SOME EXPERIMENTS WITH CONTACT INSECTICIDES. 



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SOME EXPEEIMENTS WITH CONTACT INSECTICIDES. 



By J. Chables Johnson, M.A., M.Sc, Biological Institute, 

 University College, Cork. 



Insecticides may be divided into two classes : those v/hich have to be 

 eaten to cause death, and those which kill the insect when applied to 

 its external surface. The term " contact insecticide " has been used 

 to designate the latter class. It is believed that such insecticides pro- 

 duce their fatal effect, not by penetration of the hard chitinous covering 

 of the insert, but by plugging the tracheae, or breathing pores, so that 

 the insect is deprived of air and suffocated. Some interesting experi- 

 ments lately made at the Michigan Agricultural College tend to show 

 that the toxic effects of such vapours as kerosene, creolin, and Pyre- 

 thrum result from the depression, or arrest, of oxygen-absorption by 

 the tissues. 



The presence of some typical insect pests in the plant-houses and 

 vicinity of the College during the past summer afforded an oppor- 

 tunity of comparing the value of some well-known preparations, 

 while attempts were also made to try the insecticidal effects of other 

 substances, such as formalin. 



The plants experimented with, and their respective pests, were as 

 follows : — 



Green-fly on Choisya ternata, Euonymus japonicus, Solanum Dul- 

 camara. 



Mealy bug on Streptocarpws sp., Coleus sp., Cinerarias. 

 ^ Black-fiy on Chrysanthemums. 

 BL Woodlice on Ampelopsis Veitchii. 



HpSeveral plants of each kind were sprayed, or " dipped," according 

 t6 the size, and it was found that considerable differences of resistance 

 to the same strength of solution were exhibited by different plants. 

 Choisya, and Euonymus stood severe strengths without injury, while 

 the Cinerarias were remarkably susceptible to weak solutions. The 

 others were intermediate in resistance, and each had its optimum 

 strength of insecticide. Such facts as these render spraying with in- 

 secticides rather risky for the gardener who has to use the same 

 spray for a mixed collection of plants. The results of the principal 

 experiments are given below. 



Quassia. — Quassia chips have long been known as an excellent 

 specific for most pests, and for soft-bodied insects, such as aphides, in 

 particular. I tried various strengths of quassia, and can fully endorse 

 the favourable reputation this substance has achieved. Combined with 

 soft soap or liver of sulphur, or with both, an excellent wash resulted 



