A STUDY OF THE FACTORS OF CONTROL. 89 



on 25th January ; and this green crop was dnsted with Paris green, at 

 the rate of 8 lb. per acre, and turned under, close against the cane, on 

 i4tli February. During March the adjoining cane on each side of this 

 test-plot was treated with carbon bisulphide. Since the grubs were not 

 noticeably destructive even to untreated cane, the final results of the 

 experiment were inconclusive. 



In my own preliminary experiments with arsenic during 1918, 

 however, I based my judgment as to the amount required upon the 

 above experiment, but increased the dose to 10 lb. in most instances, 

 in one case using 20 lb. The results (108) from these experimental 

 plots at Meringa were very satisfactory, but evidently not so much due 

 to the poison used as to other factors in the control, i.e. cultural 

 measures, &c. 



In subsequent field experiments I gradually increased the dose, 

 finally using only the white arsenic. I mixed the dry poison with soil 

 from the Greenhills estate, and found that it required a comparatively 

 large amount to be cpickly fatal to grubs put into it. Yet when I used 

 approximately w'hat would he 80 lb. per acre all the grubs died in 

 1-4 days. During the 1920 season we treated large areas both at ]Meringa 

 and at Greenhills, dusting the poison in a furrow made close against 

 the side of the stools, and covering it in. In these experiments we used 

 arsenic at the rate of 20 lb., 40 lb., 60 lb., and 80 lb. per acre. A 

 drought season followed, so the grubs huddled close under the stools 

 and went fairly deep, to maintain the necessary moisture. Few appeared 

 to have gotten out far enough to get the effect of the poison ; hence the 

 results of the treatment were most discouraging. 



In May, 1920, we started another series of experiments at Greenhills, 

 this time placing the poison in the drill, either just before or soon after 

 the cane was planted. In all forty-six plots, of five rows each, were 

 used, the treated plots alternating with untreated checks. In this work 

 we used arsenic at the rate of 40 lb., 60 lb., 80 lb., 100 lb., and in two 

 plots 200 lb., to see if it had any injurious effect upon the plants. One 

 of the experimental fields was in a section of the estate that usually 

 shows grub injury soonest, and this was planted to the variety D 1135. 

 It was treated with the poison on 12th May, when the drills were partly 

 filled and the cane a few inches high. On 24th February, 1921, the 

 devastation of the grubs began to be evident in the checks. We found 

 45 grubs under the first stool that we examined, and 13 of them had 

 reached the third stage. A stool in the plot with 200 lb. of arsenic only 

 had 5 living grubs, and we discovered 4 that had recently died of the 

 poison. None of these grubs had reached the third stage. A stool in a 

 check alongside gave 18 grul^s, and 4 of them had reached the third 

 stage. Digging a stool in the plot with 100 lb. arsenic we found 11 grubs, 

 2 being in the third stage. Next, in the plot where 80 lb. arsenic had 

 been applied, there were 7 grubs to a stool, 1 having reached the third 

 stage. In a plot with 60 lb. arsenic, 8 grubs were found under a stool, 

 1 having reached the third stage. Digging a stool in the plot with 40 lb. 

 arsenic, we got 9 grubs, 3 in the third stage. A check-stool alongside 

 had 18 grubs, 10 in the third stage. Hence it will be noted that there 

 was a marked decrease in the grubs in all of the treated plots, the l)est 

 results being where the greater amounts of arsenic had been used, 

 ^Moreover, in the treated cane, most of the grubs had apparently died 



