The aim of this study is to determine the Spodoptera exigua larva population in the field and factor affecting their density. Population study of S. exigua larva and its affecting factors was carried out in Sekinchan, Selangor during 2003-2004. The larval density was found fluctuating during the study, where the highest number of larvae was an averaged of 18.17 per m2, while the lowest number was an averaged of 1.5 per m2. The mean number of larvae per plant also varies from 1.83 to 5.42. It was found that the larval density was influenced by the age and availability of the host plant. A total of 1881 larvae were collected, where 18.29 and 20.31% were successfully becoming female and male moths, respectively; 20.63% was being parasitized, where 7.07, 11.43, 0.11 and 2.02% were being parasitized by Microplitis manilae, Chelonus sp., Temelucha sp. and Peribaea orbata, respectively. Besides that, other biotic factors such as fungal or bacterial infection also cause death to the S. exigua larva, where a total of 1.91 and 10.89% were infected by them, respectively. Whilst 16.64% of the collected larvae were dead due to pesticide and 7.44% were not known cause of the death. Besides that, 3.89% of the S. exigua died during pupal stage or emergence. Further, climatic factor was found not influencing the larva populations. There were no correlation between the number of larva collected with the means of temperature, relative humidity and rainfall.
The incubation period of Spodoptera exigua (Hübner) was not influenced by the host plant, whereas larval development time and pupal period were affected. Larval development time was longest on shallot and lady's finger, followed by cabbage and long bean. Larvae did not develop beyond the first instar when fed on chilli. The pupal period was longer on lady's finger than on cabbage, shallot and long bean. Overall, adult longevity was not influenced by the host plant but there was a difference between female and male longevity among the host plants. Survival of S. exigua was affected by the host plant at the larval stage. The number of larval instars varied between 5 and 8 within and between the studied host plants. Long bean was found to be the most suitable host plant and provide the best food quality for S. exigua compared to the other host plants, as it allowed faster development, fewer larval instars and a higher survival rate.