Study of Cotton Bollworm, Helicoverpa armigera Hübner (Lepidoptera: Noctuidae) using Dyar?s Rule

Abstract : Discrimination of different larval instars of insects is important in morphological, physiological and toxicological studies under laboratory conditions. The morphometric study of different parts of an insect?s body is needed to obtain an index to distinguish different larval instars. In this study, Dyar?s rule stating the ratio of size of each sclerotized body part in successive instars is in a constant range was studied on different larval instars of cotton bollworm, Helicoverpa armigera. The insects were reared at 26±2°C, 60±5% RH and a photoperiod of 16: 8 (L: D) h. for 10 generations. One hundred larvae of each instar were randomly selected and width of head capsule in widest region, as well as length and width of stigmata located on prothorax and first and last abdominal segments were measured by calibrated micrometer. A frequency distribution plot showed that due to overlap in stigmata dimension, size of the stigmata was not a suitable index for distinguishing different larval instars in this insect. The widths of head capsule for 1st to 6th larval instars were 0.257-0.314, 0.4-0.485, 0.6-0.743, 0.96-1.2, 1.5-1.8 and 2.55-2.8 mm, respectively. Dyar?s ratios for 1st to 6th larval instar intervals were obtained 1.52, 1.512, 1.631, 1.612 and 1.552, respectively. There was no overlap in the data range; therefore, head capsule width can be used with high confidence as an index to distinguish larval instars. Linear regression analysis revealed significant effect of larval instars on head capsule (R2 =0.999). Despite strong relationship between dimension of stigmata and instar numbers, some strong overlapping in stigmata dimensions in subsequent instars decreased the usefulness of stigmata dimensions for instar discrimination. Overlapping particularly in the first abdominal stigmata was more than others
Keyword : Larval instars, sclerotized, head capsule, stigmata
Author(s) : , D. Mohammadi, R. F. Pour Abad, M. R. Rashidi, S. A. Mohammadi
Downloads : 2400

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