Larval movements of Chilo partellus (Lepidoptera: Pyralidae) within and between plants: timing, density responses and survival.
Four dispersal phases occur during larval development of Chilo partellus on young food plants: ballooning of newly-hatched larvae when moving from egg batch to plant whorl (phase 1); ballooning of 1st- and 2nd-instar larvae, which leave the plant whorl in the week after egg eclosion (phase 2); walking prior to stem penetration (phase 3); and walking after stem penetration (phase 4). In laboratory experiments, larval dispersal was density dependent during dispersal phases 1, 3 and 4, and there were clear differences between maize and sorghum in the percentage of migrating larvae and time of migration. Maize plants accommodated many more pupae than sorghum plants, which suggests that the distribution of older larvae in the field can be more clumped in maize than in sorghum. Pupal weight decreased with infestation level on sorghum but seemed less affected on maize. Resistance to starvation in 4 combinations of temperature and relative humidity increased with age. All hatchlings survived starvation for 6 h but with considerably reduced crawling capacity. Seven- and 15-day-old larvae were decreasingly affected. Older larvae survived and maintained their mobility after 48 h. Successful dispersal depends on food plant, egg load, and costs of migration; these costs change with larval age.