ABSTRACT

Terminating insecticide applications at the end of a season is a very important management decision which in the COTMAN monitoring program relies on identification of the last effective boll population (NAWF=5) and the accumulation of an additional 350 heat units. Earlier evidence showed that bollworms and boll weevils stop feeding on bolls at NAWF5+350HUs, however, evidence of changes in the boll wall with age (at NAWF=5 + 350 HU’s) to support this premise is lacking. The cotton (Gossypium hirsutum L.) fruit wall structural and biochemical features were analyzed for four weeks after anthesis. At approximately 350 heat units after anthesis, the endocarp (inner layer of the fruit wall) cells possessed an increased concentration of phenolic materials, thickened cell walls, and increased lignification of cell walls. These structural changes coincide with the increased resistance to physical penetration during this time of development. In addition, the mesocarp cells had an increased concentration of tannin material, and the exocarp (outer layer) cells contained cytoplasmic microfibrils which may be oxalate crystals. Also the percent composition of free sugars of the total fruit dry weight decreased with time. These observations help to explain the reported loss of interest by insects in the fruits as feeding sites at about 350 heat units after anthesis. These results provide increased confidence in timing of insecticide termination at this stage of fruit development as is used in the COTMAN crop monitoring program.