RICE RATOONING - IRRI books - International Rice Research Institute

Insect pestsand their natural enemiesin ratoon riceC. G. dela Cruz and J.A. LitsingerDepartment of Entomology, International Rice ResearchInstitute, P.O. Box 933, Manila, PhilippinesABSTRACTA ratoon crop is potentially at risk from insect pests because it extends the time rice isavailable as a host. With its shortened vegetative stage, a ratoon crop is unsuitable toearly season insects — whorl maggot Hydrellia philippina and caseworm Nymphuladepunctalis. Stem borer numbers are severely reduced at main crop harvest, but somesurvive to attack the ratoon crop. Leaffolder infestation is higher at the vegetative stageon a ratoon crop than on a sown main crop. The lack of land preparation in establishinga ratoon crop allows a high carryover of natural rice pest enemies, particularly of thepredators which prevent significant buildups of brown planthopper, whitebackedplanthopper, and green leafhopper. Virus diseases on the main crop pose the mostserious insect-related problem in ratoon rice.As with any system that intensifies production, double rice cropping involving aratoon crop may create a higher risk of pest buildup. Intensified cropping thatextends the time rice is available allows more insect generations per year. Insectpopulations increase exponentially with time. In theory, ratoon rice should sustainhigher insect populations than a single or a double rice crop separated by a fallow orrotation with a nonrice crop (20).During a rice-free period, specializing pests must either subsist on suboptimalhosts, on which their rate of increase is low or negative; enter a quiescent or dormantstage, during which they are often subject to severe mortality; or undertake highlyrisky, long distance dispersal. The potential increase of a pest population over theannual cycle depends on the length of time rice is available.Loevinsohn et al (21) indicate that increasing the cropping intensity within acontiguous area greater than 1 km 2 (314 ha) can encourage insects that are ricespecialists. Brown planthopper (BPH) is an exception, it has a dispersal range of 2km. Using a composite index of spatio-temporal intensity/number of “hectaredays,” Loevinsohn et al showed exponential increases in populations of five riceinsect pests.The effect of a ratoon crop on pest populations could be positive, negative, orneutral. Insect groups that prefer the vegetative stage should decrease, because thevegetative stage is radically shorter in a ratoon crop. Insect groups that have noparticular preference for a crop growth stage or that prefer the reproductive orripening stages should stay the same, or increase.