The role of vector control in rice tungro disease management

N. Widiarta

AbstractTungro disease, which is spread by rice green leafhoppers, especially Nephotettix virescens, is one of the most destructive diseases of rice. The disease is successfully suppressed by planting rice at the recommended time to avoid high disease pressure and by rotating varieties with resistance to green leafhopper in synchronously planted areas, for example, in South Sulawesi. Simultaneous planting, however, is difficult to practice by farmers for various reasons. Therefore, until recently, tungro disease expansion and outbreaks have mainly occurred in asynchronously planted areas, primarily in Bali and Java. The population density of green leafhoppers in paddy fields in those areas is maintained at a low level by the dispersal activity of adults. Integrated pest management (IPM) strategies have been developed based on characteristic population dynamics of the vector to reduce the proportion of viruliferous vectors. In this study, the use of antifeedants against N. virescens to control tungro spread in synchronously and asynchronously planted fields was also examined. The antifeedant and virus transmission inhibition activities of andrographolide, a major compound of Andrographis paniculata, and commercial insecticides such as imidacloprid, pymetrozin, MIPC, and nytenpyram were tested against female adults of N. virescens. Imidacloprid and nytenpyram showed better antifeedant activities than andrographolide and pymetrozin. Imidacloprid inhibited acquisition and inoculation of tungro viruses better than the others. The results imply that antifeedants have a potential to reduce virus transmission without directly disturbing the food chain. In asynchronously planted areas, application of diacloden and MlPC successfully decreased vector population density but not tungro spread. In synchronously planted areas, application of diacloden, imidacloprid, and MlPC significantly reduced both vector population density and disease incidence.

Introduction

Rice tungro disease (RTD) has affected almost all provinces of Indonesia. Recently, the affected area has extended to the northern coastal lowland of West Java in Subang (Hasanuddin et al 1995, Widiarta et al 1997a). Therefore, the important West Java rice bowl is now threatened. RTD has been successfully controlled in Sulawesi by transplanting synchronously over wide areas at the recommended planting time (Sama et al 1991). This enables rice plants to escape peak periods of disease pressure. Synchronous planting was also combined with rotation of varieties particularly resistant to the tungro vector, Nephotettix virescens.

Rice is susceptible to tungro infection during the early vegetative stage. Infection at this stage causes severe yield loss. The older the rice plant is when infected, the lower the yield loss. The recommended planting time is based on the annual cycle of N. virescens populations and RTD incidence. In Maros, South Sulawesi, April and October are the months of highest tungro risk because population densities of green leafhopper (GLH) and RTD incidence are high then. Therefore, farmers are recommended not to have young rice plants in the fields at this time to escape damage.

The escape strategy is combined with varietal rotation between seasons. Breeders have identified seven genes with resistance to GLH. Rice varieties are categorized into five groups

based on the resistance gene of the parent. After the combined escape and rotation strategy was implemented, the tungro-infected area in South Sulawesi decreased remarkably. Before synchronous planting was established in 1984, the tungro-infected area exceeded 4,000 ha; now it is less than 100 ha.

The escape strategy is difficult to implement in asynchronously planted areas because it is difficult for farmers to plant simultaneously over wide areas. Varietal rotation reduced disease incidence in asynchronous areas as long as GLH did not adapt to the varieties being used. The problem is, however, that resistant varieties with different resistance genes also differ in eating quality. Farmers need varieties with diverse resistance genes but that arc similar in quality. Recommended varieties may not be accepted because of their inferior taste. Almost all Indonesians prefer IR64-type quality.

Following the success of tungro management in South Sulawesi, the main problem has switched from Sulawesi and Bali in 1980-85 to Java and Bali more recently. Unfortunately, Java and Bali contribute more than 60% of the total rice production in Indonesia. Consequently, tungro threatens rice production and self-sufficiency in the country. Other control strategies are therefore required, especially for asynchronously planted areas.

Population dynamics of N. virescens

The fluctuating RTD incidence in asynchronously planted areas is closely related to changes in GLH density (Suzuki et al 1992). RTD incidence increases when GLH numbers increase. To establish a control strategy for RTD in asynchronously planted fields, the population dynamics of GLH was studied intensively in Bali using a FARMCOP insect suction trap (Cariño et al 1979, Aryawan et al 1993). All arthropods caught were identified in the laboratory. It was established that the population density of GLH increased only at the early stages of rice growth and decreased thereafter (Widiarta 1992). Some studies showed that population density did not increase at all after immigrants invaded rice fields. The peak density of N. virescens was much lower than that of its sibling species, N. cincticeps, in temperate paddy fields. The population density of N. ciscticeps was observed in Okayama, Southwestern Japan, using a procedure similar to that used in Bali (Widiarta 1993). The population density of N. cincticeps increases from the time it infests rice fields until just before harvest and its peak densities are much higher than those of N. virescens.

A key factor analysis for the life tables of N. virescens and N. cincticeps was conducted. Key component mortality can be identified with reference to the biggest slope of regression coefficient. Results showed that nymphal mortality including loss of adults after emergence is the key factor for both N. virescens and N. cincticeps. Egg parasitism is the second key factor for N. virescens but not for N. cincticeps. To establish relationships between key component morality and biotic and abiotic factors, regression analysis was conducted using nymphal mortality and predators as well as nymphal mortality and rainfall. Nymphal mortality for N. virescens was not influenced by spider numbers. Nymphal mortality for N. virescens was not related to physical factors such as rainfall. Therefore, adult loss after emergence is considered to be the key factor.

RTD control strategy

The previously mentioned characteristic population dynamics of N. virescens suggest that the population density of N. virescens in asynchronously planted fields is kept low by the dispersal activity of adults. Newly emerged adults have short residential periods within fields, probably

because they move to other fields of younger rice plants. Therefore, they lay fewer eggs in fields where they emerged. Egg parasitoids also play an important role in reducing egg survival. Therefore reducing the population density of N. virescens is not the most appropriate way to control tungro. The requirement is to reduce the proportion of ineffective vectors. Virus sources can be reduced by using resistant varieties that restrict virus multiplication and by practicing selective weeding of alternative virus hosts. Antifeedants can be used to reduce the duration of virus inoculation and acquisition. This review presents the role of antifeedants in controlling vector feeding and of insecticides in controlling vector density in relation to RTD.

Control of N. virescens feeding activity

The use of an antifeedant reduces virus acquisition and inoculation without killing the insect, which helps maintain the balance with its natural enemies, especially egg parasitoids. Some substances have been tested in the laboratory (Widiarta et al 1997b). Feeding on artificial diets using the streaked parafilm method was employed. The insects tested were allowed to feed through parafilm. The feeding rate and number of stylet marks on the parafilm were observed. Insect survival after treatment was also recorded to discriminate between antifeedant activity and insecticide activity. Table 1 shows that andrographolide, an active compound of the tropical plant Andrographis paniculata, and pymetrozin insecticide supressed feeding by females at the lowest concentration of 20 ppm, while imidacloprid and nytenpyram, a neonicotinoid insecticide, did so at concentrations of 0.01 ppm. Insect survival rates at concentrations tested were generally high, but decreased markedly at the highest concentration of 40 ppm for both the andrographolide and pymetrozine. The chemicals tested showed antifeedant activity against N. virescens.

Andrographolide at 40 ppm and the insecticides pymetrozine, imidacloprid, and nytenpyram significantly reduced the number of stylet marks (Table 2). The minimum feeding acquisition and inoculation times for transmission of tungro by N. virescens were reported to be 5–30 minutes (Wathanakul and Weerapat 1969). The reduction in number- of feeding marks may be used as an indicator of the extent to which virus transmission is impeded.

Rice tungro virus transmission inhibition by antifeedants

The effect of antifeedants on virus acquisition and inoculation was investigated (Widiarta et al unpublished). RTD-affected plants were treated with antifeedant before N. virescens was given access to the plants. Healthy rice seedlings were also treated with antifeedant before infective N. virescens was given access to them to assess feeding inhibition. The test tube inoculation method was used for both tests.

Results showed that pymetrozin and andrographolide applied to tungro-affected plants significantly decreased the number of vectors that became infective (Fig. 1). Application of imidacloprid at 0.01 to 0.02 ppm completely prevented acquisition. Pymetrozin and andrographolide applications on test plants at concentrations of 20 ppm significantly reduced virus transmission by N. virescens (Fig. 2). Furthermore, increasing the concentration of pymetrozin to 40 ppm reduced virus transmission, but increasing that of andrographolide did not. Application of imidacloprid at concentrations of 0.01 and 0.02 ppm significantly reduced virus transmission. It was clear that imidacloprid greatly reduced virus transmission by N. virescens, while andrographolide and pymetrozin did so only at lower concentrations.

Control of N. virescens population

A field experiment was conducted in the 1997 dry-season crop in asynchronously planted fields in Subak Padanggalak, Bali, and in the wet-season crop for synchronously planted fields in Subak Samsam, Bali (Widiarta et al unpublished). Insecticides were applied fortnightly, starting 1 week after GLH infestations occurred. The insecticides tested were diacloden, imidacloprid, and MIPC. The experiment was conducted using a randomized block design with four replicates of each treatment. The plot size of each replicate was 5 × 8 m. The susceptible rice variety Pelita was transplanted at 25 × 25-cm spacing, 21 days after sowing. The population density of green leafhoppers was observed weekly by counting 39 hills /plot. RTD incidence was observed at 8 weeks after transplanting and before harvest.

In the asynchronous plantings, applications of diacloden and MIPC significantly reduced the population density of GLH (Fig. 3). RTD incidence before harvest, however, was not reduced significantly (Fig. 4). In the synchronous planting, applications of diacloden, imidacloprid, and MIPC significantly reduced not only GLH density (Fig. 5) but also RTD incidence (Fig. 6). Thus, reduced vector density decreased RTD incidence, especially in synchronously planted crops.

Conclusion

Low doses of insecticides such as imidacloprid, nytenpyram, and pymetrozin as well as andrographolide showed antifeedant activity against N. virescens and also inhibited transmission of rice tungro viruses. Thus, control of feeding has the potential to check tungro without disturbing the food chain. Population control of GLH by insecticides showed the limitation of this approach in reducing RTD spread in asynchronous plantings.

References

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NotesAuthor’s address: N. Widiarta, Research Institute for Rice, J1. Raya IX Sukamandi 41256.Subang, West Java, Indonesia.

Citation: Widiarta N. 1999. The role of vector control in rice tungro disease management. p. 143-151. In: Chancellor TCB, Azzam O, Heong KL (editors). Rice tungro disease management. Proceedings of the International Workshop on Tungro Disease Management, 9-11 November 1998, International Rice Research Institute, Los Baños, Philippines, 166 p.