Yield Response of Two Cotton Cultivars to Tobacco Budworm Infestation1

Abstract

Two cotton, Gossypium hirsutum L., cultivars were artifically infested with tobacco budworm, Heliothis virescens (F.), larvae for different time intervals during the first 8 weeks of fruiting to determine impact on yield reduction and maturity delay. The cultivars tested were ‘Stoneville 213’ (full‐season) and ‘Tamcot CAMD‐E’ (short‐season) grown in 1 ✕ 13.7 m long single row plots in a two‐planted one‐skip row pattern during the 1982 and 1983 growing seasons on a Leeper silty clay loam (fine, montmorillonitic, nonacid, thermic Vertic Haplaquepts). Treatments consisted of applying 12 first instar larvae in the terminal area of each plant, once per week for the following combination of weeks after fruiting had begun: none (control), Weeks 1 to 2, to 4, 1 to 6,1 to 8, 7 to 8, 5 to 8, 3 to 8, 3 to 4, to 6, and 3 to 6. When larvae were not applied in a given week, plots were protected with an insecticide. Plots were machine harvested three times for yield determinations and delays in maturity. All treatments involving larval applications the first 2 weeks of fruiting resulted in significant reductions in yield, compared with the control, for both cultivars at first harvest. Tamcot CAMD‐E with larval treatments produced a larger percentage of its total yield at first harvest than did Stoneville 213. When first and second harvest were accumulated, the same trends were apparent. Total yield of Stoneville 213 was significantly reduced only when larvae were applied for at least a 4‐week time period. The same trend was apparent with Tamcot CAMD‐E: timing of applications (rather than length of time) had the greatest effects; however, reduction in yield was not significant. Subclass regression analysis of yield as a function of larval application indicated that yield reduction trends averaged over years were significant and the responses of the two cultivars were found to be significantly different from each other as tested by homogeneity of slopes. The yield of Tamcot CAMD‐E was not reduced or delayed as much as that of Stoneville 213 when tobacco budworm larvae were applied during early‐, mid‐, and full‐season. The application of larvae during the early stage of fruiting had a greater impact on reducing yield and delaying maturity than when larvae were applied during mid‐ and late‐season.