Comparative Toxicity of <i>Helicoverpa armigera</i> and <i>Helicoverpa zea</i> (Lepidoptera: Noctuidae) to Selected Insecticides

• Main Authors: Fernando R. da Silva, Dario Trujillo, Oderlei Bernardi, Jose Carlos Verle Rodrigues, Woodward D. Bailey, Todd M. Gilligan, Daniel Carrillo
• Format: Article
• Language: English
• Published: MDPI AG 2020-07-01
• Series: Insects
• Subjects: corn earworm; insecticide; invasive species; Old World bollworm; resistance
• Online Access: https://www.mdpi.com/2075-4450/11/7/431

Until recently, the Old World bollworm (OWB) <i>Helicoverpa armigera</i> (Hübner) and the corn earworm <i>Helicoverpa zea</i> (Boddie) (Lepidoptera: Noctuidae) were geographically isolated. Both species are major pests of agricultural commodities that are known to develop insecticide resistance, and they now coexist in areas where <i>H. armigera</i> invaded the Americas. This is the first study to compare the susceptibility of the two species to conventional insecticides. The susceptibility of third instar <i>H. armigera</i> and <i>H. zea</i> larvae to indoxacarb, methomyl, spinetoram, and spinosad was determined using a diet-overlay bioassay in a quarantine laboratory in Puerto Rico. Mortality was assessed at 48 h after exposure for up to eight concentrations per insecticide. Spinetoram exhibited the highest acute toxicity against <i>H. armigera</i>, with a median lethal concentration (LC₅₀) of 0.11 µg a.i./cm², followed by indoxacarb and spinosad (0.17 µg a.i./cm² for both) and methomyl (0.32 µg a.i./cm²). Spinetoram was also the most toxic to <i>H. zea</i> (LC₅₀ of 0.08 µg a.i./cm²), followed by spinosad (0.17 µg a.i./cm²) and methomyl (0.18 µg a.i./cm²). Indoxacarb was the least toxic to <i>H</i>. <i>zea</i>, with an LC₅₀ of 0.21 µg a.i./cm². These findings could serve as a comparative reference for monitoring the susceptibility of <i>H. armigera</i> and <i>H. zea</i> to indoxacarb, methomyl, spinetoram, and spinosad in Puerto Rico, and may facilitate the detection of field-selected resistance for these two species and their potential hybrids in areas recently invaded by <i>H. armigera</i>.