LAUSR

Salvinia molesta D.S. Mitchell (Salviniaceae), a floating aquatic weed, is a menace in many water bodies, disrupting the ecological balance wherever it invades, with wide ranging economic and health impacts. Biological control using the weevil Cyrtobagous salviniae Calder & Sands has been an effective control method globally, although some infestations persist, in particular in shaded waterbodies. This study thus aimed to determine the effect of shade on the efficacy of biological control of S. molesta. Plants were grown in a greenhouse at high and low nutrients, in three varying shade levels, and half of the experiment subjected to herbivory by C. salviniae. Plant biomass was significantly greater in high‐nutrient conditions, in the absence and presence of herbivory at all levels of shade tested. Under shade and high‐nutrient conditions, plants were significantly higher in quality (i.e. lower carbon:nitrogen ratio) compared with those exposed to full sun. Plants in the shade sustained less damage, possibly due to the high insoluble nitrogen concentrations, which may have deterred herbivory by C. salviniae. However, when exposed to full sun, plants were of lower quality and consequently sustained more damage from the weevils despite lower populations. The results therefore suggest that nutrient supply is pivotal in plant growth compared with light (amount of PAR). The results from this study add to the growing body of literature that plant quality is a major factor in determining the success of aquatic weed biological control programmes.