"pass the economic test,'' which permits it to 
distribute pesticides at a cost that will allow 
their economic usage, 
The objective of applied engineering re- 
search in pesticide application is to develop 
new and improved equipment and methods for 
accurately directed and more uniform ap- 
plication ofall types of pesticides--liquid, dust, 
granular or biologic--to all important crops 
and animals, The successful culmination of 
such research will reduce also the danger of 
damage to crops or wildlife on adjacent areas, 
will improve the control of pests, and will 
reduce the cost to the producer. Engineers will 
give increasing attention, together with their 
cooperators, to developing much more specific 
information needed on many aspects of present 
control programs, There are urgent needs for 
improvements on application methods and 
equipment to obtain better performance with 
less chemical and to reduce drift of the ap- 
plied pesticide from the target site, Thereisa 
trend toward more use of highly directed ap- 
plications of pesticides, e.g., the increased 
use of band application of herbicides in row 
crops, Substantial improvements can and must 
be made in application equipment, but more 
information must be obtained through research 
before these can be effected, 
Deposit Specification 
The first thing that must be known either 
accurately or approximately is how much 
chemical is required and where and how it is 
to be applied, Work by the U.S, Department of 
Agriculture during World War II indicated that 
there was an optimum size of aerosol for use 
in some insecticide spray applications, Work 
by the U.S, Forest Service indicates that cer- 
tain drop sizes yield the best results for some 
types of forest insect spraying, Other re- 
search has shown that by directing the spray 
correctly, effective insect control may be 
obtained with greatly reduced dosages, In the 
final analysis, very little is recorded on re- 
search in deposit specifications--much more 
is needed, 
Particle Size 
It is useful to assume that a pesticide has 
a definable toxicity, In actual practice, we ap- 
ply many times the amount of insecticide 
207 
theoretically necessary to kill the insects if 
the insecticide could be distributed directly 
on the insects so that each insect received the 
minimum lethal dosage, It is interesting to 
calculate the number of insects that could be 
killed by 1 pound of insecticide if ideally 
distributed, For example, U.S, Department of 
Agriculture entomologists indicate that an 
"average" boll weevil may weigh approximately 
17 mg. and that 1 p.p.m. of a good phosphate 
poison is required to kill it, Since the ap- 
proximate amount of poison required to kill one 
boll weevil is mg, then 1 pound would 
2 
100,000 
kill approximately 23 billion weevils, Assum- 
ing a plant population of 23,000 plants per acre, 
we could kill 1,000 boll weevils per plant on 
1,000 acres, Even more startling calculations 
could be made for much smaller insects, such 
as the mosquito and the aphid, but such re- 
sults only lead to wrong impressions, since no 
field-application procedure will approach these 
efficiencies, 
It is well known that the forces on a par- 
ticle depend on its size, It is also generally 
believed that biological effectiveness of a 
pesticide often depends on the size of par- 
ticles into which it is divided, This con- 
clusion is usually based on calculations of the 
theoretical coverage obtained from dividing a 
specified volume into particles of various 
sizes, Table 1 shows how 10 gallons of liquid 
would cover a perfectly flat surface, with 
droplets having a contact angle of 90°. 
From these figures it is not difficult to ap- 
preciate the reasoning in favor of low-gallon- 
age fine-droplet spraying, However, although 
it has been assumed that pest control generally 
improves as we divide our pesticides finer 
and finer, experimental evidence does not 
completely confirm this supposition, We are 
certain that, quite often, the finer the droplets 
the more difficult it is to control their place- 
ment, 
Only a few years ago row crops, such as 
potato and tomato, were being sprayed at ap- 
plication rates varying between 100 to 200 
gallons per acre and with pump pressures from 
300 to 400 pounds per square inch, With these 
high-gallonage applications, there was less 
reason to be concerned about the size of the 
spray droplet than with the mist sprays, which 
often are used at the present time (30), 
