American Society of Naturalists

Naota Ohsaki, Masaaki Ohata, Yoshibumi Sato, and Mark D. Rausher (Oct 2020)

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Fascinating female butterflies disadvantaged in interspecific competition

A  fundamental principle of ecology states that closely related organisms that use the same resources cannot coexist, and better competitors exclude lesser competitors. However, it has been claimed that competitive exclusion in phytophagous insects is uncommon. In Hokkaido, Japan, two Pierid butterflies use different host plants. Pieris napi uses a recently introduced host, whereas P. melete uses an ancestral host. When the novel host species was introduced around 1960, the two Pierid species exhibited little preference between the two species. By the 1970s they were shown to specialize on different hosts. Around the year 2000, Ohsaki and Sato showed that the ancestral host species was less suitable than the novel host for larval growth and survival for both butterfly species, which raised the question of why P. melete had shifted to prefer the inferior host. In 2018, Ohsaki and Ohata demonstrated that courtship by P. napi males concentrated on P. melete females, whereas P. melete males ignored P. napi females in outdoor cages of Kyoto University. P. melete is a larger species than P. napi. Because larger females lay more eggs, this size difference presumably makes P. melete females more attractive to P. napi males. This study demonstrates strong, asymmetric reproductive interference between the two butterfly species, with courtship by P. napi males substantially reducing the number of eggs laid by P. melete, but not the reverse. This sexual interference appears to be responsible for sexual exclusion of P. melete from using the introduced host. Evidence suggest this shift in host use is an evolutionary change, but other explanations, such as direct effects of interactions on female oviposition behavior cannot be ruled out definitively.

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Abstract

A number of empirical studies have concluded that reproductive interference, RI, contributes to parapatric species distributions or sexual exclusion. However, the possibility that divergent host-plant use in phytophagous insects is due to sexual exclusion has seldom been considered. Here we present evidence that RI is responsible for different host-plant use by two Pierid butterfly species, Pieris napi and P. melete. When a novel host species was introduced about 50 years ago, two Pierid butterfly species at first used both the ancestral host species and the novel one. Subsequently, P. napi shifted to use only the novel host, while P. melete shifted to specialize on the ancestral host. To explain these patterns, we investigated whether the two host species differ in suitability for larval growth and survival. Additionally, we tested whether RI occurred between the two species using large outdoor field cages. Courtship of females by conspecific and heterospecific males reduces the number of eggs laid by approximately half. However, RI is asymmetric and would generate selection on P. melete females to evolve to avoid the more suitable host species preferred by P. napi. Thus, our study suggests that sexual exclusion can explain the shift in host use by these two butterfly species.