Insecticidal properties of an alkaloid from Alstonia boonei de wild

Abstract

The insecticidal properties of the alkaloid obtained from Alstonia boonei De Wild stem bark was tested against the maize stem borer, Sesamia calamistis Hampson in a laboratory bioassay. Less than one day old (less than 24 h after hatching) S. calamistis larvae were reared on artificial diet treated with the alkaloid at a rate of 0.00125 %, 0.00100 %; 0.00050 %, 0.00025 %, and 0.00000 % (Control). There was no significant difference between the treatments in the survival of the larvae (range 66.67 % to 86.67 %). at 15 days after introduction (DAI). There were significant differences (P < 0.05) in the wet and dry weights of these larvae, which ranged from 25.50 mg in the diet containing 0.00125 % of the alkaloid to 36.30 mg in the 0.00050 % and 5.90 mg in the 0.00125 % to 8.80 mg in the 0.00025 %, respectively. The natural response rate C, (OPTC) for the larval survival at 15 DAI was 0.25. Percentage pupation was significantly higher in the Control (80 %) compared with the rest treatments (range 40 – 53.33 %). Pupal weight was significantly higher in the Control (213.67 mg) compared with the 0.00100 % treatment (137.83 mg). The natural response rate C, (OPTC) for pupation was 0.42 with an LD50 of 0.02662 %. While all the pupae from the Control successfully emerged into the adults, percentage adult emergence for pupae from other treatments ranged from 0.00 to 35.00 %. Various stages of unsuccessful adult emergence were observed in the treatments containing the alkaloid. Total developmental period ranged from 45 days in the 0.00025 % to 58 days in the 0.00125 %. These results show a very high level of insecticidal property of A. boonei stem bark alkaloid against S. calamistis. We believe that this alkaloid can play a major role in the management of this insect and similar insect pests of tropical crops.