Hydrellia philippina (rice leaf miner)

H. philippina is abundant during the early stages of crop growth, but generally ceases to be a problem after the plant reaches the booting stage. This is probably due to the lack of suitable habitat for the larvae.H. philippina larvae feed on the inner margins of unopened leaves. Conspicuous linear feeding lesions are visible when the central leaf opens. Damaged leaves become distorted and may be broken off by the wind (Ferino, 1968b). Infested plants are stunted. The larvae can cause damage to the boot leaf and developing panicles (Sain et al., 1983) which can lead to partial filling of grains (Varadarajan et al., 1977). Small punctures may appear in the middle of the flag leaf and its margin may become discoloured (Basu, 1979). The chlorotic effect, coupled with disrupted sugar metabolism and poor nutrient uptake, are probably the reasons for the manifested effects on infested plants (Ramamurthy et al., 1977).

Cultural Control

Draining the water at intervals of 3-4 days during the first 30 days after transplanting reduces egg laying as the adult flies are more attracted to standing water (Salazar et al., 1993). Drained fields, however, allow more weeds to grow.

Crop establishment methods which enable the plants to cover the water surface most rapidly result in insignificant damage from H. philippina. Covering the water surface with Azolla and Salvinia molesta in Indonesia (Bangun, 1988) helped to prevent infestation. Direct-seeded rice fields or seed beds are not as attractive to adults as the transplanted crop. For autumn rice in India, increasing the plant density and nitrogen level resulted in decreasing damage by H. philippina (Yein and Das, 1988). Similarly, in Colombia, greater attack was observed by Hydrellia spp. at the lowest plant density without affecting yields (Salazar et al., 1993). Close planting decreases oviposition and subsequent damage by H. philippina (Viajante and Heinrichs, 1985b).

Varietal Resistance

One rice cultivar, IR 40, and two wild rices, Oryza brachyantha and O. ridleuyi, have been identified as resistant to H. philippina. In India, of 52 rice lines derived from 52 crosses that were screened for resistance to H. philippina during the dry season of 1985, 11 were found to be promising with a maximum of 10% damaged leaves. The most resistant were RP 2418-5, RP 2418-10 and RP 2419-3 with 3-5% damaged leaves (Sain and Hakim, 1988). Similarly, IR 9209-48-3-2 and UPR 82-1-7 also showed lowest incidence of Hydrellia sp. at Punjab, India (Jaswant Singh et al., 1990).

H. philippina is only a pest of rice in irrigated conditions. By 3-4 weeks after transplanting, rice plants in continuous standing water have more whorl maggot damage than plants in water-saturated soil. This may be due to the adults' preference of flooded fields for oviposition (Viajante and Heinrichs, 1985a).Ferino (1968b) reported an estimated yield loss due to H. philippina of 1.4 t/ha in the Philippines. In south India, Thomas et al. (1971) reported that the whorl maggot could cause 20-30% yield loss on the first crop from April to September, and that the infestation was less in the second crop. Nurullah (1979) reported that H. philippina does not cause any adverse effect on tiller production and may even increase the number of productive tillers to compensate for possible grain loss.Viajayante and Heinrichs (1986) conducted six yield loss experiments and found that H. philippina did not adversely affect yield of IR 36 variety but did delay maturity. The experiment was conducted by comparing yields of artificially infested and non-infested caged plots.The conflicting reports regarding yield losses are probably due to different experimental conditions. A possible reason for reported yield losses in some studies may be the inability to fully recover from whorl maggot damage when simultaneously stressed by soil mineral deficiencies or toxicities. Further studies should be conducted to determine the variability in whorl maggot damage.