RICE RATOONING - IRRI books - International Rice Research Institute

INSECT PESTS IN RATOON RICE 203The increase of GLH on the wet-seeded plots could be attributed to densitydependence. In wet-seeded plots, there was more plant surface to exploit, offeringless crowded conditions than in transplanted plots. Continuous flooding alsosignificantly affected GLH numbers. More GLH were attracted to an aquaticenvironment. Ghose et al (9) noted more GLH in ill-drained habitats.Like the GLH, WBPH numbers peaked at the late vegetative stage, thendeclined rapidly, regardless of planting method (Fig. 2, 3). Even though low levels ofWBPH occurred on the second crop, significantly fewer occurred on the ratoon thanon the seeded crop (Table 1). Seeding method or water regime had no effect onWBPH numbers.The transplanted main crop developed higher BPH numbers than the wetseededmain crop, but populations markedly declined with plant age in both seedingmethods (Fig. 2, 3). BPH numbers in ratoon and seeded second crops wereextremely low, which may explain why no significant differences were found(Table 1).Natural enemy population dynamicsSpider numbers closely tracked GLH and planthopper numbers in the main crop.Hopper numbers rose slowly on vegetative rice, but second generations showed arapid increase. Spiders increased in number during the reproductive stage (Fig. 2, 3).Hunting spiders were twice as abundant on the main crop as were space-web spidersand five times more prevalent than orb-web spiders. The rank order of spiderabundance on the main crop carried over to the ratoon, where spiders were moreabundant than on the seeded second crop. The ratoon, being undisturbed afterharvest of the main crop, was a refuge for spiders. It is also highly probable that thespiders foraged from the ratooned plots to the seeded second crop plots in responseto the presence of more prey, explaining why hopper numbers in general were low onall second crops (30).The hunting spiders were dominated by Lycosa pseudoannulata, whichmaintained an even population level throughout the ratoon crop. Space-web spiderAtypena (= Callifrichia) formosana numbers declined slightly as the ratoonmatured. Because it makes its webs at the base of tillers, A. formosana and the highlymobile hunting spiders can readily colonize a new crop or remain unaffected duringharvest and growth of a ratoon.Orb-web Tetragnatha spp. and Dyschiriognatha sp. spider numbers werelowest on the ratoon. They were more devastated during harvest of the main cropthan the other groups because they need a tall crop from which to suspend theirwebs. These spiders normally become abundant during later growth stages. Theydeclined in numbers at harvest of the main crop, when hoppers had dispersed tocolonize younger crops.Spider numbers did not significantly differ in transplanted and wet-seededcrops. Significantly more spiders occurred on continuously flooded than intermittentlyflooded plots (Table 1), probably reflecting the higher GLH numbers onthe flooded plots.Microvelia ripple bug and Cyrtorhinus mirid bug declined with crop age onthe main crop but were capable of rapid colonization of newly planted fields (Fig.