A life table study of the yellow rice borer, Tryporyza incertulas (Walker) in Sri Lanka

Abstract

A life table study was conducted in Sri Lanka during 1982 to contribute understanding to the population dynamics of T. incertulas in the rice agroecosystem. The determination of larval instars revealed the occurrence of 5 instars and the duration of larval period was on average 36 days. Head capsule width measurements were correlated significantly with larval instar, establishing the applicability of Dyar's principle to T. incertulas. Ecological life tables were constructed from population data for four generations of T. incertulas. An intergeneration variability of real mortality (100rx) was observed in each life stage as follows: egg stage, 45.2-63.9; 1st instar larval stage, 9.0-18.5; 2nd-5th instar larval stage, 10.8-13.9; and pupal stage, 5.7-9.1. Total generation mortality varied between 83-93%. Two egg parasites, Telenomus sp. and Tetrastichus schoenobii Ferriere, and one pupal parasite, Tetrastichus ?inferens Yoshimoto, were found associated with yellow rice borer populations. Egg parasitism contributed most to the generation mortality and ranged between 43.4-60.8%. Larval mortality by parasites or pathogens was not observed and the disappearance of larvae, defined as mortality by unidentified mortality agents, accounted for 23-29% of real generation mortality. Pupal parasitism by T. inferens contributed 5.7-9.1% to the total generation mortality. Generation survival for T. incertulas is described by a type 3 survivorship curve with more than 70% of mortality occurring prior to the 2nd larval instar. The mean percentage survival of each life stage was: eggs, 44.5; lst instar larvae, 30.4; 2nd instar larvae, 27.0; 3rd instar larvae, 24.6; 4th instar larvae, 21.8; 5th instar larvae, 18.4; and pupae, 11.6. Disappearance of 2nd-5th instar larvae was the key factor in regulation of T. incertulas populations. Adult moth populations showed a female to male ratio of 4:1. Female moths showed a high fecundity with each female producing 4.20 (+OR-) .83 egg masses and each egg mass containing 50 (+OR-) 23.1 eggs. Parasitism by Telenomus sp. and T. schoenobii and disappearance of 2nd-5th instar larvae were found to be directly density dependent and pupal parasitism by T. inferens was inversely density dependent.