugs for Bugs. Pesticides are generallysmall organic molecules that exist in the form of afree base. For this reason, they can be easily meltedand recrystallized between a microscope slide andcoverslip. The unique patterns displayed by recrys-tallized pesticides are evident in the photo micro-graphs.

MICROSCOPIC IDENTIFICATIONOF PESTICIDESFor the most part, pesticides are composed of smallorganic molecules that are generally administered inthe form of a free base or occasionally as organicsalts (1, 2, 3). Because these chemicals are re-stricted to certain discreet classes, they can often beidentified through crossed polarized microscopicexamination of recrystallized pesticides. It shouldbe stressed that pesticides are very toxic andextreme caution should be used in their handling.All work should be conducted in a fume hood andmicroscope slides should have their coverslipssealed with a suitable mounting medium (such asFisher Scientific, Inc. Permount).

To prepare pesticide crystals for examination inthe microscope, deposit a few milligrams of theappropriate pesticide on a glass microscope slideand carefully place a coverslip over the powder.Next, heat the bottom side of the microscope slidecarefully with a bunsen burner or hot plate until thepowder has completely melted. Some pesticidesdecompose upon heating and will provide poorsubjects for microscopic examination. When

Figure 2. MONURON (top right previous page)This herbicide is a member of the substituted urea class ofpesticides that are relatively nonselective and are usuallyapplied to the soil as preemergence herbicides. However,some substituted ureas have postemergence uses, whileothers are active when foliar applied. The ureas are readilyabsorbed by the roots and rapidly translocated to the upperplant segments, producing phytotoxic symptoms that aremost visible in the leaves. The primary site for biologicalactivity appears to be the Hill photosynthetic reaction,although other mechanisms have been postulated. Monu-ron has been used for preemergence control of broad-leaved and grass weeds to protect crops such as cotton,sugar cane, pineapple, asparagus, and citrus.

Figure 1. DICHLORAN (top left previous page)This fungicide is a member of the substituted aromatic classof pesticides derived from the parent compoundhexachlorobenzene. These chemicals are useful for seedtreatment and in some instances, as a wood preservative.The mechanism of action displayed by this diverse group ofpesticides is probably by chemically combining withbiological amines and thiols. The fungistatic activitygenerally results in a reduction of fungi growth rates orinterference with sporulation. Dichloran has moderate oraltoxicity to animal life and has proven useful in controllingfungal diseases in many fruits, vegetables, ornamentaldiseases, and field crops. It is particularly effective againstthe Botrytis, Monilinia, Rhizopus, Sclerotinia, and Sclero-tium fungal species.

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melted, the molten chemical will flow underneath thecoverslip and fill the entire volume between thecoverslip and microscope slide. Either allow theslide to cool slowly before examination or place themelted chemical on the microscope stage andexamine the crystallization process occurring.Figure 5 illustrates a slowly crystallizing sample ofthe insecticide, DDT (dichloro-diphenyl-trichloroethane). This sample is typical of those thatcrystallize in the form of spherulites. Spherulites area common crystallization motif that display a 3-dimensional radial growth pattern from a centralnucleus. To obtain an accurate inventory of crystallizationpatterns, it is advisable to first recrystallize all of thepesticides (and related chemicals) that will be ofinterest. Next, carefully photograph all of thecrystallization patterns that occur in these samples

Fiqure 3. CARBARYLIntroduced in 1956, carbaryl was the first carbamate to besuccessful in the insecticide industry. More of this pesticidehas been used worldwide than all other carbamatescombined. Carbaryl has two distinct properties thataccount for its popularity: it has a very low mammalianoral and dermal toxicity and possesses a rather broadspectrum of insect control. This has led to its widehousehold use as a lawn and garden insecticide. Like theother cabamates, carbaryl antagonizes acetylcholine andcompetes for binding sites on the enzyme cholinesterase.In agriculture, carbaryl is used as a contact insecticiderecommended for use against pests of fruit, vegetables,cotton, and many other crops.

Fiqure 4. PARAQUATThis powerful defoliant is a member of the bipyridyliumclass of pesticides. The two most important herbicides inthis group are paraquat and diquat, both of which are contactherbicides that damage plant tissues quickly, causing theplants to appear frostbitten because of cell membranedestruction. This rapid degeneration occurs within hours ofapplication, making these novel herbicides also useful aspreharvest dessicants for seed crops, cotton, soybeans, sugarcane, and sunflowers. Diaquat is sometimes used in aquaticweed control, and paraquat uses include stubble clearing,pasture renovation, inter-row weed control in vegetablecrops, and weed control in plantation crops. Paraquatearned a degree of notoriety several years ago when the U.S.government collaborated with the Mexican government totreat marijuana fields with the defoliant. Many drug userswho were exposed to paraquat-laced marijuana developednose bleeds, headaches, vomiting, and breathing disorders.

and catalog the photomicrographs by cross-referenc-ing the samples to both crystallization motif andsample type. Many pesticides display uniquepatterns as is illustrated in Figure 1 for the herbicide,dichloran. Unfortunately, some pesticides displayeither a common crystallization pattern such as themajor spherulite depicted in Figure 7, or crystallizein very small domains that will not allow uniqueidentification. In my laboratory, we have recrystallized over 150different pesticides and cataloged the results asdescribed above. We are able to identify, with amicroscope, approximately 100 (or two-thirds) of thechemicals. This represents a rapid alternative to GasChromatographic (GC) or High Pressure LiquidChromatographic (HPLC) techniques (5) and canactually be used as a complimentary tool.

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Figure 7. CHLORDANEThis well-known insecticide was the first member of theCyclodiene class of pesticides introduced in 1945. Othermembers of this class include Aldrin, Dieldrin, Heptachlor,Endosulfan, and Chlordecone (Kepone). The cyclodienesare persistent insecticides that are very stable in soil andrelatively stable to ultraviolet radiation from sunlight.Subsequently, they have been widely used as soil insecti-cides and for control of termites and soil-borne insectswhose immature stages (larvae) feed on the roots of plants.These insecticides are the most effective, long-lasting,economical, and safest termite control agents yet devel-oped. Unfortunately, several soil insects have developedresistance to these chemicals in agriculture, which hasresulted in a rapid decline in their use.

Figure 8. DCPAChlorthal is the common name given to this arylaliphaticacid by the International Standards Organization, but theherbicide was renamed DCPA by the Weed Society ofAmerica. The mechanism of action of arylaliphatic acidpesticides is antagonism of auxin growth hormones. Theyinduce cell elongation and tissue proliferation, produceadventitious roots, modify the arrangement of leaves andother organs, and, in some instances, cause fruit to developin the absence of fertilization. DCPA is used as a preemer-gence herbicide recommended for the control of annualweeds in many different crops. It is slowly hydrolzed insoils with-life of about 100 days in most soils.

Figure 5 DDT (dichloro-diphenyl-trichloroethane)This organochlorine insecticide can be considered as thepesticide of the greatest historical significance, due to itseffect on the environment, agriculture, and human health.First synthesized by a German graduate student in 1873, itwas rediscovered by Dr. Paul Muller, a Swiss entomologist,in 1939 while searching for a long-lasting insecticide for theclothes moth. DDT subsequently proved to be extremelyeffective against flies and mosquitoes, ultimately leading tothe award of the Nobel Prize in medicine for Dr. Muller in1948. Effective January 1, 1973 the EnvironmentalProtection Agency (EPA) officially canceled all uses ofDDT, but not before more than 1 billion kilograms of DDThad been introduced into the United States. Like Endosul-fan, DDT disrupts the delicate balance of sodium andpotassium within neurons. The pesticide was effectiveagainst a wide spectrum of insects in the agricultural arenaand also mosquitoes that transmit malaria and yellow feveras well as body lice that carry typhus.

Figure 6. ALDICARB SULFONEThis very toxic insecticide is the sulfone metabolite of thecarbamate aldicarb. Unlike most nematicides, aldicarb isprovided as a granular powder that significantly reduces thehandling hazards. Aldicarb is drilled (spiked) into the soil atplanting or during various stages of plant growth. Thecarbamate is then solubilized by ground water and isabsorbed by the roots and translocated throughout the plant,killing both insects that pierce and suck foliage as well asnematodes in and around the roots. The plants oxidize theparent aldicarb carbamate to the sulfone derivative duringtranslocation and it is this species that is responsible for theinsecticidal properties. Aldicarb is currently registered forcotton, potatoes, sugar beets, oranges, peaches, peanuts,sweet potatoes, and ornamentals.

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Figure 12. CARBOFURANCarbofuran is a member of the carbamate class of pesti-cides derived from the basic carbamic acid moiety. Themode of action of this pesticide is similar to that of theorganophosphates, which act by inhibition of the enzymecholinesterase. Carbamates were first introduced in 1951by Geigy Chemical Co. in Switzerland, but found little usedue to low effectiveness and relatively high costs. Laterstudies indicated that the N-methyl carbamates were muchmore toxic to insects. Carbofuran is a plant systemic andhas a high water solubility, which allows the pesticide to betaken into the roots or leaves. It is also a very promisingnematicide, being registered as a nematicide for alfalfa,tobacco, peanuts, sugar cane, and possibly effective insoybeans, cotton, grapes, and grains. The carbamate has arelatively short residual lifetime, rendering it useful onforage and vegetable crops.

Figure 9. CAPTAFOLThis useful foliage protectant fungicide is a member of thedicarboximide class of pesticides and was first introduced in1961. The dicarboximides are some of the safest of allpesticides that are available and are even recommended forlawn and garden use. The mechanism of action in thedicarboximide class appears to be the same for all membersof the family. They probably act nonspecifically, althoughthe nature of reactions between fungicides and fungal cellcomponents is not very clearly defined. It is generallybelieved that thiol or alkylthio groups of fungal metabolicenzymes are most likely in vivo reaction sites. In additionto home use, dicarboximides are utilized primarily as foliagedusts and sprays on fruits, vegetables, and ornamentals.

Figure 10. ENDOSULFANThis powerful insecticide is a member of the organochlorineclass of pesticides, fathered by the now banned DDT(dichloro-diphenyl-trichloroethane). It is very closely relatedchemically to dieldrin, substituting a heterocyclic sulfur inplace of the saturated bicyclic ring system. Organochlorinesalter both sodium and potassium concentrations in neurons,affecting impulse transmission and causing muscles to twitchspontaneously. Many of the organochlorine insecticideswere found to be very toxic to all forms of wildlife early intheir use history. Endosulfan earned a bad reputation for anaccidential fish kill in the Rhine River in 1969. As aconsequence of their extreme toxicity and because they alsopersist for a long time in the environment, organochlorinesare gradually being phased out.

Figure 11. DIAZONThis insecticide is a member of the heterocyclic nitrogenclass of pesticides and was introduced commercially in1952. It subsequently has been marketed under a varietyof names. Diazinon is a relatively safe insecticide thathas an amazingly good track record around the home. Ithas been effectively utilized in a wide spectrum ofapplications, including insects in the home, lawn,garden, ornamentals, around pets, and for fly control instables and pet quarters. Diazinon has been successfullyused in a slow-release formulation that allows theinsecticide to volatilize at a lower rate, killing flying andcrawling insects in the vicinity.

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ABOUT THE AUTHORSMr. Davidson is a Research Scientist, Center forMaterials Research and Technology (MARTECH) andthe Supercomputer Computations Reseacrh Institute(SCRI), The Florida State University, Tallahassee,Florida, U.S.A. Mr. Page and Mr. French are ResidueChemists, Chemical Residue Laboratory, FloridaDept. of Agriculture and Consumer Services, Talla-hassee, Florida.

REFERENCES

1. George W. Ware, PESTICIDES, Theory and ApplicationChapter 4. W. H. Freeman and Co., San Francisco, CA.(1983).

2. FARM CHEMICALS HANDBOOK, Ed. Charlotte Sine,Meister Publishing Company, Salem, MA. (1991).

3. Michael W. Davidson, Mark F. French, and Michael J.Page, “Drugs for Bugs”, American Laboratory 23, 34-38(1991).

4. Michael W. Davidson “An Introduction to Photomicrog-raphy”, School Science Review 73, 35-46 (1991).

5. Claus Schlett, “Multi-residue Analysis of Crop ProtectionAgents and Metabolites by HPLC and Diode-Array Detec-tion” American Environmental Laboratory 12, 12-17 (1990).