The host-odour preferences of the rice leaf bug Trigonotylus caelestialium between the rice plant Oryza sativa L. and four species of graminaceous weeds, Poa annua, Alopecurus aequalis, Digitaria ciliaris and Eleusine indica, were investigated with an olfactometer aiming to clarify the influence of these odours on invasion of the bug to paddy fields at the flowering stage of rice. Both female and male adults significantly preferred the graminaceous weed A. aequalis in the flowering stage to rice in the fifth-leaf stage. The bugs also significantly preferred flowering P. annua and A. aequalis to rice in the panicle-formation stage. However, the bugs showed no preferences between rice in the flowering and grain-filling stages and the flowering graminaceous weeds P. annua, D. ciliaris and E. indica. The preference of the rice leaf bug for the flowering graminaceous weeds before rice flowering coincides with the fact that these bugs mainly live on these weeds before rice flowering. It is considered that the bug's similar preference for flowering rice panicles as the flowering graminaceous weeds causes the intense invasion of the bug into paddy fields at this rice developmental stage.
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