oa Journal of the Entomological Society of Southern Africa - Tests of threshold levels based on larval counts for chemical control of cotton bollworms



Replicated field plots of cotton at Groblersdal and Rust der Winter were treated as follows during the 1980-81 growing srason and the yields were measured: (A) sprayed weekely. (B) never sprayed (T1), (T2), (T4), (T6) sprayed when one or more. two or more. four or more. or six or more bollworm larvae were found in weekly examinations of 12 plants per plot. All sprays were of cypermethrin at 35 g/ha. The predominant bollworm species was Heliothis armiger (Hubner): Diparopsis castanea Hampson and Earias spp. (prohably E. insulana (Boisduval) and E. biplaga Walkeri were also present. Yields were depressed in the plots sprayed weekly, probably because of phytotoxicity. The lowest yields were obtained from unsprayed plots and the highest from T1 plots (sprayed 5 times at both sites). The sucressin test thresholds T1 toT6 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. Some reasons for this contrast are suggested. Yields were well correlated with mean larval numbers for the season. The relevance of these findings to the establishment of economic thresholds based on larval counts is discussed.


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