Helicoverpa zea (Lepidoptera: Noctuidae) Oviposition and Larval Vertical Distribution in Bt Cotton Under Different Levels of Nitrogen and Irrigation

In some Bt cotton (Gossypium hirsutum L.) varieties, bollworm (Helicoverpa zea Boddie) larval behavior differs from non-Bt varieties. Laboratory assays indicate bollworm larvae can detect Bt proteins, which may cause behavioral differences. Plant stress from factors including fertility and water availability causes changes in plant physiology and Bt expression. Our objective was to determine whether nitrogen and irrigation influenced bollworm behavior in Bt cotton by recording the vertical distribution of eggs and larvae over time. We conducted small plot experiments with Cry1Ac + Cry1F cotton in 2016 and 2017 with three nitrogen rates, along with irrigated and nonirrigated treatments during 2017. Bollworm locations were determined by in-field examination of 10–20 cotton plants per plot over 6–8 wk. The location of each egg and larva was recorded by node, with instar estimation of each larva. Oviposition was higher in in plots receiving nitrogen; first and second instars were also more common in plots receiving nitrogen or irrigation, whereas older instars had similar numbers among treatments. Oviposition was more evenly distributed throughout the canopy earlier in the sampling period than during later weeks, with more eggs in the top third of the canopy in only three of 14-wk. Early instars were also evenly distributed throughout the canopy. Later, instars moved to the middle portions of the canopy, away from bottom nodes, and did not move toward the terminal. Understanding bollworm behavior can inform both crop scouting and resistance management decisions.