Population dynamics of the common cutworm Spodoptera litura F. and its damage to onion, Allium cepa L. in Ilocos Norte [Philippines]

Conducted in Mabusag Sur and Mabusag Norte, Badoc, Ilocos Norte during the dry season 2003 and 2004, respectively, the study aimed to determine the population density of the cutworm Spodoptera litura at different growth stages of onion, the field and post harvest in infestation levels, other host plants, and the biotic factors affecting population changes. Cutworm egg masses appeared during the reproductive and maturity stages of the crop in Mabusag Sur but only at seedling stage in Mabusag Norte. The larvae appeared at all growth stages with peak at the late vegetative and maturity stages in Mabusag Sur, and at maturity stage in Mabusag Norte. Post harvest larval infestation was observed in the drying areas in both locations, pupae were observed in the 2nd and 3rd weeks. Cutworm larvae were observed in Mabusag Sur infesting the weeds Stachytarpheia jamaicensis, Trianthema portalacastrum, Arjemum mexicana, an unidentified species, and corn, Zea mays L. In Mabusag Norte, the host weeds were T. portalacastrum, Cardiospermum halicacabum, Synedrella nodiflora, Echinochloa crusgali, Cucumis melo, and Physallis angulata. The identified mortality factors were predation, parasitism, and disease infection. In Mabusag Sur, cutworm population density was inversely related to that of the natural enemies but the reverse was true in Mabusag Norte, except at the flowering stage of onion. Solenopsis geminata was voraciously preying on cutworm egg masses and larvae in Mabusag Sur and Mabusag Norte together with another ant species. Field-collected larvae in Mabusag Sur were parasitized by species of Tachinidae, Braconidae and Ichneumonidae. Those in the drying area were parasitized by braconid and infected with bacteria and fungus. Pupae were prayed upon by S. geminata and parasitized by a tachinid. These natural enemies were observed also in Mabusag Norte.