62 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE, 1939 
cial mixtures. In preparing these dust mixtures rery elaborate pre- 
cautions have been taken by the growers to insure that the product. 
when ready for application, has the recommended rotenone content 
of approximately 1 percent. 
In many districts of the pea weevil-infested areas of [daho, Wash- 
ington, and Oregon an innovation in entomological procedure has 
developed in that the growers of peas have voluntarily examined 
their fields repeatedly to determine what portion- of the fields are 
infested by the pea weevil to such an extent as to warrant the applica- 
tion of insecticides, A.s a result of their experiences during the past 
two or three seasons many of these growers have become adept in ex- 
amining their fields and in recognizing the presence of a potentially 
destructive pea weevil infestation. Some of the larger canning com- 
panies have employed full-1 ime entomologists to aid in the pea weevil- 
control program, while others have employed trained personnel foi 
that part of the season when technical knowledge is needed. 
An outstanding accomplishment in the control activities against 
the pea weevil has been the development of large units of dusting 
equipment. Many of these units are equipped with a 50-foot boom, 
and during the early season of L939 one outfit with an 80-foot boom 
which distributes the dust mixture uniformly throughout the entire 
80-foot swath was developed bv private interests. This particular 
unit has a hopper capacity oi approximately 800 pound- of the 
rotenone-dust mixture and is capable of dusting approximately 250 
acres per day. The use of these dusting units has led to a question 
regarding the extent of mechanical damage caused by their pas 
through the pea fields. To answer this question, a study was made m 
•2 { J typical pea fields in Washington. The resulting data indicate that 
dusters with a 30-foot swath pulled by caterpillar tractors with B-inch 
treads caused sufficient damage to decrease the yield by about 5 per- 
cent as compared with that in fields not treated with insecticides. 
Horse-drawn dusters caused hall' or less than half this damage, and 
r lie 30-foot dusters pulled by trucks caused a decreased yield of ap- 
proximately 3 percent. Although no detailed studies have been made 
regarding the degree of mechanical damage caused by the 50-foot <>r 
80-foot dusters, the fact that the use of tl lese larger unit- decreases the 
number of trip- in any field has led to the general conclusion that 
they cause less damage than the -mailer unit-. The comparatively 
-mall degree of mechanical damage caused by the dusting units is 
not held by the growers t<> be important in comparison with the 
excellent control of the pea weevil obtained by their use. 
Tests in the laboratory and in the field indicated that the effective- 
ness against the pea weevil of dust mixtures containing rotenone was 
not increased by the addition of various conditioner-. In general the 
mixture- containing L.O percent of rotenone were more effective than 
those containing 0.75 or 0.50 percent Much less difference was noted in 
the toxicity t<> the pea weevil between dusts containing L.O and 0.75 
percent of rotenone than between those containing 0.75 and 0.50 per 
of this ingredient, particularly in tests made during the spring. 
These results agree in genera] with the limited held trials with dusts 
lining these percentages of rotenone. A cube-dust mixture with 
diatomaceous earth a- a carrier did not give -<> high a degree of 
mortality of the pea weevil a- cube with talc a- a carrier, particular!) 
