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Tests of threshold levels based on larval counts for chemical control of cotton bollworms



Tests of threshold levels based on larval counts for chemical control of cotton bollworms



Journal of the Entomological Society of Southern Africa 45(1): 109-121



Replicated field plots of cotton at Groblersdal and Rust der Winter [South Africa] were treated as follows during the 1980-1981 growing season and the yields were measured. (A) sprayed weekly. (B) never sprayed (T1), (T2), (T4), (T6) sprayed when 1 or more, 2 or more, 4 or more, or 6 or more bollworm larvae were found in weekly examinations of 12 plants/plot. All sprays were of cypermethrin at 35 g/ha. The predominant bollworm species was Heliothis armigera (Huebner): Diparopsis castanea Hampson and Earias spp. (probably E. insulana (Boisduval) and E. biplaga Walker) were also present. Yields were depressed in the plots sprayed weekly, probably because of phototoxicity. The lowest yields were obtained from unsprayed plots and the highest from T1 plots (sprayed 5 times at both sites). The successive test thresholds T1-T6 were associated with a large (41% overall) and significant decrease in yield at Groblersdal but not at Rust der Winter, where the difference was small (15% overall) and could have been due to chance. Reasons for this contrast are suggested. Yields were correlated with mean larval numbers for the season. The establishment of economic thresholds based on larval counts is discussed.

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Accession: 000998240

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