78 ANM AL KI ll'OFlTS OF I M il'A K T.M 1 :N'1- OF ill K 'ULTUKK. lO.'JT 
cases, only those lircpar.irioiis fiom plants containing rotenoiie sliowod indi- 
f-ations of cfloctivcness as insocticidos. Tests were also made of several hundred 
samples of American-grown pyrethrum and prei)arations from roots of Crarca 
rirf/inirnia. These were tested by the biological assay method in connection 
with i)lant breeding or selection work under way in the lUireau of Plant 
Industry. This metliod ma^cs possible the rapid determination of the insec- 
ticidal efficiency of a preparation, being about I'O times as rapid as chemical 
analysis. 
A method of evaluating <"onc(«nt rated lly sprays was devel(ti>ed. 
NirOTINE AS A FLMIGANT 
In the work witli nicotine as a fumigant it has been slutwn that this insecti- 
<'ide is more ('tTe< tive when used under dry than under liumid conditions as 
formerly supposed, though the humidity at which the insects are held after 
fumigation apparently has little etTect on the efhciency of the treatment. Eight 
kinds of aphids were used iu these tests, and nine other species of insects, 
including four species of thrips, the housefly, adult codling m(»ths, and silk- 
worm larvae. Much variation in resistance was found, the hoti.sofly l>eing 
very resistant and the aphids wry susceptible. The bean aphid was killed with 
a concentration of two-thirds of a part per million ; the adult codling moths 
and the silkworm larvae were almost as susceptible. Body size and susceptibil- 
ity of insects to nicotine vapor are not directly related. 
A gasoline-driven unit wliich atomizes nicotine solutions and circulates the 
fumigant has been tested for aphid control in greenhou.ses. Toxicity tests 
fallow that where approximately the same concentrations of nicotine vaiKjr are 
used, the cost for material for treatment by this method is about one-half that 
of some of the other well-known methods. It is planned to put [his method on 
a connnercial basis as soon as possible. 
In i»hysiological studies on the effect of nicotine on insects it has been 
shown that the primary effect of nicotine is on the nervous tissue of the 
iiisect's heart mechanism: and in comparing two insects it was indicated that 
in the southern armyworm. \\ here heart nerves have not been found, the etTtx*t 
was much less apparent than it was in the roach, where heart ntM'ves are 
present. 
RECORDING VOLUME AND AMPLITUDE OF THE INSECT HEARTBEAT 
In this W(n"k a method was devised for amplifying the heartb(\u of an insect, 
and recently a special cardiograpliic camera has been developed for making 
liermanent records of the volume and amplitude of the insect heartbeat. With 
this ai)paratus it is jiossible to study the elTect of various types of insecticides 
or pharmacological compounds on the insect heart and make accurate rec«H-ds 
of their effect so that comi)arisons can be made. This api)ararus ojumis a new 
field of investigation, and the results of this work will lead to a better under- 
standing of the effect of insecticides on insects. 
DIGESTION AND ABSORPTION IN SOUTHERN ARMYWORM 
In studies on digestion and absorption in tin* southern armywctrm. it has 
been found that pyrethrum. which is an excellent in.seciicide against many 
insects, loses its toxicity in the digestive tract of the larvai*. wheri'.Ms rotenone. 
the main toxic constituent of derris root, pas.ses through the digestive tract 
with toxicity against mosquito larvae practically unchanged. Incubation of 
these insecticides with various tissues and organs taken from the larvae results 
in little or no decrease of toxicity in the rotiMione but an appreciable detoxi- 
fication of the i)yrethrum. as tested against mosijuito larvae. This helps explain 
tlie ineffectiveness of these insecticides against this insect and in(licates an 
explanation of its behavior in oth(»r cases. A chemical analysis of the bl«»od 
of the .southern armyworm has been made as a i»reliminary to further tests 
on the absorpti(»n of food i>roducts. insecticides, or other substances from the 
alimentary tract. About .''.(> blood constituents were tjuant itat Ively (hMermiiuMl. 
ft is of interest to note that whih' cojiper is foinnl in appreciabb' (|uantities 
in both organic an<l inorganic conjbinat ion, apparently no o\ygcn-transix)rl ing 
<»i respiratory pigment is preseiU in the blo<»d of this insect. This i.s con- 
trary to the a<sumi)tioii with reg.ird lt» <-enMiii oilier insects. 
