INSECTS AND PLANT DISEASES — CARTER 337 



sects and fed to nonvirus individuals, which in turn have transmitted 

 the virus acquired in this way to plants. 



It might well be asked why some sucking insects feeding on a virus- 

 diseased plant acquire and transmit the virus while other species 

 feeding on the same plant do not. It is true that many viruses are 

 transmitted by very few species of insects, others by only one, whereas 

 some insects can transmit many viruses. The leafhoppers as a group 

 transmit the viruses with a very restricted transmission, the plant lice 

 the less restricted. The peach aphid is known to transmit 21 viruses, 

 many of which can also be transmitted by other aphid species. On the 

 other hand the leafhopper-transmitted viruses are usually limited to 

 a single species of insect, though there are a few exceptions. 



Many species do, however, take up a virus when they feed on diseased 

 plants although they are unable to pass the virus on to other plants. 

 In some of these cases, the midintestine acts as a barrier and instead 

 of passing through into the blood the virus passes on and is ejected 

 with the insect's faeces. In other cases, viruses have been shown to 

 pass through the intestine into the blood of nontransmitting insects, 

 so some other barrier, possibly the walls of the salivary glands, acts 

 to prevent the virus from passing out into the plant during the process 

 of feeding. 



It is difficult to see any but purely academic interest in these relation- 

 ships between insect and virus, but workers in this field feel that if 

 these relationships can be explained, then the nature of viruses, 

 whether living or nonliving, may be explained also. It is true that a 

 few viruses have recently been purified and concentrated into what 

 appear to be purely chemical bodies which are proteins. In this state 

 they can be measured, weighed, and their physical and chemical char- 

 acteristics determined. To those who believe that measurement is the 

 alpha and omega of science, this evidence suffices, but it would be pre- 

 mature to suppose that the last word has been said. Perhaps the safest 

 view to hold in the interim is that while certain viruses, not particu- 

 larly notable for their close relationship with insects, have been "puri- 

 fied," the viruses having close and seemingly obligatory relationships 

 with their insect carriers have not been, and the most tenable explana- 

 tion is that the word "virus" as used thus far covers a number of 

 closely allied but different categories. Whatever may be the actual 

 nature of viruses the fact is not altered that the diseases they cause 

 in plants are still with us. 



THE EFFECTS OF CLIMATE AND WEATHER 



Climate and weather exert an important influence on insects in 

 general and on insect transmitters of plant diseases no less. Climate, 

 being a general expression for all the weather encountered over a 

 relatively large region, governs the distribution of many insects. 



