-71- 
in less than 24 hours. The insecticide was as effective 3 days after 
it was applied as when fresh, and ^hen diluted 1:4 and 1:2 with gypsum 
killed the flies as readily as in pure-dust form. In tie field the 
pure powdered derris root gave 87-percent protection after treatments ~ 
with a hand duster on June 16 and 23 and July 2. The untreated plot 
suffered 78-percent injury. Derris-gypsum dust, Derrisol spray, 
sodium fluosilicate spray, and corrosive sublimate solution gave 
70 percent or more protection under the same conditions. Similar 
treatments on August 21 and 29, applied under less favorable con- 
ditions, ^ere not so effective against the late brood of maggots. 
The Massachusetts Agricultural Experiment Station ( 198 ) in 
1931 stated that in laboratory tests, derris dust, both pure and 
diluted 1:2 wi'jth, gypsum, was very effective in killing adults end 
prevented ; ovinosition, whereas eight .other materials '"ere much less 
effective. 
Gorham (.121) in 1934 reported on control la Quebec. Derris ponder 
in suspension in water and Derrisol in solution ™ere among the most 
effective insecticides. Derris dust gave results equal to the liquid 
preparations, but all other dusts tried "ere somewhat less effective 
than liquids. The critical time for insecticidel action is just after 
the larva emerges and Hiile it is still close to. the soil surface. 
The average results obtained during the lest four years ^ith the four 
most efficient materials ere sho^n in the following table: 
Material 
Average injury by larvae 
Spring 
1 . Autumn. 
Percent ' 
Percent 
0.64 
12.65 
2 . 20 
31.05" 
4.36- ■ 
28.71 
4.04 
25.52 
34.03 
51.12 
Mercury bichloride, 15 pounds per 
acre, 18 plots 
Derris, 15 pounds per acre, 20 plots 
Derrisol, 5 pounds per acre, 16 plots 
Sodium silicofluoride, 400 pounds per 
acre, 12 plots 
Checks, 48 plots - ' ' 
The Massachusetts Agricultural Experiment Station ( 200 ) • in 1938 
reported that four applications made on June 5, 12, 19, and 26, of 
cube-clay dust to carrots from seed sown May 4 -z^^e complete protection 
and can be recommended. 
^hitcomb (317) in 1938 reviewed laboratory tests of insecticides. 
In 1929 eggs were pieced on moist blotting naper in petri dishes and 
