OBSERVATIONS ON COTTON THRIPS. 99 



As the result of a number of observations and experiments it was found that the 

 length of the egg stage varied from 4 to 15 days, with an average of 8*2. In February 

 the minimum and maximum were, respectively, 9 and 15 days ; in March, 6 and 11 ; 

 and in April, 4 and 9. 



The period during which the larva remained on the plant varied from 3 to G days, 

 with an average of 4*2. From 4 to 14 days elapsed between the entrance of larvae 

 into the soil and the emergence of the adults, the greatest number of emergences 

 taking place on the sixth day. 



From this it will be seen that the developmental cycle may be as short as 11 days, 

 or may take as long as 35 days ; but the writer is of opinion that 18 days may be 

 regarded as the usual time, and the adults do not begin egg-laying until two days 

 after their emergence from the soil. 



Control Measures. 



In discussing the control of this thrips, it should be borne in mind that its be- 

 haviour after the old cotton crop is removed till the new one is well advanced is not 

 yet precisely known. Larvae were difficult to find on wild plants at the beginning 

 of August, but until the rains commenced they were numerous. 



Control may be considered under three heads, namely, suggestions, cultural 

 methods, and insecticide appHcations. 



1. Suggestions. 



(a). If this insect spends the " dead season " on vegetation, aU plants of every 

 description should be removed ; but if, on the other hand, it rests in the soil, the 

 ploughing of the old cotton land should take place as soon as convenient so as to 

 expose the aestivating stage. 



(6). Larvae and adults are very numerous on the last standing cotton, and the 

 writer suggests that, since there is so httle to be gained and so much to lose, the 

 watering of the cotton should cease shghtly earher and the cotton should be cut out 

 and burnt sooner ; by this means the number of thrips that could attack the suc- 

 ceeding cotton crop would be greatly diminished. 



(c). Observations tend to show that heavy waterings have a detrimental effect on 

 the numbers of thrips emerging from the soil. The probable reason for this is to be 

 found in the fact that the soil does not crack so quickly, and the adults, unable to 

 escape from the ground, succumb. The efiect of irrigation on this thrips is a line on 

 which further investigations are required. It is suggested that one or two heavy 

 waterings about the beginning of November might have a very beneficial efiect. 



(d). Owing to the wide range of food-plants, experiments with trap crops should 

 be tried. 



2. Cultural Methods. 



The cultivators of cotton in the Gezira appreciate the extent of the damage this 

 insect can do, and the destruction of all weeds should be the first preventive 

 measure to be adopted. Practically every plant in the vicinity of cotton has been 

 found to harbour the larvae, and all vegetation growing amongst the cotton or in its 

 vicinity should therefore be eradicated. This not only apphes to the growing season, 

 but is equally appUcable to land which will be under cotton the following year, and 

 to the banks of all canals and gadwels. 



