448  Vegetable  Powder  as  a  Larvicide.  { Am  ji™^™' 
most  cases,  as  effective  as  oil,  and  it  can  be  used  in  places  where 
oil  would  not  serve  the  purpose. 
The  larvicide  in  question  consists  of  various  vegetable  powders, 
including  the  familiar  pyrethrum  or  insect  powder  of  the  stores. 
In  fact,  all  my  earlier  work  was  done  with  the  pyrethrum  powder, 
which  gave  excellent  results.  But  it  was  so  expensive  that,  had 
it  been  generally  used  and  recommended  for  this  work,  the  cost 
would  have  become  prohibitive  within  a  short  time.  So  I  deter- 
mined to  find  some  similar  powder  possessing  the  good  qualities  of 
pyrethrum  and  at  the  same  time  being  available  to  every  one,  and 
not  subject  to  the  speculative  manipulations  of  the  market. 
As  the  larvicidal  action  of  pyrethrum  powder  is  purely  mechan- 
ical, I  considered  it  possible  to  produce  a  powder  from  the  common 
weeds  of  the  neighborhood  that  would  be  just  as  effective,  and, 
indeed,  I  have  found  this  to  be  the  case.  The  weeds  and  grasses  of 
nearly  every  species  in  the  neighborhood,  when  dried  and  reduced 
to  powder  of  the  proper  fineness,  gave  very  good  results,  and  it  is 
not  at  all  necessary  to  select  any  particular  species  in  order  to  pro- 
duce an  effective  powder. 
A  good  powder  should  spread  quickly  and  evenly  over  the  sur- 
face of  water  even  if  thrown  on  by  the  handful,  pushing  the  re- 
sulting film  to  all  parts  of  the  surface  to  be  treated.  It  is,  however, 
much  more  potent  when  applied  in  the  form  of  a  spray  over  the 
entire  area  of  the  larger  pools.  The  covering  capacity  should  be 
from  3,500  to  more  than  4,000  square  feet  per  pound  of  powder. 
Perhaps  the  greatest  disadvantage  in  the  use  of  the  powder  is  its 
short  period  of  effectiveness — probably  only  a  few  minutes — but  it  is 
so  deadly  that,  once  the  larvae  come  in  contact  with  it,  they  die  in  a 
few  minutes,  even  when  immediately  removed  to  another  receptacle. 
The  powder  loses  its  effectiveness  as  soon  as  it  is  thoroughly  wet. 
I  have  found  it  very  effective  for  killing  such  well-known  species 
as  Anopheles  quadrimaculatus,  Culex  quinquefasciatus,  Culex  abomir 
nator,  Aedes  coclumbice  and  Psorophora  cyanescens.  Of  these  the 
species  with  short,  thick  air  tubes  such  as  Anopheles,  Aedes  and 
Psorophora  are  easily  killed,  while  the  species  of  Culex  with  very 
long,  slender  tubes  are  more  resistant,  Cnlex  abominator  being  the 
most  resistant  species  I  have  had  to  deal  with. 
These  studies  have  been  made  in  connection  with  the  work  of 
this  bureau  on  the  relation  of  mosquitoes  and  malaria  to  agriculture, 
under  the  general  supervision  of  Dr.  W.  D.  Hunter,  with  Mr.  D. 
L.  VanDine  in  charge  of  the  field  work. 
