The importance of accounting for larval detectability in mosquito habitat-association studies

Distribution and dynamics of Anopheles gambiae s.l. larval habitats in three Senegalese cities with high urban malaria incidence

Urban malaria has become a challenge for most African countries due to urbanization, with increasing population sizes, overcrowding, and movement into cities from rural localities. The rapid expansion of cities with inappropriate water drainage systems, abundance of water storage habitats, coupled with recurrent flooding represents a concern for water-associated vector borne diseases, including malaria. This situation could threaten progress made towards malaria elimination in sub-Saharan countries, including Senegal, where urban malaria has presented as a threat to national elimination gains. To assess drivers of urban malaria in Senegal, a 5-month study was carried out from August to December 2019 in three major urban areas and hotspots for malaria incidence (Diourbel, Touba, and Kaolack) including the rainy season (August-October) and partly dry season (November-December). The aim was to characterize malaria vector larval habitats, vector dynamics across both seasons, and to identify the primary eco- environmental entomological factors contributing to observed urban malaria transmission. A total of 145 Anopheles larval habitats were found, mapped, and monitored monthly. This included 32 in Diourbel, 83 in Touba, and 30 in Kaolack. The number of larval habitats fluctuated seasonally, with a decrease during the dry season. In Diourbel, 22 of the 32 monitored larval habitats (68.75%) were dried out by December and considered temporary, while the remaining 10 (31.25%) were classified as permanent. In the city of Touba 28 (33.73%) were temporary habitats, and of those 57%, 71% and 100% dried up respectively by October, November, and December. However, 55 (66.27%) habitats were permanent water storage basins which persisted throughout the study. In Kaolack, 12 (40%) permanent and 18 (60%) temporary Anopheles larval habitats were found and monitored during the study. Three malaria vectors (An. arabiensis, An. pharoensis and An. funestus s.l.) were found across the surveyed larval habitats, and An. arabiensis was found in all three cities and was the only species found in the city of Diourbel, while An. arabiensis, An. pharoensis, and An. funestus s.l. were detected in the cities of Touba and Kaolack. The spatiotemporal observations of immature malaria vectors in Senegal provide evidence of permanent productive malaria vector larval habitats year-round in three major urban centers in Senegal, which may be driving high urban malaria incidence. This study aimed to assess the presence and type of anopheline larvae habitats in urban areas. The preliminary data will better inform subsequent detailed additional studies and seasonally appropriate, cost-effective, and sustainable larval source management (LSM) strategies by the National Malaria Control Programme (NMCP).

Distribution of Culex pipiens life stages across urban green and grey spaces in Leiden, The Netherlands

BACKGROUND

There is an urgent need for cities to become more climate resilient; one of the key strategies is to include more green spaces in the urban environment. Currently, there is a worry that increasing green spaces might increase mosquito nuisance. As such, this study explores a comprehensive understanding of how mosquitoes utilise contrasting grey and green habitats at different life stages and which environmental factors could drive these distributions.

METHODS

We used a setup of six paired locations, park (green) vs. residential (grey) areas in a single model city (Leiden, The Netherlands), where we sampled the abundances of different mosquito life stages (eggs, larvae, adults) and the local microclimatic conditions. In this study, we focused on Culex pipiens s.l., which is the most common and abundant mosquito species in The Netherlands.

RESULTS

Our results show that while Cx. pipiens ovipositioning rates (number of egg rafts) and larval life stages were far more abundant in residential areas, adults were more abundant in parks. These results coincide with differences in the number of suitable larval habitats (higher in residential areas) and differences in microclimatic conditions (more amenable in parks).

CONCLUSIONS

These findings suggest that Cx. pipiens dispersal may be considerably more important than previously thought, where adult Cx. pipiens seek out the most suitable habitat for survival and breeding success. Our findings can inform more targeted and efficient strategies to mitigate and reduce mosquito nuisance while urban green spaces are increased, which make cities more climate resilient.

Birds differentially prioritize visual and olfactory foraging cues depending on habitat of origin and sex

Animals interpret their environment by combining information from multiple senses. The relative usefulness of different senses may vary between species, habitats and sexes; yet, how multimodal stimuli are integrated and prioritized is unknown for most taxa. We experimentally assessed foraging preferences of great tits (Parus major) to test whether urban and forest individuals prioritize visual and olfactory cues differently during foraging. We trained 13 wild-caught birds to associate multimodal (colour + odour) cues with a food reward and assessed their foraging preferences in a cue-separation test. In this, the birds could choose between the multimodal training cue and its olfactory or visual components. Our results suggest that the birds did not perceive multimodal cues in an integrated way, as their response was not stronger than for unimodal cue components. Urban birds preferred olfactory cues, while forest birds preferred visual cues. Nevertheless, female birds preferred the multimodal cue, while males foraged more randomly with respect to which cue was present. These findings contribute to our understanding of the relative roles of vision and olfaction in bird foraging behaviour. Future work should focus on how habitat- and sex-specific sensory prioritization modifies bird foraging behaviour and foraging success in the context of urban adaptations across populations.

Viral infection changes the expression of personality traits in an insect species reared for consumption

Disease-induced personality change results from endogenous and adaptive host responses or parasitic manipulation. Within animal husbandry systems understanding the connection between behaviour and disease is important for health monitoring and for designing systems considerate to animal welfare. However, understanding these relationships within insect mass-rearing systems is still in its infancy. We used a simple repeated behavioural-emergence test to examine parasite-induced differences in group personality traits in the house cricket Acheta domesticus, by comparing the behaviours of 37 individuals infected with the Acheta domesticus densovirus (AdDV) and 50 virus-free individuals. AdDV-infected animals had a much lower emergence probability, longer times until emergence, and did not change their behaviour with experience compared to the virus-free animals. AdDV-infected animals also had lower variation in their probability of emergence within the population, most likely related to animals displaying a relatively uniform sickness response. These infected animals also had higher variation in their response to experimental trial experience; this greater variation resulted from a difference between males and females. Infected females responded to experience in a similar way as virus-free animals, while AdDV-infected males showed a response to experience in the opposite direction: i.e., while all other groups reduced emergence time with experience, infected males always increased their mean emergence time as trials progressed. Our results are important not only in the context of animal personality research, but also with regards to creating husbandry systems and disease monitoring within the insects-as-food industry that are considerate to both production traits and animal welfare.

Gene flow relates to evolutionary divergence among populations at the range margin

Background

Morphological differentiation between populations resulting from local adaptations to environmental conditions is likely to be more pronounced in populations with increasing genetic isolation. In a previous study a positive clinal variation in body size was observed in isolated Roesel’s bush-cricket, Metrioptera roeselii, populations, but were absent from populations within a continuous distribution at the same latitudinal range. This observational study inferred that there was a phenotypic effect of gene flow on climate-induced selection in this species.

Methods

To disentangle genetic versus environmental drivers of population differences in morphology, we measured the size of four different body traits in wild-caught individuals from the two most distinct latitudinally-matched pairs of populations occurring at about 60°N latitude in northern Europe, characterised by either restricted or continuous gene flow, and corresponding individuals raised under laboratory conditions.

Results

Individuals that originated from the genetically isolated populations were always bigger (femur, pronotum and genital appendages) when compared to individuals from latitudinally-matched areas characterised by continuous gene flow between populations. The magnitude of this effect was similar for wild-caught and laboratory-reared individuals. We found that previously observed size cline variation in both male and female crickets was likely to be the result of local genetic adaptation rather than phenotypic plasticity.

Conclusions

This strongly suggests that restricted gene flow is of major importance for frequencies of alleles that participate in climate-induced selection acting to favour larger phenotypes in isolated populations towards colder latitudes.

An Algal Diet Accelerates Larval Growth of Anopheles gambiae (Diptera: Culicidae) and Anopheles arabiensis (Diptera: Culicidae)

The population sizes of Anopheles gambiae Giles (Diptera: Culicidae) and Anopheles arabiensis Patton (Diptera: Culicidae) increase dramatically with the onset of the rainy season in sub-Saharan Africa, but the ecological mechanisms underlying the increases are not well understood. As a first step toward to understand, we investigated the proliferation of algae, the major food of mosquito larvae, in artificial fresh water bodies exposed to sunlight for a short period, and old water bodies exposed to sunlight for a long period, and the effects thereof on the development of these anopheline larvae. We found that an epizoic green algal species of the genus Rhopalosolen (Chlorophyta: Chlorophyceae) proliferated immediately after water freshly taken from a spring was placed in sunlight. This alga proliferated only briefly (for ~10 d) even if the water was repeatedly exposed to sunlight. However, various algal species were observed in water that remained under sunlight for 40 d or longer (i.e., in old water bodies). The growth performance of larvae was higher in sunlight-exposed (alga-rich) water than in shade-stored (alga-poor) water. Stable isotope analysis suggested that these two anopheline species fed on Rhopalosolen algae in fresh water bodies but hardly at all on other algae occurring in the old water bodies. We concluded that freshly formed ground water pools facilitate high production of anopheline species because of the proliferation of Rhopalosolen algae therein, and the increase in the number of such pools in the rainy season, followed by rapid increases in A. gambiae and A. arabiensis numbers.