Effects of microcosm scaling and food resources on growth and survival of larval Culex pipiens.

BACKGROUND: We used a simple experimental design to test for the effects of microcosm scaling on the growth and survival of the mosquito, Culex pipiens. Microcosm and mesocosm studies are commonly used in ecology, and there is often an assumption that scaling doesn't affect experimental outcomes. The assumption is implicit in the design; choice of mesocosms may be arbitrary or based on convenience or cost. We tested the hypothesis that scale would influence larvae due to depth and surface area effects. Larvae were predicted to perform poorly in microcosms that were both deep and had small openings, due to buildup of waste products, less exchange with the environment, and increased competition. To determine if the choice of scale affected responses to other factors, we independently varied leaf litter quantity, whose effects on mosquitoes are well known. RESULTS: We found adverse effects of both a lower wall surface area and lower horizontal surface area, but microcosm scale interacted with resources such that C. pipiens is affected by habitat size only when food resources are scarce. At low resource levels mosquitoes were fewer, but larger, in microcosms with smaller horizontal surface area and greater depth than in microcosms with greater horizontal surface area and shallower depth. Microcosms with more vertical surface area/volume often produced larger mosquitoes; more food may have been available since mosquitoes browse on walls and other substrates for food. CONCLUSIONS: The interaction between habitat size and food abundance is consequential to aquatic animals, and choice of scale in experiments may affect results. Varying surface area and depth causes the scale effect, with small horizontal surface area and large depth decreasing matter exchange with the surrounding environment. In addition, fewer resources leads to less leaf surface area, and the effects of varying surface area will be greater under conditions of limiting resources. This leads to smaller size, which limits fecundity and survival. Choice of container size, either by ovipositing females or researchers, interacts with a major aspect of the ecology of animals; obtaining resources in a resource-limited environment.

Effects of sodium concentration on Aedes triseriatus (Diptera: Culicidae) and microorganisms in treeholes.

A 2-factor field microcosm experiment with 2 concentrations of sodium ([Na]) was performed with and without mosquitoes to test the hypothesis that Na is a limiting nutrient for the treehole mosquito, Aedes triseriatus (Say), at concentrations observed in the field. This experiment also tested the effects of both water [Na] and mosquito larvae on cohabiting treehole microorganisms. Female Ae. triseriatus emerging from microcosms with high-[Na] were significantly larger than females emerging from low-[Na] microcosms. There was no effect of [Na] on the size of males; however, males had higher body [Na] than females at both [Na]. Densities of protozoans were lower in the presence of mosquitoes, and densities of bacteria were lower at high than low [Na]. Water [Na] may affect mosquito populations by decreasing energy expenditures to obtain Na at higher [Na], allowing for more allocation to growth. Males were not affected by the [Na]s tested here. This experiment agrees with field observations and other laboratory studies indicating that [Na] may affect populations of Ae. triseriatus under certain conditions.

Effects of pH and sulfate on insects and protozoans inhabiting treeholes.

We used laboratory microcosms designed to simulate treeholes to study the effect of changing levels of chemicals found in precipitation in central Pennsylvania (H+ and SO4=) on the treehole insects Aedes triseriatus (a mosquito), Helodes pulchella (a helodid beetle), and Culicoides guttipennis (a ceratopogonid midge), and treehole protozoans. Protozoans were tested in both the presence and absence of insects. We hypothesized that the individual insect species would have differential tolerances to abiotic stresses, and that effects of low pH on protozoans would be especially strong. Survival of helodids was higher than mosquitoes and midges. Emergence and survival of mosquitoes and midges were lower at low pH. Densities of ciliates increased the most at high pH in the absence of insects. Densities of flagellates increased the most at low pH in the absence of insects. The presence of helodids, mosquitoes, and protozoans was associated with higher final [SO4=], as was low pH. Mosquitoes at low pH caused the largest increases in cation concentrations, and protozoans at high pH caused the largest decreases in cation concentrations. It appears that the biota of treeholes are affected by ionic changes in simulated tree stemflow that can be caused by anthropogenic atmospheric deposition. The species studied here were sometimes differentially affected by the common pollutants, [H+] and [SO4=]. The resulting changes in these discrete treehole communities may allow them to be useful bioindicators of the status of forest ecosystems altered by changing atmospheric chemistry.

Insect species interactions and resource effects in treeholes: are helodid beetles bottom-up facilitators of midge populations?

The insect community living in central Pennsylvania treeholes in autumn consists primarily of larvae of two species of helodid beetles, Prionocyphon discoideus and Helodes pulchella, and larvae of one species of ceratopogonid midge,Culicoides guttipennis. We manipulated treehole volume and the densities of these insects in laboratory microcosms. We hypothesized that: (1) helodid beetle larvae, which are shredders, would enhance growth and survival of ceratopogonid midge larvae (deposit feeders) in a processing chain commensalism, and (2) the quantity of resources expressed as water volume plus leaf litter would affect helodids and protozoans directly. Intraspecific competition was not found in midges, nor was interspecific competition between the two helodid species. Protozoan population densities decreased or grew slower in the presence of insects and in smaller microcosms. Development time and adult wing length of the midge (C. guttipennis) were affected by both total microcosm volume and insect species combination. Under resource limitation, midges grew larger in the presence of helodids, and in general, midges were larger in treatments with higher ratios of helodids to midges. Water chemistry in the microcosms was affected both temporally and by insect presence. Hydrogen ion levels decreased over time, and microcosms with no insects had lower hydrogen ion levels. Specific conductance increased in all treatments over time, and microcosms with no insects had lower conductivity than most treatments. Helodid larvae have a positive effect on midges, possibly due to a processing chain facilitation. If helodids are keystone decomposers in this system, their presence could affect resource availability and affect other organisms in the community. Similar processing chain commensalisms occur in other phytotelmata. These types of interactions may therefore be important in the structure and function of detritus-based communities.

Effects of rutin-fed caterpillars on an invertebrate predator depend on temperature.

A factorial experiment examined the effects of varying concentrations of the allelochemical rutin in caterpillars and the length of time the caterpillars had fed on the behavioral interactions of predatory stinkbugs (Podisus maculiventris) and their prey (Manduca sexta). Diet had no significant effect on defensive behavior of the caterpillars. The length of time that the caterpillars had fed (1 vs. 24 h) only influenced the frequency of caterpillars knocking the attacking stinkbugs away, with caterpillars knocking the stinkbugs away more often after 24 h of feeding. A second experiment tested the effects of diet (prey fed various concentrations of rutin), temperature (18° C and 28° C) and gender on consumption and growth parameters of fifth instar stinkbugs. At the cooler temperature, the bugs ate more, gained more weight but took twice as long to complete the stadium and consequently had reduced relative consumption and relative growth rates. Diet had no significant effect on biomass gained or stadium duration, but rutin-fed caterpillars did depress the stinkbugs' relative consumption rates. The effect of food quality on relative growth rate (RGR) was temperature dependent; rutin had no significant effect at the cooler temperature, but a high dose of rutin reduced RGR at the warmer temperature. Rutin had a greater negative impact on the females than the males. The effect of rutin on these predators was different than the effect on their prey (this study compared to Stamp (1990, 1992)): the negative effects of rutin seem to impact on the stinkbug's growth rather than on molting.