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.

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“…Stenotus rubrovittatus is a serious pest of rice, causing pecky rice (Takeuchi et al . ), and migrates from Italian ryegrass and other poaceous plants to rice fields in early summer (Hayashi and Nakazawa ; Kashin et al . ; Nagasawa and Higuchi ).…”

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“…3). Takeuchi et al (2005a) reported that immigrant peaks of adult L. chinensis in rice fields were observed from 100 to 150 degree-days after the initial heading stage. These results suggested that the immigration of L. chinensis did not occur in the field where the grass plants did not bear seed head, even the dispersal of L. chinensis from hibernation sites had already begun.…”

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“…Although S. rubrovittatus shows an opportunistic tactic (i.e., polyphagy and multivoltine brood strategy), the bug exhibits a marked preference to L. multiflorum inflorescences from early to midsummer, when the plant species produces seed heads ( Yoshioka et al 2011 ). The mirid bug, however, only occurs in rice paddies during the short period when rice heads are present in midsummer ( Takeuchi et al 2005 ). They reproduce little in rice paddies ( Takeuchi et al 2005 ) but are assumed to spill over to rice fields from proximal meadows and fallow fields dominated by L. multiflorum .…”

“…The mirid bug, however, only occurs in rice paddies during the short period when rice heads are present in midsummer ( Takeuchi et al 2005 ). They reproduce little in rice paddies ( Takeuchi et al 2005 ) but are assumed to spill over to rice fields from proximal meadows and fallow fields dominated by L. multiflorum . Therefore, meadows can be regarded as source habitats for the bugs, and the paddies are sinks.…”