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Simple Summary: The aphid species Rhopalosiphum nymphaeae is a highly polyphagous pest that poses a significant threat to global agriculture. Biological control using natural enemies, such as the harlequin ladybird (Harmonia axyridis), is important for managing this pest. Our study evaluated the predation ability of H. axyridis in controlling R. nymphaeae. We examined the functional and numerical response of larva and adult H. axyridis preying on R. nymphaeae. Our results showed that the predation rate of individual H. axyridis on R. nymphaeae nymphs significantly increased as prey density increased. It was noted that fourth instar and adult H. axyridis were identified as optimal developmental stages in controlling R. nymphaeae. In addition, variations in intraspecific interference and self-interference exhibited by H. axyridis under increasing predator density were observed. The water lily aphid (Rhopalosiphum nymphaeae) 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 (Harmonia axyridis) is a well-known aphid predator and may pose a potential threat to R. nymphaeae. To study the predation ability of H. axyridis at different developmental stages on R. nymphaeae, we assessed the functional response, attack rate, and search effect of H. axyridis larvae and adults preying on R. nymphaeae. 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 H. axyridis on R. nymphaeae nymphs significantly increased as prey density increased. In contrast, the search effect of H. axyridis gradually decreased with an increase in prey density. Meanwhile, H. axyridis at different developmental stages possess varying predation abilities; fourth instar and adult H. axyridis were found to be highly efficient predators of R. nymphaeae. H. axyridis 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 H. axyridis exhibited varying degrees of intraspecific interference and self-interference influence as predator density increases. These results strongly support H. axyridis as an effective biocontrol agent for R. nymphaeae. [ABSTRACT FROM AUTHOR]

Invasive plants may be attacked both above ground and below ground. Few studies have, however, investigated the simultaneous effects of above-ground and below-ground herbivory. In the present study, we report the effects of beetle herbivory and nematode infection on alligator weed, Alternanthera philoxeroides, an invasive plant in China. We found that the root-knot nematode Meloidogyne incognita widely occurred on the plant in south China. To examine its effect on the plant in conjunction with above-ground herbivory, we conducted a field common garden experiment with a local insect defoliator, Cassida piperata. We also included the native congener Alternanthera sessilis in our experiments for a comparison of the response of invasive and native species. We found no significant effects on plant biomass of the nematode infection in conjunction with the above-ground herbivory. Further chemical analysis, however, showed that the water-soluble carbohydrate content in roots of A. philoxeroides was significantly increased in plants attacked by both the nematode and the herbivore compared with the water-soluble carbohydrate content in plants attacked by only the nematode or herbivore alone. We found no such change in the native congener A. sessilis. Together these results may suggest that A. philoxeroides tolerates joint above-ground and below-ground damage by allocating more resources to below-ground material. [ABSTRACT FROM AUTHOR]