Dispersal by Larvae of the Stem BorersScirpophaga incertulas(Lepidoptera: Pyralidae) andChilo suppressalis(Lepidoptera: Crambidae) in Plots of Transplanted Rice

We studied larval dispersal behavior of two rice stem borers, Scirpophaga incertulas (Walker) and Chilo suppressalis (Walker), to evaluate the potential of seed mixtures for resistance management in B. thuringiensis (Bt) rice. Both species showed extensive movement among plants (or “hills”) in plots of transplanted rice, during the course of larval development. On rice plants at the vegetative stage, almost all S. incertulas larvae dispersed on the day of eclosion. On plants at booting stage, most S. incertulas bored into hills on which egg masses were placed (referred to as the “release hill”). Almost all neonate C. suppressalis also bored into the release hill, at both vegetative and booting stages. At both rice growth stages, most larvae of both species dispersed to new hills between 7 and 18 d after eclosion. Both S. incertulas and C. suppressalis moved among tillers within the release hill, as indicated by an increase in dispersion among tillers over time. The distance and direction of dispersal of ballooning S. incertulas larvae was influenced by wind speed and direction. Larval recovery within plots generally declined rapidly over the first 5 d after egg hatch and then more slowly thereafter. Because many S. incertulas and C. suppressalis larvae move among tillers within hills and among hills within plots, many larvae in plots planted to seed mixtures will consume tissue from both Bt and non-Bt plants. This behavior will reduce the cumulative dose of toxin ingested and can accelerate the evolution of resistance.