Scirpus juncoides Roxb. var. ohwianus is a cyperaceous weed species found in rice fields and has been reported as an alternative substrate plant for the egg deposition of the sorghum plant bug, Stenotus rubrovittatus. The seasonal occurrence of S. rubrovittatus and its damage to rice grains were investigated in S. juncoides-infested and non-S. juncoides-infested rice fields in 2005 and 2006. In S. juncoides-infested rice fields, the immigration of S. rubrovittatus adults started prior to the heading period of rice plants, and nymphs and adults of the next generation were also observed in the field. In non-S. juncoides-infested rice fields, S. rubrovittatus adults immigrated into the fields after the rice plant heading period and very few nymphs were observed throughout the survey period. The percentages of rice grains damaged by S. rubrovittatus in S. juncoides-infested rice fields were higher than those damaged by S. rubrovittatus in non-S. juncoides-infested fields. These results suggest that weed management in rice fields is essential for reducing the extent of plant bug damage to rice grains as well as for managing weeds in the original habitat of plant bugs.

Two mirid bugs, Stenotus rubrovittatus (Matsumura) and Trigonotylus caelestialium (Kirkaldy) (Heteroptera: Miridae), are sympatric pests that infest rice in many regions of Japan. Three components, hexyl butyrate, (E)-4-oxohex-2-enal and (E)-hex-2-en-1-yl butyrate, in S. rubrovittatus and three other components, hexyl hexanoate, (E)-hex-2-en-1-yl hexanoate and octyl butyrate, in T. caelestialium, were identified as sex attractant pheromone components in these species. The synthetic pheromone lures for S. rubrovittatus do not attract T. caelestialium and vice versa. The attractiveness of the combination of two separate lures for the two mirid bugs within one trap (combination lures) and the attractiveness of a mixed lure, in which all six synthetic-pheromone components of both species were loaded onto a single septum, were compared with that of separate lures for each species in field experiments. Both the combination lures and the mixed lure were as attractive to males of both mirid bugs as the separate lures for each species in fields with gramineous plants. In paddy fields, the mixed lure was also as attractive to males of both species of mirid bugs as the separate lures for each species. These results suggest that the mixed lure and the combination lures could be useful as simultaneous monitoring tools for two sympatric major rice pests using a single trap.

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Scirpus juncoides is a noxious weed species in rice fields that intensifies damage to rice grains caused by the sorghum plant bug Stenotus rubrovittatus, by serving as the oviposition site. In this study, we examined whether herbicide application for the elimination of S. juncoides has a controlling effect on the bug. In the non-bentazon-applied fields, the bug s invasion into the rice fields prior to the heading period of rice and the nymphal emergence were observed. When bentazon was applied 41 days or 62 days after transplanting in S. juncoides-infested rice fields, the bug s invasion into the fields occurred only after the heading period of rice, and the nymphal emergence of the bugs was not observed. When bentazon was applied 77 days after transplanting, the bug s occurrence resembled that in the non-bentazon-applied fields. These results indicated that the bug s propagation could not be avoided, when the herbicide application timing is later than the occurrence of the first-generation adults. The effects of the timing on damage to rice grains were unclear. Therefore, judging from differences in the bug s occurrence between the rice fields studied, it seems reasonable to conclude that the elimination of S. juncoides with herbicides before the occurrence of the first-generation adults is effective for the control of the bug.