Response of the Heliothis virescens (F.) larvae to nitrogen fertilization and irrigation inputs in cotton

Abstract

When high inputs of nitrogen (N) fertilizer and irrigation are used in cotton production, variation in quantity and quality of the crop as food for insect herbivores can result. Field studies were conducted in 1983-85 to investigate the effects of these inputs in cotton on survival of Heliothis virescens (F.) larvae and to provide baseline biological data for use in crop-pest simulation models. Effects of N and irrigation on larval establishment were evaluated by infesting squaring cotton with newly hatched larvae and determining survival after 7 days. Preliminary studies conducted to determine appropriate release sites for artificial infestation indicated that release on the upper 2 cm of the mainstem terminal resulted in greatest levels of larval establishment compared to release on young squares (flower buds) or vegetative leaves. When larvae were released on plants that had received varying amounts of N under irrigated and dryland conditions, larval survival generally was greatest where high rates of N were used. This response to N was most consistent under irrigated conditions. Establishment levels were lowest in older plants with maturing bolls and in cotton not contained in exclusion cages. In laboratory studies, young larvae feeding on terminal leaves from irrigated plants receiving the highest amounts of N were observed to have the greatest weight gain. Levels of leaf petiole nitrates, sugars and simple phenolics were monitored in plants used in field and laboratory studies, but relationships between these variables and larval establishment or weight gain were not clear. Response of crop plants and native populations of Heliothis and boll weevil to N fertilization and irrigation was monitored over three growing seasons on two different soil types and under chemically protected and natural crop environments. In unprotected field plots, Heliothis and boll weevil damage levels were observed at times to be increased with high N and irrigation. In protected field plots, no response by Heliothis was apparent. For both protected and unprotected studies, fertilization and irrigation resulted in continued production of squares well into the crop maturation period; this resulted in continued availability of food for boll weevil and increased boll weevil damage.