RICE RATOONING - IRRI books - International Rice Research Institute

INSECT PESTS IN RATOON RICE 199seeded second crop could have attracted more ovipositing adults. It is also possiblethat the ratoon was less attractive to RWM because it has an abbreviated vegetativestage. RWM prefer young, actively tillering plants.Although RWM are attracted to flooded fields, we found no difference ininfestation levels between continuous and intermittently flooded fields. This doesnot agree with Karim’s findings (16). He reported that maintaining a flooded fieldattracted more ovipositing RWM.Transplanted rice plots provided less plant cover over the paddy water,resulting in fewer eggs and damaged leaves than the more rapid canopy developmentin wet-seeded rice. This again does not agree with findings elsewhere, indicating thatthe ovipositional preference of RWM for a less dense crop was superseded ormodified by other factors in this experiment.We posit two reasons why a wet-seeded crop, whether ratooned or seeded,appeared more attractive to RWM. First, the ratoon from a wet-seeded main cropdeveloped more basal tillers than the ratoon from a transplanted main crop. Tillersthat emerged from the base of the stubble were considered physiologically youngerthan the nodal tillers that appear immediately in the cut stubble after harvest. It islikely that ovipositing whorl maggots were more attracted to the basal than to thenodal tillers. Second, the wet-seeded second crop, established 2 wk earlier than thetransplanted rice second crop, could have been exposed to higher RWMpopulations.LF numbers which did not represent a carryover from the main crop were lowoverall (less than 1% damaged leaves), but significantly more occurred on the ratooncrop than on the seeded second crop (Table 1). It is unlikely that the reproductivestageratoon, which has larger leaves, was more attractive to ovipositing LF than thevegetative-stage seeded second crop. Neither water regime nor seeding methodaffected LF, again indicating that LF prefers older rice.SB infestation (deadhearts or whiteheads) was much less on the ratoon cropand seeded second crop than on the main crops (Fig. 1), but a significantly higherinfestation occurred on the ratoon than on the seeded second crop (Table 1 andFig. 1). Larvae were carried over to the ratoon and recolonization of ovipositingmoths was greater. Duport (7) found a ratoon crop attractive to ovipositing YSB.However, the initially high SB populations on the ratoon declined progressivelywith crop age, indicating a lack of new colonization. Harvesting the main cropfurther removed a substantial proportion of stem borer larvae and pupae, reducingtheir potential to increase in the ratoon.SSB Chilo suppressalis and dark-headed stem borers (DHSB) C. polychrysuswere equally prevalent on the main crop; populations of YSB Scirpophagaincertulas and PSB Sesamia inferens were low (Table 2). Young larvae of SSB andDHSB predominated on the young ratoon, suggesting that these insects wouldreadily oviposit on a ratoon crop. The seeded second crop had no older larvae orpupae, indicating that infestation occurred late.The transplanted rice plots had higher SB populations than the wet-seededrice plots, which may be a reflection of the lower tiller numbers in transplanted rice(Table 1).