The Air f lane in Forest-Pest Control 



473 



place dusts. The sprays were less af- 

 fected by wind and adhered to foliage 

 much better, but the quantity of in- 

 secticide needed was still too high for 

 economical application by airplane in 

 this country. Aerial distribution of both 

 sprays and dusts continued to be used 

 to some extent in European forests be- 

 cause of the higher values at stake. 



THE DISCOVERY of the astounding 

 insecticidal properties of DDT in 1943 

 revolutionized airplane spraying. Here 

 was a chemical that by previous stand- 

 ards was unbelievably toxic to many 

 insects and was therefore just what 

 was needed to make aerial application 

 practical. Soon the military forces were 

 spraying entire islands in the Pacific 

 to kill mosquitoes and other disease- 

 bearing insects, and when DDT and 

 other new organic insecticides became 

 available for civilian use, airplane ap- 

 plications were tried on crops and for- 

 ests. The results of the trials in forest 

 spraying were so encouraging that the 

 insecticides have been put to a wider 

 use each succeeding year. 



Much of the forest spraying has been 

 limited to applications covering fewer 

 than 1,000 acres, although in 1947 

 413,000 acres of western forest land 

 were successfully treated for control 

 of the Douglas-fir tussock moth and in 

 1948 more than 200,000 acres of east- 

 ern woodland were likewise treated 

 for gypsy moth. DDT sprays have been 

 so effective against those insects that 

 airplane spraying has become the 

 standard method of control. 



In experimental tests, good results 

 also have been obtained in controlling 

 the spruce budworm, hemlock looper, 

 pine sawflies, and the Saratoga spittle- 

 bug, but with bark beetles and certain 

 other insects control has been unsat- 

 isfactory. Thus, airplane spraying 

 does not solve all forest-insect prob- 

 lems, primarily because of the difficulty 

 of obtaining a uniform deposit on all 

 trees and on all parts of a tree. 



The method is most effective for the 

 foliage-feeding species that actively 

 move about in the tree crowns; with 



them, a uniform deposit is not neces- 

 sary, because their normal activities 

 eventually bring them in contact with 

 a lethal dose of insecticide. The less 

 active defoliators and those in pro- 

 tected, situations can probably be con- 

 trolled, but higher dosages or multiple 

 applications may be required to com- 

 pensate for the uneven distribution of 

 the insecticide. 



It has not been possible to obtain 

 an efficient deposit of DDT sprays on 

 tree trunks or other vertical surfaces 

 with aerial application. 



THE SPRAY MIXTURE most com- 

 monly used in forests is a solution of 

 DDT in No. 2 fuel oil. The DDT is 

 first dissolved in a naphthenic hydro- 

 carbon solvent and then diluted to the 

 desired volume with the fuel oil. The 

 usual dosage rate is 1 pound of DDT 

 in 1 gallon of liquid an acre, although 

 under favorable conditions dosages as 

 low as l /4 pound in 1 gallon or less 

 an acre have been effective for some 

 insects. In spraying watersheds where 

 fuel oil might impart an objectionable 

 taste or odor to domestic water sup- 

 plies, xylene is used as a solvent and 

 kerosene substituted for fuel oil. 



Emulsions and suspensions have 

 been used to a limited degree in ex- 

 perimental work. The former are pre- 

 pared by first dissolving the DDT in 

 a solvent as in preparing oil solutions, 

 then adding an emulsifying agent and 

 diluting with water. Emulsions have 

 two serious disadvantages: They can- 

 not be exposed to freezing tempera- 

 tures and they are more toxic to fish 

 and other aquatic animals. Suspen- 

 sions are made by dispersing wettable 

 powders or so-called colloidal prepara- 

 tions in water. Wettable powders have 

 not been satisfactory, because the sus- 

 pended material tends to settle rapidly 

 after mixing, clogging the equipment. 

 The colloidal dispersions have not 

 been adequately evaluated. 



THE AIRPLANES most commonly 

 used in 1948 to apply sprays were 

 military biplane trainer-type aircraft 



