Seasonal changes in temperature and rainfall and its relationship with the incidence of weeds and insect pests in rice (Oryza sativaL) cultivation in Sri Lanka

Abstract

Weeds and insect pests are the key yield determinants of rice (Oryza sativa L.) that can be influenced by climatic parameters. The changes of ambient temperature and rainfall regime from 1982 to 2012 during two major growing seasons, Yala (March to August) and Maha (September to February) and their relationship with incidence of weeds and insect pests of rice were studied in three major rice growing districts of Sri Lanka, namely Anuradhapura, Polonnaruwa and Kurunegala. These districts were selected based on the availability of long term meteorological data and crop statistics. Occurrence of extreme climatic events during the study period was identified in relation to the base period (1961–1990) and their relationship with El Niño and La Niña events (ENSO) was examined. Costs of weed control and insect pest control published by the Department of Agriculture were used as proxies to the changes of weed and insect pest incidence, respectively. The difference in day and night ambient temperatures has significantly increased in Polonnaruwa (Yala: 0.066°C year−1, Maha: 0.0465°C year−1) and Kurunegala (Yala: 0.034°C year−1; Maha: 0.024°C year−1) districts. Cost for weed control has significantly decreased in Polonnaruwa district during Yala and Maha seasons at a rate of 0.81 and 0.42 US$ ha−1 season−1, respectively, in spite of increasing temperature regime. All extreme rainfalls events were recorded in ENSO years at Anuradhapura, Polonnaruwa and Kurunegala districts during the Yala season. The highest weed control costs have recorded during ENSO years in Anuradhapura and Polonnaruwa districts and that of insect pest control was recorded during ENSO years in Kurunegala district. There was no distinct linear correlation between climatic variables and, weed and insect pest incidences. However, an apparent relationship was evident between occurrence of extreme rainfall events and ENSO events. Moreover, it is likely that weed and insect pest incidences would be relatively higher during ENSO years.