A mixture of putative sodium salts of camptothecin and bamboo tar is a novel botanical insecticide against rice planthoppers and stem borers

Abstract

The phytochemical camptothecin (CPT) from Camptotheca acuminate (Nyssaceae) and the byproduct bamboo tar (BT) from bamboo charcoal making are insoluble in water. Here, we prepared water-soluble CPT-Na and BT-Na by alkalization of CPT and BT and a mixture of both salts (CPT/BT-Na), and evaluated their insecticidal activities against brown planthopper Nilaparvata lugens and Asiatic rice striped borer Chilo suppressalis under laboratory and field conditions. The modeling analysis of time-concentration-mortality responses indicated that the lethal concentrations (LC50 and LC90) of CPT-Na and BT-Na sprayed together against the two rice pests were reduced, respectively, by 3.5–14.8-fold and 3.3–29.1-fold in comparison with the estimates of the two salts sprayed separately. The reduction magnitude varied with post-spray days. These highlight a strong synergism of the two salts in insecticidal activity. In two field trials, a spray of CPT/BT-Na (CPT-Na + BT-Na: 1.5–3.0 + 135–270 g ha−1) resulted in more efficacious and persistent rice pest controls than not only a doubled spray of either CPT-Na or BT-Na, but also a recommended spray of imidacloprid against N. lugens or fipronil against C. suppressalis. All together, the mixture is a promising botanical insecticide for effective management of different rice insect pests.