Dichogaster nr curgensis Michaelsen (Annelida: Octochaetidae): An earthworm pest of terraced rice in the Philippine Cordilleras

The oligochaete Dichogaster nr. curgensis Michaelsen, has become a pest of irrigated rice in the Philippine terraced highlands. Heavy erosion of the deforested Abra River watershed deposited unnaturally high numbers (13,659 m(-2)) of this epigeic earthworm along the riverbank, which subsequently became concentrated in irrigated rice fields (17,294 m(-2)) in over 20,000 ha. The earthworms' burrowing activity caused leaks in rice levees and body setae mechanically injured roots resulting in plant stunting and seedling death. Infested fields exhibited reduced plant stand and uneven growth. Rice was more sensitive to damage occurring during the vegetative than reproductive stage. The crop could tolerate earthworm densities of 140 m(-2) but suffered significant yield loss when populations were ten times higher, and complete loss at 700 m(-2). Field observations and greenhouse tests showed prolonged flooding reduces earthworm densities. Seedbeds should be located away from the rice fields and taller seedlings (45-55 days old) should be transplanted to tolerate >14 cm depth pending until the late reproductive stage. A preventative method of creating holding ponds is suggested to trap earthworms before irrigation water passes to the fields. During the off-season, infested fields may be kept flooded for 2-3 months to lower resident densities. Leaves of Gliricidia sepium (Jacq.) Steud, a popular pesticidal plant suggested by farmers, increased rather than decreased earthworm numbers when incorporated into the soil. Carbofuran granules at 0.5 kg a.i. ha(-1) soil incorporated before transplanting caused over 90% mortality in a greenhouse trial. Copyright (C) 1996 Elsevier Science Ltd.