Life on a Fast Pad: Waterlily Leaf Beetle Impact on Water Lilies

Larval and adult waterlily leaf beetles, Pyrrhalta (Galerucella) nymphaeae (Coleoptera: Chrysomelidae), feed on the upper leaf surface of water lilies, Nuphar luteum (Nymphaeaceae), which grow along parts of the Ogeechee River. The purpose of this study was to estimate secondary production of the beetle and its impact on leaf and nutrient dynamics. We studied the population densities and instar distributions of P. nymphaeae by sampling leaves from the river and a nearby pond site. We examined characteristics of larval growth by isolating water lily leaves and infesting them with egg masses of the beetle. A survey of the river margins was used to estimate densities of water lily leaves, and turnover of leaves was examined by marking newly leaves. Larval growth was rapid (hatching to pupation in 9d); the growth rate (mean with 95% CI) calculated from field studies was 0.5415 ± 0.0278 mg°mg— ¹d°— ¹ (ash—free dry mass [AFDM] per unit larval ashfree dry biomass per day). There were continuous overlapping cohorts from late spring through mid—autumn. Mean larval standing stock ash—free dry biomass per unit Nuphar leaf surface area (LSA) was 283.6 mg/m². Larval production (AFDM per unit LSA) over the study period was estimated to be 27.14 g°m— ²°yr— ¹. Annual adult (egg) production was <0.20% larval production. The annual larval production/biomass ratio was 195. These production values were for very specific habitats that represented only °140 m² within a 2.3—km stretch of river. Therefore, Pyrrhalta production (AFDM per unit river surface area) was only 47 mg°m— ²°yr— ¹. The standing crop ash—free dry biomass per unit LSA of Nuphar leaves was 26.0 g/m². Pyrrhalta grazing removes only the upper 25% of the leaf; thus, only 6.7 g/m² is available to the beetles, as calculated from standing crop biomass of the host plant. However, larval production alone, without adjustment for egestion, respiration, or adult feeding, surpassed Nuphar standing crop biomass availability. This indicates that a rapid turnover of macrophyte biomass is necessary to support the observed levels of beetle production. Nuphar leaves lasted only 17 d at the Pyrrhalta—infested river site, compared to > 6 wk at a nearby floodplain pond site where Pyrrhalta were absent. Based on estimates of Pyrrhalta food consumption and Nuphar leaf longevity, the life cycle of the beetle is closely linked to the availability of individual leaves. Nuphar has a very high nutrient content (4% N, 0.5% P, and 1.85 K on a dry mass basis). Thus, by increasing plant turnover rates, Pyrrhalta feeding potentially increases the cycling of nutrients from sediments to the overlying water column. Beetle grazing also exposes the interior of Nuphar leaves to microbial attack. These results indicate that under certain conditions: (1) aquatic macrophytes may be very heavily grazed; (2) macrophyte production may enter the detrital food chain throughout the growing season rather than only after autumn senescence; and (3) heavy grazing by animals may result in very large underestimates of net primary productivity of macrophytes when the estimates are based on maximum standing crop biomass.