Insects in the Forest: A Survey 



tical in the woods. Early attempts with 

 airplane dusting and spraying were un- 

 satisfactory. Before the development of 

 DDT, several attempts were made in 

 Canada and the United States to con- 

 trol leaf feeders with the then common 

 insecticides lead arsenate and cal- 

 cium or cryolite by dusting from the 

 airplane or the autogiro. Those experi- 

 ments were of much technical interest 

 but developed little of practical value. 

 We now know that those attempts 

 failed because of the lack of a suitable 

 insecticide. 



THE DEVELOPMENT of DDT during 

 the war and several more remarkable 

 insecticides since then among them 

 benzene hexachloride and chlordane 

 has presented an entirely new concept 

 of the practicability of insecticidal con- 

 trol of forest insects. The toxicity of the 

 arsenates or cryolite was so low that 15 

 to 30 pounds an acre was needed to 

 obtain the same degree of control as 

 can be achieved with l /^ to 1 pound of 

 DDT. During the season of 1947, more 

 than 500,000 acres of forest land was 

 treated in various parts of the United 

 States with DDT at a rate of a pound 

 an acre. Satisfactory control resulted 

 at costs ranging from $1 to $3 an acre. 

 Much more remains to be done in per- 

 fecting equipment and improving the 

 technique of application, but it seems 

 safe to generalize that the control of 

 defoliator outbreaks in the future will 

 be a sound and economical forest oper- 

 ation. On the average, from year to 

 year more than 2 million acres of forest 

 land is defoliated annually. This en- 

 tails the destruction of 10 to 75 percent 

 of the trees in outbreaks of many differ- 

 ent insects and in all cases a tremen- 

 dous reduction in growth in the trees 

 not killed outright. It does not appear 

 too optimistic to hope that more than 

 one-half of this loss can be prevented 

 by aerial spraying with the new chem- 

 ical weapons supplied by science. 



THE DIRECT METHODS of insect con- 

 trol just described are not always en- 

 tirely satisfactory for several reasons: 



411 



They are expensive; they are not al- 

 ways so effective as desired; they are 

 strictly alleviative, that is, they do not 

 alter the underlying causes of insect 

 outbreaks. Dr. Hopkins fully recog- 

 nized these disadvantages in his ear- 

 liest efforts and again and again 

 pointed out the advantages of adopt- 

 ing practices that would make condi- 

 tions unfavorable for insect attack. 

 He fully appreciated the impossibility 

 of applying the methods that were then 

 being developed for the control of gar- 

 den- and truck-crop insects to forested 

 areas. He made many suggestions for 

 the control of insects affecting crude 

 and finished forest products prac- 

 tical suggestions based on operational 

 procedures. 



As early as 1913, entomologists of 

 the Department of Agriculture pointed 

 out that infestations of the gypsy moth 

 in the New England States gained 

 momentum in stands of hardwoods 

 composed largely of "favored" species. 

 The idea was then proposed that for- 

 ests within the gypsy moth zone be 

 managed so as to increase the propor- 

 tion of unfavored host species. Similar 

 findings, published in 1924, resulted 

 from studies of the spruce budworm in 

 Canada. Basic points to consider in 

 managing the spruce-fir type to lessen 

 the destructiveness of spruce budworm 

 outbreaks were found to be the pre- 

 dilection of the spruce budworm for 

 fir, the tendency of fir to regenerate at 

 the expense of spruce ( so that fir domi- 

 nated in the cut-over stands), and the 

 ability of younger, more vigorous trees 

 to withstand defoliation. 



In studies of pine bark beetles much 

 research has been pointed toward find- 

 ing preventive control measures. So far 

 it is only with the western pine beetle 

 in the ponderosa pine that any specific 

 method of management has been found 

 and proved to be practicable and ef- 

 fective. In that case it was the predilec- 

 tion of the beetle for certain susceptible 

 trees in stands of ponderosa pine that 

 served as the key to management con- 

 trol. Studies of the characteristics of 

 many thousands of beetle-killed and 



