BASELINE SUSCEPTIBILITY OF EGGPLANT FRUIT AND SHOOT BORER, Leucinodes orbonalis Guenée IN THE PHILIPPINES TO A Bacillus thuringiensis CRY1Ac INSECTICIDAL PROTEIN
The baseline susceptibility of eggplant fruit and shoot borer (Leucinodes orbonalis) populations in the Philippines to a Bacillus thuringiensis Cry1Ac protein was evaluated using a diet-overlay bioassay. Infested fruits with larvae were collected from nine eggplant growing provinces in the Philippines during the 2012-2013 crop growing seasons. L. orbonalis larvae were reared to adults on semi-synthetic diet, allowed to mate in the laboratory and produce eggs. The neonate larvae, from eggs of the field collected parents, were exposed to eight concentrations of Cry1Ac along with an untreated control and then assessed for mortality. The results showed that all L. orbonalis populations tested were susceptible to the Cry1Ac protein obtained from a bio insecticide that was used in the assay. Probit analyses of 35 bioassays of EFSB larvae collected from nine provinces showed that the median lethal concentrations (LC50) ranged from 0.45 to 2.07 ng/cm2 while LC95 values ranged from 65.10 to 1501.00 ng/cm2. Interpopulation variation in susceptibility was 4.6-fold and 23-fold at the LC50 and LC95 values, respectively. Linear Mixed Model and pairwise mean comparison analysis allowed pooling of the 35 bioassays data into four (4) groups. Effects of season and province for lethal concentrations (LCs) and the slope for each group were insignificant. The median lethal concentrations (LC50) ranged from 0.88 to 1.54 ng/cm2 and the LC95 values ranged from 119.23 – 470.73 ng/cm2 which represented a <2-fold and 4-fold variation in susceptibility, respectively. The results obtained in this study will be continued to generate more reliable estimates of the LC99 values which can be considered for development of diagnostic concentrations in resistance monitoring.
Key words: baseline susceptibility, Bt Cry1Ac, eggplant shoot and fruit borer, Leucinodes orbonalis
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