Functional and Numerical Responses of Harmonia axyridis (Coleoptera: Coccinellidae) to Rhopalosiphum nymphaeae (Hemiptera: Aphididae) and Their Potential for Biological Control

The water lily aphid (<i>Rhopalosiphum nymphaeae</i>) is a highly polyphagous herbivore that causes severe damage to many terrestrial and aquatic plants, especially lotus. Due to environmental concerns about water pollution and other issues caused by chemical control methods, there is an urgent need to develop effective and sustainable control methods. The harlequin ladybird (<i>Harmonia axyridis</i>) is a well-known aphid predator and may pose a potential threat to <i>R. nymphaeae</i>. To study the predation ability of <i>H. axyridis</i> at different developmental stages on <i>R. nymphaeae</i>, we assessed the functional response, attack rate, and search effect of <i>H. axyridis</i> larvae and adults preying on <i>R. nymphaeae</i>. The numerical response of this process was also evaluated under a constant ladybird-to-aphid ratio and constant aphid density conditions, respectively. Our results showed that all predator stages exhibited type II functional responses. The predation rate of individual <i>H. axyridis</i> on <i>R. nymphaeae</i> nymphs significantly increased as prey density increased. In contrast, the search effect of <i>H. axyridis</i> gradually decreased with an increase in prey density. Meanwhile, <i>H. axyridis</i> at different developmental stages possess varying predation abilities; fourth instar and adult <i>H. axyridis</i> were found to be highly efficient predators of <i>R. nymphaeae</i>. <i>H. axyridis</i> adults exhibited the highest predation ability and predation rate, while both the adult and fourth-instar larvae exhibited the highest attack rate. Moreover, fourth-instar larvae exhibited the highest search effect value at initially lower prey densities, although adults surpassed them at higher prey densities. Our results also indicated that <i>H. axyridis</i> exhibited varying degrees of intraspecific interference and self-interference influence as predator density increases. These results strongly support <i>H. axyridis</i> as an effective biocontrol agent for <i>R. nymphaeae</i>.