when these materials enter food chains, A 
classical example illustrating this problem 
relates to the use of TDE or DDD (2,2-bis (p- 
chlorophenyl)-1,1-dichloroethane) to control 
the Clear Lake gnat (Chaoborus astictopus), 
This nonbiting gnat is an important pest in a 
resort area because of its incredible abund- 
ance. Herms recorded that 85 million adult 
gnats weighing about 85 pounds were caught in 
a single night at one light trap. TDE was 
chosen for application to the 41,600-acre lake 
because of its high toxicity to the gnat larvae 
and relative safety to fish. The entire lake was 
treated with TDE at 14 p.p.b. in 1949, using 
14,000 gallons of 30-percent emulsifiable con- 
centrate. This resulted in spectacular control, 
and no gnat larvae were found for nearly 2 years 
until reinfestation occurred from neighboring 
lakes. Re-treatments in 1954 and 1957 were 
equally successful. However, adverse effects 
were subsequently noticed in the populations 
of such carnivorous birds as the Western grebe 
and the California gull, Analytical studies of 
the food chains in Clear Lake showed that 2 
years after treatment plankton contained 5.3 
p.p.m, of TDE, although no insecticide could 
be found in the water. Plant-eating fish con- 
tained from 40 to 300 p.p.m. of TDE and 
carnivorous fish, such as the brown bull head, 
2,500 p.p.m. The Western grebe and the Cali- 
fornia gull feeding on these fish contained 
1,600 to 2,000 p.p.m. of TDE (13). 
The problem at Clear Lake has been solved 
to the satisfaction of both conservationists and 
property owners by the substitution of methyl 
parathion (0,0-dimethyl 0-p-nitrophenyl phos- 
phorothioate) as a larvicide. This insecticide 
has an LCso value to first-instar gnat larvae 
of 1.2 p.p.b. compared with an LCs of 115 
p.p.b. for the most susceptible fish present, 
the bluegill sunfish (Lepomis macrochirus), 
Methyl parathion is readily hydrolyzed in 
animal tissues and excreted as p-nitrophenol 
conjugates and phosphoric acid esters and thus 
does not accumulate in food-chain organisms. 
In 1962, three applications over a 2-month 
period at 2,3 to 3.3 p.p.b. gave acceptable gnat 
control and have provided a new concept of re- 
peated low-level applications to kill the imma- 
ture larvae. No adverse effects on wildlife have 
been demonstrated (12). 
The problems of environmental stability and 
biological magnification of pesticide residues 
21 
demand extensive study. In many cases, the 
highly stable chlorinated organics can be re- 
placed by esters, such as the organophosphates 
and carbamates, Malathion and carbaryl (Sevin, 
or l-naphthyl N-methylcarbamate) have gen- 
erally wide margins of selectivity and are 
rapidly degraded and excreted as water-soluble 
metabolites, so that there is no appreciable 
storage in animal fats. The metabolic pathways 
of excretion of malathion have been illustrated 
previously (fig. 1). Carbaryl is believed to be 
detoxified, essentially as shown in fig. 2 (6). 
Although carbaryl is normally effective for 
insect control from 5 to 15 days, its half-life 
in water and soil is of the order of 1 week. 
Significant residues in animal tissues do not 
persist for more than 1 week. No contamina- 
tion of cow's milk has been observed within the 
level of existing analytical methods. Thus, the 
use of this compound represents an important 
step forward in ameliorating the problem of 
environmental contamination (1). 
Despite the usefulness of malathion, car- 
baryl, and other newer esters as replace- 
ments for the very stable DDT and cyclodiene 
insecticides, there is real need for persistent 
insecticides of this type to control many insect 
pests of forests, field andforage crops, range- 
lands, and of man and animals. Methoxychlor 
(2, 2-bis(p-methoxyphenyl)-1,1,1-trichloroeth- 
ane), a pesticide closely related to DDT, is 
much more selective than the latter, with an 
oral LD59 to the rat of 6,000 mg. per kilo- 
gram as compared with 250 fay DDT, It has a 
high degree of insectidical activity, as shown 
in table 1. Most important, methoxychlor is 
much more rapidly eliminated from the ani- 
mal body and thus is free from the major 
problem of fat storage and presumably of 
ecological multiplication. The data in table 11 
contrast the behavior of DDT and methoxy- 
chlor when fed to rats. 
The methoxychlor residues stored after 
feeding at 500 p.p.m. declined very rapidly 
and could not be detected 2 weeks after cessa- 
tion of feeding. 
A recent study with human volunteers fed 
for 8 weeks with methoxychlor at 2 mg. per 
kilogram of body weight per day and supported 
by extensive clinical and histological studies 
has shown the safety of methoxychlor at 200 
times the maximum permissible dietary level 
(39). 
