Nitrogenous Fertilizer Reduces Resistance but Enhances Tolerance to the Brown Planthopper in Fast-Growing, Moderately Resistant Rice

Abstract

The brown planthopper, Nilaparvata lugens (Stål), is a key challenge to rice production in Asia. Outbreaks of planthoppers are associated with excessive fertilizer applications; consequently, we examined planthopper interactions with susceptible, tolerant and resistant varieties of rice under varying levels of soil nitrogen in a greenhouse experiment. We compared planthopper fitness (survival × reproduction) and plant tolerance (functional plant loss index) for 16 varieties at 0, 80 and 150 Kg added nitrogen ha−1. The planthoppers grew larger, developed more quickly and laid more eggs on susceptible varieties, compared with the resistant and tolerant varieties. Moreover, soil nitrogen generally increased planthopper fitness on resistant varieties, but relative resistance was maintained. Functional plant loss was highest among the susceptible varieties, but weight and growth rate reductions per mg of planthopper were often highest in the tolerant varieties. Tolerance was associated with large, fast-growing plants, with at least moderate resistance to the planthopper. Susceptibility was associated with a small size and/or an absence of resistance genes. Our results suggested that early-tillering rice plants can be both resistant and tolerant to the brown planthopper, but cannot be both susceptible and tolerant of planthoppers at high densities. This indicates that at least moderate resistance is required for tolerance against this herbivore. Furthermore, although dwarf varieties had a low tolerance of planthoppers, they could express resistance through functioning resistance genes.

Effect of nitrogen fertilizer on the resistance of rice near-isogenic lines with BPH resistance genes

“…Plants with insect resistance traits are keys to IPM programs. Since BPH-resistant rice varieties have been used in the market, the environmental impact on their resistance has been noticed (Horgan et al 2021 ; Kuang et al 2021 ). Nitrogen input is highly correlated with crop yield.…”

Interactions between Rice Resistance to Planthoppers and Honeydew-Related Egg Parasitism under Varying Levels of Nitrogenous Fertilizer

“…was often higher in field plots under high nitrogen ( Table 2 ). Rice plant growth rates increase under high-nitrogen conditions and plants under high nitrogen are more susceptible to a range of pests, particularly sap-sucking pests [ 4 , 6 , 31 , 36 ]. Although we did not quantify or qualify volatiles emitted from rice under low or high nitrogen, the higher parasitism by Oligosita spp., despite relatively constant egg numbers in exposed trap plants ( Table S1 ), suggests that eggs in high-nitrogen plants were more attractive to the parasitoids.…”

“…Variety effects may be more apparent at later crop stages, but these effects are initiated in the early crop. Therefore, a lack of apparent variety effects on planthoppers in our field plots ( Figure 1 A,B) may be due to the early crop stage at which sampling was conducted, but could also be due the low captures (BPH), and only late-stage resistance in IR62 to WBPH [ 31 ]. In contrast to planthoppers, GLH was, as expected, less abundant in plots of IR62 ( Figure 1 A,B).…”

“…IR62 is also resistant to WBPH and GLH in the Philippines [ 47 ]. However, resistance against WBPH is predominantly expressed after maximum tillering [ 31 ]. Resistance in the variety was derived from the Indian landrace PTB33 and is likely associated with the Bph3 locus, which contains the Bph3 and/or Bph32 resistance genes [ 49 , 50 ].…”

“…For example, honeydew produced from planthoppers feeding on susceptible rice is mainly derived from phloem feeding, whereas planthoppers on resistant rice often produce honeydew that is predominantly derived from xylem feeding and is, therefore, likely to contain fewer nutrients [ 30 ]. Furthermore, honeydew composition is affected by crop fertilizer levels; for example, planthoppers feeding on rice grown in high-nutrient soils increase their phloem-based feeding (compared to xylem-based) and produce larger quantities of honeydew [ 31 ]. Honeydew represents a significant nutrient source for the parasitoids of phloem-feeding herbivores, such as aphids, in other cereal systems [ 32 , 33 , 34 , 35 ]; however, to our knowledge, the role of honeydew as a food for the parasitoids of rice planthoppers or leafhoppers has not yet been studied.…”