can occur is sometimes narrow. And generally, an optimum temperature 

 is found, at which a pathogen launches its most effective attack. 



Relative coolness at application time appears to favor the attack of a 

 good many pathogens. In extensive studies with viruses at Beltsville, 

 although a number of caterpillar pests could be infected at environments 

 exceeding 100° F., both the symptoms and the death rates in these cater- 

 pillars increased rapidly as temperatures were dropped. Many bacteria 

 and fungi multiply best at around 77°to 85 F. Some have failed to multiply 

 at all in environments warmer than the low 90's or even 80's. 



A low temperature requirement of a disease organism need not rule 

 out its use in hot weather. Some larval hosts carry disease organisms 

 down to cool levels in soil. Pathogens sprayed on plants such as trees 

 often can be relied on to land in cool shade in quantities sufficient to do 

 effective pest control work. 



Moisture requirement tests have shown that some pathogens can 

 stand as much dryness as their insect hosts, or more. Viruses have 

 remained infective after severe tests in dry environments. However, 

 humidity or rainfall generally aid the persistence and spread of insect 

 pathogens, even those that can stand considerable dryness. 



In order to grow in living tissue, any pathogen requires moisture, 

 but once inside an insect host a pathogen has suitable moisture conditions 

 automatically provided. 



Time requirements are of prime importance for determining when to 

 apply a microbial material, how soon to expect results, and how long the 

 killing activity will continue. 



An insect may prove susceptible to a pathogen only at one brief stage 

 of its life cycle from egg to adult. In other cases, insects are found 

 susceptible at all stages, although usually there is a time at which 

 susceptibility is greatest. 



The interval to expect between application of a microbe and an exten- 

 sive pest kill can range from 24 hours to many days. Pathogens that 

 develop slowly can be highly valuable when speed is not critical for saving 

 a crop. Some viruses that work slowly, for example, have given outstanding 

 protection against insect pests of tree crops. 



Even chronic diseases that weaken but do not kill insects outright have 

 possible usefulness in pest control, if the application is well-timed. Some 

 protozoa interfere with normal egg production of host insects, thereby 

 reducing prospective broods. And in a European field experiment, a 

 protozoan applied to weevil-infested sugar beets weakened the weevil 

 enough to permit a reduction in amounts of chemical insecticides ordi- 

 narily required for control. 



What Equipment and Treatments Suit Field Uses 



Since microbial sprays, dusts, and granules are similar in form to 

 chemical insecticides, they can be applied with familiar hand or power 

 ground equipment or aircraft used in agricultural and forest pest control. 

 The USDA Forest Service has been giving special attention to improved 

 air and ground equipment and techniques for applying microbial sprays. 



