Native snails choose an invasive macrophyte over a native macrophyte as a food resource

Invasive species cause ecological and economic impacts on invaded ecosystems, although the presence of native species hampers the propagation of invasive species due to biotic resistance. We tested the effects of grazing by the native channeled applesnail (Pomacea canaliculata (Lamarck, 1828)) over the invasive macrophyte Hydrilla verticillata (L. f.) Royle (water thyme) and the native macrophyte Egeria najas Planch to evaluate the potential of herbivory as a mechanism to resist invasion. Both macrophyte species were offered, individually and combined, as food resources to the snail to evaluate its feeding preference. The macrophyte energy content and stiffness were also measured. The results indicate a higher H. verticillata biomass consumption by P. canaliculata when this macrophyte was available both individually and combined with E. najas, suggesting that H. verticillata is more palatable to the snail, despite this macrophyte being stiffer than the native one. Therefore, P. canaliculata may offer resistance to H. verticillata invasion. The feeding preference and high rates of herbivory by this snail on H. verticillata likely are associated with the higher energy content of the invasive, compared with the native, macrophyte. Experiments combining different food availabilities and snail densities are necessary to evaluate the biological control capacity of P. canaliculata under different scenarios.

Feeding rates of Balloniscus sellowii (Crustacea, Isopoda, Oniscidea): the effect of leaf litter decomposition and its relation to the phenolic and flavonoid content

The goal of this study was to compare the feeding rates of Balloniscus sellowii on leaves of different decomposition stages according to their phenolic and flavonoid content. Leaves from the visually most abundant plants were offered to isopods collected from the same source site. Schinus terebinthifolius,the plant species consumed at the highest rate, was used to verify feeding rates at different decomposition stages. Green leaves were left to decompose for one, two, or three months, and then were offered to isopods. The total phenolic and flavonoid contents were determined for all decomposition stages. Consumption and egestion rates increased throughout decomposition, were highest for two-month-old leaves, and decreased again in the third month. The assimilation rate was highest for green leaves. The mode time of passage through the gut was two hours for all treatments. Ingestion of leaves occurred after two or three days for green leaves, and on the same day for one-, two- and three-month-old leaves. The speed of passage of leaves with different decomposition stages through the gut does not differ significantly when animals are fed continuously. However, it is possible that the amount retained in the gut during starvation differs depending on food quality. The digestibility value was corrected using a second food source to empty the gut of previously ingested food, so that all of the food from the experiment was egested. The digestibility value was highest for green leaves, whereas it was approximately 20% for all other stages. This was expected given that digestibility declines during decomposition as the metabolite content of the leaves decreases. The phenolic content was highest in the green leaves and lowest in three-month-old leaves. The flavonoid content was highest in green leaves and lowest after two months of decomposition. Animals ingested more phenolics when consumption was highest. The estimated amount of ingested flavonoids followed the same trend as assimilation rate. Flavonoids accounted for a large portion of total phenolics, and the estimated amount of flavonoids consumed was similar for one-, two- and three-month-old leaves. Our results suggest that the high phenolic and flavonoid concentrations in green leaves are feeding deterrents. Isopods may discriminate among concentrations of flavonoids and modify their consumption rates to maintain their intake of flavonoids when ingesting leaves with lower flavonoid content.

Leaf species identity and combination affect performance and oviposition choice of two container mosquito species

1. Resource diversity can be an important determinant of individual and population performance in insects. Fallen parts of plants form the nutritive base for many aquatic systems, including mosquito habitats, but the effect of plant diversity on mosquito production is poorly understood.2. To determine the effects of diverse plant inputs on larval mosquitoes, experiments were conducted that examined how leaves of Vitis aestivalis, Quercus virginiana, Psychotria nervosa, and Nephrolepis exalta affected the container species Aedes triseriatus and Aedes albopictus.3. The hypothesis that leaf species have different effects on larval survival, growth, population performance, and oviposition choice of the two mosquito species was tested. The hypothesis that larval performance of A. albopictus responds additively to combinations of the four plant species was also tested.4. Larval survival and growth differed among the four leaf species, and oviposition preference differed among the two leaf species examined. Measurements of population performance demonstrated significant variation between leaf treatments. Larval outcomes for A. albopictus were significantly affected by leaf combination, and the hypothesis of additivity could be rejected.5. These results indicate that individual leaf species are important in determining the performance of container dwelling mosquitoes, which grow larger and survive better on mixed-species resource than expected based on an additive model of resource utilisation.