The azuki bean beetle (Callosobruchus chinensis) is a pest that primarily targets legumes in the genus Vigna, though some populations have also adapted to infest soybeans (Glycine max). Damage to soybeans caused by these beetles is becoming an increasing concern in Africa, whereas infestations in Japan remain sporadic with limited impact.

  To investigate the genetic basis of soybean infestation ability, we conducted crossing experiments with three lines of C. chinensis: one capable of infesting soybeans (soybean-infesting line) and two unable to infest soybeans (non-infesting lines). Using a Bayesian approach, we estimated the effects of Wolbachia infection, parental and offspring bean diets, and parental line on emergence rates. Our results showed that Wolbachia infection negatively affected emergence rates. F₁ hybrids from crossings between soybean-infesting and non-infesting lines exhibited reduced emergence rates on soybeans when infected with Wolbachia compared to uninfected hybrids. We also observed that consistency in bean diets between parents and offspring positively influenced emergence rates, suggesting that parental diet may affect offspring adaptability to soybeans.

  The reduction in emergence rates suggests that Wolbachia infection lowers hybrid fitness on soybeans, potentially limiting the proliferation of soybean-infesting individuals. Interestingly, the soybean-infesting line itself is infected with Wolbachia, raising questions about how it establishes and persists despite the failure of hybrids. This may involve complex interactions between Wolbachia and the host's ability to digest soybeans.

  Further experiments involving infected females and uninfected males (with reciprocal crossings rendered completely incompatible by Wolbachia) indicated that female infection likely reduces hybrid emergence rates on soybeans. While hybrid emergence is not entirely eliminated, the observed reduction could act as a weak barrier to gene flow. This barrier may restrict genetic exchange between soybean-infesting and non-infesting lines, potentially facilitating incipient speciation. As the acquisition of soybean-infesting ability by non-infesting lines poses a threat to food security, future studies should investigate the molecular mechanisms through which Wolbachia influences host traits.