44 



and the bean weevil [Bruchus obtectus]. The results of the work of 

 1916 and 1917 show that for both of these widely different pests an 

 increase in atmospheric humidity means an increase in the rate of 

 metabolismas measured by the length of the life-cycle. The optimum 

 percentage of atmospheric humidity is the highest that will not 

 encourage a heavy growth of fungi. An atmospheric humidity of 

 100 per cent, destroys by encouraging the growth of fungi, while low 

 atmospheric moisture destroys directly, probably by the abstraction 

 of water. The egg-stage, at least in the case of the bean Bruchid^ 

 is most sensitive to the efEect of low atmospheric humidity, though 

 all stages are unfavourably affected. It is thought that this fact 

 might be utilised against certain other species of insects infesting stored 

 grain. 



The study of remedial measures against the pear psylla [Psylla 

 jpyricola] [ R.A.E., A, v, 321] has been continued and extended on 

 a,ccount of the contradictory results reported by growers. Experience 

 indicates that while the numbers destroyed by the treatments at 

 present advocated may be sufficient to prevent staining of the fruit, 

 damage is likely to occur to the fohage and under certain conditions 

 to the fruit also. It is possible that spraying just before the blossoms 

 open with commercial lime-sulphur (1 to 9), to which nicotine has been 

 added at the rate of | pint to 50 U.S. gals., would destroy both the 

 eggs and surviving adults and thus form an efficient remedy. What 

 effect this mixture would have on the opening blossoms is not known. 

 Work on the control of the strawberry weevil [Anthonomus signatus] 

 has been previously described [R.A.E., A, vii, 256]. 



Bean seeds have been mined before the plants were above ground 

 by a maggot, presumably the seed corn maggot, Pegomyia fusciceps, 

 Zett. An early spring generation of flies oviposits in the soil upon 

 or near the bean seed, and the larvae enter the seed and destroy it. 

 Experiments described in this report indicate the value of a continuous, 

 wide band of tarred sand or a strip of tarred paper around the plots. 



Some study has been made of the use of hydrocyanic acid gas for 

 greenhouse fumigation. It is recognised that its use is greatly limited 

 by fear of damaging the plants and that the dosage or time of exposure 

 or both should vary with the species of plant and its state of develop- 

 ment, the construction of the house, and possibly with temperature 

 and moisture. The variation in results in different houses lies 

 primarily in the distribution and density of the gas atmosphere, 

 and some way of determining tliis density must be devised before 

 much progress with fumigation can be made. An apparatus for this 

 purpose is described. The evidence gathered from the tests indicated 

 that the gas reached its maximum general concentration within eight 

 minutes or less of the time when fumigation began. There was first 

 a swift up-rush of the gas that produced a heavy concentration entirely 

 above the plants ; the gas then distributed outwards and downwards 

 until the maximum general concentration was reached. After that 

 concentration gradually decreased until it fell below the minimum 

 dosage for insects. Irregularities in distribution occurred which are 

 at present without adequate explanation, and such irregularities may 

 be sufficient to damage the plants if concentration is markedly greater, 

 or to fail to kill the insects if the converse occurs. These irregularities 

 may be partly overcome by increasing the number of generators. 



