Linking movement and oviposition behaviour to spatial population distribution in the tree hole mosquitoOchlerotatus triseriatus

Vertical habitat stratification in Aedes triseriatus and Aedes hendersoni (Diptera: Culicidae): complications associated with sloped and flood-prone landscapes

For the sibling mosquito species Aedes triseriatus Say and Ae. hendersoni Cockerell, success of offspring is highly dependent on oviposition site selection by gravid females, and the dynamics of that selection process have been the subject of much investigation. Patterns of vertical oviposition stratification were examined in this study by placing basal and canopy level oviposition traps in relatively steep and flat terrains, as well as in flood-prone riverbank vs upland settings. Strong stratification existed in both steep and flat terrains, with Ae. triseriatus favoring basal ovitraps and Ae. hendersoni favoring canopy ovitraps. Contrary to expectations, Ae. hendersoni displayed even more marked preference for canopy ovitraps in steeper terrain. Both species avoided basal ovitraps in flood-prone locations, where Ae. triseriatus oviposited only in canopy ovitraps and greatly outnumbered Ae. hendersoni in canopy ovitraps. In contrast, the expected stratification was found in nearby upland habitats.

The use of drones for mosquito surveillance and control

In recent years, global health security has been threatened by the geographical expansion of vector-borne infectious diseases such as malaria, dengue, yellow fever, Zika and chikungunya. For a range of these vector-borne diseases, an increase in residual (exophagic) transmission together with ecological heterogeneity in everything from weather to local human migration and housing to mosquito species' behaviours presents many challenges to effective mosquito control. The novel use of drones (or uncrewed aerial vehicles) may play a major role in the success of mosquito surveillance and control programmes in the coming decades since the global landscape of mosquito-borne diseases and disease dynamics fluctuates frequently and there could be serious public health consequences if the issues of insecticide resistance and outdoor transmission are not adequately addressed. For controlling both aquatic and adult stages, for several years now remote sensing data have been used together with predictive modelling for risk, incidence and detection of transmission hot spots and landscape profiles in relation to mosquito-borne pathogens. The field of drone-based remote sensing is under continuous change due to new technology development, operation regulations and innovative applications. In this review we outline the opportunities and challenges for integrating drones into vector surveillance (i.e. identification of breeding sites or mapping micro-environmental composition) and control strategies (i.e. applying larval source management activities or deploying genetically modified agents) across the mosquito life-cycle. We present a five-step systematic environmental mapping strategy that we recommend be undertaken in locations where a drone is expected to be used, outline the key considerations for incorporating drone or other Earth Observation data into vector surveillance and provide two case studies of the advantages of using drones equipped with multispectral cameras. In conclusion, recent developments mean that drones can be effective for accurately conducting surveillance, assessing habitat suitability for larval and/or adult mosquitoes and implementing interventions. In addition, we briefly discuss the need to consider permissions, costs, safety/privacy perceptions and community acceptance for deploying drone activities.

Mosquito community composition and abundance at contrasting sites in northern South Africa, 2014-2017

Most data on species associations and vector potential of mosquitoes in relation to arboviral infections in South Africa date back from the 1940s to late 1990s. Contextual information crucial for disease risk management and control, such as the sampling effort, diversity, abundance, and distribution of mosquitoes in large parts of South Africa still remains limited. Adult mosquitoes were collected routinely from two horse farms in Gauteng Province; two wildlife reserves in Limpopo Province, at Orpen Gate in Kruger National Park (KNP) and Mnisi Area in Mpumalanga Province between 2014-2017, using carbon dioxide-baited light and tent traps. Mosquito diversity and richness are greater in untransformed natural and mixed rural settings. In untransformed wilderness areas, the most dominant species were Culex poicilipes, Anopheles coustani, and Aedes mcintoshi, while in mixed rural settings such as the Mnisi area, the two most abundant species were Cx. poicilipes and Mansonia uniformis. However, in peri-urban areas, Cx. theileri, Cx. univittatus, and Cx. pipiens sensu lato were the most dominant. Aedes aegypti, Ae. mcintoshi, Ae. metallicus, Ae. vittatus, Cx. pipiens s.l., Cx. theileri, and Cx. univittatus had the widest geographical distribution in northern South Africa. Also collected were Anopheles arabiensis and An. vaneedeni, both known malaria vectors in South Africa. Arbovirus surveillance and vector control programs should be augmented in mixed rural and peri-urban areas where the risk for mosquito-borne disease transmission to humans and domestic stock is greater.

Oviposition Behavior and Distribution of Eucryptorrhynchus scrobiculatus and E. brandti (Coleoptera: Curculionidae) on Ailanthus altissima (Mill.)

(1) Eucryptorrhynchus scrobiculatus Motschulsky (Coleoptera: Curculionidae: Cryptorrhychinae) is a major quarantine forest pest in China. It often co-occurs with E. brandti (Coleoptera: Curculionidae: Cryptorrhychinae) on a single host Ailanthus altissima (Mill.) Swingle (tree of heaven). (2) In this study, to explain the coexistence of the two weevils on a single host, we investigated the oviposition behavior of E. scrobiculatus and oviposition sites of E. scrobiculatus and E. brandti under afield and laboratory conditions. (3) The characteristic behaviors of E. scrobiculatus females prior to oviposition included searching, locating, excavation, turning, locating the oviposition cavity, egg deposition, and hiding. (4) The oviposition sites used by E. scrobiculatus and E. brandti differed. Eucryptorrhynchus scrobiculatus females laid eggs in the soil near A. altissima and compound leaf petioles, while E. brandti females laid eggs in A. altissima trunks. The eggs in compound leaf petioles did not hatch in the field. (5) Eucryptorrhynchus scrobiculatus and E. brandti utilized different oviposition sites and these differences in habitat use may reduce the competition for resources between species during the larval period, thus facilitating their coexistence on A. altissima.

Vertical Habitat Stratification in Sympatric and Allopatric Populations of Aedes hendersoni and Aedes triseriatus (Diptera: Culicidae)

Vertical habitat stratification in populations of Aedes hendersoni Cockerell (Diptera: Culicidae) and Aedes triseriatus (Say) (Diptera: Culicidae) has been observed to varying degrees throughout the species' sympatric range, and potential causes of the phenomenon, including species competition and interaction, have been debated extensively. Stratification patterns in oviposition in allopatric, sympatric, and marginally sympatric populations of both species were investigated and compared in this research to detect any pattern differences related to species composition. Expected patterns were observed in sympatric populations, with Ae. hendersoni preferentially ovipositing in canopy habitats, whereas Ae. triseriatus preferred basal habitats. Allopatric populations presented a strong shift toward basal preference in the former and a slighter but significant shift toward canopy in the latter. Marginal populations of Ae. hendersoni showed intermediate height preferences, whereas preferences of marginal and sympatric Ae. triseriatus did not differ. The convergence of habitat selection in allopatric populations and corresponding divergence in sympatric populations support interspecific competition-based hypotheses regarding the origin of the stratification phenomenon, although plausible alternative or contributing explanations are numerous and warrant further study.

Mosquitoes of field and forest: the scale of habitat segregation in a diverse mosquito assemblage

Knowledge of the distribution of arthropod vectors across a landscape is important in determining the risk for vector-borne disease. This has been well explored for ticks, but not for mosquitoes, despite their importance in the transmission of a variety of pathogens. This study examined the importance of habitat, habitat edges, and the scale at which mosquito abundance and diversity vary in a rural landscape by trapping along transects from grassland areas into forest patches. Significant patterns of vector diversity and distinct mosquito assemblages across habitats were found. The scale of individual species' responses to habitat edges was often dramatic, with several species rarely straying even 10 m from the edge. The present results suggest blood-seeking mosquito species are faithful to certain habitats, which has consequences for patterns of vector diversity and risk for pathogen transmission. This implies that analysts of risk for pathogen transmission and foci of control, and developers of land management strategies should assess habitat at a finer scale than previously considered.

Spatio-Temporal Distribution of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) and Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae) in a Cotton Production Area

Helicoverpa armigera (Hübner) and Pectinophora gossypiella (Saunders) are major pests of cotton in Greece and elsewhere. Analysing male captures in pheromone traps over two seasons, in two cotton producing sites in central Greece, the spatial and temporal variation in population dynamics were examined. In 2007, captures of male H. armigera increased in late July and maintained at high levels for 1 month and declined at the end of August. For P. gossypiella, male captures remained at low levels during summer, increased late in August, peaked at mid of September and declined toward the end of the season. In 2008, trap captures of both species increased sharply by the end of June and remained at relatively high levels until August and September for P. gossypiella and H. armigera, respectively. Spatial analysis produced a spatial trend map over space, a temporal stability map over time and a spatial and temporal trend map for both species, which could lead in separating the field into management zones, and direct control to areas that exhibit high densities of the pest population and are stable over time.

The Interactive Role of Immature Stage Competition, Cohort Overlap, and Resource Limitation on the Population Viability of the Treehole Mosquito Aedes triseriatus (Diptera: Culicidae)

The size and success of epidemiologically significant adult mosquito populations are inherently tied to the conditions of the aquatic habitat in which juvenile stages grow until eclosion. While resource competition and quality are well-established controls to juvenile growth and survival, the implications to overall population rates of increase are less understood due to the large sample sizes needed to parameterize population models for all five juvenile life stages under multiple environmental and demographic conditions. Here, we present the results of >4,300 trials of wild-caught Aedes triseriatus (Say, 1823) larvae and pupae reared under varying resource quantity crossed by the presence or absence of competition within a single cohort as well as multiple overlapping cohorts. Demographic projection was used to make predictions of the realized growth rates of simulated Ae. triseriatus populations across the range of potential Ae. triseriatus fecundity. Further, to inform control efforts on juvenile habitat, we constructed a stochastic simulation to estimate the rates of successful emergence from habitats under different resource regimes and levels of cohort overlap. We found that while Ae. triseriatus populations were robust to low resource levels and competition within a cohort, the combination of these stressors with multiple cohort overlap led to self-limitation or complete collapse of mosquito populations. Despite this importance of intraspecific competition to population viability, the stochastic simulation revealed only a modest self-limitation of adult emergence, with the clear implication that high-resource habitats are a higher value control target.

Oviposition Site Selection Structures Niche Partitioning Among Coccinellid Species in a Tropical Ecosystem

The competitive exclusion hypothesis suggests that coexisting related species using similar resources in nature should partition their realized niches. This hypothesis has direct implications for conservation strategies using biological control, taking into consideration the shifts caused by the introduction of natural enemies in a local community. Such introductions typically lead to disruptions in species interactions and interfere with community structure. In this study, we asked whether community structure of aphidophagous lady beetles is determined by the distribution of specific plants and aphids. To answer this question, we describe the distribution patterns of lady beetles (adults, larvae, and egg clusters) relative to plants and aphids in eight crop ecosystems in a central region of Brazil. We used canonical correspondence analysis to evaluate lady beetle distribution relative to selected habitat variables. Cycloneda sanguinea L., Hippodamia convergens Guérin-Méneville, Harmonia axyridis Pallas, and Eriopis connexa Germar (Coleoptera: Coccinellidae) differed in their use of plants and aphids. The association of egg clusters with specific plants/aphids was stronger than that of larvae or adults. In conclusion, lady beetle species occupied different niches, indicating different patterns of habitat use that may facilitate their coexistence in crop ecosystems. Furthermore, immature individuals had more specific environmental associations than adults, likely because female choice of oviposition sites influences their distribution and thus lady beetle community structure.

Hump-shaped density-dependent regulation of mosquito oviposition site-selection by conspecific immature stages: theory, field test with Aedes albopictus, and a meta-analysis

Oviposition site selection by gravid females is an important determinant of the distribution, abundance, and dynamics of dipteran hematophagous insects. The presence of conspecific immature stages in a potential oviposition site could, on the one hand, indicate the suitability of that site but on the other hand could indicate the potential for intraspecific competition. In this paper, we present a graphic model suggesting that the trade-off between these two opposing forces could result in a hump-shaped density-dependent relationship between oviposition rate and conspecific immature stage density (hereafter, the "Hump-shaped regulation model") with positive effects of aggregation prevailing at low densities and negative effect of intraspecific competition prevailing at higher densities. We field-tested the predictions of this model at both the egg- and the larval levels with Aedes albopictus and evaluated if and how these relationships are affected by resource enrichment. We found support for the hump-shaped regulation model at both the larval and the egg levels. Using oviposition cups containing varying numbers of conspecific larvae, we showed that the oviposition activity of Ae. albopictus first increases and then decreases with larvae number. Medium enrichment resulted in higher hatching rate, and demonstrated linear relations for the no-enrichment treatment where larvae density range was low and hump-shaped relationship for the enriched medium that had a wider larvae density range. Using pairs of oviposition cups, we showed that at low egg densities mosquitoes laid more eggs on substrates containing pre-existing eggs. However, at higher egg densities, mosquitoes laid more eggs on a virgin substrate. Based on our results and on a meta-analysis, we suggest that due to study design or methodological shortcomings the hump-shaped regulation model is often left undetected and that it is likely to be more common than currently thought.

Modelling the spread of Wolbachia in spatially heterogeneous environments

The endosymbiont Wolbachia infects a large number of insect species and is capable of rapid spread when introduced into a novel host population. The bacteria spread by manipulating their hosts' reproduction, and their dynamics are influenced by the demographic structure of the host population and patterns of contact between individuals. Reaction-diffusion models of the spatial spread of Wolbachia provide a simple analytical description of their spatial dynamics but do not account for significant details of host population dynamics. We develop a metapopulation model describing the spatial dynamics of Wolbachia in an age-structured host insect population regulated by juvenile density-dependent competition. The model produces similar dynamics to the reaction-diffusion model in the limiting case where the host's habitat quality is spatially homogeneous and Wolbachia has a small effect on host fitness. When habitat quality varies spatially, Wolbachia spread is usually much slower, and the conditions necessary for local invasion are strongly affected by immigration of insects from surrounding regions. Spread is most difficult when variation in habitat quality is spatially correlated. The results show that spatial variation in the density-dependent competition experienced by juvenile host insects can strongly affect the spread of Wolbachia infections, which is important to the use of Wolbachia to control insect vectors of human disease and other pests.

Estimation of population size and dispersal of Aedes polynesiensis on Toamaro motu, French Polynesia

Mark-release-recapture methods were used to compare Aedes polynesiensis Marks adult numbers and dispersal between dry and wet seasons in a closed population on a small island (motu) in French Polynesia. Females were more than three times more common during wet (December 2008) than dry (May 2007) season samplings although high numbers of vectors were collected during both seasons. Lincoln-Petersen estimates for Ae. polynesiensis females on the motu were 6,055 per hectare for the dry season and 18,860 per hectare for the wet season. Marked females dispersed rapidly to all parts of the motu and survived until recaptures on days 1-5 after release. Males were not adequately sampled using human sentinels or Biogent Sentinel traps.

Invasive plant alters ability to predict disease vector distribution

Risk models for vector-borne diseases rely on accurate quantification of the relationship between vector abundance and habitat, but this relationship can be obscured if habitats are modified by invasive species in ways that alter vector behavior but are undetectable in remotely sensed data. At a forest in eastern Missouri we assessed whether the presence of an invasive shrub, Amur honeysuckle, Lonicera maackii, affects oviposition by treehole mosquitoes, Aedes triseriatus, a primary vector of La Crosse virus in the United States. Oviposition significantly decreased with increasing density of L. maackii. Moreover, our results indicate that L. maackii may hinder the efficacy of models that use remotely sensed data to predict vector abundance: there was a strong relationship between landscape composition around plots and oviposition, but only in plots not invaded by L. maackii. Overlooking potentially important but cryptic effects of invasive plants on habitat selection by vectors may undermine accurate forecasting of disease risk.

Stable and unstable malaria hotspots in longitudinal cohort studies in Kenya

BACKGROUND

Infectious diseases often demonstrate heterogeneity of transmission among host populations. This heterogeneity reduces the efficacy of control strategies, but also implies that focusing control strategies on "hotspots" of transmission could be highly effective.

METHODS AND FINDINGS

In order to identify hotspots of malaria transmission, we analysed longitudinal data on febrile malaria episodes, asymptomatic parasitaemia, and antibody titres over 12 y from 256 homesteads in three study areas in Kilifi District on the Kenyan coast. We examined heterogeneity by homestead, and identified groups of homesteads that formed hotspots using a spatial scan statistic. Two types of statistically significant hotspots were detected; stable hotspots of asymptomatic parasitaemia and unstable hotspots of febrile malaria. The stable hotspots were associated with higher average AMA-1 antibody titres than the unstable clusters (optical density [OD] = 1.24, 95% confidence interval [CI] 1.02-1.47 versus OD = 1.1, 95% CI 0.88-1.33) and lower mean ages of febrile malaria episodes (5.8 y, 95% CI 5.6-6.0 versus 5.91 y, 95% CI 5.7-6.1). A falling gradient of febrile malaria incidence was identified in the penumbrae of both hotspots. Hotspots were associated with AMA-1 titres, but not seroconversion rates. In order to target control measures, homesteads at risk of febrile malaria could be predicted by identifying the 20% of homesteads that experienced an episode of febrile malaria during one month in the dry season. That 20% subsequently experienced 65% of all febrile malaria episodes during the following year. A definition based on remote sensing data was 81% sensitive and 63% specific for the stable hotspots of asymptomatic malaria.

CONCLUSIONS

Hotspots of asymptomatic parasitaemia are stable over time, but hotspots of febrile malaria are unstable. This finding may be because immunity offsets the high rate of febrile malaria that might otherwise result in stable hotspots, whereas unstable hotspots necessarily affect a population with less prior exposure to malaria.

Environmental correlates of abundances of mosquito species and stages in discarded vehicle tires

Discarded vehicle tires are a common habitat for container mosquito larvae, although the environmental factors that may control their presence or abundance within a tire are largely unknown. We sampled discarded vehicle tires in six sites located within four counties of central Illinois during the spring and summer of 2006 to determine associations between a suite of environmental factors and community composition of container mosquitoes. Our goal was to find patterns of association between environmental factors and abundances of early and late instars. We hypothesized that environmental factors correlated with early instars would be indicative of oviposition cues, whereas environmental factors correlated with late instars would be those important for larval survival. We collected 13 species of mosquitoes, with six species (Culex restuans, Cx. pipiens, Aedes albopictus, Cx. salinarius, Ae. atropalpus, and Ae. triseriatus) accounting for r95% of all larvae. There were similar associations between congenerics and environmental factors, with Aedes associated with detritus type (fine detritus, leaves, seeds) and Culex associated with factors related to the surrounding habitat (human population density, canopy cover, tire size) or microorganisms (bacteria, protozoans). Although there was some consistency in factors that were important for early and late instar abundance, there were few significant associations between early and late instars for individual species. Lack of correspondence between factors that explain variation in early versus late instars, most notable for Culex, suggests a difference between environmental determinants of oviposition and survival within tires. Environmental factors associated with discarded tires are important for accurate predictions of mosquito occurrence at the generic level.