Environmental Factors That Regulate Mosquito Physiology and Behavior

Abstract

Mosquitoes remain the greatest threat to global human health because they transmit pathogens to humans and other animals when females imbibe a blood meal. Disease transmission is restricted temporally and spatially because not all seasons or habitats support mosquito growth, development, host seeking, and reproduction. Temperate mosquitoes respond to photoperiod by entering states of arrested development to survive harsh winter conditions. Additionally, temperature profoundly influences mosquito development, host seeking, and reproductive processes, as well as pathogen replication. Recent research is uncovering how humidity affects mosquito host-seeking and oviposition behavior. Researchers are also gaining an understanding of how light pollution and high temperatures in cities impact mosquito physiology and behavior. Future studies characterizing the interactions among multiple environmental factors will allow researchers to better predict how mosquitoes are responding to increasing urbanization and climate change, to develop novel control measures, and to better direct interventions to limit disease transmission.

1. INTRODUCTION

As hundreds of thousands of people die each year from diseases caused by mosquito-borne pathogens, mosquitoes are the undisputed deadliest animals on Earth (27). More than 17% of all human infections are transmitted by arthropod vectors, and the majority are caused by mosquitoes (34, 134). Specifically, mosquitoes can transmit filarial nematodes that are causative agents of elephantiasis and canine heartworm (1, 62); eukaryotic pathogens that cause malaria (31); and multiple arboviruses, including West Nile, Zika, chikungunya, yellow fever, and dengue viruses (2, 43, 73, 111, 121). Despite more than 130 years of targeted interventions and control measures (reviewed by 100), mosquitoes continue to pose a substantial disease risk to humans, domesticated animals, and wildlife. Several emerging mosquito-borne flaviviruses are lethal to nonhuman animals (reviewed by 103), and 2024 was the worst year on record for human cases of dengue (81).

Mosquito-borne disease transmission is not uniformly distributed across time or space due to both seasonal and spatial variations in abiotic factors such as daylength, temperature, and humidity. These environmental factors profoundly influence several aspects of mosquito physiology and behavior, including the time required for mosquitoes to complete their development from aquatic larvae to terrestrial adults, to mate, and to locate vertebrate hosts and oviposition sites (75, 79, 82, 88, 104, 128). Therefore, it is not surprising that, in temperate environments, West Nile virus transmission peaks in summer, when mosquitoes are most active and abundant (65), and that, in the tropics, seasonal differences in temperature and rainfall contribute to outbreaks of malaria (92).

This article offers an overview of what we know about the environmental regulation of mosquito physiology and behavior while also highlighting exciting new developments and promising areas for future research. Specifically, I discuss research that evaluates how changes in photoperiod, temperature, humidity, and urbanization affect mosquito biology. I also showcase how this knowledge is enabling researchers to better predict spatial and temporal differences in disease transmission and has the potential to improve mosquito control.

2. EFFECTS OF PHOTOPERIOD ON MOSQUITOES

Like all organisms, mosquitoes actively grow and reproduce during seasons when environmental conditions are favorable and resources are plentiful. However, mosquitoes must also contend with inimical seasons when environmental conditions are harsh and resources are scarce or unavailable. They do so by entering either states of temporary dormancy, or quiescence, or a preprogrammed and prolonged state of developmental arrest known as diapause. Although estivation/summer diapause allows tropical mosquitoes to circumvent the dry season (reviewed by 38), diapause has been more extensively studied in temperate mosquitoes. Temperate mosquito species belonging to the genera Culex (45), Aedes (97), and Anopheles (40) enter diapause in response to short days and low temperatures to survive the low temperatures and lack of food that characterize winter. The life stage when mosquitoes enter diapause is population or species specific, but across Culicidae there are species that enter diapause in the egg, larval, and adult stages (reviewed by 38, 39). Some mosquito species diapause as eggs [e.g., the tiger mosquito, Aedes albopictus] or larvae [e.g., the pitcher plant mosquito, Wyeomyia smithii] or cease developing as eggs and/or larvae in the same generation [e.g., Northern populations of the tree hole mosquito, Aedes triseriatus (69)], while in other species only adult female mosquitoes enter diapause in a state of reproductive arrest [e.g., Northern house mosquitoes, Culex pipiens (45)]. Prior to entering diapause, mosquitoes acquire energetic reserves and/or upregulate stress responses (108, 132).

Quiescence:

a state of temporary dormancy that occurs in direct response to an environmental stressor and ceases when the stress ends

Diapause:

a preprogramed state of dormancy, occurring in response to a non-stressful, token stimulus, that persists for a predetermined time

Estivation:

a prolonged period of dormancy that animals, including insects, enter to survive hot and dry seasons

2.1. When Do Mosquitoes Initiate and Terminate Diapause?

Determining when mosquitoes initiate and terminate diapause is important for population management and preventing disease transmission. However, monitoring annual changes in mosquito populations can be difficult, as traditional traps for mosquito surveillance collect adult female mosquitoes that are either host seeking or preparing to lay eggs (32). Therefore, it is difficult to assess when mosquitoes that overwinter as eggs or larvae initiate diapause or when adult females cease host seeking and laying eggs as part of their overwintering dormancy. Surveillance data in southwestern Ontario, Canada, from 1976 demonstrate that populations of Cx. pipiens likely initiated diapause by mid-September and terminated diapause by late May because female mosquitoes were collected in traps between these periods (66). A subsequent study that measured egg follicle lengths of mosquitoes deployed in the field as larvae revealed that female Cx. pipiens in southern Ontario entered diapause by mid-July, with more than 90% of females entering diapause by early August (91). More recently, a semifield study demonstrated that Cx. pipiens enter diapause between late August and late September in Aimes, Iowa, United States (48). However, a field study in Columbus, Ohio, United States, found that nondiapausing Cx. pipiens can persist in natural environments as late as December and that female Cx. pipiens terminated diapause by mid-March (120). Therefore, diapause duration varies with latitude such that mosquito populations from higher latitudes, where winter is longer and more intense (e.g., southern Ontario), enter diapause earlier than those at lower latitudes (e.g., Iowa and Ohio, United States).

Diapause duration is also likely decreasing in response to climate change. Although Mushegian et al. (102) demonstrated that the critical photoperiod that induces diapause in populations of Ae. albopictus from both Japan and the United States is strongly correlated with latitude, these authors also found that diapause incidence in Ae. albopictus is strongly influenced by temperature-specific requirements for growth and development and is informed by the growing degree day diapause deadline that was experienced in the previous year. This suggests that, as autumn temperatures increase due to human-mediated climate change, mosquitoes will enter diapause later in the year. Indeed, later diapause entry has been demonstrated in the pitcher plant mosquito, Wy. smithii (20). Furthermore, the duration of quiescence is also predicted to decrease in the southeastern United States, leading to increased West Nile virus transmission in the spring and fall (98).

Growing degree day diapause deadline:

the latest day in the year when the temperature-specific requirement for embryos to complete development to adulthood would be met

FOXO:

a forkhead transcription factor that regulates dauer formation in nematodes and is upregulated during diapause in several insects, including mosquitoes

2.2. The Hormonal and Genetic Regulation of Diapause in Mosquitoes

Although it is not entirely clear how mosquitoes measure seasonal differences in daylength to appropriately coordinate diapause, all animals have sophisticated circadian clocks that can distinguish and predict daily changes in light and dark (125). These clocks consist of interlocking feedback loops where positive circadian transcription factors (CLOCK and CYCLE) increase the abundance of negative circadian regulators (PERIOD, TIMELESS, and CRYPTOCHROME2) that then inhibit CLOCK:CYCLE activity (reviewed by 56). Whether these same clock genes also allow mosquitoes and other insects to distinguish long, summer days from short, winter days, and thereby also properly coordinate seasonal changes in activity and development, has been studied in Cx. pipiens and Wy. smithii, and much of this work is described below. Researchers are also beginning to understand the molecular and hormonal regulation of diapause in Ae. albopictus.

Diapause in Cx. pipiens is characterized by low levels of juvenile hormone (JH) (124) and by low levels of insulin signaling (116, 118). This is important because typically insulin signaling inhibits the activity of the forkhead transcription factor FOXO such that, under short-day conditions, when insulin signaling is low, FOXO is able to upregulate genes that increase stress responses and other diapause phenotypes (119). A functional circadian clock is necessary for female Cx. pipiens to measure daylength, as knocking down transcripts of negative circadian regulators (period, timeless, or cryptochrome2) with RNA interference (RNAi) caused short-day-reared females to avert diapause (95). More recently, Dhungana et al. (41) found that the circadian transcription factor CYCLE regulates expression of the neuropeptide allatotropin, which stimulates the corpora allata to produce JH (Figure 1a). Additionally, PAR-DOMAIN PROTEIN 1 (PDP1), a transcription factor in the stabilizing feedback loop of the circadian clock, regulates several genes, including inhibitors of insulin signaling (42). Moreover, knocking down pdp1 transcripts prevented short-day-reared Cx. pipiens from accumulating fat reserves (28). Taken together, these studies provide compelling evidence that clock components are connected to hormonal regulators of diapause in Cx. pipiens (Figure 1a).

Figure 1.

Hormonal (a) and molecular (b) regulation of diapause in the Northern house mosquito, Culex pipiens. (a) Under long days, the circadian transcription factor CYC binds to and upregulates the production of the neuropeptide allatotropin, leading to an increase in JH and a reduction in insulin signaling, which then leads to ovarian development and small fat bodies. Under short-day conditions, the circadian transcription factor PDP1 upregulates production of ptp99a and ipp1, which suppress insulin signaling and JH levels, resulting in undeveloped egg follicles and fat hypertrophy. (b) FOXO is a master regulator of many diapause phenotypes. Specifically, FOXO increases transcripts of glycogen synthase, atp-bct, and low-density lipoprotein receptor chaperone; the corresponding proteins, in combination with PEPCK, allow diapause-destined female mosquitoes to store and maintain sufficient energy reserves. FOXO also upregulates transcripts of PDZ, which is involved in fortifying the midgut against low-temperature stress, and oxidor, which combats ROS. Abbreviations: atp-bct, atp-binding cassette transporter; ipp1, inositol polyphosphate 1-phosphatase; JH, juvenile hormone; PEPCK, phosphoenolpyruvate carboxykinase; ptp99a, protein tyrosine phosphatase 99a; ROS, reactive oxygen species.

RNA interference (RNAi):

a functional genetic technique wherein double-stranded RNA is introduced into an organism to specifically reduce/knock down target mRNA abundance

Wy. smithii overwinter as third- or fourth-instar larvae within pitcher plants (21). The hormonal regulation underlying diapause in this species is not clear but is likely caused by low levels of ecdysteroids that prevent molting to the next life stage, because ecdysteroids are the common mechanism by which diapause is regulated in immature life stages across diverse taxa (reviewed by 36, 37). Additionally, Wy. smithii require short photoperiods to initiate diapause as well as long photoperiods to terminate diapause (122). Initial experiments with quantitative trait loci (QTLs) in Wy. smithii uncovered specific loci correlated with the stage of diapause and time of year that larvae enter diapause (93). The researchers also found a single locus that interacted epistatically with the circadian clock gene timeless, but as this gene was not located near any QTL associated with diapause, these results suggested that the clock may not be involved in the evolution of photoperiodic responses in Wy. smithii (93).

Quantitative trait loci (QTLs):

genomic regions associated with variable traits within a population; these are identified by crossing multiple lines and generating linkage maps

Recently, Bradshaw et al. (18) explored the relationship between the circadian clock and diapause initiation in populations of Wy. smithii collected across a large geographic range. Before initiating experiments, they reared field-collected larvae for four or more generations in a common garden to mitigate field effects. Experimentally, diapausing larvae were exposed to a 10-h short day, followed by night lengths of 14, 38, or 62 h scanned in separate experiments with a 1-h or 2-h pulse of white light. These Bunsöw night-interruption experiments (25) allow researchers to determine whether the circadian clock is involved in photoperiodic time measurement because, when the light pulse occurs during a photoinducible phase, organisms misinterpret daylength as being long instead of short and inappropriately avert or terminate diapause (129). Bradshaw et al. (18) demonstrated that the circadian clock regulates diapause termination in several populations of Wy. smithii, particularly those from ancestral, southern populations. However, more recently derived, northern and mountain populations display less or no evidence for circadian regulation of the photoperiodic diapause response. These results show that the connection between the circadian clock and the photoperiodic timer varies geographically and across evolutionary time. Overall, this work underscores the importance of assessing connections between the circadian clock and seasonal responses in multiple populations of the same species.

Photoinducible phase:

a time period late in the night that, when illuminated, causes organisms to interpret a daylength as long

Adult Ae. albopictus females perceive short days and produce diapausing embryos that overwinter as completely developed, pharate first-instar larvae within the chorion of their egg (97, 141). Relative to other species, much less is known about how female Ae. albopictus measure and interpret daylength or translate this information to their offspring (reviewed by 86). However, diapausing embryos of Ae. albopictus have lower levels of dopamine and tyramine, and low levels of JHIII regulate embryonic diapause in Ae. albopictus (11, 12). Additionally, transcripts of a paralog of idgf4 (imaginal disc growth factor 4) were more abundant in Ae. albopictus embryos from temperate and tropical populations that were exposed to diapause-inducing conditions (44), suggesting that this gene may regulate diapause. Boyle et al. (16) identified 11 genes that are associated with single-nucleotide polymorphisms that differ between diapausing and nondiapausing populations of Ae. albopictus and therefore might also regulate diapause. Collectively, these studies enhance our understanding of the transgenerational control of diapause in Ae. albopictus. Future research will likely reveal the hormonal regulators of diapause in this important disease vector and whether they are connected to the circadian clock.

Single-nucleotide polymorphisms:

single, point changes in DNA sequences that can be compared across populations to identify genomic regions that have undergone selection

2.3. Molecular Changes That Allow Mosquitoes to Combat Winter Stressors

Once mosquitoes enter diapause, they undergo several physiological and behavioral changes that allow them to conserve metabolic resources and cope with external and internal stresses. These have been particularly well described by Denlinger & Armbruster (39). Here, I discuss recent studies and highlight emerging mechanisms that allow multiple mosquito species to combat stresses associated with winter.

Food resources are not available during winter. Therefore, it is not surprising that, prior to entering diapause, temperate mosquitoes upregulate genes involved in carbohydrate and lipid storage and then downregulate these transcripts during diapause (39). Specifically, diapausing embryos of Ae. albopictus have higher levels of diacyl and triglycerides (11). Additionally, knockdown of glycogen synthase, atp-binding cassette transporter, and low-density lipoprotein receptor chaperone, all of which are regulated by FOXO, prevented females of Cx. pipiens from storing sufficient levels of glycogen and lipids (105). More recently, RNAseq revealed that several transcripts, including the phosphoenolpyruvate carboxykinase (pepck) transcript, are upregulated in the fat body of diapausing female Cx. pipiens (142). This pepck transcript was also upregulated in embryos of Ae. albopictus from tropical populations that were exposed to diapause-inducing conditions (44), and pepck is upregulated during diapause in several other insects (33, 123, 149).

In addition to conserving metabolic resources, diapausing mosquitoes must also combat external (e.g., low temperatures, humidity) and internal (e.g., reactive oxygen species) stressors. The exoskeletons of diapausing mosquitoes are more resistant to water loss (14, 67). Recently, King et al. (78) found that diapausing females of Cx. pipiens upregulate expression of a protein containing PDZ and LIM domains. These domains promote protein:protein interactions and can act as scaffolds to direct signaling molecules to specific regions of cells (85), demonstrating that this PDZ protein likely allows diapausing Cx. pipiens to combat low temperatures. Indeed, knocking down pdz transcripts with RNAi reduced actin accumulation in the midgut and shortened female lifespan (78). Diapausing Cx. pipiens also mitigate and avoid damage caused by reactive oxygen species by upregulating transcripts encoding catalase and superoxide dismutase (117), as well as a transcript encoding an oxidoreduction-like protein (OXIDOR) (77). Importantly, both PDZ and oxidor are targets of FOXO (119), underscoring the importance of this transcription factor as a master regulator of the diapause program in Cx. pipiens and likely other mosquitoes (Figure 1b).

3. EFFECTS OF TEMPERATURE ON MOSQUITO BEHAVIOR AND PHYSIOLOGY

As mosquitoes are ectothermic organisms, it is not surprising that temperature profoundly affects mosquito development and life history traits (see recent reviews by 3, 99). Therefore, I offer a brief overview of how temperature interacts with other environmental factors to affect specific aspects of mosquito biology (Figure 2a) and how this information is being leveraged to enhance our predictions of how climate change will affect mosquitoes and disease transmission.

Figure 2.

(a) Seasonal changes in daylength, along with temperature, can cause species of temperate mosquitoes to enter an arrested state of development or diapause in the egg, larval, or adult phases. Temperature on its own, and in conjunction with daylength and humidity, can accelerate or suppress mosquito development, mating, blood feeding, and oviposition. (b) Light pollution, high temperatures, and low humidity in urban environments interact to increase the abundance of vector mosquitoes, to change daily and seasonal patterns of mosquito biting behavior, and to stimulate mosquito growth and pathogen replication, leading to increased disease transmission.

3.1. Temperature and Mosquito Development

Temperature can act in conjunction with photoperiod, diet, and intraspecific competition to substantially affect mosquito development (Figure 2a). Specifically, while photoperiod is the primary signal that regulates diapause entry in temperate mosquitoes, temperature can modulate the effects of daylength (reviewed by 37). For example, chilling diapausing larvae of elephant mosquitoes, Toxorhynchites rutilis, caused them to terminate diapause earlier when days were shorter (19). High temperatures increase the metabolic rate of mosquito larvae, pupae, and adults (5, 59), and therefore high temperatures in winter are expected to cause diapausing mosquitoes to deplete energetic reserves prematurely. However, higher temperatures during winter increased survival of diapausing Ae. albopictus embryos for reasons that are not entirely clear, but such temperatures may have allowed the pharate first-instar larvae to repair damage from cold exposure (126). Additionally, diet and temperature can interact to influence mosquito development time, survival, sex ratio, adult body size, heat tolerance, and teneral energy reserves such that some diets and temperatures can promote faster larval development of Ae. aegypti while others can increase developmental time and affect traits such as adult body size, thermal performance, energy reserves, and fecundity (46, 114). Furthermore, exposing mosquito embryos and larvae to high temperatures causes genotypic males to develop into females (6, 24). As female mosquitoes bite and transmit disease, the potential effects of climate change to increase the proportion of female mosquitoes are especially concerning.

3.2. Effects of Temperature on Mosquito Reproductive Physiology and Behavior

The effects of temperature on mosquito mating behavior and reproductive physiology have not been explored as extensively as the impact of temperature on mosquito development. Males of Ae. aegypti adjust their flight tones to harmonize and then mate with females (26). While sperm production and ovariole maturation occur over a broad range of temperatures (17–35°C) in Ae. aegypti (9), the wing beat frequencies of female Ae. aegypti are highly dependent and positively correlated with temperature (136). Bader & Williams (9) found that male Ae. aegypti successfully mate over temperatures ranging from 25°C to 35°C, with equal or higher numbers of inseminated females at 35°C. Although sperm number was positively correlated with temperature in Ae. aegypti, the number of ovarioles per female decreased as temperatures increased, suggesting that reproductive processes in female mosquitoes are likely more thermosensitive than those in males. A slightly different effect on the influence of temperature on male mating performance has been observed in Anopheles gambiae; specifically, large and small males that were exposed to lower temperatures (23°C) lived longer and were able to inseminate more females than males reared at higher temperatures (27°C), especially if males were provisioned with a sugar meal (55). Overall, these studies demonstrate that, although temperature can affect mating performance, well-nourished mosquitoes are equipped to reproduce across a broad range of temperatures.

3.3. Effects of Temperature on Host Seeking and Disease Transmission

Ambient temperature also affects host-seeking responses in female mosquitoes (reviewed by 107, 127). Temperature affects host seeking in part because, as ectotherms, mosquitoes operate within a range of temperatures such that energetically demanding activities, like oriented flight, occur at species- and population-specific temperatures around their thermal optimum (72). Recently, a semifield experiment (57) demonstrated that high ambient temperatures interfere with the ability of female An. gambiae to detect heat irradiation from warm-blooded hosts. Similarly, host-seeking females of Ae. albopictus collected from Guangzhou, China, were found to bite between 16.1°C and 37.1°C, where the upper temperature closely matches that of human hosts (148). Notably, female mosquitoes that do not feed on warm-blooded hosts, such as the Northern frog biting mosquito (Culex territans), do not show a thermal preference while flying (106). Therefore, the host-seeking responses of mosquitoes that feed on amphibians, reptiles, and worms may be less affected by changes in ambient temperature than in the case of mosquitoes that feed on warm-blooded vertebrates.

Higher temperatures also increase rates of pathogen replication while reducing the incubation period required before mosquitoes become infectious (reviewed by 13). Several studies incorporate the effects of temperature on mosquitoes and pathogens to calculate the thermal optimum of pathogen transmission for multiple combinations of mosquito disease vectors and their associated pathogens (29, 96, 137). Yet, the impact of temperature on disease transmission dynamics can be difficult to predict. For example, a standard model of vectorial capacity predicted that Anopheles stephensi would transmit malarial pathogens at 12–38°C with an optimal of 29°C, while an alternative model that calculated the relative force of infection using more realistic mosquito–parasite interactions predicted that transmission would occur between 17°C and 35°C, with optimal transmission occurring at 26°C (115).

Continued research on the role of temperature, mosquito biology, and pathogen transmission dynamics is necessary because it is unclear how much variation mosquito populations and/or strains of pathogens exhibit in their responses to temperature. It is also unclear whether and how mosquitoes and their pathogens are responding to selective pressures as the climate becomes warmer and more volatile. Temperature can also affect the efficacy of pesticides that are used to control mosquitoes (75). Therefore, field and laboratory-based studies, as well as more sophisticated mathematical models that better capture heterogeneous responses in mosquitoes and pathogens, will allow researchers to better predict the dynamics of mosquito-borne disease transmission in the face of climate change and in response to control interventions.

4. IMPACT OF HUMIDITY ON MOSQUITOES

In addition to photoperiod and temperature, humidity is an important but often understudied environmental factor that profoundly affects mosquito behavior, physiology, and disease transmission (Figure 2a) (reviewed by 23). Below, I briefly highlight some foundational and recent work on the role of humidity on mosquito development and host seeking, as well as new research on how mosquitoes sense water vapor.

4.1. Effects of Humidity and Rainfall on Mosquito Growth and Development

Foundational work by Lansdowne & Hacker (82) demonstrated that fluctuating temperature and humidity did not affect the lifespan, net reproductive rates, or intrinsic rate of increase of five populations of Ae. aegypti. However, de Almeda Costa et al. (34) demonstrate that high temperatures (33–37°C) and low humidity (60% relative humidity) inhibit oviposition in Ae. aegypti and that high humidity (80%) ameliorated the negative effect of increasing temperature on egg viability. Similarly, high temperatures coupled with low humidity reduce the reproductive capacity of Ae. albopictus (4). Notably, mosquito species differ in their ability to tolerate differences in relative humidity, and individuals can undergo physiological acclimation. For example, relative humidities between 60% and 100% do not affect the lifespan of An. gambiae sensu lato, while relative humidity < 10% is usually fatal within a few hours unless females have acclimated to low humidity (50, 60, 61, 147).

4.2. How Humidity and Dehydration Affect Mosquito Host Seeking

Humidity affects host-seeking responses in female mosquitoes. Specifically, even in the absence of other host cues, An. gambiae are more likely to engage in host seeking during periods of increasing relative humidity than during periods when relative humidity is stable or decreasing (128). However, Rowley & Graham (112) found that humidity generally had little impact on the flight performance of Ae. aegypti, except where low humidity in combination with high temperatures (32°C) decreased flight capacity. Thus, the effects of humidity on flight and host seeking likely differ between mosquito species and between mosquito populations.

How adult female mosquitoes respond to humidity is also likely affected by the internal state of the mosquito. After exposure to low relative humidity, dehydrated females of Cx. pipiens, Anopheles quadrimaculatus, and Ae. aegypti may host seek and thereby replenish water and/or nutrient reserves to combat dehydration stress (63). Moreover, post-blood-meal rehydration in Ae. aegypti is potentially mediated by increased expression of ion transporters and aquaporins that allow dehydrated females to rapidly restore the osmolality of their hemolymph after ingesting a blood meal (68).

4.3. Hygrosensation in Mosquitoes

As humans and other vertebrate hosts emit short-range gradients in relative humidity (87), female mosquitoes can also sense changes in relative humidity and use such changes as another cue to locate hosts (22, 79). Mosquitoes also sense water vapor to locate oviposition sites (104). Previously, it has been unclear how mosquitoes detect increases in relative humidity to locate both potential blood meal sources and oviposition sites. Laursen et al. (83) demonstrated that the hygroreceptors on the antennae of An. gambiae and Ae. aegypti abundantly express ionotropic receptor 93a (Ir93a), which is responsive to both local heat and humidity gradients. Knocking out this receptor with CRISPR/Cas9 did not interfere with host seeking but prevented female Ae. aegypti from landing on a human hand. Moreover, gravid females of An. gambiae and Ae. aegypti that lacked the Ir93a gene also displayed significantly lower levels of humidity seeking behavior (83). Collectively, this provides compelling evidence that Ir93a regulates both short-range host detection and oviposition in mosquitoes and is a promising target for future control interventions. Indeed, pentylamine, a volatile amine, inhibits humidity detection and oviposition in Ae. aegypti and An. gambiae (30). Whether pentylamine acts upon Ir93a is not known, but future research should determine whether this or other similar volatile amines could be used to repel mosquitoes from human hosts and/or oviposition sites.

5. INTERACTIONS OF MULTIPLE ENVIRONMENTAL FACTORS AFFECTING MOSQUITO BIOLOGY

As photoperiod, temperature, and humidity can significantly affect several aspects of mosquito biology, the interactions among these three environmental factors should have large-scale impacts on mosquito growth, development, reproduction, host seeking, and disease transmission. Given the complexity of manipulating multiple environmental factors simultaneously, there are no laboratory studies that dissect the interactions of photoperiod, temperature, and humidity in mosquito biology. However, several studies have uncovered the interactions between two of these three key environmental variables, enabling researchers to make inferences about how all three factors influence mosquito abundance and/or disease transmission. For example, high temperatures postpone photoperiodic diapause entry in Cx. pipiens (48) and Ae. albopictus (102) and can advance the photoperiodic termination of diapause in T. rutilus (19). Both light and temperature affect the expression of circadian clock genes in field-collected Anopheles coluzzii and biting behavior (139). Laboratory experiments that manipulate temperature and relative humidity demonstrate that both variables influence mosquito development, host seeking, blood feeding, and reproduction in several vector species (34, 63, 147, 148).

Leveraging multiyear, field collection data can also provide critical insights into the relative contributions of multiple environmental variables to mosquito populations. Notably, Roiz et al. (109) found that photoperiod, relative humidity, and temperature could predict the abundance of different mosquito species in Mediterranean wetlands. Additionally, Veronesi et al. (135) found species-specific differences in the flight activity of mosquito vectors in response to temperature, relative humidity, windspeed, and daylength.

6. ANTHROPOGENIC CHANGES TO PHOTOPERIOD, TEMPERATURE, AND HUMIDITY IN URBAN ENVIRONMENTS

Urbanization is rapidly increasing such that now more than half of the world’s population lives in urban areas, with a projected increase to approximately 67% by 2050 (131). Increasing urbanization also benefits many disease-vectoring mosquitoes because females oviposit in containers that are widely available in cities, including buckets, discarded tires, trash, and garden water butts (80, 88, 130, 144). Additionally, land use changes in the urban environment reduce total mosquito species richness while increasing the abundance of container-breeding mosquitoes such as Ae. albopictus, Ae. aegypti, and Culex quinquefasciatus (145, 150). Apart from providing increased breeding sites, urban environments also affect mosquito populations due to light pollution, higher temperatures, and changes in relative humidity (Figure 2b).

6.1. Effects of Light Pollution on Mosquito Biology

Artificial light at night (ALAN) is more prevalent in cities and profoundly affects the biology of all organisms inhabiting urban and periurban areas (reviewed by 10). This is because ALAN disrupts naturally occurring light:dark cycles that organisms, including mosquitoes, have used for eons to properly time their daily activities and seasonal development. For example, light pollution increases the biting behavior of day-active Ae. aegypti mosquitoes (113) but decreases the level of activity of crepuscular, nondiapausing Cx. pipiens (146). Additionally, although female Cx. pipiens exposed to ALAN had a lower proclivity to blood feed, they were more likely to lay eggs and produced more larvae than did unexposed females (49). In addition to changing daily behaviors of mosquitoes, ALAN also interferes with the ability of temperate mosquitoes, such as Ae. albopictus and Cx. pipiens, to measure daylength and initiate diapause (51, 143), likely by disrupting the expression of circadian clock genes (53, 70).

Artificial light at night (ALAN):

a form of light pollution caused by human-made light sources that are illuminated when it is dark

Crepuscular:

characterizing the activity of an animal that is active at dawn and dusk

6.2. Mosquito Responses to the Urban Heat Island Effect

As built surfaces (e.g., roads, parking lots, buildings) that trap and radiate heat are more prevalent in urban areas than in surrounding rural and suburban habitats, cities are also significantly warmer, a phenomenon is known as the urban heat island (UHI) effect (76). These high temperatures in cities allow some mosquito species to thrive in urban areas. For example, high temperatures in cities accelerate development and, surprisingly, extend the lifespan of Ae. albopictus (88). Additionally, high heat levels associated with the UHI effect in cemeteries in New Orleans increased the abundance of Ae. aegypti (35). UHIs have been associated with higher incidences of dengue fever in São Paulo, Brazil (8), likely because warmer urban areas lead to higher populations of mosquito vectors. UHIs are associated with areas with lower socioeconomic status (8, 35), contributing to disparities in disease risk. However, extremely high temperatures in cities can be detrimental to mosquitoes and potentially decrease disease risk (47). Additionally, higher temperatures that mimic the UHI effect in the lab postpone diapause initiation in Cx. pipiens (52) and prematurely terminate diapause in Wy. smithii (17). Either effect of temperature in nature would prolong the period of disease transmission in temperate environments. It is currently unclear how we can mitigate the effects of UHIs on mosquito abundance, especially as efforts to introduce more green areas and water features, which reduce temperatures in cities (58), can also increase mosquito abundance (89, 90).

Urban heat island (UHI) effect:

a phenomenon where cities have significantly higher temperatures than surrounding rural areas, particularly during the night

6.3. Changes in Humidity in Cities That Influence Mosquitoes

Generally, cities are characterized by lower relative humidity due to the abundance of built surfaces and lack of tree cover (64), and lower relative humidity affects mosquito physiology and behavior. A semifield study demonstrated that high temperatures and low humidity in urban sites decreased larval survival, adult body size, and growth rates of Ae. albopictus and shifted the peak of vectorial capacity from summer in rural sites to fall in urban areas (101). Additionally, higher numbers of Cx. pipiens were collected in urban green spaces that had higher levels of relative humidity (90). Furthermore, urbanization, in combination with dry climate, can increase mosquito preference for human hosts (110), suggesting that disease risk may be higher in less humid cities.

7. CONCLUSIONS AND FUTURE DIRECTIONS

Vector biologists have long recognized the important role that environmental factors, singly and in combination, play in mosquito physiology and behavior (19, 22, 45, 79, 82). Considering growing concerns over the effects of climate change and how this will impact mosquito distributions and disease risk, researchers have executed laboratory experiments with ecologically relevant conditions (126, 136), semifield experiments (48, 102), and field studies (20, 120, 139) to better understand how increasing and more variable temperatures and humidity are impacting mosquito biology. We are also gaining a better understanding of how elements of the urban environment, such as light pollution, higher temperatures, and lower humidity, affect mosquito biology (19, 49, 51-53, 70, 113, 143, 146). Furthermore, disease ecologists are incorporating these data to generate sophisticated models to predict how environmental factors influence mosquito populations and disease risk (29, 96, 115, 137, 148). This work is revealing that most mosquito vectors are thriving in the face of anthropogenic climate change and urbanization (88, 109, 110, 130, 144, 144) and that periods of mosquito activity in temperate environments are shifting earlier in the year and extending later in the autumn (20, 98, 120).

Despite the remarkable progress that we have made in understanding how the environment affects mosquitoes, much important research needs to be done so that we can test and enhance the accuracy of models and determine when and where to direct our control efforts. Specifically, mosquito biologists must replicate field conditions as closely as possible in the laboratory and carefully design experiments to evaluate complex interactions of multiple environmental factors. Because mimicking fluctuations in light levels, temperature, and humidity in the lab is difficult, scientists should also continue to leverage natural variations that occur in the field to better understand how environmental variables affect mosquitoes. For example, deploying mosquito larvae across relevant environmental and geographical gradients will provide more realistic data on how light pollution, UHIs, humidity, and other factors of interest affect mosquito growth, development, blood-feeding proclivity, diapause incidence, and reproductive capacity (for example, see 74). Moreover, correlating mosquito surveillance data collected by health departments and observatories with relevant environmental variables will also lead to powerful insights into how short-term and long-term changes in temperature, rainfall, and land cover have affected the abundance and community composition of mosquitoes as well as pathogen prevalence.

Notably, most existing models that predict how mosquito populations and/or mosquito-borne disease transmission will respond to climate change focus on periods of peak mosquito abundance (29, 96, 137). While these are periods when mosquito-borne disease is most likely to occur, we know relatively little about how environmental factors during the pre- and postepidemic periods influence mosquito abundance and interannual cycles of disease transmission. As diapausing and estivating mosquitoes can serve as reservoirs for pathogens (reviewed by 7, 71), better understanding how temperature, rainfall, and humidity affect mosquito survival and subsequent reproduction will likely lead to highly useful insights into when and where mosquito-borne disease transmission will occur.

We are also gaining exciting insights into the underlying mechanisms by which mosquitoes measure and respond to daylength (22, 41, 42), humidity (83), and light pollution (53, 70). As mosquitoes are becoming increasingly resistant to existing pesticides (reviewed by 54, 94) and as populations of beneficial, nontarget insects continue to decline (84, 133, 138), this basic research is critical to developing novel and ideally species-specific interventions that improve our ability to reduce populations of disease vectors. Insights gained into mosquito hygrosensors (83) and their potential inhibition with pentylamine (30) represent one example of an exciting novel method to control mosquitoes. Furthermore, understanding how environmental factors affect mosquito physiology and behavior will improve our ability to rear and deploy sterilized or genetically modified mosquitoes and thereby more effectively and specifically control mosquitoes and limit disease transmission (15, 140).

Unfortunately, disease-vectoring mosquitoes will likely remain the most formidable threat to human health, especially given their ability to adapt to and thrive with increasing urbanization and climate change. However, this reviewer remains optimistic that, through the continued engaged and collaborative efforts among molecular physiologists, mosquito ecologists, vector control specialists, mathematical modelers, disease epidemiologists, and public health officials, we will be able to substantially reduce mosquito populations and curtail disease transmission.

SUMMARY POINTS.

1. Daylength, temperature, and humidity affect mosquito development, reproduction, and blood feeding, creating a spatial and temporal mosaic in mosquito abundance and disease transmission.

2. Complex interactions among multiple environmental factors, as well as species- and population-specific genetic variation in response to environmental factors, make it difficult to predict how mosquitoes will respond to climate change.

3. Leveraging existing data collected by public health departments and/or observatory networks could enable mathematical modelers to better test and predict how environmental variation affects mosquito biology and disease transmission dynamics.

4. Light pollution, high temperatures, and low humidity enable vector mosquitoes to thrive in cities, but urban areas offer unique opportunities for researchers to determine how environmental factors interact to influence mosquito physiology, behavior, and disease transmission.

5. Insights gained into how environmental variables, both individually and collectively, affect mosquitoes as well as how mosquitoes sense, respond, and adapt to environmental factors should enable researchers to develop novel control interventions to suppress vector populations and improve public health.