20 Control of the Greenhouse White Fly 



It is clear that the efiect of naphthalene on white fly depends on the 

 presence of the residue of tarry acids. The finding was checked by fumi- 

 gations of an infested greenhouse with the various grades, but as it is 

 at best a poor remedy these will not be detailed. The materials give an 

 unpleasant flavour to the fruit and it must be realised that the intro- 

 duction of an unknown amount of carbolic acid among growing plants 

 is a risky proceeding. The vendors of naphthalenes state that the crude 

 forms contain a very variable amount of tarry acids in the different 

 samples, but they are unable to guarantee the strength. This would 

 make it impossible to standardise a treatment. 



(2) Tetrachlor ethane. This liquid has been occasionally used for green- 

 house fumigation during the last two years at the Experimental Station, 

 and gives good results against white fly. Its use is simple as it is merely 

 poured down the centre of the house in the evening and this is kept closed 

 for as long as possible on the following day. As it is not a very poisonous 

 substance it may be used in conservatories opening into dwellings where 

 the employment of cyanide is undesirable and it has a future as a f umigant 

 in very small greenhouses where the measurement of the small quantity 

 of cyanide required is a difficulty. As it costs about ten times as much 

 as cyanide fumigation the trade grower should take little interest in it. 



Its action on adult A . vaporariorum is doubtless toxic, but it appears 

 to kill the scales through the effect of the vapour on the wax as con- 

 siderable numbers of the flies attempt to emerge subsequent to the 

 fumigation and die when partially free from the pupa cases. It has no 

 effect on the eggs. It has been used for a wide variety of plants including 

 tomatoes and no damage has resulted except in one case when the foliage 

 of three young sycamores (Acer pseudoplanafus) growing in pots turned 

 brown the day after the fumigation and was subsequently shed. It may 

 be therefore that some greenhouse plants would suffer from it. Daylight 

 during the fumigation did not tend to damage and no preparation of 

 the plants appeared to be necessary. 



The liquid should be used at the rate of about half a pint to 1000 eft. 

 of space and the fumigation should be repeated as with cyanide. The 

 complete success of the fumigation depends on its duration, and the 

 limiting factor to this is the weather as the house can be kept closed 

 longer in dull than in sunny weather. A dull period should therefore be 

 chosen when possible. The mortalities obtained in four experiments, with 

 varying amounts of tetrachlorethane and varying durations, when in- 

 infested plants were sleeved after the fumigations and kept under 

 observation for two to three weeks, are shown in Table V. 



