552 



[December, 1908. 



PLANT SANITATION. 



LEAD ARSENATE AS AN 

 INSECTICIDE. 



During the present cotton-growing 

 season lead arsenate will be tried on 

 several estates in Barbados as an in- 

 secticide in combating the cotton worm. 

 For the past five years Paris green has 

 been the standard material for this 

 purpose, and all other insecticides have 

 been compared with it, in judging their 

 value. It has been almost universally 

 applied in a dry condition, mixed with 

 air-slaked lime in the proportion of 1 lb- 

 to 6 lb. of lime, although many other 

 proportions have been tried. 



Paris green has been used with good 

 results in the fight against the cotton 

 worm, but there are certain disadvan- 

 tages connected with the use of this 

 material. It is in the hope of getting 

 an equally effective^ insecticide, and at 

 the same time of doing away with some 

 of the disadvantages attendant on the 

 employment of Paris green, that lead 

 arsenate is being tried this year. The 

 hope that this insecticide will prove 

 satisfactory for the purpose is streng- 

 thened by the results of rather extensive 

 trials carried out on one estate at 

 Barbados last season. 



The advantages attending the use of 

 Paiis green are that it is quick in its 

 action, easy of application, and cheap in 

 cost. It may be applied in a dry form 

 to the cotton plant by means of very 

 simple apparatus. The disadvantages 

 connected with the use of this material 

 are, that when applied as a dust, it is 

 easily washed off by showers of rain ; 

 that when large amounts are used, it 

 often burns or scorches the leaves of 

 the cotton plant, and that it sometimes 

 has an irritating effect on the skin of 

 the labourers, which frequently makes 

 them unwilling to do the work of 

 dusting. This is a serious matter when 

 the attacks of the cotton worm are so 

 severe that it is necessary to apply Paris 

 green immediately. Much may be done 

 to obviate this last difficulty by care on 

 the part of the overseers to ensure that 

 the labourers keep a distance of several 

 rows from each other in the field, and 

 on the part of the labourers themselves 

 to keep as much as possible to windward 

 of the poison they are applying. 



Lead C arsenate was first used as an 

 insecticide by the Gypsy Moth Com- 

 mittee in Massachusetts. It had been 

 found t that when Paris green was ueed 



as a spray at ordinary strength (1 lb. to 

 100 gallons of water), the gypsy moth 

 caterpillars were not killed, and when 

 the strength of the mixture was in- 

 creased, the foliage of the tree was 

 injured. Lead arsenate completely re- 

 placed Paris green in this work as soon 

 as it was found that when the arsenate 

 was used at ordinary strength the 

 caterpillars were killed, and that what- 

 ever the strength at which it was 

 applied, it did not injure the foliage on 

 which it was sprayed. 



There are several advantages to be 

 expected from the use of lead arsenate 

 in combating the cotton worm. The 

 fact that, as already stated, this sub- 

 stance does not injure the foliage of 

 plants is of considerable importance. 

 Lead arsenate also has a very great 

 power of adhesion to the leaves of plants, 

 and this is one of the greatest induce- 

 ments to its use. It will be found that 

 after the lapse of two or three hours of 

 suushine from the time of application 

 only a very little of the arsenate would 

 be washed from the leaves by a really 

 heavy shower. 



On the other hand, there are certain 

 features that are likely to seem disadvan- 

 tageous when considered from the point 

 of view of the planter. In the first 

 place, lead arsenate, up to the present 

 time, has been sold only in the form of 

 a heavy paste, and can be applied only 

 as a spray. This necessitates the pur- 

 chase of machines, in place of the bags 

 and tin dusters, which have been used 

 for the application of dry Paris green. 

 Secondly, lead arsenate is much slower 

 in action than Paris green. This is 

 likely to be a disadvantage which will 

 be severely felt unless a very sharp 

 look-out is kept for the first appearance 

 of the cotton worm. In past seasons it 

 has happened frequently that the cotton 

 worm has not been noticed until many 

 of the caterpillars were full-grown. In 

 such a case, as every cotton grower 

 well knows, it is very difficult to prevent 

 damage to plants even by the most 

 prompt application of Paris green. 



It will probably be found best to 

 spray at regular intervals to ensure that 

 a sufficient amount of poison is always 

 on the plant, and especially to ensure 

 that the young leaves are protected as 

 they develope. 



The form of spiayer most in favour 

 with the cotton planters of Barbados is 

 the ' Auto-Spray,' which is a compressed 



