Abstract:
Since Sri Lanka is a developing and agriculture based country, it is crucial to minimize the post-harvest losses. The insect infestation of stored grains and their products is a serious problem. Nowadays controlling insects is mainly practiced by using synthetic insecticides and fumigants. But their widespread use has led to some serious problems. This study was carried out to screen the contact and fumigant toxicity of underutilized and highly abundant herbaceous plants on storage pest Tibolium castaneum. For this study methanol crude extract of dried leaves and fresh flowers of Bidens sulphurea, Helliunthus annuus, Tagetes erecta and Tithonia diversifolia were obtained and experiments were conducted with Randomized Complete Block Design and two factor factorial designs. Contact toxicity and fumigant toxicity were tested for T castaneum with five different doses and three different doses respectively. Highest mortality values were found even at the low doses in T. erecta dry leaves, H. annuus dry leaves and T. erecta flowers for contact toxicity. T erecta dry leaves and T diversifolia fresh flowers show high fumigant toxicity. T. erecta dry leaves show\qne highest both toxicity effects on T castaneum. Any mortality was not observed in control, even without food and air supplement. According to the ANOVA, there was no significant interaction effect between dose and crude type to the mean mortality % (p > 0.05). The best selected plants were re¬examined under randomly selected dose series to calculate the median lethal doses (LD50 value) using probit analysis. LD 50 values of T erecta dry leaves, H. annuus dry leaves and B. sulphurea fresh flowers are 13.129 pg/cm2, 38.925 1.tg/cm2 are 51.716 p,g/cm2 respectively for contact toxicity. LD50 values of T erecta dry leaves and T. diversifolia fresh flower were 109.044 i.tg/cm2 and 196.368 gg/cm2 respectively for fumigant toxicity. Hence those two types of extracts can be used in developing a fumigant to eradicate the infestation of Tribolium castaneum.
