Assessments of crop losses in rice ecosystems due to stem borer damage (Lepidoptera: Pyralidae)

A database created from insecticide control experiments conducted under the All-India Coordinated Rice Improvement Project from 1965 to 1992 was used to derive empirical estimates of yield losses caused by stem borers. Each unit percent damage due to white earhead damage had a much greater impact on rice yield in the irrigated ecosystem than did damage due to dead heart. White earhead damage occurs later in the season and results in direct loss of a yielding panicle, and thus, no compensation (or very little) is possible. The grain yield loss from the two phases, dead heart and white earhead damage to rice, is more than additive. Based on 770 experimental units from 28 years data, our projections for damage over rice ecosystems due to 1% dead heart or white earhead, or to both phases of stem borer damage are 2.5%, 4.0%, and 6.4% yield loss, respectively. In terms of grain production loss over ecosystems, 1% dead heart, or white earhead, or both phases of stem borer damage would be 108, 174 and 278 kg/ha, respectively. In irrigated ecosystem, 1% dead heart resulted in 0.3% or 12 kg/ha loss whereas, 1% white earhead caused 4.2% or 183 kg/ha loss in grain yields; the loss due to 1% infestation in both phases of stem borer damage was 4.6% or 201 kg/ha. In rainfed lowlands, for 1% dead heart or dead heart and white earhead caused 2.3% or 76 kg/ha yield loss. Even at levels below the currently used economic threshold considerable losses can occur. This perception on losses assumes more importance because of the inadequate host-plant resistance to stem borer in rice. Although no insecticide gave total control of stem borer damage, many increased grain yields significantly. Emulsifiable concentrates of monocrotophos and chlorpyriphos appeared more economical for adoption by farmers as their application caused maximum mortality of larvae and unhatched eggs. Insecticide granules such as diazinon and carbofuran were equally efficient in preventing stem borer damage.