Work on the metabolism of these compounds has been limited. It has been established 
that aldrin is converted to dieldrin in the animal body but dieldrin is apparently not 
metabolized (Bann, et al. 1956). In mice lindane is rapidly broken down to a number 
of products, but they have not been identified (Van Asperen and Oppenoorth 1954), In 
insects a dehydrochlorinated derivative of lindane, pentachlorocyclohexene, has been 
isolated (Sternburg and Kearns 1956). 
The problems associated with the use of chlorinated hydrocarbon insecticides have 
caused a shift of interest to the organophosphorus insecticides, and by far the greatest 
amount of work on systemic materials has been done with this class of compounds, 
There are several reasons for this. Although some of them are highly toxic, others, 
contrary to popular belief, are among the safest insecticides available. Very favorable 
toxicity ratios occur; for instance, ronnel is about 1,000 times more toxic to houseflies 
than to mammals (O’Brien 1959). Moreover, most of them are rapidly metabolized and 
eliminated in mammals. 
Although several compounds in this group have been found to be active as systemics 
against various insect pests, only two, ronnel and Co-Ral, are currently recommended, 
Both are used against cattle grubs, ronnel being administered orally as a bolus and 
Co-Ral applied in spray form, Several other promising materials are under investigation, 
and it is almost certain that better and safer ones will be found, 
This paper consists of a review of the methods used in studying the metabolism and 
determining the residues associated with the administration of systemic insecticides 
to animals. Such information forms the basis for recommending the way they may be 
used without leaving residues in meat or milk, or for establishing the tolerances needed, 
in the event a tolerance is permitted. In addition, a more detailed account will be given 
on the metabolism of the two recommended insecticides and some of those being evalu- 
ated at the present time. 
METHODS USED 
Use of Radioactive Insecticides.--Many of the studies on the metabolism of organo- 
phosphorus compounds in animals have utilized radioactive samples of the test insecticide. 
Phosphorus-32 has usually been the isotope used, although some materials have been 
synthesized with carbon or sulfur labels. It is doubtful that the vast amount of informa- 
tion accumulated could have been obtained without the use of labeled tracers, which 
allow for extremely sensitive, accurate, and rapid methods of analysis. 
Animals are sprayed or treated orally or intramuscularly with labeled samples 
of the test materials as far as possible by the methods previously found effective in 
field tests. Following treatment, animals are held in metabolism stalls to facilitate the 
collection of the many samples required, Excretory samples must be collected quantita - 
tively and numerous blood samples taken, Other samples, such as milk, hair, and fat, 
may be needed. 
Toxicity to Mammals.--The first study is that of toxicity. Organophosphorus com- 
pounds are toxic to mammals because of their ability to inhibit cholinesterase (ChE), 
an enzyme involved in controlling transmission of nerve impulses. The percent of normal 
ChE activity in the blood of treated animals usually reflects the amount of poisoning 
that has occurred, Determinations may be made by manometric, colorimetric, or 
electrometric methods. The enzyme is progressively inhibited by increasing dosages, 
but the degree of inhibition does not indicate the presence or absence of symptoms, 
Indeed, many animals die with 50 percent or more of the activity remaining in the blood, 
whereas others exist comfortably with no detectable activity in the blood, Results ob- 
tained with several insecticides are presented in table l,. 
Blood,--Information on the metabolism of the insecticide itself can also be gained 
by studying blood samples from treated animals. Determinations of total radioactivity 
161 
