selective of all insecticides, and its casualuse 
by uninstructed personnel is hazardous. The 
dimethyl analog of parathion has a higher de- 
gree of insect selectivity, but this is far from 
optimal. However, several simple derivatives 
of this type of compound have an amazing de- 
gree of insect selectivity as shown in table 3. 
Although these parathion analogs have had 
some use as insecticides for specific pur- 
poses, especially in the household or on ani- 
mals, they are generally more specific in their 
action than parathion. This characteristic and 
their considerably greater cost have unfor- 
tunately limited their application. 
More recently two other types of safer para- 
thion-type materials have appeared, which 
promise better insect selectivity together with 
broader scale utility. The employment of the 
methylthio group as a potential activating (elec- 
tron withdrawing) group for the phenyl dialkyl 
phosphates was described by Fukuto and Met- 
calf (9). They showed that the subsequent in 
vivo oxidation of the methylthio moiety to 
sulfoxide and sulfone was a type of lethal 
synthesis in the animal organism, The para- 
oxon and parathion-type derivatives incor- 
porating this group have been found to be highly 
effective as insecticides and nematocides and 
to possess systemic insecticidal action. How- 
ever, they have little or no selective toxicity 
to insects (2). Schrader (36) showed that the 
incorporation of a methyl group ortho to the 
methylthio group as in fenthion (0,0-dimethyl 
0-3-methyl-4-methylthiophenyl phosphoroth- 
ioate) produced a remarkable increase in in- 
sect selectivity. This selectivity was expanded 
to truly dramatic proportions by the addition 
of two ortho-methyl groups. 
Nishizawa et al. (30) made the equally valu- 
able discovery that the insecticidal specificity 
of methyl parathion could be greatly improved 
by the similar incorporation of an ortho-methyl 
group. The resulting compound ‘Sumithion' 
('Folithion', 0,0-dimethyl 0-3-methyl-4-nitro- 
phenyl phosphorothioate) appears to be a 
highly versatile insecticide with a greatly 
reduced mammalian toxicity (table 3), Careful 
biochemical studies have not revealed dramatic 
differences between the processes of intoxica- 
tion and detoxication with Sumithion and methyl 
parathion. The studies suggest that Sumi-oxon 
(P=O) is relatively a better inhibitor of insectan 
than mammalian cholinesterase and that Sum- 


12 
ithion and Sumi-oxon may be somewhat more 
rapidly detoxified in mammals. Thenet effects 
of the several differences in rates of reaction 
are toxicity for insects and safety to mammals. 
Selectivity to Beneficial Insects 
It has been said most aptly (21) that ''there 
is no doubt that the greatest single factor in 
keeping plant-feeding insects from overwhelm- 
ing the rest of the world is that they are fed 
upon by other insects."' The importance of this 
insect internecine warfare is well recognized 
by entomologists, as indicated by much of the 
context of this symposium, and chemical con- 
trol measures for insects are generally to be 
regarded as emergency measures to be applied 
only where biological equilibria have failed to 
curtail pest numbers below economic thres- 
holds. The concept of integrated control (40) 
therefore involves the application of specific 
or selective pesticides in a manner that will 
supplement or augment the activities of bene- 
ficial insects. Selective insecticides will also 
have a major role in protecting the honey bee 
and a variety of other wild bees, syrphid flies, 
and other pollinating insects, which are indis- 
pensable in helping the farmer to produce more 
than $4,500 million worth annually of our com- 
monest fruits and vegetables. 
These types of selective action appear to be 
entirely feasible. There are ways, i.e., by the 
application of systemic insecticides to plant 
roots and through the use of toxic baits, where- 
by broad-spectrum insecticides may be ren- 
dered highly specific. This discussion, how- 
ever, is more properly concerned with the 
intrinsic selectivity of toxicants by virtue of 
their chemical properties and the biochemical 
capacity of both pest and beneficial organisms. 
As an example of possibilities, the behavior of 
isopropyl parathion (0,0-diisopropyl 0-p- 
nitrophenyl phosphorothioate) is interesting. 
A number of years ago the writer noted that 
this close relative of the broad-spectrum in- 
secticide parathion (0,0-diethyl O-p-nitro- 
Phenyl phosphorothioate) was virtually non- 
toxic to the honey bee, although it was highly 
toxic to the house fly. Subsequent study (24) 
showed that the selectivity of isopropyl para- 
thion extended to the Opius parasites of the 
oriental fruit fly (Dacus dorsalis), which was 
