Morphology of immature stages of Aphelinus maculatus Yasnosh (Hymenoptera: Aphelinidae) related to host aphid characteristics and larval taxonomic significance

Abstract

The aphid parasitoid, Aphelinus maculatus Yasnosh (Hymenoptera: Aphelinidae), first described as a new record in China in 2016, is one of the most important natural enemies of the Chinese wolf-berry aphids. In order to relate its larval development stages to morphological changes of both the parasitoid and its host aphid during the parasitoid development, and to explore the larval taxonomic significance, a laboratory dissection study was conducted under the light microscope. There are three larval instars during immature development of A. maculatus. Instars differ in body shape, mandibles and numbers of respiratory spiracles. The first instar larva of A. maculatus is elongate in shape, the second instar larva is thick spindle-shaped to spindle-shaped, and the third instar larva is spherical-shaped with a small tail. Mandibles of the first and second instar larvae of A. maculatus are almost invisible, and their pharynxes are well developed. Mandibles of the third instar larva are well developed and are made of a pair of dark yellow sharp pointed spines each projecting from a transparent broad triangular base. The third instar larva possesses seven pairs of respiratory spiracles with one pair on the mesothorax and other six pairs successively on the second to seventh abdominal segments. As the larval development proceeds, a brown patch (the midgut of parasitoid larva) in the center of host aphid abdomen becomes larger in size and darker in color. The host aphid turns entirely black as a mummy and dies at the prepupal stage of the parasitoid, pine-nut shaped meconium is excreted at this time. The life cycle of A. maculatus from eggs to adults lasts 11.5–14 days at the temperature of 26 ± 1 °C. Morphological features of mandibles, respiratory spiracles and meconium are useful in the larval taxonomy of aphelinids.