16 INSECTICIDES, FUNGICIDES, AND WEED KILLERS. 



rotten and wormy fruit on which receptacles containing the spores of 

 fungi serve as refuge to grubs and chrysales and as shelter to masses 

 of insect eggs. The destruction of the parasites and their hiding-places 

 by this simple means causes them to disappear completely after a 

 certain time. 



Birdlime traps preventing insects and their larvae from reaching 

 points that they might ravage, are likewise useful auxiliaries. The 

 most usual trap is the ring of tar or of birdlime with which the trunk 

 of tx'ees is surrounded. The going and coming of apterous parasites 

 between the leafy portion and the soil being along the trunk, the ring 

 of sticky substance drawn round the trunk is intended to stop these 

 often daily journeys and to retain all these parasites stuck fast. An 

 examination of the habits of parasites demonstrates that almost all 

 are forced to use this road ; some to seek a refuge in the soil for the 

 night, others to ascend nightly from the soil in which they had 

 taken refuge during the day. Thus the grub descends along the 

 trunk to place itself as a chrysalis in the soil, and the butterfly, even 

 when it is not apterous, ascends along the trunk to deposit the eggs 

 which weigh down the female. The grey worm and many moth 

 grubs go every morning to find a refuge in the soil, to re-ascend the 

 trunk in the evening. This method, now very common, gives perfect 

 results. In arboriculture it is a powerful auxiliary to the liming of 

 the tree, but it is necessary to watch that this sticky substance pre- 

 serves its adhesive qualities and to renew the ring when these have 

 disappeared. Young fruit trees being sensitive and liable to perish 

 after an application of a ring of tar or birdlime, it is well to fix round 

 the trunk a strip of cardboard well fitted and to coat it with the sticky 

 svibstance. The same result is thus obtained without injuring the 

 health of the tree. 



Preventive Treatment by Means of Chemical Agents. — The 

 general conditions as regards the properties of the chemical agents 

 used in the preventive treatment of plant diseases are the same as in 

 the curative treatment. The chemical products must destroy the 

 parasites and be more poisonous to it than to the plant ; they must 

 adhere and preserve their poisonous power for a certain time, and 

 enter into intimate contact with the parasite or with its elements of 

 propagation. When such treatments are applied, as is often the case 

 during the repose of vegetation, the comparative insensibility of the 

 plant enables them to be used in doses, deadly to the parasite without 

 injuring the plant. Most fungi living protected in the interior of the 

 tissue are sheltered from the action of the poisons spread on the 

 surface of the organ attacked, and are evolved in spite of the curative 

 chemical treatment. The important point in plant diseases is to 

 destroy the spores which propagate the disease. To attain this result, 

 different spores must be attacked by different methods. If it be a 

 case of destroying winter spores, very energetic treatment must be 

 applied in winter, for these spores have an extraordinary power of 

 resisting chemical agents. If it is a case of killing summer spores, 

 which, on the contrary, are very sensitive and delicate, a treatment 

 with dilute, anticryptogamic solutions will suffice. Preventive winter 



