Leafhopper control by insecticides is not the solution to the tungro problem

S. Villareal

AbstractGreen leafhoppers (GLH) are one of the most abundant canopy arthropods in irrigated rice systems throughout much of South and Southeast Asia. Populations are rarely large enough to cause direct feeding damage to rice, but in some areas they are important pests as vectors of rice tungro disease. In some countries in Asia, chemical control of GLH based on threshold numbers of the insect is still recommended. In the majority of rice-growing areas, however, tungro disease is absent or occurs infrequently. Consequently, routine insecticide applications against GLH cannot be justified. Moreover, spraying insecticides to control GLH does not always result in effective tungro disease management. Insecticides may be harmful to human health and to the environment and indiscriminate use of certain compounds in rice has been shown to cause outbreaks of secondary pests. Alternative methods of managing tungro disease in endemic areas are available and these should be used for GLH control. Currently, virus-resistant varieties are being developed and it is hoped that they will soon be available to rice farmers and that these varieties will provide them with a further option for managing tungro.Introduction

The main leafhopper vector of tungro disease, Nephotettix virescens (Distant), can be found in almost all irrigated and rainfed rice systems in tropical Asia. In a study conducted at five irrigated sites in the Philippines in 1989, N. virescens was the most abundant phytophagous insect in all locations (Heong et al 1989). In the absence of sources of inoculum, however, this insect does not harm the rice crop through direct feeding. There have been reports of “hopperburn” caused by large populations of GLH, especially from India, but these cases are extremely rare. In most rice-growing areas, inoculum sources are few or absent so that attempting to control leafhoppers would be wasting farmers’ time and money.

GLH abundance and tungro disease incidence in endemic areas

In studies conducted in experimental plots on the IRRI farm in 1991-91, there was no correlation between numbers of N. virescens and tungro disease (Chancellor et al 1996a). In one season, high tungro incidence uas recorded in the presence of low numbers of leafhoppers. The availability of inoculum was considered to be the key determinant of the disease level. Once primary inoculum had been introduced into field plots. rapid secondary plant-to-plant spread occurred. An analysis of factors affecting tungro incidence in endemic areas in Mindanao, Philippines, however, revealed that increases in disease incidence were associated with rising vector numbers as well as with the number of viruliferous vectors (Savary et al 1993). Similar findings were recorded in an endemic area in South Luzon, Philippines, during a survey conducted in 1993-94 (Chancellor et al 1996b). In view of the impracticability of reducing vector numbers to levels needed to significantly reduce tungro spread, however, the most appropriate tungro management strategy in such areas would be to minimize the amount of initial inoculum by planting resistant varieties or by reducing the variation in planting dates.

Efficacy of insecticide sprays against GLH in managing tungro

Many field trials have been conducted on research stations to evaluate the efficacy of various insecticides against GLH and their effectiveness in reducing tungro disease. Chancellor et a1 (1997) reviewed the results from some of these trials and concluded that successful control of GLH and tungro disease can be achieved through insecticide sprays but that this may not be

possible under certain conditions. A modeling study by Holt (1996) provides an explanation for this. In his study, the author examined the effect on final tungro incidence of applying a single insecticide at 20 d after transplanting, assuming an extremely high mortality rate of 95% to N. virescens. The model output indicated that, at very low immigration rates of GLH, a reduction in disease incidence of up to 60% could be achieved. As the immigration rate of infective vectors increased, however, the reduction in tungro incidence became extremely small. Although this case refers to a situation with a single insecticide application, the assumed mortality rate of 95% is unrealistically high and this level of mortality could probably only be achieved with the use of two or more insecticides. Batay-an and Mancao (this volume) report on an insecticide trial carried out in North Cotabato, Philippines, in which good control of GLH and tungro disease was achieved using synthetic pyrethroids. Where there are widespread sources of inoculum and active movement of GLH between fields, however, such an approach may not be effective. Similar conditions were observed during a tungro epidemic in Negros, Philippines, in the 1998 wet season, where farmers resorted to weekly sprays of cypermethrin in an attempt to control the disease. The strategy was not effective in reducing tungro incidence and the sprays caused the resurgence of populations of the brown planthopper, Nilaparvata lugens (Stål), which led to “hopperburn” (Tiongco 1998). Similarly, in an insecticide trial conducted in North Cotabato in 1997, weekly spraying was not successful in preventing tungro from spreading in experimental plots. Tungro disease incidence in surrounding rice fields was very high and presumably there was continuous recolonization of the trial plots by viruliferous leafhoppers similar to the process described by Schoenly et al (1996).

Environmental and health concerns associated with insecticide applications in rice

The cartoon illustrated in Figure 1 was used in training courses for rice farmers on tungro management conducted in India and the Philippines in 1996-98. The cartoon illustrates two of the unwelcome effects of spraying insecticides in rice. First, insecticide application methods in rice are still very primitive and, with the types of sprayers and nozzles available, the efficacy is doubtful. Most people doing the spraying do not wear protective clothing and they mix the chemical without wearing gloves. They spray in front of them so that they have to walk through the sprayed area. Unfortunately, category 1 and 2 insecticides are still widely available in many Asian countries so the risk to human health through insecticide spraying is serious. Second, most insecticides that are used for insect control in rice are still nonselective and are harmful the many predators and parasites that are so important for regulating populations of insect pests in rice.

Conclusions

Applying insecticides to control leafhopper vectors of tungro disease cannot be justified in areas where inoculum sources are not present. Even in endemic areas, insecticide applications are often not effective. In a survey conducted in 1994 in the Philippines. some farmers in tungro-endemic areas said that they continued to apply insecticides to control tungro even though they knew that this approach did not work (Warburton et al 1997). They sprayed because they did not know what else they could do and they thought the insecticides might help to control some other insect pests. Our challenge is to promote the adoption of more effective and environmentally safe tungro management strategies in such areas.

References

Chancellor TCB, Cook AG, Heong KL. 1996a. The within-field dynamics of rice tungro disease in relation to the abundance of its major leafhopper vectors. Crop Protection 15: 439–449.

Chancellor TCB, Tiongco ER, Holt J, Villareal S, Teng PS, Fabellar N. Magbanua MGM. 1996b. Risk factors for rice tungro disease in endemic areas. In: Rice tungro disease epidemiology and vector ecology: the development of sustainable and cost-effective pest management practices to reduce yield losses in intensive rice cropping systems. IRRI Discussion Paper Series No. 19. Manila (Philippines): International Rice Research Institute.

Chancellor TCB, Heong KL, Cook AG. 1997. The role of leafhopper control in the management of rice tungro disease. In: Chancellor TCB, Thresh JM (editors). The epidemiology and management of rice tungro disease. Chatham (UK): Natural Resources Institute.

Heong KL, Aquino GB, Barrion AT. 1989. Population dynamics of plant- and leafhoppers and their natural enemies in rice ecosystems in the Philippines. Crop Protection 11:371–379.

Holt J. 1996. Spatial modeling of rice tungro disease epidemics. In: Rice tungro disease epidemiology and vector ecology: the development of sustainable and cost-effective pest management practices to reduce yield losses in intensive rice cropping systems. IRRI Discussion Paper Series No. 19. Manila (Philippines): International Rice Research Institute.

Savary S, Fabellar N, Tiongco ER, Teng PS. 1993. A characterization of rice tungro epidemics in the Philippines from historical survey data. Plant Disease 77:376–382.

Schoenly KG, Cohen JE, Heong KL, Arida GS. Barrion AT, Litsinger JA. 1996. Quantifying the impact of insecticides on food web structure of rice-arthropod populations in a Philippine farmer’s irrigated field: a case study. In: Polis GA, Winemiller KO, editors. Food webs: integration of patterns and dynamics. New York: Chapman and Hall. p. 343–351.

Tiongco ER. 1998. Unpublished trip report to Negros, Philippines. August 1998. Nueva Ecija (Philippines): Philippine Rice Research Institute.

Warburton H, Palis FL, Villareal S. 1997. Farmers’ perceptions of rice tungro disease in the Philippines. In: Heong KL, Escalada MM, editors. Pest management of rice farmers inAsia. Manila (Philippines): International Rice Research Institute. p. 129–141.

NotesAuthors’ address: S. Villareal, International Rice Research Institute, MCPO Box 3127, MakatiCity 1271, Philippines.

Citation: Villareal S. 1999. Leafhopper control by insecticides is not the solution to the tungro problem. p. 138-142. 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.