Larval movements of Chilo partellus (Lepidoptera: Pyralidae) within and between plants: timing, density responses and survival

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Abstract
Four dispersal phases occur during larval development of Chilo partellus (Swinehoe) on young host plants, phase 1, ballooning of newly hatched larvae when moving from egg batch to plant whorl, phase 2, ballooning of first and second instars which leave the plant whorl in the week after egg eclosion, phase 3, walking prior to stem penetration, and phase 4, walking after stem penetration. 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 lavae 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 descreased with infestation level on sorghum but seemed less affected on maize. Resistance to starvation in four combinations of temperature and relative humidity increased with age. All hatchlings survived six hours′ starvation but with considerably reduced crawling capacity. Seven- and 15-day old larvae were descreasingly affected. Older larvae survived and maintained their mobility after 48 hours. Successful dispersal will depend on host-plant, egg load, and costs of migration; these costs will change with larval age.

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