Population development of rice black bug, Scotinophara latiuscula (Breddin), under varying nitrogen in a field experiment

© 2017 E. The effects of nitrogenous fertilizers on black bug (Hemiptera: Pentatomidae) colonization and population development in rice fields has not been studied previously. This study reports on the distribution of adults, eggs and nymphs of the black bug, Scotinophara latiuscula (Breddin) during a seasonal outbreak in a rice (Oryza sativa L.) yield experiment that varied nitrogen application rates. Plots at the edge of the experimental field had higher black bug densities. Adult distribution was only weakly affected by nitrogen level. Densities of egg-masses and nymphs increased linearly with increasing nitrogen rates. Nymph development was also more advanced under high nitrogen. Oviposition was highest on larger plants with high grain numbers (actively filling grain), but was otherwise unaffected by the varieties in the experiment. Our results indicate that the build-up of black bug populations in rice is influenced by nitrogen fertilizer rates mainly acting on egg-laying and nymph development

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

© 2018 The Authors 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

Geographic and research center origins of rice resistance to asian planthoppers and leafhoppers: Implications for rice breeding and gene deployment

2017 by the authors. 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

Reducing seed-densities in rice seedbeds improves the cultural control of apple snail damage

Several cultural methods are known to reduce the densities of exotic apple snails (Pomacea spp.) and the damage they cause to rice in Asia. However, one aspect of seedling production - seedbed seed-density - has been largely overlooked and could compromise popular cultural control methods such as delayed transplanting. We conducted experiments to examine the effects of seedbed seed-density on hill survival in snail-infested paddy fields in the Philippines and to examine the interactions between seedbed seed-density and other cultural methods (delayed transplanting, 3 seedlings per hill and hand-picking). Seedbed seed-density determined seedling weight and stem thickness at the time of transplanting. Hill survival was highest where cultural methods (delayed transplanting and 3 seedlings per hill) were combined with low seed-density seedbeds (60-120gm-2). Furthermore, reduced seedbed seed-density was directly related to increased hill biomass in field plots 32 days after transplanting. Hand-picking of snails together with delayed transplanting and 3 seedlings per hill eliminated hill mortality due to snail herbivory. Farmers adopting cultural snail control methods, but without adhering to low seedbed seed-densities risk increased losses due to snails because of poor quality seedlings. We suggest that seedbed seed-densities should not exceed 120gm-2 with better results at even lower densities. © 2014 Elsevier Ltd

Seedling broadcasting as a potential method to reduce apple snail damage to rice

Exotic apple snails (Pomacea spp.) are a major threat to the productivity and profitability of rice farming globally. Cultural methods that are applicable to traditional rice transplanting are often successful in reducing snail damage to rice. However, high labor and irrigation costs associated with transplanting highlight the need to develop modern rice crop establishment methods to replace traditional, labor-intensive methods. This study examined four broad categories of rice crop establishment for their vulnerability to apple snail damage. Seedlings from dapog nurseries and wet-direct seeding were highly vulnerable to damage and produced no grain in snail-infested ponds in the Philippines. Rice transplanted from dry bed nurseries at 21 days after sowing (DAS) had high mortality (85%) and consequently low yields. In contrast, seedling broadcasting (21 DAS) significantly reduced rice vulnerability (22% seedling mortality) to snail damage compared to all other methods and resulted in the highest grain yields per plot in our experiments. We attribute lower vulnerability to snail damage and successful stand development to reduced transplanting shock at the time of seedling broadcasting and to the generally good condition of seedlings even after 21 days in polyvinyl chloride trays. We suggest that seedling broadcasting be considered as a crop establishment method with potential to sustainably manage apple snails in irrigated rice. © 2014 Elsevier Ltd

Planthopper-rice interactions: unequal stresses on pure-line and hybrid rice under similar experimental conditions

Hybrid and pure-line (inbred) rice [Oryza sativa L. (Poaceae)] varieties have distinct physiologies, particularly as related to their nutrient requirements. These differences could confound the results and interpretation of experiments that compare rice varieties grown in pots for their resistance and responses to herbivores. In this study, a series of experiments was conducted to identify potentially confounding interactions between pot size (soil volume), fertilizer regime, and the use of acetate insect cages with rice line type (hybrid, fertile parental inbred, and male sterile inbred) during bioassays with the brown planthopper, Nilaparvata lugens (Stål) (Hemiptera: Delphacidae). The growth of hybrid rice was often limited (relatively low biomass and low grain production) compared to fertile inbred lines even in large pots (7 200 ml) and when grown without added fertilizer. Several interactions between the effects of growth conditions and line type were detected. Acetate cages caused a significant reduction in grain yield in hybrids, but not in inbreds, mainly resulting from a cage-induced decrease in grain weight (smaller grains). Hybrids and male sterile lines often had higher root or above-ground biomass in the largest caged pots compared with fertile inbred lines, but biomass was similar in smaller pots, indicating that the large pots allowed longer unimpeded growth of fertile inbreds, but not other line types. There were no interactions between the presence or absence of planthoppers with line type or experimental conditions. All line types were equally susceptible to planthoppers. Often the effects of planthopper feeding on plant fitness (i.e., tolerance) were apparent when plants were grown in large pots but not in small pots, particularly for hybrid lines and under high nitrogen regimes. On the basis of these results we recommend that researchers ensure that plants are not unequally stressed by inadequate growth conditions during comparative studies with herbivores on physiologically distinct rice varieties or rice species. We recommend the use of large pots (soil volume) and lower fertilizer levels with young, non-reproductive plants during comparative bioassays with planthoppers. Field cages are recommended for hybrid-inbred comparisons during older plant stages but these are subject to a range of other confounding variables. © 2013 The Netherlands Entomological Society Entomologia Experimentalis et Applicata © 2013 The Netherlands Entomological Society

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

© 2018 The Authors 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

Responses by the brown planthopper, Nilaparvata lugens, to conspecific density on resistant and susceptible rice varieties

© 2016 The Netherlands Entomological Society. This study examines the nature of intraspecific interactions among Nilaparvata lugens (Stål) (Hemiptera: Delphacidae) planthoppers feeding on resistant and susceptible rice varieties. Planthopper nymphs produced less honeydew and gained less weight when feeding on rice variety IR62 (resistant) compared to susceptible rice varieties. A series of bioassays was conducted that varied N. lugens nymph densities on IR62 and IR22 (susceptible). Increasing nymph density facilitated feeding by conspecifics; however, intraspecific competition increased mortality of nymphs on IR62 (but rarely on IR22). Furthermore, nymph weights declined with increasing conspecific density on IR22, and the effects were weak on IR62. More female nymphs than males survived on IR62 but this was not affected by density. Nitrogenous fertilizer increased competition among N. lugens on young plants of IR22, but not on IR62. Results indicate that nymphs have a low efficiency in accessing resources when feeding on IR62, even where the plants have received fertilizer. Female-biased survival and biomass compensation for mortality may promote population recovery after development on the resistant plant and accelerate adaptation to the resistant variety. Entomologia Experimentalis et Applicat

Reduced efficiency of tropical flies (Diptera) in the decomposition of snail cadavers following molluscicide poisoning

© 2018 Elsevier B.V. Decomposition is a key ecosystem service that reduces non-living organic matter, returning nutrients to the soil. In this study, we examine the responses by fly (Diptera) communities to molluscicide-poisoned apple snail (Pomacea canaliculata) cadavers in a tropical rice production landscape. Fewer molluscicide-poisoned cadavers were colonised by decomposer flies (niclosamide = 61% of cadavers with fly larvae; methaldehyde = 53%; Camellia sp. seed extract [saponin] = 50%) compared to control (freezer-killed) cadavers (81%). Metaldehyde, niclosamide and saponin reduced the abundance (average 51% reduction), biomass-density (average 63% reduction) and species richness (average 38% reduction) of flies emerging from the snail cadavers. The decay of control cadavers was generally faster (57% more tissue removed over 3 days) than molluscicide-treated cadavers. We suggest that poisoned carcasses potentially affect ecological communities across a range of trophic levels