Management of rice tungro disease by chemical control of the green leafhopper vector

E.H. Batay-An and S.C. Mancao

AbstractThe field efficacy of five synthetic pyrethroid insecticides was evaluated for the control of green leafhoppers (GLH) and rice tungro disease (RTD) and for their effect on natural enemy populations and yield of IR64. In each of two seasons, pyrethroid-treated plots significantly reduced GLH populations and had significantly lower RTD incidence. Among the five treatments, plots treated with cypermethrin and ethofenprox had the lowest GLH population and RTD incidence at all sampling dates. Yields were higher in the treated plots than in the control. Populations of spiders, Cyrtorhinus lividipennis, Conocephalus longipennis, Agriocnemis pygmaea, and coccinellids were significantly affected by insecticide applications 1 d after treatment at 5, 20, and 35 days after transplanting (DAT). Cypermethrin, however, did not affect C. lividipennis populations at 20 DAT. Likewise, none of the insecticides reduced A. pygmaea populations at 35 DAT in the 1989 wet season or populations of any of the natural enemies at 35 DAT in the 1990 dry season. Sprays of cypermethrin and ethofenprox did not significantly affect the population of spiders, C. lividipennis, and A. pygmaea at 20 DAT in the 1990 dry season.

Introduction

Mindanao is one of the largest islands located at the southernmost part of the Philippines. Large areas of rice are grown on the island, which has a relatively even distribution of rainfall throughout the year and is free from typhoons. One of the major rice production constraints in Mindanao is the occurrence of insect pests and diseases. These are brought about by favorable climatic conditions coupled with asynchronous planting, continuous cropping of rice, and planting of susceptible cultivars. Thus, the year-round availability of rice and the warm humid climate are conducive to insect proliferation and survival.

Among the pests in Mindanao, rice tungro disease, which is transmitted by the green leafhopper, Nephotettix virescens, is the most destructive disease of rice. Outbreaks of RTD in the Philippines occurred in 1957, 1963, 1969, 1971, 1975, and 1977 (Bergonia 1978). In Central Luzon, IR36 and IR42, both moderately resistant to green leafhopper (GLH), were highly infected with RTD in 1984. Likewise, in Mindanao, high RTD incidences were observed in South and North Cotabato in 1985 and 1986 and again in 1993-98 (Truong et al, “Rice tungro disease in the Philippines,” this volume).

At present, the best way to prevent RTD is by using vector-resistant varieties because there is still no variety resistant to the tungro virus. Continuous planting of these vector-resistant cultivars, however, may cause them to succumb to RTD because of the development of virulent GLH populations. Therefore, one possibility for reducing RTD incidence in the field is to control the insect vector with insecticides.

Rice yield increases have been attributed to insecticide use under various conditions in nearly all rice-growing countries (Lim and Heong 1984). Greater use of insecticides by farmers is expected because other control strategies such as using resistant varieties and intensifying the role of biocontrol agents take time and are limited in scope. Under laboratory conditions, some commonly used insecticides such as those in the organophosphate and carbamate groups applied as foliar sprays caused 90- 100% GLH mortality 24 hours after treatment. They cannot prevent infection with tungro viruses (IRRI 1984, 1985), however, because of their slow effect—GLH can transmit virus particles before they die. Thus, rapid-acting insecticides are needed.

Synthetic pyrethroids offer some advantages because they are fast-acting and effective at low dosages; thus, residue levels are likely to be low on crops (Ozaki et al 1984). The Midsayap branch of PhilRice conducted this study to evaluate the field efficacy of five synthetic pyrethroid insecticides against GLH and RTD incidence. The objectives were to determine the best insecticide for GLH control, to assess the impact of different insecticides on natural enemies, and to determine the impact on the yield of IR64.

Materials and methods

The experiment was conducted at PhilRice-Midsayap, Bual Norte, Midsayap, Cotabato, during the 1989 wet season (WS) and 1990 dry season (DS). It was laid out in a randomized complete block design consisting of six treatments with a plot size of 5 × 5 m replicated four times. The treatments were T1 = plots sprayed with cypermethrin, T2 = plots sprayed with monocrotophos + cypermethrin, T3 = plots sprayed with deltamethrin, T4 = plots sprayed with lambdacyhalothrin, T5 = plots sprayed with ethofenprox, and T6 = untreated control.

Pre-germinated seeds of susceptible IR64 were uniformly sown on the seedbed raised under the dapog method and immediately covered with nylon mesh after sowing for protection against birds and early insect pest infestation. Ten-day-old dapog seedlings were transplanted at 3 to 5 seedlings /hill spaced at 20 × 20 cm. Starting 5 days after transplanting (DAT), three insecticide applications by knapsack sprayer were made at 15-days intervals following the manufacturer’s recommended dosage with a spray volume of 300 L /ha. Fertilizer was applied at a rate of 60-0-0 kg NPK /ha. One-half of the nitrogen was applied at planting whereas the remaining half was applied 5 to 7 days before panicle initiation. Preemergence herbicide (butachlor) was applied at 3 DAT for weed control. GLH and natural enemy populations were estimated 1 day before and 1 day after every spray application by 10 sweeps of an insect net per plot. RTD incidence was recorded at 60 DAT by counting the number of infected hills per plot. Rice yield was taken from a 10-m2 harvest area per plot, threshed, and dried at 14% moisture content. Data were analyzed statistically using analysis of variance, and Duncan’s multiple range test was used for comparison among treatments.

Results and discussion

GLH populations were significantly lower in all plots treated with cypermethrin, monocrotophos + cypermethrin, deltamethrin, lambdacyhalothrin, and ethofenprox than in the untreated control plots at all sampling dates in the 1989 WS (Table 1). Similarly, RTD incidence was significantly lower in the treated plots than in the untreated control plot (Table 1). There was no significant difference between plots sprayed with cypermethrin, ethofenprox, and lambdacyhalothrin. This result is consistent with findings reported from India (Satapathy and Anjaneyulu 1984, Krishnaiah and Ghosh 1990, Anjaneyulu and Bhaktavatsalam 1986) and from the Philippines (Macatula et al 1987).

The populations of spiders, Cyrtorhinus lividipennis, Conocephalus longipennis, Agriocnemis pygmaea, and coccinellids were considerably reduced by pyrethroid insecticide applications at 5 and 20 DAT in the 1989 WS (Tables 2-6). Cypermethrin and lambdacyhalothrin applications, however, had no effect on C. longipennis numbers at 20 DAT (Table 4). Likewise, none of the synthetic pyrethroids tested significantly affected the population of A. pygmaea at 20 and 35 DAT (Table 5). Synthetic pyrethroid insecticides applied at 20 and 35 DAT significantly reduced GLH numbers 1 day after spray application compared with the untreated control in the 1990 DS (Table 1). At 5 DAT, only cypermethrin and ethofenprox significantly reduced GLH numbers.

Percent RTD infection obtained visually at 60 DAT was significantly lower in treated plots than in the untreated control in the 1990 DS. There were no significant differences among the five synthetic pyrethroids tested (Table 7). Yields obtained from plots sprayed with cypermethrin, ethofenprox, lambdacyhalothrin, monocrotophos + cypermethrin, and deltamethrin ranged from 3.6 to 2.7 t /ha. These yields were significantly higher than the 1.9 t /ha yield of the untreated control. This result supports the findings of Macatula et al (1987).

The application of five synthetic pyrethroid insecticides on IR64 by foliar spray at 5 and 20 DAT considerably affected the populations of spiders. C. lividipennis, A. pygmaea, and coccinellids in the 1990 DS (Tables 2-6). Ethofenprox, however, did not reduce the numbers of spiders and coccinellids, and A. pygmaea and C. lividipennis adults were not affected by most pyrethroid applications at 20 DAT. Moreover, none of the five pyrethroid insecticides tested significantly affected natural enemy populations at 35 DAT (Tables 2-6).

Conclusions

Five synthetic pyrethroid insecticides were evaluated for their effect on GLH numbers, RTD incidence, and natural enemy populations at PhilRice-Midsayap. The effect of insecticide application on the yield of IR64 was also assessed.

The evaluation results are summarized as follows:1. Foliar sprays of cypermethrin, monocrotophos + cypermethrin, deltamethrin, lambda-cyhalothrin, and ethnofenprox significantly reduced GLH populations and RTD incidence in both seasons.2. Among the five pyrethroids, cypermethrin and ethofenprox were the most effective in lowering GLH numbers and percent RTD incidence.3. All plots treated with synthetic pyrethroids yielded significantly higher than the untreated control in the 1990 dry season.

4. In the 1989 wet season, foliar sprays of five pyrethroids significantly affected the populations of spiders, C. lividipennis, C. longipennis, A. pygmaea, and coccinellids at 5 and 20 DAT, except for cypermethrin on C. longipennis at 20 DAT. None of the pyrethroids affected the A. pygmaea population at 35 DAT.5. In the 1990 dry season, foliar sprays of cypermethrin and ethofenprox did not affect the populations of spiders, C. lividipennis, and A. pygmaea at 20 DAT. Moreover, none of the pyrethroids significantly reduced natural enemy numbers at 35 DAT. The five synthetic pyrethroid insecticides tested were all effective against GLH. Synthetic pyrethroid insecticide application should be started within 5 DAT and repeated at least two times at 15-days intervals to reduce GLH populations and RTD incidence.

References

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NotesAuthors’ address: E.H. Batay-an and S.C. Mancao, Philippine Rice Research Institute, Maligaya, Muñoz, 3119 Nueva Ecija, Philippines.

Citation: Batay-An E.H. and S.C. Mancao. 1999. Management of rice tungro disease by chemical control of the green leafhopper vector. p. 158-166. 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.