In many places they prevent cattle from grazing and the constant 
irritation of bloodsucking reduces weight gains and milk yields. 
With the large increase in irrigated pastures in the West, 
mosquitoes have multiplied and created problems to the livestock 
owner. No estimates are available on the losses that mosquitoes 
cause to livestock, but they are sizeable and are likely to in~ 
crease. 
Three species of bot flies attack horses, causing them to be 
nervous and difficult to manage. The larvae, which inhabit the 
stomachsof horses, no doubt cause loss of weight and unthrifti- 
nesse 
Screw=-worms are found from southern California to South Carolina. 
They cause the most injury to stock in the South, but nearly 
every year they invade the Midwestern States as far north as 
South Dakota. Screw-worms will kill animals if they are not 
cared for and their wounds treated. The largest loss to the 
grower arises from the constant vigil necessary (5-8 months in 
Texas) to find infested animals and care for them. The ranchers 
must employ riders to look over the stock every few days and 
care for infested animals. This extra labor costs many times 
as much as the chemicals used in treatment of wounds. Exclusive 
of this cost, the losses due to screw-worms (death, permanent 
injury, poor weight gains) are estimated at $20 million annually. 
Black flies are bloodsucking insects of livestock that at times 
actually kill animals because of huge swarms present. They are 
present in many places over the United States, but usually they 
are unnoticed because they feed on stock during the night. No 
doubt these bloodsucking pests have a deleterious effect on 
animals. 
Horn flies are distributed over the entire United States. They 
suck blood, causing irritation, and cattle fight them with a 
consequent loss of weight gains and milk flow. Stock suffer 2 to 
6 months of the year from this pest, depending on the latitude. 
Experiments by Laake 9/ showed that beef cattle treated with DDT 
gained 50 pounds more in one month than similarly infested animals 
not treated. Knipling 10/ estimated an annual loss of at least 
$100 million in beef cattle and $50 million in milk production 
9/ Laake, E. W. 19h6. DDT for the control of horn flies in 
Kansas. Jour. of Econ. Ent. 39:65<68. 
10/ Knipling, E. F. 195. The need for controlling livestock 
pests. Amer. Chem. Soc., Advances in Chemistry Series. (In press). 
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