narrow-spectrum insecticides of low toxicity 
to higher animals and with a wide margin of 
safety to beneficial pollinators, parasites, and 
predators. The fundamental aspects of such 
selective action will also lead us toconsidera- 
tions of the most fascinating area of selectivity 
between susceptible and resistant races of the 
same species. 
Selectivity by Restricted Absorption 
From many viewpoints, the DDT-type in- 
secticides can scarcely be improved upon as 
selective insecticides, The data of table 1 ill- 
ustrate the inability of DDT and methoxychlor 
to penetrate the selective barrier of the mam- 
malian cuticle. However, for the house fly and 
the cockroach, absorption through the insect 
cuticle is extremely efficient, so that the ex- 
ternal and internal lethal dosages are virtually 
equivalent. This is, however, not the case for 
all insects--the LDsg for the grasshopper 
(Melanoplus differentialis) is 9,380 mg. per 
kilogram topically and 2 mg. per kilogram 
injected (22), Other types of insecticides are 
readily absorbed by the mammalian cuticle as 
shown by the high cutaneous toxicity of para- 
thion to the rat (table 2), 
Very little careful study seems to have been 
made of the physiological and biochemical 
phenomena involved in the selective rejection 
or acceptance by the living cuticle of insects 
and mammals, Since cuticular absorption is, 
under conditions of normal use, the most 
important avenue for the ingress of insecti- 
cides, investigations in this area should be 
particularly rewarding in the development of 
selective insecticides. 
Selective Toxicity of Parathion-Type 
Compounds 
Parathion (0,0-diethyl 0-p-nitrophenyl phos- 
phorothioate) was the first of the broad- 
spectrum organophosphorus insecticides, and 
its discovery by Schrader (37) marked an im- 
portant milestone in the development of in- 
secticides. The use of this material and its 
dimethyl analog has grown by leaps and bounds, 
and it is estimated that approximately 20 mil- 
lion pounds of these materials are produced an- 
nually, Because of their low cost and high de- 
gree of insecticidal activity, the parathion 
compounds continue to represent the most 
efficient insecticides available. However, as 
shown in table 2, parathion is one of the least 
Table 1.--Selective toxicity of DDT and methoxychlor (29) 
all mg. per kg.| 




i ate cine enzo eee 97 
Insecticide 






| cutaneous | 


3,000 
Methoxychlor.... 

>6,000 (rabbit) 

Periplaneta 
» topical 
Musca 


a 
Table 2,--Comparative toxicity of parathion and malathion (29) 
[ LDs9 mg. per kg.] 








Insecticide 


Eavabieton. |. eee +. 
Matathniions. ss. .'s. 






Musca Periplaneta Protoparce 
domestica, americana, sexta, 
Topical injected topical 

