Assessing the optimal release density of Lepidelphax pistiae for biological control of water lettuce ( Pistia stratiotes )

Invasions of the aquatic weed Pistia stratiotes L. (Araceae) challenge the conservation of urban wetlands, as mechanical and chemical control are inefficient under eutrophic conditions. Lepidelphax pistiae Remes Lenicov (Hemiptera: Delphacidae) is a native South American phloem-feeding herbivore that feeds exclusively on this weed and has potential as a biological control agent. This study aimed to determine the initial L. pistiae density required to reduce P. stratiotes vigor and invasiveness under controlled conditions. Laboratory experiments evaluated plant biomass, number of leaves, and daughter plants, using four initial insect densities (3, 6, 9, and 12 individuals per plant) and a control over 30 days. In addition, L. pistiae populations were maintained in outdoor pools for six months under eutrophic conditions. Under laboratory conditions, 3 and 6 insects per plant showed lower biomass, fewer leaves and daughter plants, than the control. Plant biomass increased by 152.5% (± 15.58 SE) in the control but by only 8.5% (± 14 SE) with six insects per plant. These findings suggest that intermediate L. pistiae densities may reduce plant biomass under eutrophic conditions. Outdoor observations showed a decline in plant cover and vigor potentially related to L. pistiae. These qualitative observations provide valuable information about L. pistiae–P. stratiotes interaction under more realistic conditions. Our results support future augmentative biological control strategies in subtropical urban wetlands and contribute toward understand the potential of L. pistiae as a biological control agent of water lettuce.