51 
slowly, a portion of the active principle of the pyrethrum. The slight 
advantage of the emulsified cold oil extract over the simple emulsion 
as already indicated is, therefore, corroborated by the decided advan- 
tage of the emulsified hot oil extract preparation. 
The effect of pyrethrum upon larva' is to throw them into convul- 
sions or paralyze the muscles so that they have no power to direct their 
movements. None of the emulsified extracts applied to the larvae pro- 
duced such effects until we come to the emulsified cold oil applications. 
In these, the characteristic effects are rather uncertainly indicated in 
the stronger applications. With the hot oil-extract emulsion such ac- 
tions w r ere already manifested in the weaker 4£ per cent dilution, and 
very decidedly in the stronger applications. For example, in experi- 
ments 17 and 18, fifteen minutes after the application the full-grown 
larvae had utterly lost control of themselves, and it merely became a 
process of dying from that time. No chance for pupation, as in some 
of the other experiments. 
These facts show that there was really an additional insecticidal 
effect acquired by the hot oil decoction process, the extract of which 
was subsequently emulsified. 
ADVANTAGES OF THE EMULSIFIED HOT-OIL EXTRACT OF PYRETHRUM. 
The experiments above summarized again prove that the ordinary 
methods of extracting the active principle of pyrethrum are question- 
able, or at least unsatisfactory. The hot oil experiments show con- 
clusively that this method does to some extent draw out the insecticidal 
element of the powder, and retains it in the emulsion. However, its 
use upon host plants which are able to resist without injury an oil 
emulsion application of sufficient strength to destroy the insect is more 
expeditious and, perhaps, more economical than the use of the pyre- 
thrum emulsion. But plants which are injured by such an emulsion can 
be successfully treated with a weaker solution of the pyrethrum emul- 
sion, not injuring the foliage, and destroying the pest as effectually. This 
is shown by Table XIV, where, with a 4.] per cent pyrethrum emulsion 
in experiment 16, we have practically the same effect upon the growing 
larvae that a 13 per cent oil emulsion has in experiments 3, W or 12. 
The two latter can be regarded as purely oil emulsions, since it has 
been shown that the aqueous decoctions of the powder really contained 
no insecticidal properties. 
During high temperatures and bright sunshine it is well known that 
more or less danger of injury fo the plant is risked by the use of an oil 
emulsion when the strength which must be applied comes very near the 
maximum which the foliage will hear. This risk can be greatly lessened 
by using the pyrethrum emulsion, because the maximum strength which 
the plant will withstand need not be approached so closely. This ad- 
vantage should not be interpreted as a protective effect of the pyre 
thrum to the foliage, but as an additional insecticidal factor making 
the usual quantity of oil unnecessary. 
