28 



Usinger: Introduction 



TABLE2 (Introduction) 



Toxicity Range and LD-50 of Various Insecticides Against Colonized Culex quinquefasciatus Say Larvae (4th Instar) 



Larvicide 



1 



.2 



.1 



.04 



.03 



.02 



.015 



.01 



.008 



.005 



.004 



.003 



.0025 



.002 



LD-50* 



EPN 





















93 



74 



44 



25 



17 



.0031 



Parathion 

















100 



99 



63 



52 



15 



6 





.0039 



Aldrin 









100 





92 





72 





39 







4 





.0060 



Colloidal aldrin 









100 





90 



80 



73 



44 



36 



31 



16 









.0062 



Heptachlor 









100 





99 





76 





17 













.0070 



Dieldrin 







100 



95 





79 





49 





22 











.01 



DDD 





100 



95 



82 





40 



28 



10 















.023 



DDT 





100 



84 



79 





16 





















.028 



Q-137 







100 



58 





37 



28 





4 













.029 



Chlordane 





100 



98 



50 





9 



2 

















.040 



Lindane 





100 



98 



38 





10 



















.0454 



Toxaphene 



100 



89 



(5 



27 





4 



















.055 



Now DDT may be applied at as low a figure as half 

 a gallon (% pound) per acre. 



With such small amounts of material the method of 

 application becomes very important. In general, mate- 

 rials are applied as dusts, usually wettable, as sprays 

 in solution, or as finely divided mists (aerosols) or 

 smoke particles. Applications may be made by knap- 

 sack sprayer (intro. fig. 38), hand duster (intro. fig. 39), 

 or by various means from a boat, jeep (intro. fig. 40), 

 motorcycle (intro. fig. 41), or airplane (intro. fig. 42). 

 Special methods include the addition of larvicides 

 to irrigation water. 



In spite of the spectacular progress with new larvi- 

 cides, control has been complicated and made difficult 

 by the development of resistant strains of several of 

 our most important mosquitoes. Gjullin and Peters 

 (1952) reported on recent studies of mosquito resist- 

 ance as follows: "Aedes nigromaculis larvae from the 

 treated areas were found to be from three to seven 

 times as resistant to DDT as larvae from the untreated 

 areas (in the San Joaquin Valley). Resistance to 

 toxaphene was less than twice that of larvae from the 

 untreated area . . . Little or no resistance to lindane 



and aldrin was shown ... In a few tests with A. 

 dorsalis, resistance to DDT was 3 to 12 times that 

 of larvae from the untreated area, but no resistance 

 to the other insecticides was indicated. Culex tarsalis 

 larvae from a duck club in the Kern District where 

 applications of toxaphene and aldrin were failing were 

 found to be 10, 33, 11, 215, and 1300 times as resist- 

 ant respectively, to DDT, toxaphene, lindane, aldrin, 

 and heptachlor as larvae from an untreated area." 



Just what the answer to the resistance problem will 

 be is difficult to say. Certainly, it reemphasizes the 

 importance of permanent control measures, and it 

 also suggests that reliance cannot be placed on a 

 single panacea but rather on a sequence of larvicides 

 including, no doubt, materials that have not as yet 

 been formulated. 



Finally, a word should be said as to the timing of 

 control programs. Salt>marsh mosquitoes occur as 

 larvae in the winter and early spring. Therefore drain- 

 age and larviciding should be completed before mid- 

 March. Flood water species such as Aedes vexans 

 must be treated as soon as pools begin to form when 

 the water recedes in spring or early summer. Control 

 in irrigated pastures must be adjusted to water sched- 

 ules during the spring and summer months. Tree-hole 

 mosquitoes can be treated with wettable dusts during 

 the breeding season, and filling with sand and asphalt 

 or concrete may be done during the winter months. 



Intro, fig. 38. Knapsack sprayer for distributing mosquito 

 larvicides (U.S.P.H.S., C.D.C. photo). 



Intro, fig. 39. Hand duster for distributing mosquito larvicides 

 (U.S.P.H.S., C.D.C. photo). 



