The influence of host plant odors on paddy field invasion of rice-ear bugs

Rice-ear bugs are the most serious pests of rice in Japan, causing economically devastating pecky rice. "Pecky rice" is the name given to grains that become stained by rice-ear bug feeding. The invasion of the adult bugs into paddy fields rapidly increases with rice flowering and decreases abruptly afterwards. For many years, it had been unclear why the bugs' invasion increases with rice flowering, and our laboratory succeeded in demonstrating that the rice plant odor plays an important role in rice-ear bug invasion. We conducted experiments with the two most common species of rice-ear bugs in Japan, the rice leaf bug Trigonotylus caelestialium and the sorghum plant bug Stenotus rubrovittatus. Here I describe the mechanism of the bugs' invasion into paddy fields, showing our experimental results. Both bug species were attracted to whole plants of rice in the panicle-formation stage and panicles in the flowering stage. However, they were not attracted to any rice structures before the panicle-formation stage or after the flowering stage except for panicles in the full-ripe stage. Although T. caelestialium were not attracted to panicles in the full-ripe stage, S. rubrovittatus were attracted to them. T. caelestialium were also attracted to gramineous weeds Poa annua, Digitaria ciliaris, Alopecurus aequalis, and Eleusine indica. The host odor preferences of T. caelestialium between rice plants and gramineous weeds changed according to the rice developmental stage. The bugs tended to prefer flowering rice panicles to gramineous weeds, whereas they preferred gramineous weeds to rice plants before the rice flowering stage. After the flowering stage, they showed no preference between rice plants and weeds. It is thought that invasions of rice-ear bugs into paddy fields are caused by the attractiveness of flowering rice panicles. In the case of S. rubrovittatus, it is known that the appearance of Scirpus juncoides, one of the major paddy weeds in Japan, advances the invasion period of the bugs and increases the bugs' invasion into paddy fields. S. rubrovittatus were attracted to the flowering spikelets of S. juncoides. It is thought that the attractiveness of flowering spikelets of S. juncoides encourages the bugs' invasion. Our results indicate that attractiveness of rice plants to the bugs differed with the rice growth stage and plant part, and that the attractiveness of odors from rice plants, paddy weeds, and gramineous weeds (pastures) around paddy fields may play a strong role in the bugs' invasion into paddy fields.