The Rice Whorl Maggot, Hydrellia philippina Ferino (Diptera:Ephydridae) in the Philippines: A Review
Authors: Litsinger J. A. , Barrion A. T. , Canapi B. L. , Lumaban M. D. , Pantua P. C. , Aquino G. B. 
Published: January 2025

The rice whorl maggot (RWM), Hydrellia philippina Ferino (Diptera: Ephydridae), was discovered as a pest of rice only in 1961. Its damage was not distinguished from that of other early season insect pests before the Green Revolution. The spread of irrigation and double-cropping allowed its density to increase and become noticeable. It is just as abundant on modern rice varieties as on traditional ones. Other rice-feeding Hydrellia species are leaf miners but the damage from RWM is different as it mines the unfurled leaf before it expands, thus creating a unique necrotic lesion along the margin of the leaf blade. RWM feeds on wetland rice and on a number of other Poaceae and some sedges in the rice field. Most of the feeding is done during the first four weeks after transplanting but damage is rare in direct seeded rice. It is most abundant in flooded, but not drained fields. Damage occurs most during the vegetative stage where new leaf unfurls and damage peaks at 4-5 weeks after transplanting. Minor damage can occur on developing spikelets resulting in unfilled grains. Injury results in reduced plant height and root length delaying crop maturity. These effects occur unevenly hill to hill. In four irrigated, double-cropped rice locations, namely, Calauan, Laguna; Guimba, Nueva Ecija; Zaragoza, Nueva Ecija and Koronadal, South Cotabato, damaged leaves (DL) averaged 15% per cropping season over a 13 year period. Of these, two sites-Zaragoza, Nueva Ecija [DL: 20% in the wet season (WS), 19% dry season (DS)] and Koronadal, South Cotabato [DL: 25% WS, 20%DS] had economic damage levels averaging 21% DL, whereas the other two-Calauan [DL: 11%WS, 6% DS] and Guimba [DL: 7%WS, 12% DS] at 9% DL did not. However, one individual field in Koronadal, South Cotabato registered the highest incidence of damaged leaves at 68% DL. The higher damage occurs in areas of high asynchronous planting and short dry-fallows. In three rainfed wetland sites, the levels of infestation were 8% DL in the most drought prone site and 21% to 29% DL in the more rainfall favored sites. Rainfed wetland areas typically have more staggered planting and farmers grow longer maturing varieties which have a longer vegetative stage that favors RWM. In general, significant yield loss occurred in sites which had 1) high populations of noctuid defoliators Naranga and Rivula, 2) seedlings with severe transplanting shock, and 3) older transplanted seedlings. Higher losses occur the more the crop is under stress from any cause during the vegetative stage. RWM has a wide array of natural enemies ranging from egg and adult predators to larval and pupal parasitoids. Egg parasitism, however, is< 1%, whereas larval and pupal parasitoids contribute 35% parasitism. The most effective natural enemy, however, is another ephydrid fly Ochthera sauteri Cresson which preys on adult RWM ambushing them on the foliage or in flight, and stabbing them with elongated tibial spurs. In a simulated rice field, prey consumption by five Ochthera over a five- day period steadily rose from 27 to 48 adults per day, indicating that they learned how to recognize RWM over time. Cultural control methods that limit loss include planting longer maturing varieties or older seedlings, dense spacing (particularly direct seeding), early planting, splitting. N application, and field drainage during the vegetative stage. We hypothesize that the mechanism for good drainage and dense planting enhances Ochthera predation as the grayish mud provides good camouflage for the predatory fly to effectively and efficiently hunt RWM adults undetected. Presence of water creates a "mirror-effect" reflection signal from the water surface that warns RWM flies of incoming predators, who thus escape predation. Timely application of N, on the other hand, increases the ability of the rice plants to tolerate RWM injury. No rice accession from the IRRI Germ Bank was found to be resistant to RWM, however IR40 variety is moderately tolerant. Many wild rices, however, show ovipositional non ­preference or low larval survival but these traits have yet to be incorporated into cultivated rice. RWM is difficult to control with insecticides and normally it is not economical. Foliar sprays are most popular with farmers and, although relatively expensive, two low dosage sprayings before 10 OT are needed for adequate control. Other effective methods were soil incorporation of insecticide granules, root soaking, and root coating. Action thresholds have been determined to guide insecticide application. In areas with a history of high RWM infestation farmers can use earlier planted fields as a guide to insecticide usage based on the% DL level. Egg densities per hill can be used in locations with historically low levels of infestation.