Oviposition of Culex tarsalis (Diptera: Culicidae) Differs on Water Conditioned by Potential Fish and Insect Predators

Identification of Chemicals Associated Gambusia affinis (Cyprinodontiformes: Poeciliidae), and Their Effect on Oviposition Behavior of Culex tarsalis (Diptera: Culicidae) in the Laboratory

The western mosquitofish, Gambusia affinis (Baird & Girard), has been used worldwide for the control of larval mosquitoes for more than 100 yr. We found that the western encephalitis mosquito, Culex tarsalis Coquillett (Diptera: Culicidae), can detect the presence of G. affinis in oviposition sites based on associated chemicals, leading to a decrease in the number of egg rafts laid. Three volatile chemical compounds were identified in the headspace above the water where G. affinis had been held for 24 h. Oviposition bioassays conducted using standards of the volatile compounds identified (dimethyl disulfide [DMDS], dimethyl trisulfide [DMTS], and S-methyl methanethiosulphonate) found that females reduced oviposition only when low concentrations of DMTS were present, but this response was not consistent across all trials and concentrations tested. DMDS, DMTS, and S-methyl methanethiosulphonate are known bacterial metabolic waste products and may be the source of the compounds. Two nonvolatile compounds of interest were found to be present in the Gambusia-exudate water. After tasting Cx. tarsalis were deterred from ovipositing onto Gambusia-treated water from which the bacteria had been removed by filtration, indicating that the kairomone may consist of nonvolatile compound(s). One of the nonvolatile compounds isolated from the Gambusia-treated water has a benzene ring structure similar to that of cholesterol but the structure of the two nonvolatile deterrents remains to be fully characterized. Our research shows that three volatile compounds and two nonvolatile compounds are present in water associated with G. affinis (Poeciliidae: Gambusia) and affect the oviposition behavior of Cx. tarsalis in laboratory bioassays.

Mosquito oviposition and larvae development in response to kairomones originated by different fish

Predator-released kairomones indicating the presence of predatory fish are known to alter the behaviour and life-history traits of several aquatic herbivores. Culex mosquitoes (Diptera: Culicidae) respond to such cues by altering oviposition habitat selection and larval development time. These responses differ among fish species indicating composition differences among fish-released cues, but the recognition pattern is not clear. This study tested the dependence of fish recognition to co-evolution and the level of threat to larvae. We compared Culex responses to three fish, the invasive larvivorous Gambusia affinis (Cyprinodontiformes: Poeciliidae), the native larvivorous Aphanius mento (Cyprinodontiformes: Cyprinodontidae) and the native algivorous Garra rufa (Cypriniformes: Cyprinidae). The effects of cues released by the three fish on Culex oviposition and life-history shifts were compared in field mesocosms and lab tests. Our results showed that ovipositing females were significantly repelled only by cues originated from G. affinis, while developing larvae response was a more general one. Our results support the idea that fish-released kairomones differ in composition or quantities among different species.

Contamination of stream fish by plastic waste in the Brazilian Amazon

Pollution by plastics is a global problem, in particular through the contamination of aquatic environments and biodiversity. Although plastic contamination is well documented in the aquatic fauna of the oceans and large rivers of the world, there are few data on the organisms of headwater streams, especially in tropical regions. In the present study, we evaluated the contamination of small fish by plastics in Amazonian streams. For this, we evaluated the shape and size, and the abundance of plastics in the gastrointestinal tracts and gills of 14 fish species from 12 streams in eastern Brazilian Amazon. We used a Generalized Linear Mixed Model (GLMM) to compare the levels of contamination among species and between organs. Only one individual of the 68 evaluated (a small catfish Mastiglanis cf. asopos) contained no plastic particles, and no difference was found in the contamination of the gills and digestive tract. However, Hemigrammus unilineatus presented less contamination of both the gills and the digestive tract than the other species, while Polycentrus schomburgkii had less plastic in the gastrointestinal tract, whereas Crenicichla regani and Pimelodella gerii both had a larger quantity of plastic adhered to their gills in comparison with the other species. Nanoplastics and microplastics adhered most to the gills, while plastic fibers were the most common type of material overall. This is the first study to analyze plastic contamination in fish from Amazonian streams, and in addition to revealing high levels of contamination, some species were shown to possibly be more susceptible than others. This reinforces the need for further, more systematic research into the biological and behavioral factors that may contribute to the greater vulnerability of some fish species to contamination by plastics. Amazonian stream fish show contamination by plastics. The species respond differently. The smaller the particle, the easier it is to adhere to the gills.

The Influence of Larval Stage and Density on Oviposition Site-Selection Behavior of the Afrotropical Malaria Mosquito Anopheles coluzzii (Diptera: Culicidae)

In the selection of oviposition sites female mosquitoes use various cues to assess site quality to optimize survival of progeny. The presence of conspecific larvae influences this process. Interactive effects of oviposition site selection were studied in the malaria mosquito Anopheles coluzzii Coetzee & Wilkerson in dual- and no-choice assays, by exposing single gravid mosquitoes to oviposition cups containing 1) larvae of different developmental stages, 2) larvae-conditioned water (LCW), and 3) cups where visual cues of conspecific larvae were absent. Early-stage conspecific larvae had a positive effect on the oviposition response. By contrast, late stages of conspecific larvae had a negative effect. Oviposition choice was dependent on larval density. Moreover, in oviposition cups where larvae were hidden from view, late-stage larvae had a significant negative effect on oviposition suggesting the involvement of olfactory cues. LCW had no effect on oviposition response, indicating involvement of chemicals produced by larvae in vivo. It is concluded that the presence of larvae in a breeding site affects the oviposition response depending on the development stage of the larvae. These responses appear to be mediated by olfactory cues emitted by the larval habitat containing live larvae, resulting in the enhanced reproductive fitness of the females.

Oviposition Behavior of Culex tarsalis (Diptera: Culicidae) Responding to Semiochemicals Associated with the Western Mosquitofish, Gambusia affinis (Cyprinodontiformes: Poecilliidae)

Considerable previous research has focused on predator-associated semiochemicals and how they affect mosquito oviposition behavior. However, most of this work has been done without taking into consideration either the natural density of the predators or how other semiochemicals in aquatic environments might affect the responses of gravid mosquitoes. The influence of mosquitofish density, source water (tap vs pond), presence of freshly laid egg rafts, and removal of a putative source of semiochemicals (bacteria) on oviposition by Culex tarsalis Coquillett (Diptera: Culicidae) was studied in laboratory bioassays. Culex tarsalis females were deterred from laying egg rafts on water that contained semiochemicals associated with Gambusia affinis (Baird & Girard), but this deterrence was not strongly associated with the density of fish used to condition aged tap water. The number of egg rafts laid onto Gambusia-exudate water made with either tap water (density < 1 fish per liter) or pond water was typically ≥ 50% of that onto water that did not house mosquitofish. Gravid mosquitoes tested individually did not reduce oviposition onto Gambusia-exudate water as compared to controls. Likewise, oviposition by females with ablated wings did not differ significantly between Gambusia-exudate water and controls. Oviposition onto filter-sterilized Gambusia-exudate water was reduced relative to unfiltered water, suggesting that semiochemicals deterring egg-laying were still present after bacteria were removed. Taken together, these findings suggest that the responses of gravid Cx. tarsalis to chemicals from habitats containing mosquitofish are complex and the origin of the semiochemicals present in the Gambusia-exudate water needs to be elucidated.

Oviposition Behavior of Culex annulirostris (Diptera: Culicidae) Is Affected by the Recent Presence of Invasive Gambusia holbrooki (Cyprinodontiformes: Poeciliidae)

Constructed wetlands are popular tools for managing threatened flora and fauna in urban settings, but there are concerns that these habitats may increase mosquito populations and mosquito-related public health risks. Understanding the interactions occurring between mosquitoes of public health concern and co-occurring organisms is critical to informing management of these habitats to mitigate potential health risks and balance the multiple values of urban wetlands. This study examined how oviposition behavior of Culex annulirostris Skuse, the most important pest mosquito species associated with freshwater wetland habitats in Australia, is influenced by the presence of Gambusia holbrooki Girard, a widespread invasive fish. Water was collected from urban wetlands that are intensively managed to reduce G. holbrooki populations to assist conservation of locally threatened frogs, and adjacent unmanaged wetlands where G. holbrooki was abundant. Laboratory experiments were conducted to examine the oviposition response by Cx. annulirostris to water samples from these two habitats. Experiments were conducted on two occasions, once in February following draining and refilling of the urban wetlands, and repeated following a substantial rainfall event in March. The results clearly demonstrate that ovipositing mosquitoes were able to detect and avoid water derived from habitats containing fish, even in the absence of the fish themselves. Understanding how invasive species affect the behavior and spatial distribution of pest species such as Cx. annulirostris will enable future wetland design and management to maximize benefits of urban wetlands and minimize potential public health risks.

Management of Arthropod Pathogen Vectors in North America: Minimizing Adverse Effects on Pollinators

Tick and mosquito management is important to public health protection. At the same time, growing concerns about declines of pollinator species raise the question of whether vector control practices might affect pollinator populations. We report the results of a task force of the North American Pollinator Protection Campaign (NAPPC) that examined potential effects of vector management practices on pollinators, and how these programs could be adjusted to minimize negative effects on pollinating species. The main types of vector control practices that might affect pollinators are landscape manipulation, biocontrol, and pesticide applications. Some current practices already minimize effects of vector control on pollinators (e.g., short-lived pesticides and application-targeting technologies). Nontarget effects can be further diminished by taking pollinator protection into account in the planning stages of vector management programs. Effects of vector control on pollinator species often depend on specific local conditions (e.g., proximity of locations with abundant vectors to concentrations of floral resources), so planning is most effective when it includes collaborations of local vector management professionals with local experts on pollinators. Interventions can then be designed to avoid pollinators (e.g., targeting applications to avoid blooming times and pollinator nesting habitats), while still optimizing public health protection. Research on efficient targeting of interventions, and on effects on pollinators of emerging technologies, will help mitigate potential deleterious effects on pollinators in future management programs. In particular, models that can predict effects of integrated pest management on vector-borne pathogen transmission, along with effects on pollinator populations, would be useful for collaborative decision-making.