Mosquito Biology and Control

Mosquitoes - Culicidae

Mosquitoes and Human Deaths

  • Mosquitoes are the deadliest animal, causing the most human deaths annually.
  • Mosquitoes: 1,000,000+
  • Snakes: 30,000
  • Freshwater snails: 20,000
  • Assassin bugs: 12,000
  • Tsetse Flies: 10,000
  • Ascaris roundworms: 2,500
  • Crocodiles: 1,000
  • Tapeworms: 700
  • Hippopotamuses: 500
  • Elephants: 500
  • Lions: 250
  • African Cape Buffalos: 200
  • Deer: 100
  • Bees: 100
  • The high number of deaths caused by mosquitoes is primarily due to the transmission of diseases like Malaria and arboviruses.
  • Sand flies/Leishmaniasis cause approximately 20,000 deaths per year but were not included in the list.

Systematics and Taxonomy

  • Traditional classification of mosquitoes placed them in "Nematocera," but this term is outdated.
  • Modern classification: Culicomorpha (Soghigian et al 2023) includes mosquitoes, black flies (BF), and Ceratopogonids, but excludes sandflies.

Family Culicidae

  • The earliest mosquito fossil is approximately 100 million years old.
  • Phylogenetic dating suggests mosquitoes may be as old as 270 million years.
  • There are approximately 3600 species of mosquitoes.
  • The family Culicidae is divided into two subfamilies: Anophelinae and Culicinae.
  • Anophelinae: Contains about 430 species in 3 genera; most are vectors of malaria.
  • Culicinae: Contains about 3000 species in 108 genera; most are vectors of arboviruses.

Medical Importance

  • Three genera of mosquitoes are primarily responsible for disease transmission.
  • Approximately 100 species are medically important vectors.
  • Around 200 additional species have the potential to become vectors.

Life History

  • Mosquitoes undergo holometabolous development, with distinct larval and adult stages that feed on different food sources.
  • The life cycle includes:
    • Eclosion (egg hatch)
    • Larval development

Eggs

  • Several factors influence egg development and hatching:
    • Humidity
    • Temperature
    • Changes in barometric pressure
  • Oviposition sites vary by species:
    • Culex and Anopheles: often lay eggs on the water surface.
    • Aedes: typically lay eggs near the water's edge.

Larval Mosquito Habitats

  • Determined by oviposition preferences (Getachew et al 2015).
  • Many important vectors can utilize a variety of sites, including man-made containers.
  • Habitats are often ephemeral or temporary, with stagnant water.
  • Mosquitoes are rarely found in lakes or other large bodies of water except in shallow edges.
  • Ephemeral water bodies promote faster mosquito development.
  • These habitats are often free of predators.
  • Larvae can persist for extended periods in oligotrophic environments (e.g., tree holes), awaiting nutrient accumulation.

Species Specificity

  • Some species exhibit high specificity in their habitat.
  • Wyeomyia smithii: Only lives in the purple pitcher plant (Sarracenia purpurea).
  • S. purpurea is less carnivorous compared to other pitcher plants.
  • Larvae shape the microbial community of the plant via grazing.

Larval Stage

  • Eggs hatch, and larval mosquitoes emerge.
  • Larvae are filter feeders, consuming detritus and microbes.
  • Typically, there are 4 larval instars.
  • Without microbes, mosquitoes cannot develop.
  • Mosquitoes consume microbes indiscriminately to obtain B vitamins and other nutrients.
  • The mosquito gut is highly alkaline (pH 11), and digestive enzymes work to lyse bacterial cells.
  • Mosquitoes do not have gills; they breathe at the surface using a siphon (culicine) or anal papillae (anopheline) - (Coon et al 2014).

Pupal Phase

  • Pupae are motile but still require access to the surface to breathe.
  • They use "trumpets" behind their head to access air.
  • Pupae are non-feeding.
  • Eclosion marks the emergence of the adult mosquito.

Life History Cycle

  • Mating
  • Host-seeking
  • Blood feeding
  • Oviposition
  • Terrestrial phase
  • Aquatic phase

Mating

  • Males typically eclose first and form swarms near areas where females emerge.
  • Males attract females by matching their wing beat frequencies.
  • Aedes albopictus was introduced to the SE US in the 1980s.
  • It has largely displaced Ae. aegypti in much of its range, including Georgia.
  • Both species will mate with one another, but Ae. aegypti and Ae. albopictus do not produce viable offspring.
  • Only Ae. aegypti populations were significantly affected because Ae. albopictus mate multiple times, while Ae. aegypti mate only once.
  • Males transfer proteins to females in addition to sperm.
  • In some Anopheline species, this triggers a permanent recalcitrance to future matings (monogamy).

Host Seeking

  • Mosquitoes rely on multi-modal signals to find hosts.
  • Attractive scents include lactic acid and 1-octen-3-ol.
  • Skin microbiome influences attractiveness based on the ratio of certain bacteria.
  • Females typically start host-seeking 2-3 days post-eclosion.
  • Odors bind to receptors on sensilla.
  • Silencing individual receptors only partially limits host finding, as mosquitoes rely on multiple signals.
  • The more specialized a mosquito is on humans, the more difficult it is to disrupt its host-finding behavior.
  • In some species, males are attracted to hosts even though they don’t feed.

Blood Feeding

  • Key components of mosquito anatomy related to blood feeding:
    • vertex
    • frons
    • eye
    • scape
    • pedicel
    • clypeus
    • maxillary palpus
    • flagellum
    • proboscis
    • mandible
    • tentorium
    • labium
    • labrum
    • lacinia ('maxilla')
    • hypopharynx
    • broad decumbent scales
    • erect scales of occiput
    • narrow decumbent scales
    • labellum

Blood Feeding Challenges

  • (Piermarini et al. 2017) Major issues associated with blood feeding:
    • Heme from lysed red blood cells resulting from the degradation of hemoglobin, producing hemozoin and biliverdin.
    • Temperature change: is a significant issue, requiring specific adaptations.
    • Water excretion: Mosquitoes must fly away from the host, so the weight of ingested blood can be limiting. And rely on Malpighian tubules, which are analogous to kidneys and useful for studying renal disease. This maintains osmotic balance by removing water/ions from hemolymph and transporting them to the hindgut for excretion.

Hormonal Control of Reproduction

  • Ecdysone, Yolk proteins, Ovary Ecdysteroidogenic Hormone (OEH), and Insulin-Like Peptide 3 (ILP3) play critical roles.
  • Mosquito reproduction is controlled by hormones; blocking OEH and ILP3 prevents egg maturation (Matsumoto et al. 1989, Brown et al. 1998, Graf et al 1997, Riehle and Brown 1999, Brown et al. 2008).
  • Two phases of reproduction:
    • Pre-vitellogenic: high JH, low 20E
    • Vitellogenic: low JH, high 20E. 20E production in ovaries, triggered by ILPs and OEH
  • After egg development, JH elevates in preparation for the 2nd cycle of egg production.

Oviposition Site Selection

  • Female mosquitoes use several cues to select oviposition sites, particularly olfactory cues.
  • These cues include:
    • Food (decaying plant material)
    • Microbial activity
    • Presence of predators
    • Presence of other mosquito larvae
  • Different species have different attractants.
  • Many species will only oviposit in water infused with the "right" leaf material.

Toxorhynchites sp. (Elephant Mosquitoes)

  • Adults exclusively feed on nectar, having a curved proboscis not suited for blood feeding.
  • Macronutrient content in nectar varies; they obtain resources to produce eggs from larval teneral reserves.
  • Toxorhynchites larvae are predatory and preferentially eat other mosquito larvae.
  • They serve as natural predators that can limit mosquito populations.
  • Several species are autogenous and can produce the first clutch of eggs without a blood meal, using egg nutrients from larval teneral reserves.

Mosquito Control Strategies

  • Disrupt mating
  • Disrupt host seeking/feeding
  • Kill adults (terrestrial and aquatic methods).
  • Disrupt egg laying
  • Kill larvae

Historical Mosquito Control in the US

  • Malaria was once a major threat in the US and was eliminated in the 1950s.
  • Army training bases used in WW2 were often in areas where malaria was endemic.
  • Insecticides (area spraying/fogging, IRS) spurred the elimination efforts.

Repellants

  • DEET: Active ingredient, up to 95%; however, no additional protection is provided above 50%.
  • Picaridin/Icaridin: Similar protective effects, highly effective at ~20%, but no protective effect below 7%.
  • Lemon Eucalyptus oil (Corymbia citriodora): Essential oils have significant repellency.
  • Other repellents include indoor diffusers and outdoor candles.

DEET Mechanism and Safety

  • (Dennis et al 2019) - Multi-modal repellant
  • Stimulates bitter taste receptors (aversive)
  • Sensed by taste receptors on feet
  • Also sensed by taste receptors in labellar lobes
  • DEET will degrade plastics.
  • Fewer than 50 documented cases of serious toxic effects since 1960.
  • Side effects in some cases of long-term, heavy, daily use (insomnia, mood disturbances, cognitive function).

Indoor Residual Spraying (IRS)

  • 1940s Sardinia, Italy – spraying of DDT within houses.
  • Success led to WHO’s Global Malaria Eradication Programme (1955-1969).
  • Eliminated malaria in many countries (including the USA), but not Sub-Saharan Africa (SSA).
  • Poorly organized SSA program led to resistance to DDT, causing the program's abandonment.
  • The burden shifted to endemic countries, leading to a massive increase in malaria.
  • Restarted in the 2000s, using non-pyrethroid insecticides, estimated to have averted ~60 million cases of malaria.
  • Mosquitoes rest on nearby surfaces after feeding, contacting the residual pesticide.
  • Killing mosquitoes after they’ve fed prevents them from laying eggs, reducing the mosquito population.
  • Advantages: effective, targets only anthrophilic species.
  • Disadvantages: costly, labor-intensive, resistance, side effects.

Area Treatments

  • Target adult mosquitoes, previously with DDT, now with organophosphates and neonicitinoids.
  • Ultra-low volume sprays: aerosols that remain suspended in the air for a long time, killing on contact.
  • Considered safe for humans at the applied concentrations.
  • Advantages: natural control already reduces mosquito numbers, can be highly effective.
  • Disadvantages: environmental impact (incidental kills), resistance, persistence, widespread/low specificity, cost/coordination.

Insecticide Treated Bed Nets (LLIN)

  • Long-lasting insecticide-treated bed nets use an "attract and kill" strategy.
  • Nets are embedded with pyrethroids and neonicitinoids (safer for humans).
  • Mosquitoes are attracted to humans, land on the net, and die quickly from insecticides.
  • Advantages: highly effective at preventing bites, reduces populations, only impacts target species, inexpensive ($2/net).
  • Disadvantages: resistance to insecticides, longevity (3 yr/20 wash cycles), distribution, need to be properly used.
  • Estimated to have prevented 600 million cases and reduced mortality of kids < 5yo by 5%.

Pyrethroid Susceptibility Status

  • Resistance is a growing concern, with varying levels of pyrethroid susceptibility reported for malaria vectors worldwide (2010-2014).

Investment in Malaria Control

  • Less than half of the target US$ 6.86.8 billion was invested in 2020 for malaria control and elimination.
  • To reach global malaria targets, annual investments need to more than triple by 2030 to 10.310.3 billion per year.
  • The US and national malaria programs currently account for 70% of the total funding toward malaria eradication.
  • The COVID-19 pandemic severely disrupted malaria control efforts.

Biological Control - Predators

  • Mosquitofish, Gambusia affinis: doesn’t actually eat many mosquito larvae.
  • Mosquitoes are not major sources of food for bats.
  • Most important predators of larvae are aquatic predatory arthropods like waterscorpions and dragonfly/damselfly nymphs.
  • Most tadpoles aren’t insectivorous.