Phytophagous insects are quite sensitive to the nutritional status of their host plants. A increase in nitrogen availability to a plant by fertilization can affect food quality and therefore result in changes in an insect's preferences, fecundity, developmental time, and survival. Resulting microclimate changes also affect those functions. There are numerous examples of these phenomena in both natural and crop ecosystems (see Onuf 1978, Matteson 1980); Scriber (1984) estimates that there have been over 200 studies since 1930 on insect responses to N fertilization.

A current research initiated at Texas A&M University (TAMU) is directed at quantifying the relation of N fertilization and Heliothis spp. outbreaks in cotton. One set of experiments from this project is designed to determine how N fertilization may be related to establishment and survival of young larvae. Weight gain of larvae in feeding studies was one means used to investigate this relationship. Additional experiments were conducted to determine if other crop production inputs might affect the weight response. This included using plants grown under water stress, or treated with the insecticide chlordimeform. This chemical has been demonstrated to have some plant growth regulator properties (Bauer and Cothren 1986).

The procedure is a modification of that of Parrott et al. (1978) for evaluation for antibiosis in cotton to Heliothis spp. Newly eclosed larvae are fed excised terminal leaves, and weights determined after 5 days. When oviposition occurs in the terminal area of cotton, these young leaves are the primary larval diet for the first few days. This is a critical period in the insect's development and often is the stage tested in host plant resistance studies (Dilday 1983).