The insecticidal potential of Piper ribesioides (Piperales: Piperaceae) extracts and isolated allelochemicals and their impact on the detoxification enzymes of Spodoptera exigua (Lepidoptera: Noctuidae)

The dried twigs of Piper ribesioides were used to prepare various extracts in different solvents. Each extract was evaluated for their toxicity against 2nd instar larvae of Spodoptera exigua. The highest acute toxicity was observed for ethyl acetate extract with an LD50 of 3.25 and 2.50 μg/larva and LD90 of 9.19 and 8.15 μg/larva after 24 and 48 h post-treatment after topical application. Piperine, methyl piperate, N-cinnamoyl-(2-phenylethyl) amine, pinostrobin, pinocembrin, cinnamic acid, lemairamin, N,N-Diphenylcinnamide, and methyl cinnamate were isolated from this extract that was toxic to S. exigua larvae. Cinnamic acid was the most active compound with a LD50 of 0.17 μg/larva after 48 h of topical treatment. Piperine was significantly similar in activity (LD50 = 0.19 μg/larva), but toxicity decreased with the change in structural configuration as observed with methyl piperate (LD50 = 0.73 μg/larva). Other compounds were 3 to 11-fold less toxic than cinnamic acid and piperine. Growth of survived S. exigua larvae until adult emergence after topical application was recorded and maximum inhibition of about 70% was observed after the application of cinnamic acid. Egg hatch was only 29.11% after cinnamic acid treatment, compared to 97.38% in controls. Neither induction nor inhibition of detoxification enzymes due to treatment of extracts were statistically significant, however, individual compounds like cinnamic acid, methyl cinnamate, piperine, pinostrobin, pinocembrin, and lemairamin did show a moderate impact on these enzymes.