by Gaylen Neufeld
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Published by: The Kansas State Teachers College of Emporia
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Editor: Robert J. Boles
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This issue of The Kansas School Naturalist was written by Dr. Gaylen Neufeld, Associate Professor of Biology at KSTC. He is a cellular physiologist with a research interest in pesticide biochemistry and physiology.
Illustrations by Robert Boles.
The Cover: Man is faced with a dilemma when confronted by crop-destroying insects, competing plants called "weeds" and other pests. The use of pesticides provides a way to deal with this competition but they may also affect organisms which are beneficial such as the robin, earthworm, praying mantid, ladybug beetle and many others.
by Gaylen Neufeld
The usage of pesticides is an emotional issue which has generated bitter controversy particularly in the last decade and beginning with the publication of Silent Spring by Rachel Carson. Some contend that the use of pesticides provides the only way to meet the food needs of an increasing population and to allow the agriculturist to remain economically viable. On the other hand, many see pesticides as an agent of destruction which is leading to the extinction of wildlife and may in time have a similar effect upon humans. Unfortunately, many of the arguments made for or against pesticides are made out of ignorance. A rational approach needs to be followed which takes into consideration the dangers of pesticides to the world ecosystem as well as the economic necessity which an affluent society has created . This issue of The Kansas School Naturalist will discuss some of the aspects of the pesticide problem. Hopefully this will help clarify the issue and allow the reader to recognize the ways that pesticides exert their effects and what measures can be taken to provide a safe environment for future generations.
History of Development and Use
Extensive use of chemicals against insect pests began less than 100 years ago with the use of arsenic-containing mixtures against the Colorado potato beetle. Crude chemicals were used for insect control long before this however. Such uses were local and sporadic and the concoctions were often applied in desperation. The major insecticidal use of arsenic in insect control was begun as the sulfide in the late 1500's. Subsequent but still early uses of insecticides consisted largely of arsenical baits, for ants, grasshoppers and snails, tobacco plant preparations for aphid and lace bug control, powdered pyrethrum flowers for household insects and sulfur as a dust or fumigant for preservation of stored products.
Early modern insecticides were largely inorganic in nature and contained compounds of antimony, arsenic, mercury, selenium, sulfur, thallium and zinc as active ingredients. These compounds are effective insecticides with long residual action. They affect chewing insects only but may also accumulate in soils to the point where they become toxic to plants.
Contact insecticides date back into ancient Chinese history when certain plant extracts were used. About 300 years ago crude tobacco preparations were used for control of a lace bug on pear trees in France. The active component in these preparations was nicotine which is quite effective and is still widely used as the active ingredient in certain commercial preparations.
Fumigants to rid homes and commodities of annoying pests were known to ancients. Homer in the Odyssey mentions the use of sulfur fumes. Hippocrates refers to the use of fumigation by burning various gums and resins.
Modern insecticide use did not flourish until the late 1930's and early 1940's. The Second World War stimulated research on an unprecedented scale and led to a new category of pesticides, the synthetic organic compounds. DDT was introduced in about 1942 and was used with spectacular success against fleas, flies, lice, mosquitos and ticks. The U.S. Army was the major user in its fight to protect the soldier from the ravages of malaria, typhus and other insect-borne diseases. This compound was first developed by Othmar Zeidler, a young chemistry student at Strasbourg, Germany in 1874. However, its insecticidal qualities remained unknown for about sixty years. Then in 1934, Dr. Paul Miiller, who was unaware of
the earlier work of Zeidler, synthesized DDT in Switzerland and discovered its potential as an insecticide. Its first major use saved the potato crop of Switzerland from the Colorado potato beetle.
Organophosphorus compounds were introduced into world-wide use in 1946. These are highly effective chemicals with short residual activity which are used extensively today.
Despite the widespread acceptance and use of modern insecticides, there are three factors which may actually accentuate the insect problem. These are as follows:
History abounds with accounts of sufferings and deprivations visited upon human populations by competing hordes of pests. Present living standards and practices dictate that many forms of pesticides must be used in the control of pests. Benefits to man fall generally into four categories.
Chemicals for Pest Control
Chemicals that are used for the control of pests vary widely as to type and origin. They may be categorized by the type of pest controlled, for example, fungicides, herbicides, insecticides, rodenticides, etc. Another listing categorizes them as to origin or chemical structure.
Table 1. Representative list of arthropod-borne diseases.
|African sleeping sickness
Rocky Mountain spotted fever
A rat flea
The human louse
|Man, domestic animals
Man, rodents, fowl
Man, many animals
Man, horse, bird
Sales of pesticides in the United States, 1962-69, (Modified from: Metcalf, Robert L. 1971. Pesticides. Journal of Soil & Water Conservation. March-April, pp. 57-60.).
Presence in the Environment
Since the time that pesticides have been extensively used, an enormous amount has been introduced into the world's ecosystem. It should be emphasized that for the most part, these chemicals are new to the biosphere and hence there has been little time for natural systems to develop degradative enzymes that
remove them. Therefore, many of these chemicals tend to remain for many years before the levels return to pre-application values. The accompanying figure (Fig. 1) illustrates the total sales of fungicides, herbicides and instecticides in the United States for the period 1962-69. The U.S. Department of Agriculture in 1966 estimated that of 350 million acres of land cultivated, herbicides were applied to 27%, insecticides to 12% and fungicides to 2.6%. However the rate of application is much higher in intensively farmed areas.
The use of pesticides has become virtually indispensable to modern agriculture. Yet for the majority of the chemicals, we only have superficial knowledge concerning the effects of long-term use in the environment. A veritable mountain of data has been accumulated concerning the contamination of animals, crops, soil, foods and even humans. In relation to the total problem however, inquiry needs to be made concerning rates of accumulation in soils and water, effects on soil microflora, the effects upon food chains and the possibility that some of the compounds may in fact be mutagenic, teratogenic or carcinogenic in humans.
Since most of the controversy concerning pesticides has been centered around the chlorinated hydrocarbons and in particular DDT, much of the remaining discussion will deal with this group of chemicals.
Residues of these chemicals have sometimes been found in organisms, soil, and even ice thousands of miles from the point of application. A basic comprehension of the physical properties of these chemicals (chlorinated hydrocarbons or organochlorines) will enable the reader to better understand some of the effects upon the ecosystem.
1. These compounds are said to be persistent in that they are stable and remain in toxic form long enough to contaminate non-target organisms, often many miles from the site of use.
2. These compounds are mobile. They may form suspensions in air and water or adsorb to particulate matter present in air and water. Furthermore the vapor pressure is such that they codistil with water and may escape from wet soil through evaporation of moisture. They may thus circle the globe and come down in the precipitation elsewhere.
3. They have a broad spectrum of biological activity. The effects may extend to many non-target organisms.
4. These chemicals have a low solubility in water and high solubility in lipid material. Biological membranes contain large quantities of lipid compounds. Pesticides are thus easily taken up by living organisms.
The cycling of pesticides in the environment is illustrated in figure 2. The movement through the various compartments of the environment precludes their remaining a local problem. Tests in Maine and New Brunswick have shown that DDT sprayed from airplanes to control the spruce budworm in forests did not all necessarily end up at the site of application. Even in the open, away from trees, only about one-half of the DDT reached the ground. The rest was presumably dispersed as crystals in the air.
One of the greatest concerns of biologists is the effects of these persistent pesticides as they accumulate in greater amounts at the higher trophic levels in food chains. Energy flow in an ecosystem is illustrated by the food web shown in figure 3. Analysis by scientists in New York have shown the complexity of this particular ecosystem. It is evident that most of the consumers feed on several different organisms. In other words, the food chains are interlinked. Complexity is believed to be in part responsible for the stability of an ecosystem. The more crosslinks there are, the more chances there are for the ecosystem to
compensate for stresses imposed upon it. The cross-connecting links also mean that any toxic substances entering the web is distributed across it. The accumulation at higher trophic levels in a food chain as shown by the numbers is referred to as biological magnification. Figure 4 illustrates how this occurs. As biomass or living material is transferred from one level to another usually more than half is lost in respiration or by excretion. The remainder forms new biomass in the next trophic level. Losses of DDT residues in this case, are not proportional to the loss of biomass. For this reason higher concentrations are found in the carnivores.
Food web in a Long Island estuary. Numbers indicate parts per million of DDT and its derivatives on a wet weight. whole-body basis. Arrows represent energy flow. (From: Woodwell. George M. 1967. Toxic Substances and Ecological Cycles. Scientific American. March, p. 24).
Other ecosystems have been analyzed and similar results are found. Lake Michigan, for instance, contains DDT in the water at about 2 ppt (parts per trillion). Bottom samples, however, contain an average of 0.014 ppm (parts per million), amphipods, 0.41 ppm; fish 3-6 ppm; and herring gulls at top of the food chain, as much as 99 ppm. This represents an approximate five million-fold concentration from the water.
The environmental toxicity of mercury is well documented. The initial evidence for this came from the tragedy at Minamata Bay, Japan. Fish and other marine organisms represents a large portion of the diet for the inhabitants of this area. In the period 1953-60, 111 persons were reportedly poisoned with 44 deaths after eating fish loaded with mercury discharged by industries on the shores of the bay. Mercury poisoning is now often referred to as Minamata disease. In 1970, it was found that there was considerable mercury pollution in Lake Erie. Fish from this lake contained mercury at levels up to 3.5 ppm, well in excess
of Federal Drug Administration safety standards. Wild game in the western part of the United States and Canada have been shown to be contaminated with mercury. Wild game in Sweden presented the first warning that industrial discharges of mercury was contaminating their environment. Prompt action by the Swedish government brought restrictions on the uses of mercurials.
Microbial metabolism of mercury yields methyl mercury which is volatile and lipid soluble. This provides a mechanism for the circulation in the environment and accumulation by living organisms.
Analysis of various organisms shows that there is widespread contamination by DDT and other persistent pesticides. Data such as this correlates closely with the increased usage over the years. Furthermore, organisms far from areas of usage contain appreciable quantities indicating again the transport through the biosphere. Innumerable studies show that. this contamination affects virtually all organisms including humans (Table 2). It has been shown that pesticides in humans crosses the placental barrier and thus the fetus becomes exposed as well. Human milk in certain areas of the world reportedly contains pesticide residues and would be declared unfit for human consumption if it were cows' milk.
A great deal of concern has been expressed for particularly the birds of prey that occupy the top of food chains. The greatest danger for these animals is that chlorinated hydrocarbons alters their metabolism in such a way that reduces the amount of calcium in the shell of the egg. With the reduction of this vital element, the eggshell is thinner and more fragile. Breakage of eggs during incubation is extensive and reproductive success is thus decreased. There have been dramatic decreases in the hatches of many birds and hence a population decrease results.
Table 2. Concentration of DDT residues and its derivatives in various living organisms. (From: Woodwell, George M. 1967. Toxic Substances and Ecological Cycles. Scientific American. March, p. 24).
(parts per million)
|California||Grebes||Visceral Fat||Up to 1,600|
|New Zealand||Trout||Whole Body||0.6-0.8|
One of the things to be considered when a pesticide is to be used is the effect it might have upon the balance that exists between different organisms of the habitat. A reduction in population or activity of beneficial predators can lead to an enormous increase in a pest problem. Such an upset in the equilibrium may occur when the beneficial predators are more affected by the pesticide than the pests themselves. Such a situation is illustrated in figure 5. This represents a test plot containing predatory mites and their prey, the springtail. Organophosphorus insecticide was used in the test. The predatory species suffered
immediately while the springtail population was scarcely affected. In fact, there was an increase in numbers due to the reduction of predators.
A computer model of DDT flow in the environment shows that the problem may exist even after the usage has been halted (Fig. 6). The graph illustrates what would happen if in 1970 the world DDT application reached its peak and thereafter decreased until finally it reached zero usage in the year 2000. Because of
the inherent delays in the system, made possible by the persistence of the chemical and the flow through different compartments of the environment, the level in fish continues to rise for approximately 11 years after the application of DDT has begun to decline. Furthermore, the level in fish does not reach 1970 levels until more than two decades later.
It should be noted here that the United States has banned the use of DDT effective December 31, 1972. Public health and quarantine considerations, a few minor crop uses, and export are exceptions.
Ecosystem modeling of DDT movement in the biosphere by scientists at Brookhaven National Laboratory in New York suggest that the residues flow from the land through the atmosphere into the oceans and finally into the oceanic abyss. The model further suggests that the maximum concentration in the air may have occurred in 1966 and in the mixed layers of the ocean in 1971.
Effect of organophosphate insecticide on predatory-prey balance in a test plot. (From: Edwards, Clive A. 1969. Soil Pollutants and Soil Animals. Scientific American. April, p. 88).
Physiological and Biochemical Effects
It has been said that people understand acute poisoning but they find subtle physiological changes difficult to gra sp. A great deal is known about the accumulation. concentration and toxiCity of pesticides but relatively little is known concerning its metabolic effects. Probably more is known about the eggshell thinning phenomenon in birds than any of the other biochemical effects. The chlorinated hydrocarbons induce a thinning of the eggshell in a variety of birds including the brown pelican, bald and golden eagle, black ducks, mallard, Alaskan falcons and hawks, Japanese quail, ringdoves, Western grebe, osprey,
peregrine falcon. sparrow hawk, Bermuda petrel and Herring gulls. On the other hand. chlorinated hydrocarbons do not seem to affect eggshell thickness in the domestic chicken. Related chemicals, the polychlorinated biphenyls (PCB's) used in industry, do adversely affect hatchability of eggs in the domestic chicken. Scientists have shown that the chlorinated hydrocarbon pesticides may affect reproduction in at least two ways. Firstly, they induce the liver to produce steroid hydroxylase enzymes which alters the
structure of steroid reproductive hormones. The hormones become more easily excreted, blood levels are thus lowered and reproductive behavior, such as delayed breeding, is affected. Also, these chemicals appear to inhibit the enzyme carbonic anhydrase which makes the supply of calcium carried in the bloodstream available to the oviduct where the eggshell is formed. Without the supply of calcium, a
thinner eggshell results which is more fragile and thus susceptible to breakage.
Canadian biologists have shown that trout when exposed to DDT (20 ppb in the water) show inability to learn to avoid electric shock. Rats fed DDT following a learning experience took longer to relearn the maze than did animals not so exposed. Estrogenic activity of DDT has been shown in rats and monkeys. DDT also induces steroid hydroxylating enzymes in the liver of the rat. The ultrastructure of liver cells in such animals show morphological changes that may be associated with altered function. Research in the cellular physiology laboratory at KSTC shows that DDT markedly affects the respiratory capacity of mouse and chicken liver mitochondria. In this case it appears that the mitochondrial membrane may be altered to produce this inhibitory effect.
Computer calculation of DDT in the environment. DDT application rate is historically correct up to 1970. The assumption was that the usage would then begin to decline. (From: Meadows, D. H. , et aI. 1972. The Limits to Growth. Universe Books, New York).
There are some life processes that do not appear to be affected by these chemicals. DDT had no apparent effect upon aggressiveness in laboratory mice. In another study, DDT had no apparent adverse effects upon reproduction and lactation in the rat. A 15 week period of feeding DDT to turkeys did not cause alterations of blood pressure, gross structure of body tissues, histology of internal organs, or plasma levels of calcium, cholesterol and protein.
Human organophosphate exposure produced disorientation in space and time, a sense of depersonalization, and hallucinations. Heavy exposures produce convulsions. Electroencephalographic tracings (EEG's) implied abnormal activity in the temporal cortex of the brain.
The herbicide 2,4,5-T (2, 4, 5 trichlorophenoxyacetic acid) has been shown to be teratogenic, that is, it induces fetal malformations in the golden hamster. The effects were less severe for the related herbicide, 2,4-D. Both of these herbicides have been used as defoliants during the Vietnam War. The United States government in 1970 imposed curbs on the usage of 2,4,5-T both for domestic purposes and as a defoliant in the combat zone.
The significance of pesticide contamination to human health is difficult to assess. For many pesticides such as parathion and dieldrin, the effects are rapid and distressingly final. However experiments have shown that chronic DDT intakes amounting to 1, 20, and 200 times the intake of the general population had no deleterious effects as shown by careful clinical follow-up and laboratory testing. Therefore it seems that any effects which might appear will be due to more subtle changes over a longer period of time.
New Approaches to Pest Control
Alternative methods of pest control are in many stages of development and several require more basic research before their potential can be evaluated.
Future of Pesticides
Nonchemical methods of control are not expected to supplant entirely the use of chemical pesticides in the foreseeable future. It is imperative however that some of the practices be changed that have contributed to
the erosion of environmental quality and public confidence. Dr. Robert L. Metcalf, Professor of Zoology at the University of Illinois, suggests that the sale be by prescription and the use supervised by trained plant pest control specialists or phytopharmacists. Professor Metcalf further suggests that successful pest management requires:
a. coordination of programs by highly trained specialists;
b. replacement of "routine" applications by applications based on an assessment of the problem;
c. recognition that some crops can tolerate substantial pest damage without economic loss;
d. abandonment of unnecessary pesticide treatments;
e. changes in agricultural practices to utilize selected crop varieties, cultivation practices, planting patterns, crop rotation, etc., to minimize the pest problem.
Suggestions for the Home Gardener
No panaceas are offered in this section which will allow the gardener to completely discontinue the use of pesticides. Most of the alternatives to chemical control discussed in a previous section are not practical for the backyard garden. As a matter of fact, it is my opinion that pesticides are still needed for use in the garden and on ornamental shrubs. However, ecologically oriented practices can minimize their use.
Use gardening practices that aid beneficial species. Providing hiding sites such as lumber, stones and rank vegetation encourage these natural enemies to take up residence. Flowering weeds should be encouraged where they aren't overly competitive. Many beneficial species need a continual pollen source for protein from which to develop their eggs. Try collecting or buying beneficial species and releasing them in your garden. Green lacewings, praying mantids, ladybugs and others are all voracious predators.
Insect attack can sometimes be avoided by planting as early or late as possible. If the plant does not fit into the life cycle of the pest, a build-up of the population can be minimized. Furthermore, maintain as much diversity as possible. Monoculture (single-crop planting) usually encourages pest build-up. A void planting any crop in blocks. The following plants interspersed among the rest generally help repel harmful insects-mint, onion family, nasturtium and strong-smelling marigolds. Provide a water supply for birds and insects and nest boxes for insectivorous birds. Maintain organically rich soil.
If it becomes necessary to use pesticides, they should be used with extreme care. A discussion concerning some of the safer insecticides follows. A fuller explanation is given in the booklet Pesticides: A Guide to Safe Garden Use by Robert Dingwall.
a. Black Leaf 40 is a nicotine compound that is effective for sucking insects.
b. Powdered sulfur can be used for insect and fungus control.
c. Sevin is a carbamate with a short residual effect.
d. Malalhion is the least toxic of the organophosphates to mammals.
e. Methoxychlor is the least toxic of the chlorinated hydrocarbons but is still quite potent and persistent.
Product List (Taken from The Living Garden, An Environmental Calendar, 1973)
1. Oil Sprays
dormant oil - such as Scalecide
summer oil - such as Ortho Volck, Sunoco Summer Oil
2. Biological Controls
Milky spore disease - a specific control for Japanese beetle grubs, available as "Doom" (Fairfax Biological Control Laboratory, Clinton Corners, N.Y. 12514). Preferably apply after ground thaws in early spring.
Bacillus thuringiensis - a bacterium which attacks only lepidopteran larvae, such as gypsy moth and diamondback moth larvae, tent caterpillars, tobacco bud and hornworms. Apply to specific pest caterpillars as they emerge. "Thuricide" (InternaLional Minerals & Chemical Corp. , P.O. Box 192, Libertyville, Ill. 60048) or "Biotrol" (Thompson-Hayward Chemical Co., Box 2383. Kansas City, Kans.
Some beneficial predatory insects can be bought for release in the garden. They cannot be precisely managed, but the following will help control pests:
ladybugs - feed on aphids, scale insects, mealy bugs, bollworms, leafworms, leafhoppers, corn earworms, etc.
praying mantids - feed on scale aphids, caterpillars, etc.
Tricnogramma - microscopic insect, harmless to humans, which lays its eggs inside those of butterflies and moths and destroys them. Repeated liberations can control codling moth, Oriental fruit moth and imported cabbageworm. (All three are available from Mincemoyer Nursery, County Line Road, Jackson. N.J . 08527; ladybugs and praying mantids from Montgomery Ward Farm Catalogue or Bio-Control , Rt. 2, Box 2397, Auburn, Calif. 95603; Trichogramma and praying mantids from Gothard, Inc., P.O. 370, Canutillo, Tex. 79835.)
3. Botanical Sprays and Dusts
pyrethrum - such as D-con House and Garden Spray. Raid House and Garden Spray, Ortho Home and Garden Insect Spray, Aerosect. Certain additives in pyrethrum sprays have possible hazardous side effects; avoid repeated exposure.
rotenone - available as spray or dust, such as Ortho Rotenone Spray and Dust; toxic to swine, very toxic to fish.
rotenone, ryania, pyrethrum - such as B.D. Tree Spray for orchards (Peter Escher, Threefuld Farm, Spring Valley, N.Y. 10977; Tri-Excel DS. The Natural Development Co., Bainbridge, Pa. 17502).
nicotine sulfate - Black Leaf 40. Use with extreme caution, very toxic to humans; but like other botanicals, it breaks down into harmless compounds soon after application.
4. Safer Fungicides
Bordeaux mixiure - copper sulfate and lime; can be toxic to fish and other aquatic organisms.
sulfur - may burn plants on hot, sunny days; apply in evening.
NOTE: Apply dusts and sprays with care and on a limited and identified target. READ AND FOLLOW DIRECTIONS. Apply when tempera ture is under 85 degrees or in evening to avoid damaging plants.
5. Homemade Sprays for Aphid Control
onion spray - grind or chop green onions, add equal amount of water, strain.
garlic spray - mix one part of garlic extract or powder with four parts water.
pepper spray - grind several capsules of hot pepper, add an equal amount of water, strain.
6. Barrier Bands
"Tree Tanglefoot" (The Tanglefoot Co., 314 Straight Avenue, S.W., Grand Rapids, Mich. 49500). Available as spray or wrapping material. Keeps webworm, gypsy moth and other caterpillars from reaching tree foliage.
7. Other Sources
W. Atlee Burpee Co., Box 6929, Philadelphia, Pa. 19132: compost maker, herb seeds, rotenone, "Tree Tanglefoot."
Geo. W. Park Seed Co., Greenwood, S.C. 29646: cover crop and herb seeds, rotenone, soil test kits.
Wayside Gardens, Mentor, Ohio 44060: Wayside Organic Plant Food, Sea-Born (dehydrated seaweed), Fertosan (organic compost maker).
Cosmic View, Inc., 4822 MacArthur Blvd. N.E., Washington, D.C. 20007: Cosmic Dust organic fertilizer.
Audubon Bookshop, 1621 Wisconsin Ave., N.W., Washington, D.C. 20007: bluebird and other houses and building plans.
Trio Mfg. Co., Griggsville, Ill. 62340: martin houses - aluminum. Purple martins eat flies and mosquitos.
Carson, Rachel. 1962. Silent Spring. Houghton-Mifflin, Boston, Mass.
Edwards, Clive A. 1969. Soil Pollutants and Soil Animals. Scientific American. April, pp. 88-99.
Edwards, Clive A. 1970. Persistent Pesticides in the Environment. CRC Press. 18091 Cranwood Parkway, Cleveland, Ohio 44128.
Graham, Frank Jr. 1970. Since Silent Spring. Houghton-Mifflin, Boston, Mass.
Metcalf, Robert L. 1971. Pesticides. Journal of Soil and Water Conservation. March-April, pp. 57-60.
Moats, Sheila A. and William A. Moats. 1970. Toward Safer Use of Pesticides. Bioscience. April 15, pp. 459-463
Peakall, David B. 1970. Pesticides and the Reproduction of Birds. Scientific American. April. pp. 72-77.
Rudd, Robert L. 1964 . Pesticides and the Living Landscape. University of Wisconsin Press, Madison.
Woodwell, George M. 1967. Toxic Substances and Ecological Cycles. Scientific American. March, pp. 24-31.
The following are available at reasonable cost
Dingwall, Robert J. Pesticides: A Guide to Safe Garden Use. Missouri Botanical Garden, 2315 Tower Grove Ave ., St. Louis, Mo. 63110, 75 cents. The Living Garden, An Environmental Calendar, 1973. Available from Concern/ANS, 2233 Wisconsin Ave., N.W. Washington, D.C. 20007. $3.00 each or $2.50 each for orders of ten or more.
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