Changes In Day Length Research Articles

The expression of circadian clock genes in Daphnia magna diapause

Diapause is a mechanism necessary for survival in arthropods. Often diapause induction and resurrection is light-dependent and therefore dependent on the photoperiod length and on the number of consecutive short-days. In many organisms, including the microcrustacean Daphnia magna, one functional entity with the capacity to measure seasonal changes in day-length is the circadian clock. There is a long-standing discussion that the circadian clock also controls photoperiod-induced diapause. We tested this hypothesis in D. magna, an organism which goes into a state of suspended animation with the shortening of the photoperiod. We measured gene expression of clock genes in diapause-destined embryos of D. magna in the initiation, resting and resurrection phases and checked it against gene expression levels of continuously developing embryos. We demonstrate that some genes of the clock are differentially expressed during diapause induction but not during its maintenance. Furthermore, the photoreceptor gene cry2 and the clock-associated gene brp are highly expressed during induction and early diapause, probably in order to produce excess mRNA to prepare for immediate resurrection. After resurrection, both types of embryos show a similar pattern of gene expression during development. Our study contributes significantly to the understanding of the molecular basis of diapause induction, maintenance and termination.

Stepwise increases in FT1 expression regulate seasonal progression of flowering in wheat (Triticum aestivum).

Flowering is regulated by genes that respond to changing daylengths and temperature, which have been well studied using controlled conditions; however, the molecular processes underpinning flowering in nature remain poorly understood. Here, we investigate the genetic pathways that coordinate flowering and inflorescence development of wheat (Triticum aestivum) as daylengths extend naturally in the field, using lines that contain variant alleles for the key photoperiod gene, Photoperiod-1 (Ppd-1). We found flowering involves a stepwise increase in the expression of FLOWERING LOCUS T1 (FT1), which initiates under day-neutral conditions of early spring. The incremental rise in FT1 expression is overridden in plants that contain a photoperiod-insensitive allele of Ppd-1, which hastens the completion of spikelet development and accelerates flowering time. The accelerated inflorescence development of photoperiod-insensitive lines is promoted by advanced seasonal expression of floral meristem identity genes. The completion of spikelet formation is promoted by FLOWERING LOCUS T2, which regulates spikelet number and is activated by Ppd-1. In wheat, flowering under natural photoperiods is regulated by stepwise increases in the expression of FT1, which responds dynamically to extending daylengths to promote early inflorescence development. This research provides a strong foundation to improve yield potential by fine-tuning the photoperiod-dependent control of inflorescence development.

Simulation of a hybrid system of solar-microturbines in cold climate regions

In this study, a hybrid system of solar-microturbine with and without a combustion chamber, was investigated in a cold climate region (Edmonton, Canada). We developed a thermodynamic model to analyse the effects of environmental conditions on the system performance and power output during the year considering monthly changes in temperature, daylength and solar radiation using real climate and geographical data. The results showed that for a 30 m2 dish collector aperture area, the cycle outlet power was estimated from 3.70 kW in winter to 9.87 kW in summer, while the lowest and the highest cycle efficiencies were 19.44% and 35.07%, respectively for sunny days. The performance of the cycle was also compared with different climates and latitudes in Toronto and Phoenix. The total efficiencies in Edmonton were similar as that in Phoenix in summer but much lower in winter. However, the total electricity output in summer was higher in Edmonton than other two cities. It is found that the highest electricity output in winter is only a half of that in summer day in Edmonton. Furthermore, the cycle could achieve the highest total daily electricity output and fuel consumption in Edmonton in summer due to longer daylength, and higher latitude despite a colder climate if including a combustion chamber. Particularly, more than a half day has no electricity output in Edmonton in winter day if without combustion chamber. Therefore, it demonstrates that this state-of-the-art hybrid system can produce electricity and recover heat in a cold climate region but the climate effects should be considered.

Early Detection of Daylengths with a Feedforward Circuit Coregulated by Circadian and Diurnal Cycles

Light-entrained circadian clocks confer rhythmic dynamics of cellular and molecular activities to animals and plants. These intrinsic clocks allow stable anticipations to light-dark (diel) cycles. Many genes in the model plant Arabidopsis thaliana are regulated by diel cycles via pathways independent of the clock, suggesting that the integration of circadian and light signals is important for the fitness of plants. Previous studies of light-clock signal integrations have focused on moderate phase adjustment of the two signals. However, dynamical features of integrations across a broad range of phases remain elusive. Phosphorylation of ribosomal protein of the small subunit 6 (eS6), a ubiquitous post-translational modification across kingdoms, is influenced by the circadian clock and the light-dark (diel) cycle in an opposite manner. To understand this striking phenomenon and its underlying information processing capabilities, we built a mathematical model for the eS6 phosphorylation (eS6-P) control circuit. We found that the dynamics of eS6-P can be explained by a feedforward circuit with inputs from both circadian and diel cycles. Furthermore, the early day response of this circuit with dual rhythmic inputs is sensitive to the changes in daylength, including both transient and gradual changes observed in realistic light intervals across a year, because of weather and seasons. By analyzing published gene expression data, we found that the dynamics produced by the eS6-P control circuit can be observed in the expression profiles of a large number of genes. Our work provides mechanistic insights into the complex dynamics of a ribosomal protein, and it proposes a previously underappreciated function of the circadian clock, which not only prepares organisms for normal diel cycles but also helps to detect both transient and seasonal changes with a predictive power.

Chasing the sun: When to migrate, when to breed

Chasing the sun: When to migrate, when to breed

Cuscuta australis (dodder) parasite eavesdrops on the host plants’ FT signals to flower

Many plants use environmental cues, including seasonal changes of day length (photoperiod), to control their flowering time. Under inductive conditions, FLOWERING LOCUS T (FT) protein is synthesized in leaves, and FT protein is a mobile signal, which is able to travel to the shoot apex to induce flowering. Dodders (Cuscuta, Convolvulaceae) are root- and leafless plants that parasitize a large number of autotrophic plant species with varying flowering time. Remarkably, some dodder species, e.g., Cuscuta australis, are able to synchronize their flowering with the flowering of their hosts. Detailed sequence inspection and expression analysis indicated that the FT gene in dodder C. australis very likely does not function in activating flowering. Using soybean host plants cultivated under inductive and noninductive photoperiod conditions and soybean and tobacco host plants, in which FT was overexpressed and knocked out, respectively, we show that FT-induced flowering of the host is likely required for both host and parasite flowering. Biochemical analysis revealed that host-synthesized FT signals are able to move into dodder stems, where they physically interact with a dodder FD transcription factor to activate dodder flowering. This study demonstrates that FTs can function as an important interplant flowering signal in host-dodder interactions. The unique means of flowering regulation of dodder illustrates how regressive evolution, commonly found in parasites, may facilitate the physiological synchronization of parasite and host, here allowing the C. australis parasite to time reproduction exactly with that of their hosts, likely optimizing parasite fitness.

Brief light exposure at dawn and dusk can encode day-length in the neuronal network of the mammalian circadian pacemaker.

The central circadian pacemaker in mammals, the suprachiasmatic nucleus (SCN), is important for daily as well as seasonal rhythms. The SCN encodes seasonal changes in day length by adjusting phase distribution among oscillating neurons thereby shaping the output signal used for adaptation of physiology and behavior. It is well-established that brief light exposure at the beginning and end of the day, also referred to as "skeleton" light pulses, are sufficient to evoke the seasonal behavioral phenotype. However, the effect of skeleton light exposure on SCN network reorganization remains unknown. Therefore, we exposed mice to brief morning and evening light pulses that mark the time of dawn and dusk in a short winter- or a long summer day. Single-cell PER2::LUC recordings, electrophysiological recordings of SCN activity, and measurements of GABA response polarity revealed that skeleton light-regimes affected the SCN network to the same degree as full photoperiod. These results indicate the powerful, yet potentially harmful effects of even relatively short light exposures during the evening or night for nocturnal animals.

Growing in time: exploring the molecular mechanisms of tree growth.

Trees cover vast areas of the Earth’s landmasses. They mitigate erosion, capture carbon dioxide, produce oxygen and support biodiversity, and also are a source of food, raw materials and energy for human populations. Understanding the growth cycles of trees is fundamental for many areas of research. Trees, like most other organisms, have evolved a circadian clock to synchronize their growth and development with the daily and seasonal cycles of the environment. These regular changes in light, daylength and temperature are perceived via a range of dedicated receptors and cause resetting of the circadian clock to local time. This allows anticipation of daily and seasonal fluctuations and enables trees to co-ordinate their metabolism and physiology to ensure vital processes occur at the optimal times. In this review, we explore the current state of knowledge concerning the regulation of growth and seasonal dormancy in trees, using information drawn from model systems such as Populus spp.

MRG1/2 histone methylation readers and HD2C histone deacetylase associate in repression of the florigen gene FT to set a proper flowering time in response to day-length changes.

Day-length changes represent an important cue for modulating flowering time. In Arabidopsis, the expression of the florigen gene FLOWERING LOCUS T (FT) exhibits a 24-h circadian rhythm under long-day (LD) conditions. Here we focus on the chromatin-based mechanism regarding the control of FT expression. We conducted co-immunoprecipitation assays along with LC-MS/MS analysis and identified HD2C histone deacetylase as the binding protein of the H3K4/H3K36 methylation reader MRG2. HD2C and MRG1/2 regulate flowering time under LD conditions, but not under short-day conditions. Moreover, HD2C functions as an effective deacetylase in planta, mainly targeting H3K9ac, H3K23ac and H3K27ac. At dusk, HD2C is recruited to FT to deacetylate histones and repress transcription in an MRG1/2-dependent manner. More importantly, HD2C competes with CO for the binding of MRG2, and the accumulation of HD2C at the FT locus occurs at the end of the day. Our findings not only reveal a histone deacetylation mechanism contributing to prevent FT overexpression and precocious flowering, but also support the model in which the histone methylation readers MRG1/2 provide a platform on chromatin for connecting regulatory factors involved in activating FT expression in response to daylight and decreasing FT expression around dusk under long days.

Seasonal effects on testes size and sustained semen quality in captive golden-headed lion tamarins, Leontopithecus chrysomelas

The golden-headed lion tamarin (Leontopithecus chrysomelas) is an endangered primate that, according to timing of births, there seems to have seasonal reproductive patterns in captive populations inhabiting the Southern Hemisphere. The extent to which male tamarins have circannual cyclic fluctuations in testicular functions, however, remains unknown. Changes in testis size and seminal variables, therefore, were evaluated during the rainy and dry seasons of the year in seven males. There was estimation of mating and birth seasons from the breeding colony using a 27 year database (229 birth records). Births were distributed throughout the year with peaks occurring in August-October and January-March (39.7 % and 30.5 % of all births, respectively). Semen collection using penile vibrostimulation was successfully induced in more than 96 % of the attempts regardless of the season (total of 75 ejaculates). Body mass did not vary significantly between seasons, but relative testes size was larger during the dry season. Values for none of the seminal variables (total sperm count, total and progressive motility, plasma membrane and acrosome integrity, and total sperm defects), however, were different during the rainy and dry seasons. These results indicate that testicular function in golden-headed lion tamarins may not be affected by daylength changes, and that seasonal patterns of female reproduction is perhaps more relevant for the reproductive timing of the species. Furthermore, the possibility of year-round production of ejaculates containing viable sperm broadens our perspective of preserving genetic diversity within the species because there is a greater opportunity for semen collection and freezing.

Circadian activity patterns of red foxes (Vulpes vulpes) in montane forests under different culling regimes

Mammals display a great variety of activity patterns. Although red foxes (Vulpes vulpes) are generally well studied, differences in their activity patterns related to seasonality and anthropogenic impacts remain not well known. Hunting is a major anthropogenic influence on fox populations across the globe. In this paper, we assess how circadian activity patterns of red foxes in a central European montane forest change throughout the year and test whether activity patterns differ between areas with different culling regimes. We found pronounced activity peaks around dawn and particularly dusk and high levels of activity throughout the night. Foxes generally avoided daylight hours, except in wintertime, potentially indicating an avoidance of human presence. The onset and cessation of activity tracked changes in day length. Circadian activity patterns were highly similar at sites with and without targeted culls during all seasons, not suggesting a direct reaction of red foxes to hunting. Red fox activity patterns in our study were thus overall comparable with those in other environmental contexts.

The circadian clock and darkness control natural competence in cyanobacteria

The cyanobacterium Synechococcus elongatus is a model organism for the study of circadian rhythms. It is naturally competent for transformation—that is, it takes up DNA from the environment, but the underlying mechanisms are unclear. Here, we use a genome-wide screen to identify genes required for natural transformation in S. elongatus, including genes encoding a conserved Type IV pilus, genes known to be associated with competence in other bacteria, and others. Pilus biogenesis occurs daily in the morning, while natural transformation is maximal when the onset of darkness coincides with the dusk circadian peak. Thus, the competence state in cyanobacteria is regulated by the circadian clock and can adapt to seasonal changes of day length.

Phenotypic Variation in Growth and Gene Expression Under Different Photoperiods in Allopatric Populations of the Copepod Tigriopus californicus

Daylength is a major environmental condition that varies seasonally and predictably along a latitudinal cline, where higher latitudes exhibit greater ranges in total daylengths. Generally, the circadian clock acts as a network of genes whose expression dynamics are known to control daily rhythms in response to daylength, and it enables the control of many physiological processes such as growth and development. While well studied in many model animals, the influence of daylength variation on phenotypic evolution is poorly examined in marine species. In this study we demonstrate that two allopatric populations of the intertidal crustacean Tigriopus californicus exhibit plastic and divergent phenotypic responses to changes in daylength. Using common-garden experiments, we discovered that shorter daylengths promoted decreased adult body size and faster growth rates in the two divergent populations, suggesting a plastic response to shortened days. In addition, the higher-latitude population exhibited a faster growth rate at any daylength condition, indicating a fixed response, possibly as a result of adaptation to respective natural light regimes. Gene expression profiles of several circadian clock genes, monitored throughout the day by quantitative polymerase chain reaction, revealed that the key core clock genes reach higher daily transcription maxima in the southern population compared to the northern population, pointing to divergent strategies used to respond to changes in daylength. Many modifier genes to the circadian clock showed similar plastic responses to the different daylengths, supporting the existence of at least some conserved gene expression across both populations. Ultimately, our results suggest that photoperiod and daylength exert a potent selective pressure underexplored in marine systems and warranting further future research.

Annual rhythms of milk synthesis in dairy herds in 4 regions of the United States and their relationships to environmental indicators

The annual rhythms of milk and milk component yields are not well described and are important to dairy management. Recent analysis of federal milk marketing orders in the United States observed that the amplitude and time at peak (acrophase) of the rhythms of milk fat and protein concentration differ among regions, but the rhythms of milk and milk component yields are not well described. Our objective was to determine the annual rhythms of milk and milk component production from 4 US regions at the herd level and examine potential environmental factors entraining these rhythms. Monthly Dairy Herd Improvement Association records of all available herds in Pennsylvania (PA), Minnesota (MN), Texas (TX), and Florida (FL) from the years 2003 to 2016 were obtained from Dairy Records Managements Systems. Milk yield, fat and protein yield, and fat and protein concentration were fit to the linear form of the cosine function with a 12-mo period using a linear mixed effects model. Additionally, the fit of models containing either the cosine function or environmental temperature were compared using an F-test. Milk yield and fat and protein yields and concentrations fit a cosine function in all 4 states, indicating an annual rhythm. The amplitude (peak to mean) of the rhythm of milk yield varied by state and was lower in PA (1.2 kg) and MN (1.2 kg) compared with TX (3.1 kg) and FL (3.3 kg). Fat and protein yields similarly showed greater amplitudes in the southern versus northern states. The amplitudes of the rhythms of fat and protein concentration were opposite by region, with greater amplitudes occurring in MN and PA than in TX and FL. The acrophases of milk yield and milk fat and protein yields and concentrations also varied by state, but all peaked between October and March. An annual rhythm fit the data better than changes in environmental temperature for all responses in all states, except for fat and protein concentrations in FL, which exhibited lower amplitude seasonal rhythms. The yearly pattern of milk yield closely followed the fixed yearly pattern of the day to day changes in day length, whereas the rhythms of milk fat and protein concentrations followed the yearly pattern of absolute day length. Results suggest that the region of the United States in which a herd is located affects their annual rhythms of production, with a greater yearly variation in milk, fat, and protein yields occurring in the southern United States. The consistency of annual rhythms across years and herds allowed development of regression equations to adjust expectations across the year to account for the annual rhythm.

The effects of extended photoperiod and warmth on hair growth in ponies and horses at different times of year.

Photoperiod is considered the most dominant environmental cue allowing animals to anticipate and adapt to seasonal changes. In seasonally breeding mammals, changes in daylength alter pineal melatonin secretion and pituitary prolactin secretion. During the seasonal transition to shorter winter daylengths, increased production of melatonin and declining prolactin are associated with triggering winter coat growth in many animals. Similarly, studies have shown that artificial extension of photoperiod suppresses melatonin secretion and lifts prolactin inhibition to activate moulting. Four longitudinal cohort studies were conducted to determine if extended photoperiod and warmth, provided by mobile light masks and rugs (horse blankets), could reverse the onset of winter coat growth, maintain the summer coat and accelerate winter coat shedding in horses and in ponies. Studies began at dates corresponding to the autumnal equinox, one month post-summer solstice, one month pre-winter solstice and one month post-winter solstice, respectively. To extend photoperiod to approximately 15h of light, commercially available head-worn light masks provided low intensity blue light to one eye until 11pm daily. Coat condition and shedding rate were scored and hair samples collected, measured and weighed bi-weekly. Data from control and treatment groups were analysed by repeated measures ANOVA. Results revealed that extended photoperiod 1) did not reverse winter coat growth when initiated at the autumnal equinox, 2) effectively maintained the summer coat in stabled horses when initiated one month post-summer solstice, 3) accelerated shedding in outdoor living horses when initiated one month pre-winter solstice and 4) did not accelerate shedding in indoor or outdoor living ponies when initiated one month post-winter solstice. To successfully manage equine coat growth while also preserving optimal thermoregulation in both competition and breeding stock correct timing of light application is crucial and requires careful monitoring of environmental temperature. Further studies are needed where variations in breed and management are considered.

Molecular Processes Underpinning Seasonal Regulation of Flowering in Bread Wheat ( <i>Triticum Aestivum L.</i>)

Plants integrate seasonal progression in daylength and temperature to determine the optimal time to flower and produce seed. In cereals, genes that regulate responses to changing daylengths and temperature have been well-studied using controlled conditions; however, the molecular processes underpinning flowering in nature are poorly understood. Here, we investigate the molecular pathways that coordinate flowering and inflorescence development in wheat as daylengths extend naturally in the field. We find that discrete photoperiods contribute to the initiation of flowering and control of yield-related traits as days becomes longer from winter into spring. This process involves step-wise increases in the expression of FLOWERING LOCUS T1 (FT1). Photoperiod-1 (Ppd-1) partially regulates the seasonal induction of FT1, which responds dynamically to changes in daylength to influence the rate of inflorescence development. The incremental rise in FT1 expression is overridden in plants that contain a photoperiod-insensitive allele of Ppd-1, which hastens the completion of spikelet development and accelerates flowering time. The completion of spikelet formation is promoted by FLOWERING LOCUS T2, which influences spikelet number and is activated by Ppd-1. This research advances our understanding of the seasonal regulation of flowering in wheat and provides a strong foundation to improve yield potential by fine-tuning the photoperiod-dependent control of inflorescence development.

Diapause incidence and critical day length of Asian corn borer (Ostrinia furnacalis) populations exhibit a latitudinal cline in both pure and hybrid strains

The Asian corn borer, Ostrinia furnacalis, is a serious pest and is widely distributed in corn-producing regions throughout China. This insect enters facultative diapause as fully grown larvae in response to short day lengths. Here, we evaluate the changes in diapause incidence and critical day length (CDL) by crossing a tropical strain with five subtropical and temperate strains from a 2897-km latitudinal gradient to investigate the latitudinal variations in diapause induction and CDL and their heredity. Photoperiodic responses of diapause induction in the different geographical strains at different temperatures showed that diapause incidence and CDL increased with increasing latitude and decreased with increasing temperature. Similarly, diapause incidence and CDL for the hybrid strains also showed an increase with increasing latitude or decreasing temperature. Conversely, the diapause incidences were significantly lower and the CDLs were significantly shorter in hybrid strains than their parents with high diapause incidence. Furthermore, the males had significantly more influence than the females on the incidence of diapause in subsequent progeny. Diapause incidence and CDL in O. furnacalis are strikingly different among pure strains and hybrid strains. The photoperiodic response controlling diapause is strongly heritable, with diapause being a partial dominant character over nondiapause, with the incidence of diapause mainly determined by the male parent. These results can help us to predict the time course of diapause induction in nature and may provide a genetic means for pest management by providing information on the overwintering capabilities of the hybrids.

Interannual Oscillations in Earth Rotation

AbstractAt seasonal and shorter time scales, polar motion and length‐of‐day (LOD) change are dominated by mass redistribution in the Earth climate system, including the atmosphere, oceans, and hydrosphere. Long‐term polar motion and LOD change are believed to be forced by solid Earth geophysical process, such as glacial isostatic adjustment. Decadal LOD variations are also connected to core‐mantle coupling. This study provides a comprehensive analysis of interannual oscillations in all three components of Earth rotation using accurately measured polar motion and LOD time series, and geophysical excitations computed from atmospheric, oceanic, and hydrological models over the period 1962 to 2018. Strong interannual oscillations remain in all three components after detrending and band‐pass filtering to retain variations at periods between 2 and 8 years. Polar motion Y is dominated by a 3.65‐year oscillation, plus others at periods of 2.5 and 5.9 years. Polar motion X shows a strong oscillation near 5.9 years, plus smaller variations at shorter periods. A 5.9‐year LOD variation has been recognized in earlier studies, but oscillations at periods of 2.36, 3.65, and 4.9 years are also observed. Atmosphere, ocean, and hydrosphere (AOH) sources largely account for 2.5‐ and 3.65‐year components in polar motion Y, but not for the 5.9‐year component in polar motion X and Y. The 2.36‐, 3.65‐, and 4.9‐year components in LOD can largely be accounted for by AOH sources. The 5.9‐year LOD component becomes more prominent after AOH sources have been subtracted. The presence of a 5.9‐year variation in all three components of Earth rotation suggests a source in Earth's interior, probably from the core.

Onset of seasonal metabolic depression in the Antarctic midge Belgica antarctica appears to be independent of environmental cues

AbstractSeasonal progression is tracked in most animals by changes in daylength, thus allowing reliable synchrony with abundant food and favourable developmental conditions. In polar regions, daylength varies extensively, fluctuating at the highest latitudes from persistent light to persistent dark. The Antarctic midge Belgica antarctica has a narrow seasonal window in which to feed and develop, and previous work shows that this insect, despite having the elements of a circadian clock, remains continuously active when temperatures are permissive. The present study aims to clarify seasonal tracking in B. antarctica during the austral summer by monitoring oxygen consumption rates in a field population and in experimental groups exposed to shortened daylength, dehydration and chilling. Remarkably, during March, coordinated decreases in oxygen consumption are observed, ranging from 18% to 42%, in all treatment groups, indicating an anticipatory response to seasonal change regardless of the environmental cues. These results suggest that B. antarctica relies on an intrinsic mechanism to program metabolic depression at the onset of the long austral winter.

Metabolic rate does not vary with seasonal change in Anopheles arabiensis adults in South Africa

An important component of South Africa’s malaria elimination agenda is identifying the entomological drivers of residual transmission, especially those that present opportunities for enhanced vector control. Seasonal mosquito density correlates directly with malaria transmission in South Africa. Transmission is highest during the warm rainy season and lowest but not entirely absent during the cooler dry season. The factors that sustain dry-season mosquito survival remain unknown. The aim of this project was therefore to investigate seasonal change in metabolic rate to determine the presence or absence of winter dormancy in malaria vector mosquitoes. Metabolic rate, determined by CO2 production during closed-system respirometry, was measured from wild anophelines collected from KwaZulu-Natal Province, South Africa. Monthly sampling spanned all four seasons (summer, autumn, winter, and spring) in 2017. Anopheles arabiensis and An. parensis specimens formed the majority of the total 437 identified specimens (n = 216 and n = 162, respectively). Metabolic rate data from wild-caught mosquitoes showed no significant seasonal disparities for An. arabiensis and An. parensis males and females. Further laboratory experiments assessed the effect of manipulated photoperiod, representing seasonal day-length changes, on the metabolic rate of colonized An. arabiensis mosquitoes. Simulations of midwinter (10 h:14 h light dark) and midsummer (14 h:10 h) day-length showed no significant effect on the metabolic rate of these mosquitoes. Age (in days) had a significant effect on the metabolic rate of both male and female colonized adult An. arabiensis mosquitoes which may be linked to developmental factors during maturation of adults. These data suggest that the South African populations of the malaria vector species An. arabiensis and An. parensis do not curtail their breeding and foraging activities during the colder and drier winter months. Overwintering by diapause does not appear to be triggered in the adult mosquito stage in An. arabiensis. However, their respective population densities do decrease considerably during winter leading to reduced malaria transmission and the opportunity for control by winter larviciding of known breeding sites.