2 BULLETIN 1160, U. S. DEPARTMENT OF AGRICULTURE. 
the plant is also included. The plant tolerance of a compound is 
considered to be the maximum concentration at which no appreciable 
destruction of plant tissue is caused. No special effort nas been 
made to ascertain the exact plant tolerance for each compound, and 
the values given are usually less accurate than those obtained for the 
insect. 
PROCEDURE. 
The black aphis, infesting nasturtium {Tropaeolum majus) and 
many other plants, was used exclusively in these experiments. It 
was identified as Aphis rumicis L. by Dr. A. C. Baker, of the Bureau 
of Entomology. 2 This species is easily reared on nasturtium plants 
in the greenhouse at all times of the year except the hottest parts of 
the summer, when reproduction appears to be retarded. In the 
present study it has been found much less frequently attacked by 
hymenopterous parasites than the other species reared in the green- 
house. Furthermore, this aphis has a tendency to remain on the 
plants even when sprayed with toxic or irritating substances, a habit 
of considerable convenience to the experimenter. 
Dwarf nasturtium plants (Tropaeolum majus) grown in small 
flowerpots were used as food plants for the aphids. This plant seems 
to be relatively sensitive to chemicals, and it is probable tnat concen- 
trations of many compounds which will kill nasturtium would be 
harmless to the foliage of orchard trees. 
The compounds were prepared for application in a number of 
ways. Immiscible liquids were applied as emulsions, fish-oil soap, 
capryl alcohol, and in a few instances amyl alcohol being the emulsi- 
fying agents. Liquids soluble in water were mixed with a small 
amount of fish-oil soap to enable the solution to wet and spread over 
the foliage. If the liquids were basic or acidic compounds they 
were applied as soluble salts; the basic compounds were converted to 
sulphates or chlorides and the acid compounds to their ammonium 
salts. The solids tested were either soluble as such in water or were 
made so by conversion to the salt form, as stated for the liquids. 
A spreader, usually fish-oil soap, when applicable, was also used 
with these solutions. 
The solutions or emulsions were sprayed on the plants by means ' 
of a small hand atomizer. Usually a pot containing several plants 
well infested with aphids was used for each test. In some experi- 
ments a portion of a plant was placed in a bottle containing water 
and sprayed, care being taken to see that the stem was surrounded 
by cotton to prevent the dead insects from falling in. An untreated 
plant, immersed in water in this fashion, will support a colony of 
aphids in a normal condition for several days. Eighteen or 24 hours 
alter spraying the percentage of aphids killed was determined and 
the condition of the plants noted. 
The toxic concentration was arbitrarily considered to be the mini- 
mum capable of killing about 95 per cent of the aphids, or, to state 
it in a different way, it t represents the concentration which leaves 
an average of 5 per cent alive. In many instances this figure was 
2 According to Doctor Baker there is still some confusion regarding the number of species of aphids in- 
cluded under the name rumicis. He states, however, that the black aphis found on our plants belongs to 
a single species, whatever that may ultimately prove to be. The name rumicis is therefore tentatively 
applied to the aphis used in these experiments. 
