by all insects. For example, the muscoid flies 
Phormia regina, Callitroga macellaria, and 
Muscina stabulans, although they frequent the 
same environment as the house fly, have not 
developed any appreciable resistance to DDT, 
under conditions where the house fly rapidly 
developed total immunity (3). 
Strains of insects highly resistant to one 
insecticide seldom, if ever, are tolerant to a 
wide variety of pesticides. This is graphically 
shown by the data in table 9, where the toxicity 
of a number of insecticides is compared to a 
fully susceptible Sy atpm, 2 DDT- and cyclo- 
diene-resistant strain Rep, and a malathion- 
and Chlorthion-resistant strain Rsq. It is 
evident that strains of flies totally immune to 
a given insecticide remain fully susceptible to 
compounds with differing modes of action (com- 
pare DDT and trimethyltin chloride), More 
importantly, minor structural changes, suchas 
the addition of a single o-chlorine to DDT, al- 
most completely overcome the DDT resistance 
by blocking the dehydrochlorination of the 
compound by the detoxifying enzyme DDT'ase 
(27), Similarly a-deutero-DDT has been pro- 
posed as a remedial insecticide for DDT- 
resistant Aedes aegypti, the carbon-deuterium 
bond proving less susceptible to enzymatic 
action than the carbon-hydrogen bond of DDT 
(34), Although the carbamate o-isopropyl- 
phenyl N-methylcarbamate is highly toxic to 
susceptible house flies, it was completely in- 
effective against the resistant strains selected 
by DDT and the cyclodienes and by malathion 
and Chlorthion (table 9), However, the closely 
related m-propargyloxyphenyl N-methylcar- 
bamate was toxic to all three strains and the 
thiolane oxime ‘N-methylcarbamate showed no 

Table 9.--Comparative toxicity of some insecticides to susceptible and 
resistant house flies! 
Insecticide 
DDT + 0.2 pt. N,N-dibutyl-p-chlorobenzenesulfonamide......... 
(o-C1-DT) 2-(4-chlorophenyl) -2-(2,4=dichlorophenyl) -1,1,1- 
Pra EO RO eI NaMeraycratersvere elect lelateholelohetelere viel eltel.ere 
CIDOB CNT ON Meter eters clsuel seveleile, cere everouskerercnedoretedWerereyere! « 
O-ethyl 0-2,4,5-trichlorophenyl N-ethylphosphoramidothioate.. 
MreimethyttanmchVOrTGe’, sicrcte siete eieie cleleys ele! die e/e sete 
o-Isopropoxyphenyl N-methylcarbamate......... 
o-Isopropoxyphenyl N-methylcarbamate + 5 pt. piperonyl 
WiEGRIICD 6 Soro Hd oO OU COCO OO COs OOOO OOO DOOOOD 
m-Propargyloxyphenyl N-methylcarbamate....... 
1,3-Dithiolane-2-oxime N-methylcarbamate..... 
aa Metcalf, R.L., unpublished data. 
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0),0) 0) elete (¢ 6/6) ¢, 28 \07e. ee 
oe) eb) 0)\u)v 6 6's 6 wv 8) sale 
Topical LD5q Mg. per fly 
SyAIDM Rgp Rsc 
.43 
.16 
>100 
. 39 
. 38 

025 
