Sex allocation in animals is predicted to be skewed from a 1:1 ratio if sons and daughters yield different marginal fitness returns per unit maternal investment. We tested this prediction using the gall‐forming aphid Tetraneura sorini, in which lethal fighting is common among females, whereas male–male competition is moderate. Mothers (autumnal winged females) parthenogenetically produce females and males in their abdomen and can control their sizes and numbers. The females and males do not feed after birth. After mating, females produce single eggs, from which only females (foundresses) hatch and fight intensely with one another during the galling process. Larger‐sized foundresses have overwhelming advantages in fighting. If mothers invest more in individual females, they can produce larger foundresses (granddaughters), which yields greater fitness returns. This situation is contrary to the original Trivers–Willard hypothesis. Thus, we predicted that more fecund mothers in this species would invest more in females. The cost of producing one female was found to be 3.0–3.2 times the cost of producing one male. Population sex ratios were male‐biased (54.8%–58.2% male), but population sex allocation was highly female biased (68%–72% females). Larger mothers biased their progeny sex ratios more towards females and produced larger females, thus supporting our prediction. Mothers produced two types of brood: all‐female broods and mixed‐sex broods. Mothers with an all‐female brood produced larger females than those with a mixed‐sex brood, thereby offering their granddaughters (foundresses) an advantage in fighting. Local mate competition has been postulated to explain female‐biased population sex allocation in gall‐forming aphids. However, we concluded that competition among foundresses has led to the evolution of female‐biased sex allocation at the population level and dimorphism in brood sex composition.

In most terrestrial arthropods, cuticular hydrocarbons (CHCs) function to assist in desiccation tolerance and chemical communications. However, few studies have clarified whether CHC profiles change among developmental stages or among different morphs in non-social insects. In the present study, we evaluated how CHC profiles change in accordance with polyphenism in the host-alternating aphid Prociphilus oriens, which exhibits a complex life cycle and five distinct morphs. These morphs are sexual or asexual and adapt to different host plants. We found that all generations of P. oriens shared high proportions of n-alkanes, but its composition varied among morphs. Three morphs that are attended by ants were characterized by relatively high proportions of n-C25 to n-C27, whereas two morphs that are not attended by ants had higher proportions of longer-chain n-alkanes, such as n-C27 and n-C29. The CHC profiles of sexual females were largely different from those of males. Considering that sexual females of Prociphilus spp. lack organs that secrete sex pheromones (scent plaques), the CHCs of sexual females are likely to function as a sex attractant. High proportions of methyl-branched alkanes were detected in the long and flocculent waxy substances of autumnal migrants. These methyl-branched alkanes are considered a cue to recognize conspecifics. We concluded that the functions and components of CHCs differ among morphs, and that those of sexual females differ from those of males and asexual generations because of their function in sexual communication.

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Juvenile mermithid nematodes were found to parasitize winged females (sexuparae) of Eriosoma auratum and Tetraneura radicicola. The morphological characteristics of mermithid nematodes are briefly described. The 18S rDNA and 28S rDNA obtained from one nematode were sequenced and used to construct a Bayesian phylogenetic tree, on which the host ranges of mermithid nematodes were represented. Our study indicated that mermithid parasitism of sexuparae led to fewer and smaller sexual female embryos. This is the first record of a mermithid in relation to eriosomatine aphids and the fourth record with respect to Aphididae.