Is elytral color polymorphism in sweetpotato weevil (Coleoptera: Brentidae) a visible marker for sterile insect technique? Comparison of male mating behavior.

The sterile insect technique (SIT) is widely used for suppressing or eradicating target pest insect populations. In the eradication programs using the SIT, a large number of sterile insects are marked and released in the field. In Japan, Cylas fonnrmicarius (F.) group (Coleoptera: Brentidae) weevils are marked with a fluorescent powder dye to monitor the progress of such programs. However, this monitoring technique is not fully effective because of the disappearance or contamination of the dye. Therefore, an alternative marking method is required. Currently, a rare color morph such as piceous elytra (PE) is used for visible marking of C. formicarius group weevils. A PE-monomorphic strain has previously been established by artificial selection from a small locally distributed population; this can lead to reduced survival and genetic changes in behavioral traits due to inbreeding depression. In this study, we evaluated the survival rate and mating behavior of PE males of C. formicarius group. The characteristics of the PE males were similar to those of the wild strain (WS) males. Thus, we considered that PE males were suitable for visible marking in the eradication programs using the SIT.

Biological role of Nardonella endosymbiont in its weevil host.

Weevils constitute the most species-rich animal group with over 60,000 described species, many of which possess specialized symbiotic organs and harbor bacterial endosymbionts. Among the diverse microbial associates of weevils, Nardonella spp. represent the most ancient and widespread endosymbiont lineage, having co-speciated with the host weevils for over 125 million years. Thus far, however, no empirical work on the role of Nardonella for weevil biology has been reported. Here we investigated the biological role of the Nardonella endosymbiont for the West Indian sweet potato weevil, Euscepes postfasciatus. This insect is an experimentally tractable pest insect that can easily be reared on a natural diet of sweet potato root as well as on an agar-based artificial diet. By larval feeding on an antibiotic-containing artificial diet, Nardonella infection was effectively eliminated from the treated insects. The antibiotic-treated insects exhibited significantly lighter body weight and lower growth rate than the control insects. Then, the antibiotic-treated insects and the control insects were respectively allowed to mate and oviposit on fresh sweet potatoes without the antibiotic. The offspring of the antibiotic-treated insects, which were all Nardonella-negative, exhibited significantly lighter body weight, smaller body size, lower growth rate and paler body color in comparison with the offspring of the control insects, which were all Nardonella-positive. In conclusion, the Nardonella endosymbiont is involved in normal growth and development of the host weevil. The biological role of the endosymbiont probably underlies the long-lasting host-symbiont co-speciation in the evolutionary course of weevils.

The neogregarine protozoan Farinocystis sp. reduces longevity and fecundity in the West Indian sweet potato weevil, Euscepes postfasciatus (Fairmaire).

The number of West Indian sweet potato weevils, Euscepes postfasciatus, being mass-reared in a facility for use in sterile insect technique (SIT) eradication programs has undergone a drastic reduction. A neogregarine protozoan pathogen Farinocystis sp. (an undescribed species) was detected in vivo in the mass-reared E. postfasciatus. We investigated the effects of this disease on the longevity and fecundity of host weevils and the incubation time of the disease in the host body under mass-rearing conditions. Our results demonstrated that infection by this Farinocystis sp. decreased both longevity and fecundity in E. postfasciatus. In particular, the pathogen severely limited the production of progeny by infected females compared to healthy females. Therefore, we consider this protozoan infection to be the major cause of the decreased E. postfasciatus production in the mass-rearing facility.