Galerucella nymphaeae (Col., Chrysomelidae) grazing increases Nuphar leaf production and affects carbon and nitrogen...

The grazing effects of the waterlily beetle Galerucella nymphaeae on Nuphar lutea stands were studied in three ponds in Central Finland. Production of floating leaves of N. lutea and growth in the G. nymphaeae population were investigated in the ponds and bioenergetics of the beetle larvae in the laboratory. Combination of field and laboratory data enabled estimation of the effect of the beetle on the production of floating leaves of N. lutea and the consequences of grazing for the input of detritus from Nuphar into the ponds. Adults and larvae of G. nymphaeae consumed 3.0-6.1% of the net annual floating leaf production during the growing period. In addition to consumption losses, feeding accelerated the degradation rate of the leaves. This was associated with an increased flow of detrital material of Nuphar origin, and also with increased production of floating leaves in the ponds. These increments were estimated to be up to 3 times greater in the presence of grazing than without it. Grazing by G. nymphaeae releases substantial amounts of carbon and nitrogen bound in Nuphar, particularly in ponds with a dense Nuphar vegetation. It is hypothesized that feeding by this beetle may markedly affect the structure and functioning of such small aquatic systems.