1124 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



are Pistia Stratiotes, Utricular ia, Azolla, Ceratopteris, and Aldrovanda, 

 the latter a tiny plant allied to Drosera, which catches small aquatic 

 animals by means of its bilobed leaves, after the manner of Dionaa. In 

 Bengal 1 Ambatch ' is represented by the closely allied JEschynomcne 

 aspcra, Azolla nilotica is represented by .4. pinnata, and of the identical 

 plants are Vossia procera and Pistia <tc. In America the Sadd is of 

 entirely different composition. The 'Water Hyacinth' (Eiclilwrniu 

 spcciosa) is the main factor in the block of the St. John's River. It was 

 originally introduced from tropical South America. — B. I. L. 



Deceease of Helpful Paeasites. 



Scale Insects, Limitations of Parasites in the Destruction of. 



By Walter W. Froggatt, F.L.S., Government Entomologist (Agr. Gaz. 

 N. S. W. pp. 1087-93 ; November 1902).— An article on the question of 

 destroying noxious insects by other means than the mechanical processes 

 of spraying, fumigating, or poisoning, such as the introduction of parasitic 

 insects that destroy the injurious ones. It is pointed out that the 

 Eastern entomologists, including Dr. L. 0. Howard, chief of the 

 Entomological Division of the U. S. Department of Agriculture, and 

 the majority of the trained men, declare that while predatory and 

 parasitic insects are most valuable as destroyers of noxious insects, ihey 

 cannot, unless in exceptional cases, check their ravages, unless the farmer 

 or orchardist also lends a hand. 



The other school of entomologists, among whom those from California 

 are the most prominent, claim without any reservation that every 

 noxious insect has a parasite which destroys it, and if we can find the 

 native country of any particular insect, we shall there find the parasite that 

 keeps it in check ; these would stop all mechanical methods and leave 

 everything to the introduced or native parasites. 



The latter method certainly appeals to the orchardist who does not 

 intend to do anything towards keeping down pests, but lets his more 

 energetic neighbours worry at those he breeds in his neglected orchard 

 and which are sufficient to damage all the surrounding trees. He 

 would sit on his fence and wait for the parasites to clean his trees, for 

 if he were to interfere he would damage the insects working on his behalf. 



There is just enough truth in this theory to make it plausible to the 

 ordinary individual who has not gone further into the matter, that every 

 insect has its parasite, internal or otherwise. That parasite will also 

 have its particular foe to keep up the balance of power that we know 

 exists in all branches of the animal kingdom. In a wild country, where 

 man has not interfered with the surroundings, the struggle for existence 

 works out its own end, and the hardy plant or animal best adapted to the 

 situation survives. 



Everything is changed, however, when man appears with axe and 

 plough, clearing off the forest flora to make room for crops and trees, 

 succulent and delicate in comparison with the original vegetation they 

 replace, through long years of cultivation under artificial conditions. 

 The supply of food is increased perhaps a thousandfold, and insects that 

 were restricted on account of a limited food supply also multiply in 

 proportion. 



