Abstract
Simple SummaryPlanthopper outbreaks in rice are associated with excessive fertilizer applications. Public research has focused on developing resistant rice to combat these outbreaks. However, to preserve ecosystem resilience, natural enemy efficacy should be maintained on resistant and susceptible rice. We examined the impact of egg parasitoids on planthoppers (Nilaparvata lugens (Stål) [BPH] and Sogatella furcifera (Horváth) [WBPH]) and a leafhopper (Nephotettix virescens (Distant) [GLH]) in field plots of resistant (IR62) and susceptible (IR64) rice under low and high nitrogen. GLH and WBPH were more abundant in low-nitrogen plots during dry (GLH) and wet (GLH, WBPH) season sampling at an early crop stage. GLH were also more abundant on IR64. Parasitoids killed between 24 and 52% of planthopper eggs during exposures in trap plants. Parasitism by Oligosita and Anagrus wasps was higher on IR64 (BPH eggs) and in high-nitrogen plots (Oligosita spp. on BPH and WBPH eggs; Anagrus spp. on BPH eggs). Parasitism by Anagrus spp. was associated with the presence of honeydew and was highest where honeydew was derived from BPH feeding on IR62; these effects were only observed under high nitrogen. Results suggest that honeydew from IR62 favors parasitoids when plants are most vulnerable (i.e., under high nitrogen), thereby countering nitrogen-induced declines in host resistance.Host plant resistance is the most researched method for the management of planthoppers and leafhoppers in tropical rice. For optimal effects, resistance should be resilient to fertilizer inputs and work in synergy with natural enemies. In field plot experiments, we examined how rice resistance and fertilizer inputs affect mortality of planthopper and leafhopper eggs by hymenopteran parasitoids. We used IR62 as a variety with resistance to Nilaparvata lugens (Stål) [BPH], Sogatella furcifera (Horváth) [WBPH] and Nephotettix virescens (Distant) [GLH], and IR64 as a susceptible control. The herbivores were more abundant during wet season sampling in low-nitrogen plots. During this study, parasitoids killed between 31 and 38% of BPH eggs and 24 and 52% of WBPH eggs during four days of field exposure. Parasitism, mainly due to Oligosita spp., was generally higher in high-nitrogen and IR64 plots. Similar densities of eggs in exposed plants suggest that these trends were mediated by semiochemicals and therefore support the Optimal Defense Hypothesis. Honeydew from BPH on IR62 had more xylem-derived wastes than honeydew on IR64. We applied honeydew from both varieties to sentinel plants. Parasitism by Anagrus spp. was higher on plants of either variety treated with honeydew derived from IR62; however, the effect was only apparent in high-nitrogen plots. Results suggest that Anagrus spp., by responding to honeydew, will counter the nitrogen-induced enhancement of planthopper fitness on resistant rice.
Highlights
We examine interactions between rice resistance to planthoppers (Nilaparvata lugens (Stål) (BPH) and Sogatella furcifera (Horváth) (WBPH)) and a leafhopper (Nephotettix virescens Distant (GLH)) and mortality due to egg parasitoids under varying soil nitrogen regimes
For Oligosita spp., the effect was independent of the presence or composition of honeydew on the plants (Table 2); for Anagrus spp., there was evidence that honeydew is associated with higher parasitism rates, and the effect was most apparent for IR62-derived honeydew under high-nitrogen conditions (Table 2)
1: BPH = brown planthopper, WBPH = whitebacked planthopper, GLH = green leafhopper, and WS = wet season; 2: ns = p > 0.05, * = p ≤ 0.05, ** = p ≤ 0.01, *** = p ≤ 0.001; ‘-’ = factor involved in significant interaction; levels associated with highest proportions of eggs parasitized are indicated for each significant factor; 3: levels are not highlighted for significant three-way interactions
Summary
Rice is the main staple food for over 50% of the world’s population and is mainly produced in Asia [1]. Much of that production occurs in intensified lowland agroecosystems [1,2]. Pressures to increase rice production to feed Asia’s growing population have led to further intensification through the expansion of rice production into previously natural areas, the adoption of high-yielding rice varieties, a high use of chemical fertilizers, and an increase in the frequency of pesticide applications [3]. High fertilizer inputs increase the susceptibility of rice to several insect herbivores, and to phloem-feeding herbivores [4–6]. Several studies have associated insecticides with increases in the frequency of planthopper and leafhopper outbreaks across Asia [7–12], with insecticide-related mortality of natural enemies representing a major mechanism underlying these outbreaks [9,13–16]. Sustainable pest management strategies that maintain or enhance natural enemy abundance and activities, under a high use of inorganic fertilizers, are required for intensified rice production systems
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