Insects in the city: Determinants of a contained aquatic microecosystem across an urbanized landscape

Cities can have profound impacts on ecosystems, yet our understanding of these impacts is currently limited. First, the effects of the socioeconomic dimensions of human society are often overlooked. Second, correlative analyses are common, limiting our causal understanding of mechanisms. Third, most research has focused on terrestrial systems, ignoring aquatic systems that also provide important ecosystem services. Here we compare the effects of human population density and low-income prevalence on the macroinvertebrate communities and ecosystem processes within water-filled artificial tree holes. We hypothesized that these human demographic variables would affect tree holes in different ways via changes in temperature, water nutrients, and the local tree hole environment. We recruited community scientists across Greater Vancouver (Canada) to provide host trees and tend 50 tree holes over 14 weeks of colonization. We quantified tree hole ecosystems in terms of aquatic invertebrates, litter decomposition, and chlorophyll a (chl a). We compiled potential explanatory variables from field measurements, satellite images, or census databases. Using structural equation models, we showed that invertebrate abundance was affected by low-income prevalence but not human population density. This was driven by cosmopolitan species of Ceratopogonidae (Diptera) with known associations to anthropogenic containers. Invertebrate diversity and abundance were also affected by environmental factors, such as temperature, elevation, water nutrients, litter quantity, and exposure. By contrast, invertebrate biomass, chl a, and litter decomposition were not affected by any measured variables. In summary, this study shows that some urban ecosystems can be largely unaffected by human population density. Our study also demonstrates the potential of using artificial tree holes as a standardized, replicated habitat for studying urbanization. Finally, by combining community science and urban ecology, we were able to involve our local community in this pandemic research pivot.

Aquatic islands in the sky: 100 years of research on water-filled tree holes

Water‐filled tree holes are unique ecosystems that may occur high up in tree crowns and are essentially aquatic islands in the sky. Insect larvae, mesofauna, and other organisms colonize the waterbodies and feed on the accumulating detritus. Water‐filled tree holes are not only important habitats for these species but have been used as model systems in ecology. Here, we review more than 100 years of research on tree‐hole inhabiting organisms and show that most studies focus on selected or even single species (most of which are mosquitoes), whereas only few studies examine groups other than insects, especially in the tropics. Using a vote counting of results and a meta‐analysis of community studies, we show that the effects of tree‐hole size and resources on abundance and richness were investigated most frequently. Both were found to have a positive effect, but effect sizes were modulated by site‐specific environmental variables such as temperature or precipitation. We also show that parameters such as the height of the tree holes above ground, tree‐hole density, predation, and detritus type can be important drivers of organism abundance or richness but are less often tested. We identify several important research gaps and potential avenues for future research. Specifically, future studies should investigate the structure, functions, and temporal dynamics of tree‐hole food webs and their cross‐system interactions, for example, with terrestrial predators that act as a connection to their terrestrial surroundings in meta‐ecosystems. Global observational or experimental tree‐hole studies could contribute pivotal information on spatial variation of community structure and environmental drivers of community assembly. With a better understanding of these unique aquatic habitats in terrestrial ecosystems, natural and artificial tree holes can not only serve as model systems for addressing fundamental ecological questions but also serve as indicator systems of the impacts of environmental change on ecosystems.

Predators as Control Agents of Mosquito Larvae in Micro-Reservoirs (Review)

The article reviews predators that are able to control populations of mosquito larvae (Culicidae) in phytotelmata and their anthropogenic analogs. The spectrum of mosquito larva consumers in micro-reservoirs is listed. It includes flatworms, crustaceans, arachnids, insects, vertebrates, and carnivorous plants. The biology and practical use of the two most effective biological control agents, predatory mosquitoes Toxorhynchites and copepods, are considered in more detail. Prospects of invertebrate predators as controlling agents for the mosquito larvae in micro-reservoirs in temperate climate zones are briefly discussed.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1134/S1995082922010138.

Comparative Skip-Oviposition Behavior Among Container Breeding Aedes spp. Mosquitoes (Diptera: Culicidae)

Container Aedes mosquitoes are the most important vectors of human arboviruses (i.e., dengue, chikungunya, Zika, or yellow fever). Invasive and native container Aedes spp. potentially utilize natural and artificial containers in specific environments for oviposition. Several container Aedes spp. display 'skip-oviposition' behavior, which describes the distribution of eggs among multiple containers during a single gonotrophic cycle. In this study, we compared individual skip-oviposition behavior using identical eight-cup testing arenas with three container Aedes species: Aedes aegypti (Linnaeus), Aedes albopictus (Skuse), and Aedes triseriatus (Say). We applied the index of dispersion, an aggregation statistic, to individual mosquitoes' oviposition patterns to assess skip-oviposition behavior. Aedes aegypti and Ae. albopictus utilized more cups and distributed eggs more evenly among cups than Ae. triseriatus under nutritionally enriched oviposition media (oak leaf infusion) conditions. When presented with a nutritionally unenriched (tap water) oviposition media, both Ae. aegypti and Ae. albopictus increased egg spreading behavior. Aedes albopictus did not modify skip-oviposition behavior when reared and assessed under fall-like environmental conditions, which induce diapause egg production. This study indicates specific oviposition site conditions influence skip-oviposition behavior with 'preferred' sites receiving higher amounts of eggs from any given individual and 'non-preferred' sites receive a limited contribution of eggs. A further understanding of skip-oviposition behavior is needed to make the best use of autodissemination trap technology in which skip-ovipositing females spread a potent larvicide among oviposition sites within the environment.

The Effects of Larval Habitat Quality on Aedes albopictus Skip Oviposition

Aedes albopictus is an invasive mosquito species that transmits human-disease-causing pathogens. It is a container-inhabiting species that oviposits in resource-limited habitats. To mitigate larval competition, Ae. albopictus females may choose to distribute eggs from a single gonotrophic cycle among multiple containers through skip oviposition. With the use of individual females released in indoor and outdoor caged trials, we evaluated the oviposition choices made by gravid Ae. albopictus offered larval habitats with different qualities. Our results demonstrate that Ae. albopictus performs skip oviposition and that the degree of egg distribution is related to the quality of the larval habitat. In a 4-choice arena, individual Ae. albopictus oviposited in fewer containers when presented with ovisites of high-quality larval habitat (uncrowded conditions) compared with oviposition in low-quality (crowded conditions) larval habitats. Additionally, the females selectively oviposited in high-quality habitats when offered both low- and high-quality habitats, but distributed eggs more evenly among multiple high-quality habitats. Our results have important implications for mosquito management plans that include the use of lethal ovitraps, as well as the role of this behavior in distribution of disease-causing pathogens.

Container Type Influences the Relative Abundance, Body Size, and Susceptibility of Ochlerotatus triseriatus (Diptera: Culicidae) to La Crosse Virus

Ochlerotatus triseriatus (Say), the primary vector of La Crosse virus (LAC), develops in a variety of natural and artificial aquatic containers where it often co-occurs with larvae of other mosquito species. We conducted a field study at two woodlots (South Farms and Trelease Woods) in Urbana, IL, to examine how container type influences vector abundance, body size, and susceptibility to LAC. Mosquito pupae were collected from tree holes, plastic bins, and waste tires, and eclosing adults were identified to species morphologically. Oc. triseriatus and Ochlerotatus japonicus (Theobald) females were orally challenged with LAC and midgut infection rate, disseminated infection rate, and body titer were determined by reverse-transcriptase real-time PCR. Oc. triseriatus was the dominant species collected in tree holes while Oc. japonicus and Culex restuans (Theobald) were mostly dominant in artificial containers. Female Oc. triseriatus and Oc. japonicus collected from plastic bins were significantly larger than those collected from tree holes or waste tires. Oc. japonicus females from South Farms were also significantly larger than those from Trelease Woods. Oc. triseriatus females collected from plastic bins and waste tires were significantly more susceptible to LAC infection relative to females collected from tree holes. In addition, Oc. triseriatus females from waste tires had significantly higher LAC titer relative to Oc. triseriatus from tree holes. For each container type and study site, wing length was not correlated to infection or dissemination rates. These findings suggest that the container type in which Oc.triseriatus develop may contribute to the spatial and temporal dynamics of LAC transmission.

Does autocthonous primary production influence oviposition by Aedes japonicus japonicus (Diptera: Culicidae) in container habitats?

Aedes (Finlaya) japonicus japonicus (Theobald) (Diptera: Culicidae) is recently invasive in North America and has expanded its range rapidly since 1998. Throughout its native and expanded range, Ae. j. japonicus larvae are commonly observed in many types of natural and artificial water-filled containers that vary in organic matter content and exposure to sunlight. Larvae are most often found in containers with decaying leaf material or algae, and we postulated that the added autocthonous primary production from algae could be both an important food source for larvae and an influential oviposition attractant to adult Ae. j. japonicus. We tested this hypothesis by placing plastic containers with varied levels of shading to manipulate algal density in the field, and then monitored oviposition by natural populations of Ae. j. japonicus. Over 99% of larvae hatching from eggs laid on the walls of our containers were Ae. j. japonicus, indicating that this species is a dominant colonizer of artificial containers in the study areas. Although full shading treatments effectively reduced algal biomass (significant reduction in chlorophyll a levels), at only one of three sites did this appear to affect Ae. j. japonicus oviposition. We conclude that algae in larval habitats are not a major factor in oviposition choices of adult Ae. j. japonicus females except when in situ primary production is high enough to substantially alter overall organic matter content cues.