25 



that the asexual forms can continue living- on beans in a green- 

 house through the Winter and flourish vigorously during the 

 following Summer, thus forming a further source of infestation. 

 This is of importance in certain branches of the glass-house 

 industry. 



Mr. J. G. H. Frew has made a study of the biology of the gout 

 fly, and it appears ix)ssible that the severity of the attack can be 

 diminished by appropriate manuring. The relation of the time of 

 sowing to the probability of attack is being studied. 



Another method of control under investigation in the Entomo- 

 logical Department is through the agency of the natural enemies 

 of injurious insects. Parasites of certain pests — the earwig, pear 

 slug larva, and pear leaf midge — are being bred by Mr. Altson for 

 supply to the New Zealand Government. 



The discovery and suppression of winter or alternative hosts 

 is connecting the entomological work with the weed investigations 

 which have for some years been made by Dr. Brenchley in the 

 Botanical Department. 



While one hopes for the fullest possible measure of success of 

 these methods of controlling pests, it remains highly probable 

 that control by spraying will always be of great importance. 

 Serious efforts to improve this are therefore being made by Mr. 

 Tattersfield, in conjunction with Dr. Imms and Mr. Morris. 



For insect pests the spray fluids may be of two kinds — contact 

 poisons and stomach poisons. Of the latter, arsenic in one or 

 other of its combinations is well known and is quite effective, but 

 unfortunately it is poisonous to man and animals. Of the contact 

 insecticides, nicotine is at present the best, but it is subject to the 

 disadvantages of restricted source of supply and high price. 

 Systematic attempts to find substitutes are steadily yielding 

 results ; the method consists in finding the toxicity of an organic 

 compound towards certain test organisms (bean aphis, the larvae 

 of the common Lackey moth and of Selenia illiimaria) , then pre- 

 paring derivatives to see which groups and positions tend to the 

 greatest increase in toxicity. The experimental difficulties are 

 great but it is believed that they are now overcome ; some of the 

 new substances are sufficiently promising to justify study on the 

 field scale. 



Considerable attention has been given by Messrs. Tattersfield 

 and Roach to the extraction of toxic substances from tuba root 

 {Derris elliptica), and as the percentage of toxic material in 

 different consignments may vary between 7 and 22, a method of 

 evaluation has been devised (p. 45) . 



Fungi are controlled by spraying just as insects are, but little 

 is known of the processes involved. Dr. Henderson Smith finds 

 that the number of spores of the fungus (Botrytis cinerea) killed 

 by a solution of phenol of given strength, is for short exposures 

 small ; for longer exposures it rapidly increases, but there is 

 always a residue of spores that die very slowly. The results are 

 expressible by a sigmoid curve. One practical result is that an 

 experimenter can ascertain the strength of a fungicide which, in 

 the steeping of seed, would cause the maximum injury to the 

 fungus with the minimum injury to the grain. 



Heat acts much in the same way as phenol, with the distinction 



