real world of indu~trialchemi~try
edited by
W. C. FERNELIUS Kent State Uorersity Kent. OH 44242
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HAROLD 303 South Chem Systems. WITTCOFF Broadway Inc Tarrytown, NY 10591
The Challenge of Herbicides for Aquatic Weeds Dean F. Martin and Barbara B. Martin University of South Florida, Tampa, FL 33620 An aquatic weed is a matter of opinion as well as a matter of location. One aquatic plant is on the endangered plant list in ~ a s s a c h u s e t t and s ~ that same species is-regarded as a nuisance plant, an aquatic weed, in the rice fields of Italy. Aouatic ~ l a n tnrovide s a means of removine excess nutrients in water, provide oxygen through photosynthesis, and provide a habitat for small fish against predators. So do aquatic weeds, but they also may grow in such abundance that they constitute a threat to navigation. Thev mav actuallv nlace bottom-dwelling plants i n t h e shade a n d thus obtain the competitive advantage that causes native plants to be "crowded out" in preference to exotic plants. Two aquatic plants are particularly troublesome in the southeastern United States. The first, water hyacinth, Eichhornia crassipes, was introduced into the United States, probably from Argentina, during a New Orleans horticultnral exposition of the 1880's. The plant has a beautiful lavender blossom and forms an interconnected dense mat. Despite about 100 years of management through chemical, hiological, and mechanical control, the plant still thrives. The second, hydrilla, Hydrilla uerticillata, looks like an aquarium plant and in fact is. I t was accidentallv introduced into ~ l o r i d acanals about 1960, and through mm's efforts, accidental and deliberate, has spread throughout the southeast, to California, and even to a reflecting pool in Washington, DC, where i t had been deliberately planted by employees of the Park Service, who had been assured that they were planting "oxygen grass." Little did they realize that this plant can grow to a length of 20 feet, form a dense, tangled mat, nearly completely occupying the top meter of water. There are a t least six consequences of invasions by water weeds. Navigation is restricted. Recreational uses of water bodies are restricted, so economic losses associated with decreased tourism are severe. Water flow is restricted: up to 90% retardation of water flow has been observed in one hydrilla-filled canal 80 feet wide and 22 feet deep (I). Emersed plants (those with leaves and stems above water) such as water hyacinth, send water into air (transevaporation), and such water loss can he significant-nearly 2 million acre-feet in 17 western states of the United States (I). Some stands of aquatic weeds are potentially dangerous: Price (2) noted that deaths had occurred because swimmers became entangled in dense mats of aquatic weeds and drowned. Maintenance costs can he expensive as a result of the need to remove aquatic weeds periodically, up to 5500lmile annuallv for some canals. There are disadvantages with all three methods used to control aquatic weeds (mechanical, biological, and chemi-
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1006
Journal of Chemical Education
cal). For example, some scientists have believed that mechanical harvesting of hvdrilla can actuallv cause the d a n t to spread unless e;ery sprig that is cut is captured. 0Iganisms used for biocontrol can get out of hand. For example, an organism imported to stresswater hyacinths, once released, does not distinguish between the undesirable water hyacinths and the desirable ones that are used as Dart of a sewage treatment process. Chemical control through the use of herhicides has some special problems as well. Some Problems of Herblcldes Selecting the correct herbicide for use in controlling aquatic weeds has a number of ~roblems.a few of which mav he considered here. Specificity Initially, it might he argued fairly, herbicides were designed for terrestrial weed control, so it was necessary to adapt existing herhicides to use with aquatic weeds or to design new herhicides. Either approach has some special prohlems. Also, overcoming prohlems of "drift" and dilution have been challenging. Size of the Market Market size is a significant problem. Unless the demand for a control chemical is sufficientlv ereat. whv would a manufacturer he inclined to develop a"new'her6icide that was specificallv adapted for a target for - species. . . hvdrilla . example? Silvex, a moderately popular herbicide used for control of aquatic weeds had an annual use in 1979 of 300,000 lhs. ($7.5 million) (3); for comparison, synthetic ethanol (the 50th of the top 50 U S . chemicals) had an annual production of 1.3 billion pounds in the same year with a commercial value of about $350 million. Thus, herbicides for aquatic weeds seem to fall between the top 50 chemicals and orphan drugs in terms of dollar volume. Fear of Trace contaminants The use of Silvex (2,4,5-trichlorophenoxypropionicacid) was cancelled in 1979. On January 28th of that vear. the Administrator of the U S . ~nvir&mental ~genc; (EPA) ordered emergency suspension of Silvex and 2,4,5-T (2,4,5trichlorophenoxyacetic acid) on "forests, rights of way and pasture, home and garden, aquatic ditch hanklweed control, and commercial/ornamental turh uses" (3). In part, the emergency orders were based on data and information developed for and through the EPA rebuttable presumption reg-
istration of 2,4,5-T. Both this substance and Silvex contained the highly toxic contaminant 2,3,7,8-tetrachlorodioxin, (2,3,7,8-TCDD). The background of the 2,3,7,8TCDD is too complex for discussion here, hut excellent reviews are available elsewhere (4). The Silvex cancellation was reviewed in the light of cost/ benefit considerations by Gangstad (3).For example, "marginal-herbaceous weed control" varies, but the overall United States navigation benefits were estimated a t over $7 million, or about 25% of the operating costs for river and barge traffic for the acres controlled. Agricultural benefits (over $4 million) were about 2% of the cost of farm products sold in the areaof control. Other benefits were calculated. The sumtotal benefits of Silvex-based control was in excess of $40 million, and the benefit-cost ratio was about 11to 1. Against this is the unknown cost associated with a highly toxic contaminant (2,3,7,8-TCDD) of unknown impact and uncertain origin (4). Herbicide Resistance in Weeds The age of the synthetic herhicides probably dates from the end of World War I1 because of the development of phenoxyacetic acids under conditions of military secrecy. By 1956,some terrestrial weeds bad developed tolerance to 2,4D, and i t appears that the development of herbicide-resistant terrestrial plants has become increasingly apparent (5). Herbicide tolerance may become a problem with some aquatic weeds, because of the notable physiology that plants like hydrilla seem to have. Successful Herblcldes Some of the successful herbicides that have been developed are summarized in the table together with their mode of action. Delivery of the herbicide is obviously of concern because of the problems of dilution, problemsof spreading to nontarget species, problems of maintaining an effective concentration of herbicide in the target region, and the problem of delivery of the herbicide to the target plant.
Many of these are solved through chemical approaches. Timed-release substances are finding favor so that herbicides trapped in a solid dropped next to plant roots can be slowly released and maintained in effective concentration for prolonged periods. Air boats and inverted systems help deliver herbicide to target plants. An inverted system has the aqueous solution of herbicide suspended inside an organicdroplet. Sometimrsa aeighingagent isadded toadjust the density. In addition, spray in^ ran be done from a helicopter equipped with a nozzle sprayer system, and the herbiride is contained in a pulymer whose viscusity can he adjusted so that srrinr of herl~icide-ladenmaterial can be added. The markeifor aquatic herbicides, as noted earlier, is far from impressive, but it is a necessary product, and will become an increasingly important one. A herbicide can be directed to a tareet plant with considerable accuracv. Alternative approaches, such as mechanical control and biological control. have attractive features but also have disadvantages. In summarv. the manaeement of aquatic weeds through but the frustrause of se1ecti;k herbicides offers tions of limited market plus the problems that arise form a need for specificity, good control of application to target species and ever-present public sensitivity when a potential water supply is treated with herbicides. The solution to these problems demands the talents of many persons in chemistry and in other fields. The ultimate solution may be utilization of aquatic weeds (7) and altered perspectives of chemists in seeking naturally occurring herhicides (8). ~~~
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Literature Clted
isi T ~ ~ ; ~ < D~.d, u o t i r~i u n e1983, p. 14. (7) "Mskin~AquaticWeedsUseful:SomePerspedlveforDevelopingCountriea."Nationd ~eademyof Sciences, Washington, DC, 1976. (81 Dmris. P. M., and Martin. D. F., "Control af Macrophytie Growth by Naturally
Produced Suhstanced'in "Weed Control Methods for Recreation Facilities Mana~goment" (Editor: ~ ~ ~ g sE.t0.) ~ CRC d , Press, Bocs Raton,FL. 1982, Chapter 3. ~
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Mode of Action 01 Some Herblcldes Used lor Control of Aquatic Plants* Herbicide Copper sulfate
Deveioped/First Used 1900'5
Chelated copper Phenoxy acids (2,4dlchioraphenoxy) acetic acid
Endothail 7-oxabicyclo[2.2.1] heptan~2,3dicarboxylicacid Diquat 6,7dihydrodipyrido [1,2-a: 2'.t'-c]pyrarinidiium ion Fluridone 1-methyl-bphenyi-5[3-(trifluoromethyl~ phenyil-q1H)pyridinone
Glyphosate N(phosphonomethy1k glycine
1940's
ca. 1948
1955
1976
Mode bf Action Membrane disruption: affens enzymes associated with photosynthesis. Same, but copper persists even in hard water. Growth accelerators, absorbed on leaves, by roots, translocated 10 active growth areas whem abnormal, twisted. curtailed growth. Behaves like a contact herbicide leaves turn brown, have deslccated appearance. Contact herbicide, rapidly a b sorbed owing to charge. Responsiblefor photodynamic action, production of hydrogen peroxide. Absorbed onto faliage/roots, translocated sites of active growth, Inhibits synthesis of carotenoids. (quenches) leading to photodynamic action and destruction 01 chlorophyll. Action slaw (1-2 months lor control). Inactivated in water: must be applied to emersed plants. Interfereswith amino acid
Degradation Precipitates in the sediment.
Chiefly microbial degradatlon within 30-70 days, depending upon history of treatment.
Chiefly microbial degradation
-2 weeks in water column, faster in sediment. Photodegradation, ilmited microbial degradation; because of charge. can be absorbed onto suspend~d,negatively charged clays, soils. Chiefly through photodegradation.
Microbial degradation.
' d .mayerib.
Volume 62
Number 11 November 1985
1007
