The relationship between developmental stages of rice spikelets and the incidence of the rice bugs Leptocorisa chinensis, Lagynotomus elongatus, and Stenotus rubrovittatus in rice fields

Abstract

To clarify the relationship between the occurrence patterns of three species of rice bugs and the developmental stage of panicles in rice fields, we carried out two experiments. In the first experiment, we observed the development of spikelets in three paddy fields (Plots A, B, and C). The rice variety ‘Sainohana’ was used in Plot A and ‘Koshihikari’ was used in Plots B and C. Dates of transplanting were on April 25, May 8, and June 19; for Plots A, B, and C, respectively. Spikelets were classified into three stages based on ovarian development: Stage I (initial), Stage II (middle), and Stage III (fullness). After the initial heading stage, changes in the average number of each stage of spikelet development were expressed in days or cumulative degree-days above a base of 10°C. In the second experiment, the incidence of each rice bug species was monitored in rice fields. Then, the relationship between rice bug incidence and cumulative degree-days was determined. Variations in the average number of spikelet development stages among the three plots expressed in cumulative degree-days were smaller than those expressed in days after the initial heading stage. The peaks of incidence of adult rice bugs were near the peak of Stage I at 50–150 degree-days. The incidence of nymphs began to increase near the peak of Stage II at 200–250 degree-days. The occurrence patterns of the rice bugs were found to be closely related to the abundance of the three stages of spikelets. Cumulative degree-days can be used to uniformly express the changes in the incidence of rice bugs among different rice varieties, seasons, and fields.