Bottom-up effects of nitrogen fertilizer on cotton growth and population expansion of <i>Aphis gossypii</i> (Hemiptera: Aphididae)

Bottom-up effects, triggered by fertilization regimes, can be key ecological forces regulating pest populations. In this study, we investigated the effects of 5 different concentrations of Ca(NO3)2 (0, 2, 4, 6, and 8 mM) on cotton plants and the performance of Aphis gossypii. Our results demonstrated that nitrogen application significantly enhanced the growth indices of cotton seedlings (eg plant height, aboveground fresh weight) and plant biochemistry parameters (eg nitrogen, chlorophyll, and tannin). Nitrogen also affected the life history parameters of A. gossypii (eg longevity, reproductive days, fecundity, intrinsic rate of increase, finite rate of increase, and net reproductive rate). Notably, the positive effects on cotton seedling growth and A. gossypii performance observed at the 6 mM level diminished at the 8 mM level. Additionally, the highest soluble sugar content was observed in the unfertilized plants (the 0 mM treatment). Electrical penetration graph analysis revealed that A. gossypii spent less time on intercellular probing and more time feeding on phloem with increasing nitrogen levels, suggesting improved nutrient acquisition from phloem, which supported the observed increase in fecundity. The above results indicate that the bottom-up effects of nitrogen fertilizer could trigger outbreaks of A. gossypii. These findings provide a scientific basis for optimizing nitrogen fertilizer within integrated pest management programs so as to protect yields, reduce the risk of aphid outbreaks, as well as the costs for labor and ecological environment.