Understanding insect performance on nontarget native plants is critical in the risk assessment of weed biological

control agents. A combination of laboratory and field host tests could help us better understand how abiotic and biotic environments interact to determine insect feeding and development on potential nontarget plants. In this study, we examined the performance of Agasicles hygrophila Selman and Vogt, a beetle used for biological control of the invasive plant alligator weed, Alternanthera philoxeroides (Mart.) Griseb., on a nontarget native plant, Alternanthera sessilis (L.) R.Br. ex DC. Our laboratory tests with various sized A. sessilis stems showed that larvae could pupate and successfully develop into adults on A. sessilis regardless of the stem size, but with larger stems improving the pupation rate. The insect was also found to pupate, and the pupae survive, when there was no stem available under laboratory conditions. When rearing pupae in a Petri dish, increased moisture improved pupal and adult survival, while most pupae were unable to develop to adults under dry conditions. In our field surveys, however, pupae were only found in stems, and no pupae appeared to survive without stems. Our predation tests further showed that pupae exposed to ambient environments (with no stems) had the lowest emergence rate, relative to pupae developing in A. sessilis stems. Together, these results indicate that A. sessilis stems facilitate pupal survival by providing moisture and protecting pupae from attack by natural enemies, explaining the difference in pupation rates between laboratory tests and field observations. Our study provides strong evidence that successful pupation of A. hygrophila on A. sessilis in nature is jointly determined by the environment and the host plant quality. Thus, considering abiotic and biotic factors that influence plant quality, insect survival and development could help to improve the assessment of the ecological host range of the insect biological control agent.