The influence of the rice bug Leptocorisa oratorius on rice yield.
Abstract
Leptocorisa oratorius, which feeds on the panicle of rice, is a major target for insecticide applications by farmers in Indonesia. The management of this insect has been insufficiently addressed because of unknown damage relationships in modern rice varieties. Hence, a study was conducted to assess the impact of L. oratorius on the yield of variety IR64. Sources of bias typically associated with studies on yield loss (spray effects, caging, plant compensation for damage) were avoided by studying the effect of differences in natural infestation levels of L. oratorius on rice yield at a large number of field sites. Several parameters, including the number of spikelets m-2 prior to panicle emergence (i.e. prior to attack by L. oratorius), were measured at each site to account for local conditions. The effect of these variables on yield was analysed in a statistical model. After removing non-significant variables from the model, the effect unambiguously attributable to L. oratorius density was determined. The median density of L. oratorius in farmers' fields (pooled from panicle emergence until the milky stage of rice) was 0.10 adults hill-1 (i.e. 2.6 adults m-2) in the dry season (75 sites) and 0.14 adults hill-1 (3.5 adults m-2) in the wet season (94 sites). The analysis showed that L. oratorius populations caused no measurable reduction in yield of rice. If a linear relationship in the range of 2-10% yield loss for every added adult per hill was assumed, under the measured field populations of L. oratorius in East Java the median loss would have been in the range of 0.2-1.0% in the dry season of 1997 and 0.2-1.3% in the wet season of 1997/98. Additional field cage experiments were conducted to compare the susceptibility of different stages of rice development to L. oratorius attack and to study yield loss components. In field cages with a bug/panicle ratio substantially higher than observed in the field, L. oratorius reduced yield when feeding at peak flowering of rice, indicating that rice is most susceptible to L. oratorius attack at flowering. The presence of L. oratorius at peak flowering and at the peak milky stage, moreover, caused empty seed. There was, however, no indication that L. oratorius caused partially filled seed. Further study is needed of the possible mechanism by which the plant, under moderate attack from L. oratorius, redirects assimilates from damaged spikelets to those that would have gone unfilled. In conclusion, L. oratorius did not cause an important reduction in rice yield in this study, and we conclude that the application of insecticides targeted at L. oratorius is mostly unwarranted. Farmers need practical training on the biology of L. oratorius and its feeding on rice to appreciate why chemical control is generally not required.