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Egg Vs. Escaped Worm Thresholds for Control of Bollworm in B.t. Cotton in South Carolina

M. J. Sullivan, S. G. Turnipseed, D. M. Robinson and J. T. Walker


Use of escaped worm (less than 0.25 inch) thresholds for bollworm (Helicoverpa zea) in transgenic B.t. cotton in South Carolina during 1996 led to unacceptable levels of boll damage and subsequent yield loss. Larvae developed from eggs deposited within the plant canopy, particularly blooms and squares, and damaged small bolls by feeding underneath bloom tags. These larvae were difficult to control because of their location and size. Because of these problems, a treatment threshold based on eggs was established for 1997; treat at 75 eggs OR 30 small larvae(less than 0.25 inch) OR 3 large larvae(greater than 0.25 inch) OR 5% boll damage per 100 plants. Four geographic locations in the southern half of the state were selected to test this threshold. Two sets of studies were established in all locations; in one set, a broad spectrum insecticide was applied in early July to disrupt beneficial species and the other set was left undisturbed. Hand picked yield data, from both disrupted and undisrupted sets, indicated that the egg threshold increased lint yields by 58 and 83 lbs. respectively, when compared to the escaped worm threshold. Machine picked yields indicated similar differences. Undisrupted sets required an average of 0.25 fewer pyrethroid treatments for bollworm, treatment dates were delayed, and these plots matured earlier. We will continue to use this same egg threshold in South Carolina in 1998.

Reprinted from Proceedings of the 1998 Beltwide Cotton Conferences pp. 1037 - 1038
©National Cotton Council, Memphis TN

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Document last modified Sunday, Dec 6 1998