Discrimination among larval instars in Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) based on head-capsule width.

The relationship between larval instar and head-capsule width in Helicoverpa armigera (Hubner) was examined in laboratory-reared and field-collected insects from Ibaraki and Yamanashi in eastern Japan. Each of the first three and each of the last three instars could reliably be distinguished using head-capsule width for both strains fed on an artificial diet. In the laboratory, 91.8% of larvae from Ibaraki had five instars, and 8.2% had six or seven instars, while 36.1% of the larvae from Yamanashi had five instars, and the rest had six instars. Pupation occurred when the larval head-capsule width reached about 2.6 mm, and the development time for each instar was independent of the total number of instars. Consequently, head-capsules were typically larger at each instar for larvae with fewer instars. The distribution of head-capsule widths in larvae obtained from sunflowers in Ibaraki showed peaks corresponding to the first, penultimate, and ultimate instars of laboratory-reared larvae. However, the widths supposedly representing the second and third instars were smaller than those found in the laboratory, suggesting that H. armigera larvae molt more times in the field than in the laboratory and require a longer time to complete development on sunflowers. These results may enable improvement of H. armigera forecasting and insecticide bioassay tests.