Further Observations on the Persistence of Larvicides against Culicoides and a Discussion on the Interpretation of Population Changes in the untreated Plots

Abstract

This paper presents further observations on the continued persistence of larvicidal treatments applied to small plots (10 yd. × 10 yd.) on Soutra Hill, Midlothian, to control larvae of the biting midge, Culicoides impunctatus Goetgh. Before doing so, consideration is given to the nature of the observed fluctuations in larval population encountered in the untreated plots. Evidence is produced which indicates that this is not due either to delayed hatching of eggs (this, if it occurs, is of minor importance), to change in technique or to the application of insecticide to neighbouring plots, but is due to the larvae responding to an oscillating factor (or factors) in the breeding site. It is suggested that one such factor might be soil water level. Therefore, it follows that changes in the untreated larval population must be taken into account when assessing the degree of control achieved by treatments, especially when the larval population is declining.As regards insecticidal persistence three years after application: —(i) Nine of the eleven DDT treatments (wettable powder, water-miscible concentrate and dust at 12, 50 and 200 mg. p,p′DDT/sq. ft.) gave 100 per cent. control. The other two (both wettable powder at 12 mg./sq. ft.) produced 75 and 22 per cent. reduction.(ii) Only one of seven γ BHC treatments (wettable powder at 50 mg. γ BHC/sq. ft.) gave complete control and one treatment (w.p. 12 mg./ sq. ft.) was completely ineffective.(iii) DDT retained its superiority to γ BHC as a larvicide against Culicoides, achieving 93 per cent. control compared with 55 per cent. for γ BHC.(iv) The lowest dosages of dieldrin (3 mg./sq. ft.) and chlordane (6 mg./ sq. ft.) had begun to lose their larvicidal power, control declining from 76 to 53 per cent. and 67 to 29 per cent., respectively. There was no change at 6 mg. dieldrin/sq. ft. but the other treatments (12 and 25 mg. dieldrin and 12, 25 and 50 mg. chlordane/sq. ft.) had improved. Dieldrin at 12 and 25 mg./sq. ft. and chlordane at 50 mg./sq. ft. attained complete control.The spray-volume treatments, now in their second year, showed an increase in activity. This was most marked in the wettable-powder treatments at 25 mg. p.p′DDT/sq. ft. which a year ago were completely ineffective and now five out of six trials gave complete control and the other 97 per cent. Improvement in the treatments at 50 mg./sq. ft. was less (from 43 to 83%) due in the main to the complete failure of one treatment (10 gal./acre).It is confirmed that spray volume in itself is unimportant in larvicidal applications providing the coverage is adequate.