Stem borers revisited: Host resistance, tolerance, and vulnerability determine levels of field damage from a complex of Asian rice stemborers.

Yield losses from rice stem borers depend on the nature of the rice variety, the timing of attack, and the composition of the stem borer species assemblage. This study uses a range of phenotyping methods to distinguish different categories of herbivore-rice interaction that determine relative damage levels (dead heart and whitehead-panicles) and yield losses to varieties exposed to stem borers. Phenotyping studies were conducted in a greenhouse, screen house and field using two stem borer species (Scirpophaga incertulas [yellow stem borer - YSB] and Chilo suppressalis [striped stem borer - SSB]) and 12 rice lines. Only YSB displayed oviposition preferences across rice varieties. Both stem borer species performed better (greater survival, shorter development times, heavier pupae) on rice at vegetative compared to reproductive growth stages, and SSB was less capable than YSB of developing on rice at reproductive growth stages. Stem borer larval survival, body weight, development time, and tiller damage across different rice varieties in greenhouse experiments was poorly correlated between the two stem borer species and for each of the species on rice at two different plant growth stages (vegetative and reproductive). In contrast, rice tillering and yield responses to the two stem borer species were often highly correlated, but only when plants were attacked at the reproductive stage. Short-term, controlled experiments revealed aspects of host resistance and relative changes in plant biomass and yield in response to damage (condition change). However, the results from controlled environments and field-plots were not generally correlated because plant vulnerability, i.e., relative exposure to stem borer attack due to crop duration, largely determined field damage. We recommend that phenotyping studies should differentiate between the resistance, tolerance and vulnerability of rice lines to complexes of stem borers in a given region. Single stem borer species experiments under controlled environments are useful to gain knowledge of the nature of rice-stem borer interactions; however, field testing with comparative treatments, particularly under high soil fertilizer levels that increase plant attractiveness, are better for assessing the relative propensities of rice varieties to incur damage and yield losses due to complexes of stem borers.

Unanticipated benefits and potential ecological costs associated with pyramiding leafhopper resistance loci in rice.

We tested the hypotheses that increasing the number of anti-herbivore resistance loci in crop plants will increase resistance strength, increase the spectrum of resistance (the number of species affected), and increase resistance stability. We further examined the potential ecological costs of pyramiding resistance under benign environments. In our experiments, we used 14 near-isogenic rice lines with zero (T65: recurrent parent), one, two or three resistance loci introgressed through marker-assisted selection. Lines with two or more loci that were originally bred for resistance to the green rice leafhopper, Nephotettix cincticeps, significantly reduced egg-laying by the green leafhopper, N. virescens. Declines in egg-number and in nymph weight were correlated with the numbers of resistance loci in the rice lines. To test the spectrum of resistance, we challenged the lines with a range of phloem feeders including the zig-zag leafhopper, Recilia dorsalis, brown planthopper, Nilaparvata lugens, and whitebacked planthopper, Sogatella furcifera. There was an increase in the number of tested species showing significant declines in egg-laying and nymph survival on lines with increasing numbers of loci. In a screen house trial that varied rates of nitrogenous fertilizer, a line with three loci had stable resistance against the green leafhopper and the grain yields of infested plants were maintained or increased (overcompensation). Under benign conditions, plant growth and grain yields declined with increasing numbers of resistance loci. However, under field conditions with natural exposure to herbivores, there were no significant differences in final yields. Our results clearly indicate the benefits, including unanticipated benefits such as providing resistance against multiple herbivore species, of pyramiding anti-herbivore resistance genes/loci in crop plants. We discuss our results as part of a review of existing research on pyramided resistance against leafhoppers and planthoppers in rice. We suggest that potential ecological costs may be overcome by the careful selection of gene combinations for pyramiding, avoidance of high (potentially redundant) loci numbers, and introgression of loci into robust plant types such as hybrid rice varieties.

Resistance and tolerance to the brown planthopper, Nilaparvata lugens (Stål), in rice infested at different growth stages across a gradient of nitrogen applications.

High resource availability can reduce anti-herbivore resistance (a plant's ability to defend against herbivores and reduce damage) in rice, Oryza sativa L, but may also increase tolerance (a plant's ability to withstand damage by, for example, compensatory growth). Through a series of greenhouse, screenhouse and field experiments, this study examines fitness (survival and development × reproduction) of the brown planthopper, Nilaparvata lugens (Stål), on resistant (IR62) and susceptible (IR22) rice varieties and age-related rice tolerance to planthopper damage under varying resource (nitrogenous fertilizer) availability. Planthoppers reared on IR62 in the greenhouse had lower fitness than planthoppers on IR22. IR62 became increasingly resistant as plants aged. IR22 was generally more tolerant of planthopper damage, and tolerance increased in IR22, but declined in IR62, as the plants aged. Rice plants infested at pre-tillering stages (3-4 leaf stage) in the screenhouse had greater losses to root, shoot and grain yield per unit weight of planthopper than plants infested at tillering stages, particularly in IR22. These trends were mainly due to the impact of planthoppers during pre-tillering stages and the length of exposure to the planthoppers. High nitrogen compromised IR62 resistance, particularly in tillering plants in the greenhouse study; however, high nitrogen did not increase planthopper biomass-density on IR62 in greenhouse or field cages. Tolerance to damage in IR62 at mid-tillering stages declined under increasing levels of nitrogen, but nitrogen increased tolerance during late-tillering stages. Planthopper damage to IR22 in field cages was severe and hopperburn (plant death) occurred in 83% of IR22 plants under high nitrogen (60-150 kg N ha-1). In contrast, despite planthopper infestations, damage to IR62 was low in field-grown plants and productivity (tillers, roots, shoots and grain) increased in IR62 under increasing nitrogen. Our results indicate that, whereas nitrogenous fertilizer increases planthopper fitness on susceptible and resistant varieties, the net effects of high nitrogen on IR62 include decreased planthopper biomass-density (apparent in all experiments) and higher tolerance to damage during later growth stages (observed in the greenhouse, and during one of two seasons in field cages).

Virulence adaptation in a rice leafhopper: Exposure to ineffective genes compromises pyramided resistance.

Pyramiding resistance genes is predicted to increase the durability of resistant rice varieties against phloem-feeding herbivores. We examined responses by the green leafhopper, Nephotettix virescens (Hemiptera: Cicadellidae), to near-isogenic rice lines with zero, one and two resistance genes. The recurrent parent (T65) and monogenic lines (GRH2-NIL and GRH4-NIL) with genes for resistance to the green rice leafhopper, Nephotettix cincticeps (Hemiptera: Cicadellidae), were susceptible to the green leafhopper, but the pyramided line (GRH2/GRH4-PYL) was highly resistant to the green leafhopper. We selected green leafhoppers, N. virescens, from five sites in the Philippines for over 20 generations on each of the four lines. Populations selected on GRH2/GRH4-PYL gained partial virulence (feeding and development equal to that on T65) to the pyramided line within 10 generations and complete virulence (egg-laying equal to that on T65) within 20 generations. After 20 generations of rearing on the susceptible monogenic lines, green leafhoppers were also capable of developing and laying eggs on GRH2/GRH4-PYL. Furthermore, green leafhoppers reared on the susceptible GRH4-NIL for 20 generations showed equal preferences for T65 and GRH2/GRH4-PYL in choice bioassays. Our results indicate that previous long-term exposure to ineffective genes (including unperceived resistance genes) could dramatically reduce the durability of pyramided resistance. We suggest that informed crop management and deployment strategies should be developed to accompany rice lines with pyramided resistance and avoid the build-up of virulent herbivore populations.

Effects of nitrogen on egg-laying inhibition and ovicidal response in planthopper-resistant rice varieties.

A series of experiments was set up to examine the effects of nitrogen on rice (Oryza sativa L.) resistance against Nilaparvata lugens (Stål) and Sogatella furcifera (Horváth). Egg laying by N. lugens was reduced on the indica variety IR60. Nymph biomass (N. lugens and S. furcifera) was also lower on IR60: this was associated with low honeydew production and a high proportion of xylem-derived honeydew in N. lugens but not in S. furcifera. Nitrogen increased egg-laying by S. furcifera and increased N. lugens nymph biomass on all varieties tested. Oviposition and egg mortality in both planthopper species were examined on plants at 15, 30 and 45 days after sowing (DAS). Sogatella furcifera laid more eggs on plants at 15 DAS, but laid few eggs during darkness; N. lugens continued to lay eggs on older rice plants (30 DAS) and during darkness. Egg mortality was high on cv. Asiminori, highest at 45 DAS, and higher for S. furcifera than for N. lugens. Mortality of S. furcifera eggs was associated with lesions around the egg clusters. These were more common around clusters laid during the day and suggested induction by Asiminori of an ovicidal response. Egg mortality declined under higher soil nitrogen levels. Results are discussed in the light of improving rice resistance against planthoppers and reducing rates of planthopper adaptation to resistance genes.

Heterosis for Interactions between Insect Herbivores and 3-Line Hybrid Rice under Low and High Soil Nitrogen Conditions.

Hybrid rice results from crossing a male-sterile line (the A line) with a pollen doner (the restorer or R line). In 3-line hybrid breeding systems, a fertile B line is also required to maintain A line populations. Heterosis is defined as a condition of traits whereby the hybrid exceeds the average of the parental lines. Heterobeltiosis is where the hybrid exceeds both parents. Hybrid rice may display heterosis/heterobeltiosis for growth, yield and resistance to herbivores, among other traits. In a greenhouse experiment, we assessed the frequency of heterosis for resistance to the brown planthopper (Nilaparvata lugans (BPH)), whitebacked planthopper (Sogatella furcifera (WBPH)) and yellow stemborer (Scirpophaga incertulas (YSB)) in eight hybrids under varying soil nitrogen conditions. We also assessed plant biomass losses due to herbivore feeding as an approximation of tolerance (the plant's capacity to compensate for damage). Nitrogen reduced resistance to all three herbivores but was also associated with tolerance to WBPH and YSB based on improved plant survival, growth and/or yields. Plant biomass losses per unit weight of WBPH also declined under high nitrogen conditions for a number of hybrids, and there were several cases of overcompensation in rice for attacks by this herbivore. There was one case of nitrogen-related tolerance to BPH (increased grain yield) for a hybrid line with relatively high resistance, likely due to quantitative traits. Heterosis and heterobeltiosis were not essential to produce relatively high herbivore resistance or tolerance across hybrids.

Heterosis for Resistance to Insect Herbivores in a 3-Line Hybrid Rice System.

Three-line hybrid rice is produced by crossing male sterile (A line) rice with a fertility-restorer (R line). Fertile lines (B lines) are also required to maintain A line seed for breeding programs. We used a range of hybrids and their parental lines to assess the frequency and nature of heterosis for resistance to the whitebacked planthopper (Sogatella furcifera), brown planthopper (Nilaparvata lugens) and yellow stemborer (Scirpophaga incertulas). Heterosis is defined as trait improvement above the average of the parental lines as a result of outbreeding. Based on the results from a greenhouse study that challenged hybrids and their parental lines with each herbivore species, we found that susceptibility to planthoppers was associated with one of the eight A lines tested, but resistance was improved by crossing with a relatively resistant restorer. Higher frequencies of heterosis for susceptibility in comparisons between hybrids and their B lines suggest that susceptibility was not related to the cytoplasmic genomes of the associated sterile A lines. Furthermore, because none of the parental lines possessed currently effective resistance genes, improved resistance against planthoppers was probably due to quantitative resistance. In a related field trial, hybrids had generally higher yields than their fertile parents and often produced larger grain; however, they were often more susceptible to stemborers, leaffolders (Cnaphalocrocis medinalis) and other caterpillars (Rivula atimeta). This was largely a consequence of hybrid heterosis for plant biomass and was strongly affected by crop duration. We make a series of recommendations to improve hybrid breeding to reduce the risks of herbivore damage.

Adaptation by the Brown Planthopper to Resistant Rice: A Test of Female-Derived Virulence and the Role of Yeast-like Symbionts.

The adaptation by planthoppers to feed and develop on resistant rice is a challenge for pest management in Asia. We conducted a series of manipulative experiments with the brown planthopper (Nilaparvata lugens (Stål)) on the resistant rice variety IR62 (BPH3/BPH32 genes) to assess behavioral and bionomic changes in planthoppers exhibiting virulence adaptation. We also examined the potential role of yeast-like symbionts (YLS) in virulence adaptation by assessing progeny fitness (survival × reproduction) following controlled matings between virulent males or females and avirulent males or females, and by manipulating YLS densities in progeny through heat treatment. We found virulence-adapted planthoppers developed faster, grew larger, had adults that survived for longer, had female-biased progeny, and produced more eggs than non-selected planthoppers on the resistant variety. However, feeding capacity-as revealed through honeydew composition-remained inefficient on IR62, even after 20+ generations of exposure to the resistant host. Virulence was derived from both the male and female parents; however, females contributed more than males to progeny virulence. We found that YLS are essential for normal planthopper development and densities are highest in virulent nymphs feeding on the resistant host; however, we found only weak evidence that YLS densities contributed more to virulence. Virulence against IR62 in the brown planthopper, therefore, involves a complex of traits that encompass a series of behavioral, physiological, and genetic mechanisms, some of which are determined only by the female parent.

Differences Between the Strength of Preference-Performance Coupling in Two Rice Stemborers (Lepidoptera: Pyralidae, Crambidae) Promotes Coexistence at Field-Plot Scales.

Two stem-boring moths, the yellow stemborer (YSB) Scirpophaga incertulas (Walker), and the striped stemborer (SSB), Chilo suppressalis (Walker), damage rice in Asia. YSB is the dominant species in much of tropical Asia. Both species are oligophagous on domesticated and wild rice. We investigated the roles of host plant preferences and larval performance in determining the larval densities of both species in rice plots. In screenhouse experiments, YSB showed significant preference-performance coupling. Adults preferred high-tillering rice varieties during early vegetative growth. In contrast, SSB did not demonstrate oviposition preferences under the same screenhouse conditions, but did oviposit less on the wild rice Oryza rufipogon Griff. than on domesticated rice varieties during a choice experiment. Despite differences in preference-performance coupling, larval survival and biomass across 10 varieties were correlated between the two species. YSB and SSB larvae occurred in relatively high numbers on rice varieties with large tillers (IR70, IR68, and T16) in wet and dry season field experiments. However, whereas YSB was the dominant species on IR68 and IR70, it was relatively less abundant on T16, where SSB dominated. Results suggest that YSB preferentially attacked fast-growing rice varieties with high tiller numbers early in the crop cycle. Meanwhile SSB, which has weak preference-performance coupling, occurred in rice plants with large tillers that were relatively free of YSB later in the crop cycle. These factors may allow the species to coexist. We discuss the implications of proximate and ultimate factors influencing stemborer co-occurrence for the sustainable production of rice in tropical Asia.

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

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.

Efficacy and Cost-Effectiveness of Phenotyping for Rice Resistance and Tolerance to Planthoppers.

The standard seedling seedbox test (SSST) is the most prevalent phenotyping test in research on the genetics and breeding of planthopper-resistant rice. Using 16 rice lines that included plants susceptible, resistant and tolerant to the brown planthopper (Nilaparvata lugens), we compared the SSST to modified seedling seedbox tests (MSSTs) and the days-to-wilt (DTW) test. We also conducted a series of performance tests to assess nymph survival and development; adult longevity and egg-laying; egg survival; honeydew production; and plant weight loss. We also assessed the relative costs of the different phenotyping tests to better recommend test protocols that are suitable for high-throughput phenotyping. The SSST was found to be highly robust but fails to identify late-stage resistance; tolerance; or ovicidal responses. MSSTs improved phenotyping by identifying plants with low damage from planthoppers at later growth stages. Herbivore performance tests such as population or biomass build-up tests reduce space requirements and reduce setup and evaluation costs compared with bulk tests. They can also facilitate the assessment of plant tolerance; albeit with added costs. The DTW test most clearly segregates resistant and susceptible plants, thereby facilitating gene discovery and marker-assisted selection. We recommend that bulk testing be improved by switching from the SSST to a suitable MSST and that donor variety and pre-release lines be assessed for the nature of rice-planthopper interactions using biomass build-up tests-including the DTW test.

Changes in reflectance of rice seedlings during planthopper feeding as detected by digital camera: Potential applications for high-throughput phenotyping.

Damage to grasses and cereals by phloem-feeding herbivores is manifest as nutrient and chlorophyll loss, desiccation, and a gradual decline in host vigour. Chlorophyll loss in particular leads to a succession of colour changes before eventual host death. Depending on the attacking herbivore species, colour changes can be difficult to detect with the human eye. This study used digital images to examine colour changes of rice seedlings during feeding by the brown planthopper, Nilaparvata lugens (Stål) and whitebacked planthopper, Sogatella furcifera (Horváth). Values for red (580 nm), green (540 nm) and blue (550 nm) reflectance for 39 rice varieties during seedling seed-box tests were derived from images captured with a digital camera. Red and blue reflectance gradually increased as herbivore damage progressed until final plant death. Red reflectance was greater from plants attacked by the brown planthopper than plants attacked by the whitebacked planthopper, which had proportionately more green and blue reflectance, indicating distinct impacts by the two planthoppers on their hosts. Analysis of digital images was used to discriminate variety responses to the two planthoppers. Ordination methods based on red-green-blue reflectance and vegetation indices such as the Green Leaf Index (GLI) that included blue reflectance were more successful than two-colour indices or indices based on hue, saturation and brightness in discriminating between damage responses among varieties. We make recommendations to advance seed-box screening methods for cereal resistance to phloem feeders and demonstrate how images from digital cameras can be used to improve the quality of data captured during high-throughput phenotyping.

Microbiome responses during virulence adaptation by a phloem-feeding insect to resistant near-isogenic rice lines.

The microbiomes of phloem-feeding insects include functional bacteria and yeasts essential for herbivore survival and development. Changes in microbiome composition are implicated in virulence adaptation by herbivores to host plant species or host populations (including crop varieties). We examined patterns in adaptation by the green leafhopper, Nephotettix virescens, to near-isogenic rice lines (NILs) with one or two resistance genes and the recurrent parent T65, without resistance genes. Only the line with two resistance genes was effective in reducing leafhopper fitness. After 20 generations on the resistant line, selected leafhoppers attained similar survival, weight gain, and egg laying to leafhoppers that were continually reared on the susceptible recurrent parent, indicating that they had adapted to the resistant host. By sequencing the 16s rRNA gene, we described the microbiome of leafhoppers from colonies associated with five collection sites, and continually reared or switched between NILs. The microbiomes included 69-119 OTUs of which 44 occurred in ≥90% of samples. Of these, 14 OTUs were assigned to the obligate symbiont Candidatus sulcia clade. After 20 generations of selection, collection site had a greater effect than host plant on microbiome composition. Six bacteria genera, including C. sulcia, were associated with leafhopper virulence. However, there was significant within-treatment, site-related variability in the prevalence of these taxa such that the mechanisms underlying their association with virulence remain to be determined. Our results imply that these taxa are associated with leafhopper nutrition. Ours is the first study to describe microbiome diversity and composition in rice leafhoppers. We discuss our results in light of the multiple functions of herbivore microbiomes during virulence adaptation in insect herbivores.

Effects of Vegetation Strips, Fertilizer Levels and Varietal Resistance on the Integrated Management of Arthropod Biodiversity in a Tropical Rice Ecosystem.

Integrated biodiversity management aims to conserve the beneficial species components of production ecosystems and reduce the impacts of pests. In 2011 and 2013, experiments were conducted at Los Baños, Laguna, Philippines, to compare arthropod communities in rice plots and on levees with and without vegetation strips. Vegetation strips included spontaneous weeds, sesame and okra (2011), or mung bean (2013). The plots were treated with one of three nitrogen levels and in one experiment were planted with planthopper-resistant (IR62) and susceptible (IR64) rice varieties. Parasitoids and predators of lepidopteran pests and of the ricebug, Leptocorisa oratorius, were more abundant in high-nitrogen rice plots where their prey/hosts also had highest densities. Planthoppers and leafhoppers were more abundant in low-nitrogen plots. Weedy and sesame/okra bunds provided habitat for a range of natural enemies including spiders, parasitoids and predatory bugs, but did not have higher pest numbers than cleared bunds. Higher abundances of the predator Cythorhinus lividipennis and higher parasitism of planthopper (Nilaparvata lugens) eggs by Anagrus sp. were associated with sesame/okra bunds in late season rice plots. Mung bean also provided habitat for key predators and parasitoids that spilled over to adjacent rice; however, mung bean was also associated with higher numbers of lepidopteran and grain-sucking pests in the adjacent rice, albeit without increased damage to the rice. For ricebug in particular, damage was probably reduced by higher parasitoid:pest ratios adjacent to the vegetation strips. Varietal resistance and mung bean strips had an additive effect in reducing abundance of the planthopper Sogatella furcifera and the leafhopper Nephotettix virescens. Reduced numbers of these latter pests close to vegetation strips were often compensated for by other plant-sucking bugs, thereby increasing the intensity of potentially stabilizing interspecific interactions such as competition. We highlight the benefits of diversifying rice landscapes and the need to optimize vegetation strips, e.g., by including lepidopteran trap-plants, for intensive rice production systems.

Geographic and Research Center Origins of Rice Resistance to Asian Planthoppers and Leafhoppers: Implications for Rice Breeding and Gene Deployment.

This study examines aspects of virulence to resistant rice varieties among planthoppers and leafhoppers. Using a series of resistant varieties, brown planthopper, Nilaparvata lugens, virulence was assessed in seedlings and early-tillering plants at seven research centers in South and East Asia. Virulence of the whitebacked planthopper, Sogatella furcifera, in Taiwan and the Philippines was also assessed. Phylogenetic analysis of the varieties using single-nucleotide polymorphisms (SNPs) indicated a clade of highly resistant varieties from South Asia with two further South Asian clades of moderate resistance. Greenhouse bioassays indicated that planthoppers can develop virulence against multiple resistance genes including genes introgressed from wild rice species. Nilaparvata lugens populations from Punjab (India) and the Mekong Delta (Vietnam) were highly virulent to a range of key resistance donors irrespective of variety origin. Sogatella furcifera populations were less virulent to donors than N. lugens; however, several genes for resistance to S. furcifera are now ineffective in East Asia. A clade of International Rice Research Institute (IRRI)-bred varieties and breeding lines, without identified leafhopper-resistance genes, were highly resistant to the green leafhopper, Nephotettix virescens. Routine phenotyping during breeding programs likely maintains high levels of quantitative resistance to leafhoppers. We discuss these results in the light of breeding and deploying resistant rice in Asia.

Do rice hybrids have heterosis for insect resistance? A study with Nilaparvata lugens (Hemiptera: Delphacidae) and Marasmia patnalis (Lepidoptera: Pyralidae).

Antibiosis-based resistance to two insect pests of rice, Nilaparvata lugens (Stål) (Hemiptera: Delphacidae) and Marasmia patnalis Bradley (Lepidoptera: Pyralidae), was compared in 11 F1 hybrids and their parental lines. Our objective was to determine whether hybrids show heterosis (hybrid vigor) for insect resistance or susceptibility. Heterosis is defined as the amount by which a hybrid exceeds its midparent value or its better parent. Overall, we did not find evidence of heterosis or heterobeltiosis (a type of heterosis in which a hybrid exceeds its better parent) for antibiosis-based resistance or susceptibility to either of the insects. One hybrid, IR64616H, seemed more resistant to M. patnalis than its better parent but none of the other hybrids showed heterobeltiosis for resistance or susceptibility to either insect. Three hybrids had resistance to N. lugens that exceeded their midparent value, possibly due to dominant resistance in one of the parents. The increased frequency and severity of insect outbreaks on hybrid rice that have been reported in China may be attributable to factors other than diminished antibiosis in hybrids, such as greater attractiveness of hybrids to migrating or dispersing insects or differences in agronomic practices applied to hybrids and inbred rice cultivars.

Planthopper "adaptation" to resistant rice varieties: changes in amino acid composition over time.

The brown planthopper, Nilaparvata lugens, shows considerable geographic and temporal variability in its response to varieties of cultivated rice. N. lugens has repeatedly "adapted" to resistant rice varieties; however, the physiological changes underlying planthopper adaptation are poorly understood. Endosymbionts within planthoppers, such as yeast-like endosymbionts (YLS) could play a role as they produce essential amino acids for planthoppers. We used a full factorial study to determine how natal rice variety, exposed rice variety, YLS presence, and the number of reared generations affected nymphal development, planthopper total nitrogen content, and planthopper hydrolyzed amino acid profiles. Nymphal development was strongly influenced by a four-way interaction between the exposed rice variety, natal rice variety, number of reared generations, and YLS presence. While symbiosis improved nymphal performance in the 8th generation, it appeared to be a drain on nymphs in the 11th generation, when the aposymbiotic nymphs actually showed higher performance than the symbiotic nymphs. This suggests that the symbiotic relationship may be acting beneficially in one generation while acting as a drain during another generation. Aposymbiotic planthoppers reared for 11 generations had a higher proportional concentration of rare amino acids than those reared for 8 generations, indicating that the planthopper itself appears to improve its ability to acquire rare amino acids. Therefore, the change in amino acid composition of planthoppers suggests an underlying change in protein expression or amino acid metabolism over time.