Trans-stadial effects and larvicidal potential of jatropha (Jatropha curcas) and castor (Ricinus communis) oils on Spodoptera frugiperda in laboratory

Résumé

Over the past decade, outbreaks of the fall armyworm (Spodoptera frugiperda) have had devastating impacts on agriculture in Africa.

This study aimed to evaluate the properties of jatropha and castor oils on S. frugiperda. The oils were specifically applied to eggs and tested for egg hatching, subsequent larval mortality, and pupae emergence. Thus, two bioassays were conducted in the laboratory. On one hand, the effect of insecticidal oil application on fall armyworm eggs was measured. On the other hand, direct application effect of the oils on fall armyworm’s second instar larvae was determined. Four treatments (T1-4) were used for each assay in a complete randomized block design: T1: Water; T2: Water + Omo (laundry detergent); T3: Jatropha oil (1.25% v/v), and T4: Castor oil (1.25% v/v). For the trans-stage effect bioassay, batches of 10 eggs were sprayed with 1 mL of the treatment using a micropipette. The experimental eggs monitored to the pupal stage, to calculate egg hatching, larval mortality, and pupation rate. On the direct insecticidal oil effect bioassay, batches of 10 second instar individuals were sprayed with 2.5 mL of each treatment. The larval mortality was recorded. The results showed that castor and jatropha oils had a trans-stage effect, resulting in significant larval mortality (df= 3, p< 0.01) and reduced pupation rate (df= 3, p< 0.01). Unexpectedly, larval mortality in the trans-stage assay was higher than that obtained by direct application of biopesticides on the larvae. This study suggests that an application of the jatropha or castor oils on the fall armyworm’s egg stage is associated with  high larvicidal effect.The oils were applied both on larvae and eggs, so the general term “insecticidal”.