Abstract
Fertilizer with different ratios of nitrogen (N) to phosphorus (P) can influence crop plant performance and defense against herbivores. Spodoptera exigua is an important agricultural pest that has caused serious economic loss, especially in recent decades. In the present study, we explored effects of different intensities and durations of S. exigua herbivory on host plant biomass and on S. exigua enzyme activities in response to five fertilizer treatments with different N: P ratios of 1: 5, 1: 3, 1: 1, 3: 1 and 5: 1. The results showed that fertilizer type can significantly influence interactions between caterpillars and its hosts. Compensatory growth of leaf biomass was detected under fertilizer with N: P = 3: 1. Fertilizer with a higher proportion of N appears to maintain stem biomass in defoliated seedlings similar to controls that are not exposed to herbivory. There was no significant difference in root biomass under most conditions. High proportion of N also enhanced the activity of two antioxidant enzymes, catalase (CAT) and superoxide dismutase (SOD) in low density of beet armyworm. However, with increased herbivorous intensity, a higher proportion of P played a more important role in increasing the activities of CAT and SOD. Higher P likely enhanced acetylcholine esterase (AChE) activity at lower degrees of defoliation, but a higher N proportion resulted in higher AChE activity at higher degrees of defoliation. Higher N proportion contributed to reduced carboxylesterase (CarE) activity at high intensity, short-term defoliation. However, when defoliation intensity increased, the difference in CarE activity between fertilizer categories was little. The study explored the interaction between the damage of S. exigua and the biomass accumulation of its host plant Brassica rapa, and the influence of the N/P ratio in plant fertilizer on this interaction. Systematic analysis was provided on the biomass of B. rapa and the activity of metabolic enzymes of S. exigua under different treatments.
Highlights
Many aspects of herbivorous insect-plant interactions have been studied substantially, such as co-evolution of plants with insects, plant chemistry, insect physiology, behavior and ecology [1]
The present study indicated that fertilizer application, defoliation intensity and duration might mediate the leaf biomass of rape seedlings
After defoliation by beet armyworm caterpillars for two days, there was complete compensation of rape seedlings growth in the N: P = 3: 1 fertilizer group
Summary
Many aspects of herbivorous insect-plant interactions have been studied substantially, such as co-evolution of plants with insects, plant chemistry, insect physiology, behavior and ecology [1]. Many studies have proved that suboptimal diet is of disadvantageous to Lepidopteran by altering their foraging decisions to affect survivorship, growth, development and fecundity [3,4,5]. Plants have developed several resistant mechanisms to respond to herbivore damage. Many studies have documented that fertilization enhances the growth, development, fecundity, survival, and/or density in several kinds of plant pests [17] such as caterpillars [18,19], leaf beetles [20,21], leaf miners [22,23] and browsing mammals [24,25]. Some other studies have found differently that fertilization enhances or has little effect on pest resistance [26,27,28,29]. The widely accepted paradigm that fertilization enhances insect resistance is not true in all cases
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