412 BIOLOGICAL CONTROL 



negative, further experiments are necessary under the new environ- 

 mental conditions wherein it is proposed to colonise them. 



5. It will be readily understood that, under the conditions 

 demanded by noxious weed control, parasites assume the opposite 

 role to the one they perform in the repression of insect pests. In 

 other words, parasites require to be rigidly excluded in all cases 

 where insects are utilised in weed repression. Under a new and 

 favourable environment the unforeseen entry of such agents might 

 well result in the activities of a valuable phytophagous insect 

 being permanently nullified to a degree which, in an extreme case, 

 might almost amount to totality. 



6. It needs to be recollected, as W. R. Thompson has pointed 

 out, that, although insects entail a vast amount of destruction to 

 economic plants, they rarely cause sufficiently vital damage which 

 will affect the survival of wxll-established species to a marked 

 degree. Measured in economic terms, such damage frequently 

 assumes vast proportions, but in the biological sense it is not 

 necessarily of the same importance. Furthermore, the recupera- 

 tive powers of plants are so great that they are much less liable 

 to be killed outright as the result of insect attack than insects 

 themselves, which almost always succumb from the effects of 

 their parasites. 



In the pages which follow an account is given of the principal 

 examples of the biological control of pest-plants. 



Control of Lantana. The first attempt to control a noxious 

 2)lant by its insect enemies was made in the Hawaiian Islands. 

 Nearly seventy years ago Dr. Hillebrand, the distinguished 

 botanist of the Islands, introduced the thorny species of Lantana, 

 L. camara, for ornamental purposes. This shrub subsequently 

 became a veritable scourge to the pastures of low-lying regions, 

 especially on the leeward side of the Islands. It is a native of 

 the warmer parts of America, and during a visit made by Koebele 

 to Mexico in 1898 it was observed in its habitat, and some insects 

 were bred from Lantana seeds. In 1902 Koebele again visited 

 Mexico, this time with the object of securing any promising 

 insect enemies of the plant for purposes of introduction into the 

 Islands. The result of his work was that 23 species of insects 



