Christopher J. Paradise

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

Abstract 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.

The pesticide malathion reduces survival and growth in developing zebrafish

Malathion is an organophosphorous pesticide widely used to control mosquitoes in urban areas and pests, such as boll weevils, in agricultural areas. Zebrafish, Danio rerio, are model organisms for developmental toxicology research because they are readily available, produce large numbers of clear embryos, and are sensitive to environmental changes. The nonlethal effects of malathion on developing zebrafish embryos, however, previously have not been analyzed quantitatively. We exposed zebrafish embryos to sublethal malathion concentrations to determine malathions effects on a developing vertebrate. Zebrafish exposed to 0.5, 1.0, or 1.5 mg/L of malathion consistently elicited more rapid hatching from the chorion than zebrafish exposed to 2.0-, 2.5-, or 3.0-mg/L malathion concentrations. In addition, exposure to 2.0, 2.5, or 3.0 mg/L of malathion resulted in significantly shorter body length and eye diameters, indicating that malathion had teratogenic effects on zebrafish embryos. Malathions action as an acetylcholinesterase inhibitor and the toxicity of the metabolites of malathion may be responsible for malathions teratogenic effects on fish development.

Relationship of water and leaf litter variability to insects inhabiting treeholes

Abstract I surveyed treeholes in central Pennsylvania for 7 mo in 1995 to investigate relationships among insect communities and water and leaf litter resources. I used water volume, essential to growth of treehole larvae, as an indicator of habitat size. Leaf litter is the basal food resource in treehole communities, and litter volume is related to the amount of energy available. Insect species richness and larval mosquito (Ochlerotatus triseriatus) and ceratopogonid midge (Culicoides guttipennis) densities were higher in treeholes that maintained high water volumes than those with low volumes throughout the study. Treeholes with high litter volume, irrespective of water volume, had the highest densities of C. guttipennis, and higher total larval densities than those with lower litter volumes; however, litter volume did not affect insect species richness. Scirtid beetle larvae were common in all treeholes, although their densities were unaffected by both litter and water volume. Insect species richness was not related to litter volume, unlike some other studies of phytotelmata. However, my study supported earlier conclusions that both habitat size and resource availability influence structure of treehole communities and maintain species populations. The influence of each resource on particular species presence and density may be related to physical aspects of treeholes, biotic interactions, and natural history of individual species.

Effects of Land Use and Disturbance on Benthic Insects in Headwater Streams Draining Small Watersheds North of Charlotte, NC

Abstract Increasing development north of Charlotte, NC, threatens aquatic life in streams by reducing riparian zones and increasing runoff. Runoff, sedimentation from erosion, and poor construction practices are principal sources of pollution. We asked how land use and disturbance affected benthic insects. We visited nine streams from May to October 2001, collected data on insect diversity, chemistry, and physical habitat. We used a Geographic Information System to delineate watersheds and land use patterns. Watersheds were categorized based on land use, abiotic variables, and disturbance. Insect communities were more diverse in streams draining low disturbance watersheds than in streams draining highly developed watersheds. Sensitive taxa were found in streams with extensively forested watersheds, but were nonexistent in extensively developed watersheds. Disturbances occurring in streams caused declines in diversity, often eliminating sensitive taxa. Aquatic insect diversity is related to land use patterns and disturbances, and anthropogenic alteration of habitat has negative consequences to that diversity.

Interactive effects of resources and a processing chain interaction in treehole habitats

The degree to which the outcome of biotic interactions is sensitive to resource levels is a fundamental issue in ecology. Conversely, resource availability may be affected by processing chain interactions, where organisms specialize on a resource in different states of decay. An organism upstream in the decay sequence can affect availability of resources to downstream consumers. Experimental microcosms that mimic tree-hole habitats were used to test the hypothesis that leaf-shredders process resources and facilitate populations of downstream consumers under conditions of resource limitation in a detritus-based community. Resources were varied independently of presence of leaf-shredding scirtid beetles (Helodes pulchella and Prionocyphon discoideus) to examine interactions among resource level, scirtids, and the mosquito Aedes triseriatus. Scirtids facilitated growth of A. triseriatus only when resource quantity was low. Survival of mosquitoes was negatively affected by the lowest resource level only when scirtids were absent. At the highest resource levels, scirtids hindered growth of mosquitoes in comparison to high resources without scirtids. Leaf-shredders affect resource availability, and resource processing by scirtids has significant repercussions on populations of A. triseriatus that are dependent upon resource level. Although resources alone had large effects on mosquito growth, these bottom-up effects were conditionally mediated by shredders.

Prey recognition time of praying mantids (Dictyoptera: Mantidae) and consequent survivorship of unpalatable prey (Hemiptera: Lygaeidae)

When juvenile praying mantids (Tenodera sinensis)were exposed to unpalatable prey (the milkweed bug Oncopeltus fasciatus),they attacked, sampled, and then usually rejected the prey. About 70% of the handling time was spent feeding. When offered a second milkweed bug, the mantids usually attacked the prey. However, the overall time required for the mantids to sample, recognize, and then reject the unpalatable prey decreased by half. The proportion of handling time that was spent feeding remained the same as in the first encounter. In contrast, when the second prey individuals encountered by mantids were Drosophila melanogaster,the flies were completely consumed and the proportion of handling time that was spent feeding significantly increased. When praying mantids were exposed to the milkweed bugs for the first time, up to 33% of the bugs survived attack by the mantids. Survival of milkweed bugs increased to 55% when mantids had been previously exposed to the bugs. In contrast, flies that were caught never survived.

Episodes of unpalatable prey reduce consumption and growth of juvenile praying mantids

Third-instar praying mantids (Tenodera sinensisSaussure: Mantidae) were fed either a sequence of unpalatable milkweed bugs (Oncopeltus fasciatusDallas: Lygaeidae) and palatable fruit flies (Drosophila melanogasterMeigen: Drosophilidae) or a control diet of palatable flies only. Mantids fed a sequence of 3 days of unpalatable bugs and 4 days of palatable flies took, on average, 5 days longer to develop to the fourth instar than the controls, and consequently, their growth rate was less than that of the controls. They ate 10–15 unpalatable bugs in that time, but also, because the stadium was prolonged, they ended up eating as many flies as the faster-growing controls and therefore attained similar biomass. Mantids subjected to episodes of 2 days with unpalatable bugs followed by 4 days of flies had reduced growth rates that were a function of both more time spent in the stadium and less weight gained than the controls. Mantids subjected to episodes of 1 day with unpalatable bugs followed by 4 days of flies did not gain as much weight as the controls but had similar growth rates. Mantids fed unpalatable bugs on Days 1 and 6 and palatable flies on the other days and mantids fed flies for 4 days and then 1 day without food grew at the same rate. In this case, eating unpalatable prey for 2 days of 7 (with 7 days = average stadium duration) slowed weight gain as much as missing food for a day. We conclude that, depending on the sequence, episodes (in this case, 1 or more days) of eating unpalatable prey can reduce the daily rate of consumption sufficiently to have a negative impact on biomass gained, stadium duration, and growth rate.

Colonization and development of insects in simulated treehole habitats with distinct resource and pH regimes

AbstractI used replicated artificial treehole habitats to determine how initial colonization and subsequent growth of treehole insect larvae were affected by water volume, leaf litter mass, and pH. Low leaf litter mass had the most significant and recurring effect on the communities. Densities of the common insects, Aedes triseriatus (Say), Culicoides guttipennis (Coquillett), and two scirtid beetle species (Helodes pulchella [Guerin] and Prionocyphon discoideus [Say]) were all higher in low leaf mass than high leaf mass microcosms. Species richness early in the experiment was higher in high water volume and low leaf resource microcosms, suggesting earlier colonization in those microcosms. Growth and survival of C. guttipennis and the syrphid Mallota posticata (Fabricius) were adversely affected by low pH. More scirtid beetles were attracted to low leaf mass habitats, yet scirtids grew to a larger size in high leaf litter treatments. This suggests that oviposition preferences of females, intraspecific com...

Variable quantities of toxic diet cause different degrees of compensatory and inhibitory responses by juvenile praying mantids.

Juvenile praying mantids are faced with a wide variety of prey types, including physiologically harmful ones. How they respond to these prey, behaviorally and physiologically, is examined in this study. By adapting a technique developed by Blau et al. (1978) for herbivorous insects, we determined the deterrence and toxicity of unpalatable prey. Artificial diets containing different percentages of cardenolide‐containing prey were offered to third instar juveniles. When fed diets containing small proportions of unpalatable milkweed bugs (Oncopeltus fasciatus Dallas), juvenile mantids (Tenodera sinensis Saussure) showed a slight decrease in consumption and growth. In contrast, with an intermediate proportion of milkweed bugs in the diet, feeding was not inhibited and, correspondingly, a marked reduction in growth rate occurred. Yet higher proportions of milkweed bugs in the diet acted primarily as a feeding deterrent. Mantids from different egg cases responded differently to the same range of milkweed bug diets. This variation was evident both in acceptance of the novel diet and in relative consumption rates.

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