Investment Trade-Off between Mating Behavior and Tonic Immobility in the Sweetpotato Weevil Cylas formicarius (Coleoptera: Brentidae)

Simple Summary Tonic immobility (TI) is an essential anti-predator behavior to reduce the effect of predators. To know the relationship between mating behavior and TI, the effect of TI on courtship and copulation, as well as the effect of courtship and copulation on TI, were investigated in the sweetpotato weevil (SPW), Cylas formicarius. In the present study, we found the duration of TI was significantly reduced in the stage of courtship and copulation. The pairs with males from the L-strain (SPW with longer period of TI) showed lower frequency and longer duration of courtship than pairs with males from the S-strain (SPW with shorter duration of TI). Similarly, males from L-strain pairs showed a longer period of copulation than pairs with males from the S-strain. However, there is no significant difference in the frequency of copulation and the success of insemination. The results of our study clearly demonstrate that SPW mating behavior and TI are negatively correlated. Numerous studies have confirmed that the trade-off between anti-predator behavior and mating behavior occurs in certain insect species. This suggests that insects invest more in anti-predator behavior, and fewer resources or time can be used in mating behavior. However, few studies focus on tonic immobility, an important anti-predator behavior in nature, and different stages in mating behavior. Tonic immobility (TI) is considered to be an important anti-predator behavior. Herein, we investigated the relationship between TI and mating behavior in the sweetpotato weevil (SPW), Cylas formicarius. As the first step, we artificially selected SPWs for the longer duration of TI (L-strain) and the shorter duration of TI (S-strain). The effect of courtship and copulation on the duration of TI in two artificial selection strains was tested. Furthermore, we compared the frequency and duration of two mating behaviors in four kinds of pairs (LFxLM, LFxSM, SFxLM, and SFxSM: LM-L-strain male; SM-S-strain male; LF-L-strain female; SF-S-strain female). Finally, we tested insemination success in four kinds of pairs (male and female SPWs from the L-strain or the S-strain). The courtship and copulation significantly reduced the duration of TI. Pairs with males from the L-strain showed lower frequency and longer duration of courtship than pairs with males from the S-strain. Similarly, males from L-strain pairs showed a longer period of copulation than pairs with males from the S-strain. However, there is no significant difference in the frequency of copulation and the success of insemination. These results support that there was a significant trade-off between TI and courtship as well as copulation in the SPW.