Abstract:

Objective: This study aims to find a suitable bioindicator of chemotaxonomic characters reflecting the developmental stage of the larvae of Yunnan shoot borer Tomicus yunnanensis (Coleoptera: Curculionoidea: Scolytidae) in order to further improve the comprehensive ability of scientifically integrated management on the insect pest and the forecasting accuracy. Method: The instar numbers were determined based on morphological indexes, including head capsule width, body length, maxilla length and width. Cuticular hydrocarbons were extracted by immersing larval individuals of different instar numbers in the pure n-hexane. The hydrocarbon extracts were analyzed by using gas chromatograph mass spectroscopy (GC-MS). The kinds and contents of compounds were determined by matching MS library and confirmed by comparison of Kovat's index (KI) and C8-C40 alkane mixture standards. The principal component analysis and random forest analysis were used to analyze the chemical composition and compare the changes of the composition of cuticular blends among age classes. The content among the larval instars was also compared by Kruskal-Wallis test. Result: The cuticular hydrocarbons of T. yunnanensis larvae were composed of nineteen compounds that contained n-alkanes, branched alkanes and alkenes. Their amounts are orderly dominated by N-pentadecane was the representative of n-alkanes, accounting for 55.02%±8.20%, the content of 13-methyl-docosecane accounted for 94.04%±14.02% of the total branched alkanes, the highest, and the main alkenes were 1, 21-docosadiene, accounting for 63.03%±0.39%. With the development of larval instars, both length and kind of carbon chains increased significantly. The content increased exponentially (y=e^{0.086 5x-0.629}, R=0.911). There were 8 kinds of hydrocarbon components in the 1^st instar larvae, and the content was (2.32±0.59) ng·ind^-1. There were 8 hydrocarbon components in the 2^nd instar larvae, and the content was (44.40±11.46) ng·ind^-1. There were 17 hydrocarbon components in the 3^rd instar larvae, and the content was (280.75±72.48) ng·ind^-1. The combined contribution rate of the first principal component and the second principal component was 81.7%, which could distinguish the larva of different instar numbers of T. Yunnanensis. There was non-overlap between the instar classes, indicating that the classification standard was obviously better than the morphological index method. The 2, 6, 11-trimethyl-dodecane, n-pentadecane, n-nonadecane, n-tricosane and n-pentadecane were relatively important in the given variables, and their contents were significantly different among larval instars (P < 0.000 1). Among them, 2, 6, 11-trimethyl-dodecane could be regarded as an existential candidate for the 1^st instar larvae, n-tricosane and n-pentacosane could be used as a predicated criterion for the 2^nd and 3^rd instar larvae, respectively. Conclusion: The kinds and contents of the cuticular hydrocarbons in different larval instars of T. yunnanensis are significantly different. The 2, 6, 11-trimethyl-dodecane can be used for identification of the 1^st instar larvae, n-tricosane and n-pentacosane can be used to identify the 2^nd and 3^rd instar larvae, respectively. Our results indicate that the cuticular hydrocarbon of T. yunnanensis larvae is an optimal index for the taxonomic identification of instar numbers.

Key words: Tomicus yunnanensis, larvae, cuticular hydrocarbon, minor compound, instar number, Dyar's rule