CLASS INSECTA 



289 



flea seems to be the agent in the transmission of bubonic plague from 

 rats to man. 



318. Combating Injurious Insects. — Injurious insects may be com- 

 bated in many ways. Mandibulate insects can be destroyed by placing 

 poisoned food where they will get it or by spraying poisons in solution on 

 the leaves or fruit of plants which they eat. This method can be applied 

 in the case of cockroaches, crickets, earwigs, ants, beetles, and lepidop- 

 terous larvae. Suctorial insects may be sprayed with oily substances 

 which close the spiracles and cause suffocation, or if they are in a room 

 where they can be reached with poisonous gases, they may be destroyed 

 by fumigation. Insect powders usually produce their effects by being 



Flo. 188. — Australian lady-beetle, Rodolia cardinalis Mulsant, and the scale insect on 

 which it feeds, Icerya purchasi Maskell. A, the adult beetle; B, the pupa; C, larvae feeding 

 on the scale insect; D, a twig of orange, showing the scale lice, and larvae and adults of 

 the beetle. A to C, X 4; Z), natural size. {From Marlatt, Year-book, U. S. Dept. Agr., 

 for 1896.) 



breathed into the tracheal tubes. In some cases insect pests may be 

 secured in numbers by jarring the plants and collecting them in appro- 

 priate hoppers, after which they may be killed in the manner most 

 convenient. Insects may be trapped in a number of ways and may be 

 combated by the spreading of infectious diseases among them. The most 

 important factor in insect control, however, is their destruction by birds, 

 and to this end insectivorous birds should be zealously protected. 



319. Beneficial Insects. — Both the larvae and the adults of the order 

 Odonata (o do na' ta; G., odontos, tooth), which includes the dragon flies 

 and damsel flies, are carnivorous, the adults being beneficial because of 

 their destruction of mosquitoes, which form a large part of their food. 



Some beetles are beneficial because they attack injurious insects. 

 Among these are the tiger beetles and ground beetles, which live upon the 

 ground and destroy cutworms and other noxious insects which they can 

 capture. Ladybirds, or lady beetles, are very beneficial because of their 



