Abstract
The biological activity of a Bacillus thuringiensis serovar kurstaki formulation (KM301WP) was investigated in a bioassay with the common cutworm, Spodoptera litura, using a diet incorporation method. Mortality occurred first at days 1-2, increased at days 3-4 and reached a plateau at days 4-5 after larval infection. The time-mortality curves were similar between larval instars except for degrees of susceptibility. Younger larvae were more sensitive. The first instar was most sensitive, followed by the second instar but there was no large variation between the third to fifth instars. Larval development and growth were greatly suppressed at sub-lethal doses. The EC50 based on the suppression in weight gain for 7 days was about 20% less than the LC50 of mortality on day 7. The susceptibility of neonates differed between bioassays and also between egg masses. Low temperature (15°C) affected responses of S. litura to the B. thuringiensis formulation, showing twice the sensitivity compared to the LC50 in the 20°-30°C range. The present study showed that neonates of S. litura can be used for bioassay of B. thuringiensis formulations using the diet incorporation method because they are the most susceptible larval stage, easy to handle in small vials, easy to rear without changing diet for 7 days, and reliable for estimating LC50 and EC50. It is important to mix neonates from different egg masses before use to reduce variation in susceptibility.
