Tritrophic effects of nitrogen on cotton ecosystem
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Nitrogen (N) fertilization is one of the most common agronomic practices in crop production, and can have profound impacts on tritrophic interactions, affecting both bottom-up and top-down forces in pest management, and consequently on community structures of ecosystems. In the study the effects of N fertilization (42, 112, 196, and 280 ppm N) on tritrophic inteactions among cotton plants, Gossypium hirsutum L., beet armyworm, Spodoptera exigua (Hübner), and the parasitoid, Cotesia marginiventris (Cresson), were investigated in the laboratory, greenhouse and field. N addition increased plant biomass and plant nutritional quality as indicated by total N content of leaf blade and petiole nitrate-N. N enhancement decreased the major non-volatile plant defensive compounds (hemigossypolone and heliocides 1-4) of cotton leaf tissue. Spodoptera exigua larvae developed faster on plants receiving high N fertilization and significantly fewer S. exigua larvae underwent a sixth larval instar than those reared on low N plants. In dual-choice tests, both S. exigua larvae and adult females preferentially chose cotton plants with high N fertilization for feeding and oviposition, respectively. Increased nutritional quality and weaker defense of cotton plants receiving high N fertilization might both contribute to faster development and feeding preference of S. exigua larvae, and oviposition preference of S. exigua adult females on high N plants compared to low N plants. N fertilization of host plants subsequently fastened the development of C. marginiventris, possibly due to more balanced protein:carbohydrate (P:C) ratios, or less defensive compounds in the hemolymph of the hosts. N enhancement ameliorated the production of plant hormones (jasmonic acids and salicylic acids) and most of the herbivore-induced volatile plant secondary metabolites, except for the major two green leaf volatiles (Z-2-hexenal and Z-3-hexenal), the releasing of which was increased. The parasitisms of early instar S. exiuga larvae by C. marginiventris were not significantly affected by N treatments. The slowed growth of S. exiuga larvae feeding on low N plants did not translated into higher mortality inflicted by C. marginiventris mainly due to a shift of timing of susceptibility. N fertilization in the field increased plant growth, and some populations of pests and predators. N fertilization had variable effects on cotton lint yield.