Do Riparian Buffers Protect Stream Invertebrate Communities in South American Atlantic Forest Agricultural Areas?

We investigated the influence and relative importance of insecticides and other agricultural stressors in determining variability in invertebrate communities in small streams in intensive soy-production regions of Brazil and Paraguay. In Paraguay we sampled 17 sites on tributaries of the Pirapó River in the state of Itapúa and in Brazil we sampled 18 sites on tributaries of the San Francisco River in the state of Paraná. The riparian buffer zones generally contained native Atlantic forest remnants and/or introduced tree species at various stages of growth. In Brazil the stream buffer width was negatively correlated with sediment insecticide concentrations and buffer width was found to have moderate importance in mitigating effects on some sensitive taxa such as mayflies. However, in both regions insecticides had low relative importance in explaining variability in invertebrate communities, while various habitat parameters were more important. In Brazil, the percent coverage of soft depositional sediment in streams was the most important agriculture-related explanatory variable, and the overall stream-habitat score was the most important variable in Paraguay streams. Paraguay and Brazil both have laws requiring forested riparian buffers. The ample forested riparian buffer zones typical of streams in these regions are likely to have mitigated the effects of pesticides on stream invertebrate communities. This study provides evidence that riparian buffer regulations in the Atlantic Forest region are protecting stream ecosystems from pesticides and other agricultural stressors. Further studies are needed to determine the minimum buffer widths necessary to achieve optimal protection.

Species at Risk (SPEAR) index indicates effects of insecticides on stream invertebrate communities in soy production regions of the Argentine Pampas.

We investigated relationships among insecticides and aquatic invertebrate communities in 22 streams of two soy production regions of the Argentine Pampas over three growing seasons. Chlorpyrifos, endosulfan, cypermethrin, and lambda-cyhalothrin were the insecticides most frequently detected in stream sediments. The Species at Risk (SPEAR) pesticide bioassessment index (SPEARpesticides) was adapted and applied to evaluate relationships between sediment insecticide toxic units (TUs) and invertebrate communities associated with both benthic habitats and emergent vegetation habitats. SPEARpesticides was the only response metric that was significantly correlated with total insecticide TU values for all three averaged data sets, consistently showing a trend of decreasing values with increasing TU values (r2=0.35 to 0.42, p-value=0.001 to 0.03). Although pyrethroids were the insecticides that contributed the highest TU values, toxicity calculated based on all insecticides was better at predicting changes in invertebrate communities than toxicity of pyrethroids alone. Crustaceans, particularly the amphipod Hyalella spp., which are relatively sensitive to pesticides, played a large role in the performance of SPEARpesticides, and the relative abundance of all crustaceans also showed a significant decreasing trend with increasing insecticide TUs for two of three data sets (r2=0.30 to 0.57, p-value=0.003 to 0.04) examined. For all data sets, total insecticide TU was the most important variable in explaining variance in the SPEARpesticides index. The present study was the first application of the SPEAR index in South America, and the first one to use it to evaluate effects of pesticides on invertebrate communities associated with aquatic vegetation. Although the SPEAR index was developed in Europe, it performed well in the Argentine Pampas with only minor modifications, and would likely improve in performance as more data are obtained on traits of South American taxa, such as pesticide sensitivity and generation time.

Species traits and environmental constraints: entomological research and the history of ecological theory.

The role that entomology has played in the historical (1800s-1970s) development of ecological theories that match species traits with environmental constraints is reviewed along three lineages originating from the ideas of a minister (Malthus TR. 1798. An Essay on the Principle of Population. London: Johnson) and a chemist (Liebig J. 1840. Die Organische Chemie in ihrer Anwendung auf Agricultur und Physiologie. Braunschweig: Vieweg). Major developments in lineage 1 focus on habitat as a filter for species traits, succession, nonequilibrium and equilibrium conditions, and generalizations about the correlation of traits to environmental constraints. In lineage 2, we trace the evolution of the niche concept and focus on ecophysiological traits, biotic interactions, and environmental conditions. Finally, we describe the conceptual route from early demographic studies of human and animal populations to the r-K concept in lineage 3. In the 1970s, the entomologist Southwood merged these three lineages into the "habitat templet concept" (Southwood TRE. 1977. J. Anim. Ecol. 46:337-65), which has stimulated much subsequent research in entomology and general ecology. We conclude that insects have been a far more important resource for the development of ecological theory than previously acknowledged.

Wetland management strategies that enhance waterfowl habitats can also control mosquitoes.

Two studies in California wetlands and a third in Minnesota wetlands indicate that management practices designed to enhance habitat quality for waterfowl can concurrently reduce mosquito problems. In a seasonally flooded pickleweed wetland in Suisun Marsh, Solano Co., CA, we demonstrated that reducing plant-cover by 50% increased benthic densities of chironomid midge and dytiscid beetle larvae; these insects can be important in waterfowl diets. This manipulation also concentrated Aedes melanimon and Culiseta inornata mosquito larvae along wetland perimeters; thus, the need for control measures was greatly restricted spatially. A study in 9 experimental ponds in Suisun Marsh demonstrated that higher water levels could enhance populations of the macroinvertebrates important in waterfowl diets; general macroinvertebrate densities were higher at 60 cm depths than 20 cm or 40 cm depths. In contrast, Cs. inornata densities were lowest at 60 cm depths and highest at 20 cm depths. A study conducted in a perennial-water cattail wetland in Minneapolis-St. Paul, MN, demonstrated that a temporary water-level drawdown, designed to enhance waterfowl habitat quality of perennial-water wetlands, also reduced densities of Coquillettidia perturbans mosquito larvae. These mosquitoes disappeared immediately after the drawdown, but even after water depths were restored to pre-drawdown levels, significant numbers did not reappear until 4 years post-drawdown. Studies in 202 other Minnesota wetlands also demonstrated the susceptibility of Cq. perturbans populations to drawdown, but the impact of drawdown was greater in stands of emergent cattail than in floating cattail.

Influence ofMyriophyllum aquaticum cover onAnopheles mosquito abundance, oviposition, and larval microhabitat.

The surface cover produced by aquatic macrophytes is the primary habitat for immature stages (eggs, larvae, and pupae) ofAnopheles mosquitoes. We hypothesized that both the abundance of immatureAnopheles and the recruitment ofAnopheles (from oviposition or larval movement) is positively related to the amount of surface cover present. Field sampling documented a positive correlation betweenAnopheles egg and larval abundance and the amount of vegetative cover present (measured as the number of emergent stems m-2) in monospecific beds ofMyriophyllum aquaticum in a California, USA, wetland. Experiments conducted to determine the influence ofMyriophyllum stem density on selection of oviposition sites by adultAnopheles females clearly indicate that oviposition rate (eggs m-2 d-1) increases as stem density increases from 0 to 1000 stems m-2 but decreases as stem density approaches 2000 stems m-2. In selecting microhabitats,Anopheles larvae preferred patches with high stem densities over patches with few or no plant stems; this preference correlates with differences in habitat quality (e.g., increased refuge from predation and enriched food sources). The optimal habitat for anopheline mosquitoes apparently occurs above a threshold plant density of approximately 500Myriophyllum stems m-2. Habitat heterogeneity produced by variability in the distribution and structure of aquatic vegetation strongly influences the local distribution and abundance of anopheline mosquitoes.

Where is mosquito research published?

To identify the core journals that publish articles on mosquito research, the Agricola database (covering 1985-89 citations) was analyzed. This source contained 3,007 citations to mosquito research, and the citations were found in 404 different serials. Whereas 10 serials (2.5% of total serials) in Agricola produced 50% of the citations, almost one-half of the serials in this source contained only one citation to mosquito research. Fifteen journals (the core journals) contained greater than 1% of total citations. The number of citations found among 126 different databases and abstracting services that were examined varied: 39 had no citations to mosquitoes, but 13 (including life-sciences, medical and even popular-literature databases) had greater than 100 citations. The choice of core journals will be an increasingly important decision by biological libraries as costs for journal subscriptions rise.

Experimental test of the influence of aquatic macrophyte cover on the survival of Anopheles larvae.

Emergent or submergent macrophytes can enhance the survival of Anopheles larvae by providing favorable microhabitat and refuge from predation. The relationships among the amount of aquatic macrophyte cover, the density of Gambusia affinis, and survivorship of Anopheles freeborni larvae were tested experimentally. Larval survivorship was positively related to amount of plant cover and negatively related to Gambusia density. Larval survivorship was lowest in treatments with low plant cover and high fish density (0-15% survivorship) and highest in treatments with high plant cover and no fish (41-76% survivorship). Similar patterns of larval survivorship were found in enclosure experiments conducted at 3 sites, using 3 species of aquatic macrophytes (Potamogeton pectinatus, Myriophyllum aquaticum and M. spicatum).

Herbivory and intraspecific competition in a stream caddisfly population.

Field experiments were conducted to assess the effects of different population densities of the herbivorous caddisfly Helicopsyche borealis on periphyton biomass and on its own growth rate in Big Sulphur Creek, a third-order stream in northern California, USA. Stream enclosures were used to vary grazer density from one-eighth to twice natural density (1/8X-2X) in two experiments (35 d and 60 d), which spanned the period of most rapid larval growth. Periphyton biomass and chlorophyll a were inversely related to grazer density. Grazer densities of 1/8X-1/2X moderately reduced periphyton when compared to an ungrazed control, whereas densities of 1X-2X greatly depleted periphyton. The growth rate of H. borealis larvae declined with increasing larval density. Growth rate was highest at densities of 1/8X-1/2X; larvae grew more slowly at 1X and showed no growth at 2X. Weekly supplementation of periphyton to 1/4X, 1X, and 2X densities significantly increased the final body mass of larvae at 1X and 2X (by 26% and 50%, respectively, compared to unsupplemented larvae) but did not change the body mass of larvae at 1/4X. These results suggest that periphyton is a limiting resource in Big Sulphur Creek and that H. borealis larvae compete exploitatively for that resource. Intraspecific competition may be an important, but often overlooked, feature of many herbivore populations in streams.

Demonstration of sex pheromones in caddisflies (Trichoptera).

Field tests with live, caged females and whole-body extracts of females of the caddisflyGumaga griseola (McLachlan) (Trichoptera: Sericostomatidae) demonstrate, for the first time, the existence of a chemically mediated sexual communication system in this insect order. Both live females and extracts are significantly more attractive to conspecific males than either empty control traps or traps baited withGumaga nigricula (McLachlan) females. Anatomical structures suggest that semiochemicals are widespread in Trichoptera.