Insects Associated with <i>Stevia rebaudiana</i> Bertoni (Bertoni), and Toxicity of Compounds from <i>S. rebaudiana</i> against <i>Spodoptera frugiperda</i> (J.E. Smith) Larvae

Stevia rebaudiana (Bertoni) Bertoni (Asterales: Asteraceae: Eupatorieae), a plant native to Paraguay, produces glycosides used as zero-calorie sucrose alternatives. Despite being a relevant cash crop, little information is available on its suitability to Tennessee or on potential insect pests that may affect Stevia production in Tennessee. In addition, the market value for Stevia products could be enhanced by additional uses. One example is the potential use of Stevia glycosides as insecticides. To resolve these current knowledge gaps, the primary research goals of this project were to: 1) determine if differences in the number of insect visitations exist among three cultivars (‘Candy,’ ‘Sownatural,’ and ‘Stevie’) of S. rebaudiana in Tennessee throughout vegetative and reproductive stages; 2) identify the Orders and Families of insect visitors to Stevia, and assess differences among cultivars; and 3) assess insecticidal activity of Stevia compounds against larvae of the fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), as a relevant pest model considering its polyphagy and cases of resistance to currently available pesticides.

Significant differences in insect visitation were evident, with the highest number of visitors found on ‘Stevie’, which also had the highest average visitation per observation. The lowest average visitation per observation and the fewest total visitations were found on ‘Sownatural’. Among the visiting insects, six Orders and 30 Families were identified. Hymenoptera was the most observed Order, followed by Diptera and Hemiptera. No seriously damaging pests were observed.

Bioassays of FAW larvae indicated toxicity of Stevia glycosides, as well as an impact on weight gain and development. Stevioside had the lowest LC₅₀ [LC50], while whole leaf powders demonstrated the most growth reduction. Similar results were observed when performing bioassays with a colony of FAW with resistance against transgenic corn producing the Cry1Fa protein from Bacillus thuringiensis (Berliner) (Bt).

The outcome of this research is an insect profile of Stevia which may be used for the development of insect management plans for use in large-scale production. The use of Stevia glycosides as insecticides is also supported and should increase the value of this crop.