Controlling Mosquito Larvae in Urban Drainage Structures in Arizona

Mosquitoes are important human disease vectors worldwide. In Arizona, the primary species of concern are Culex tarsalis and Culex quinquefasciatus, both of which can transmit West Nile virus (WNV) and St. Louis encephalitis virus (SLEV). Aedes aegypti is also a common urban vector with the potential of transmitting Zika, chikungunya and dengue viruses, although local transmission in Arizona has been rare. Treatment options for these viruses in humans are limited, so controlling the mosquito vector and reducing human/vector contact remain the best disease prevention strategies.

Before they develop into the adult stage, all mosquitoes are aquatic (Figure 1). Therefore, controlling the mosquito population at the immature (larval) phase is ideal as the mosquitoes are confined to a water source during this stage. Larval control in the water source can be achieved either by removing the source or treating the water with insecticides or biological agents. The goal is to prevent the mosquitoes from reaching the adult stage when they can acquire and transmit pathogens causing human and animals diseases. C. quinquefasciatus and Ae. aegypti utilize water sources found in common urban drainage structures for immature development. This article describes how to survey and control these mosquito larvae and pupae in drainage systems.

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Larval habitats

Different mosquito species use various types of nonflowing water bodies for immature development. The particular water body is chosen by the adult female mosquito 48 to 72 hours after taking a bloodmeal, when egg development is complete and the female lays her eggs (oviposits). Like all insects, mosquito development time is influenced by temperature, so higher temperatures accelerate development and reduce the time from egg to adulthood. For more information about mosquito biology, see Mosquitoes: Biology and Integrated Mosquito Management.

Culex quinquefasciatus (Southern House Mosquito)

This species typically lays clutches of 150-200 eggs (Suman et al., 2011). These eggs are assembled as a raft that floats on the surface of the water. Figure 1 shows a diagram of the life cycle of Culex mosquitoes (CDC, 2024). The eggs hatch within 24 to 48 hours (Fig. 2B). Development from the egg to the adult stage takes approximately 7 days under the Sonoran Desert summer temperatures (Moser et al. 2023). Cx. quinquefasciatus mosquitoes lay eggs in still water bodies with high organic content, such as open septic tanks, storm drains and unmanaged swimming pools. As an urban species, immatures can develop even in small water containers such as pet water dishes and saucers under potted plants (Walker et al. 2011).

Culex tarsalis (Western Encephalitis Mosquito)

Similar to Cx. quinquefasciatus, Cx. tarsalis lays eggs in floating rafts and the larvae hatch shortly after oviposition. Unlike Cx. quinquefasciatus, this species prefers permanent or semi-permanent water bodies to oviposit, such as ponds or irrigation ditches. In southern Arizona it is associated with agricultural lands and is less common in urban areas (Williamson et al. 2025).

Aedes aegypti (Yellow Fever Mosquito)

This species exhibits different oviposition behavior from Culex mosquitoes, as it cements individual eggs on the substrate of the water-holding container just above the water line (Figure 2A). The egg must stay damp for about 48 hours to allow for embryo development after which it can remain viable in dry conditions for months before hatching in the presence of water. Female mosquitoes only oviposit in small, temporary water containers such as plastic buckets, tires and saucers under potted plants. They will not oviposit on soil-lined waterbodies such as ditches but will utilize cement-lined drainage structures (personal observation, D. Williamson). Ae. aegypti and Cx. quinquefasciatus mosquitoes may both use the same small containers for oviposition sites.

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Drainage structures can provide a suitable environment for mosquito larvae to develop. Urban drainage systems in the desert southwest of the United States typically do not contain the intricate storm drain networks that are common in wetter regions of the country. For example, the drainage features present in the Phoenix metropolitan area are designed to collect and absorb water within each 1-mile square block. This requires the utilization of multiple types of drainage features that lead the water to the lowest points in the watershed, usually a wash, greenbelt, park, or retention area. The drainage feature at the end of the watercourse is called a dry well and is typically an underground vertical water structure that disposes of unwanted runoff or stormwater. Most dry wells are cement-lined cylinders with an average diameter of about 4 feet and depths ranging from 2 to 85 feet (Edwards et al., 2016). They normally have a gravel bottom, and a slotted manhole cover at the surface (Figure 3). Dry wells differ from storm drain collection reservoirs in that there are no discharge pipes that transfer water from the dry well to a larger storm drain network. The water instead stays within the dry well until it either evaporates or permeates into the groundwater through infiltration (Edwards et al., 2016). In principle, dry wells should not retain water for more than 72 hours (New Jersey Stormwater Best Management Practices Manual, 2004). Maricopa County specifically recommends drywells that cease to drain a retention basin with 36 hours should be replaced or refurbished (Maricopa County, 2014). Unfortunately, dry wells that are not maintained or that were poorly constructed may hold water for much longer, creating excellent habitat for Culex quinquefasciatus and Aedes aegypti larvae. Many counties throughout Arizona utilize dry wells as a component of water management.

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The primary method for sampling a water source for the presence of immature mosquitoes is the use of a standardized dip cup. The dip cup is a cup roughly 6” in diameter with a pole attached to one end and holds ~ 350 mL of water. The cup is dipped into the water and then inspected for the presence of larval and pupal mosquitoes. This provides a simple and standardized approach to sample open water sources.

However, sampling dry wells and other drainage structures can be challenging for vector control professionals. Standard dip cups do not have the reach to sample low water levels in deep dry wells. Additionally, dip cups require the removal of the manhole cover which requires training and specialized tools. Some dip nets may be malleable enough to make it through the grate slot, but pole length and maneuverability through a grate slot may not be practical.

A novel sampling technique developed by our team is a siphon pump. An inline hand siphon pump is fitted with an appropriate length of hose for the sampling location on the intake side, and a small length of hose on the output side leading to a bucket. The end of the hose on the intake side should be fitted with a flexible funnel to aid in drawing surface larvae into the intake hose. The siphon pump method allows simple, rapid and consistent sampling of the water without removing the manhole cover. Once the flexible funnel is fed through the grate slots in the manhole cover (Fig. 3B), the line can be lowered to the water level. The initial disruption of the water surface will likely cause the larvae to dive, but the continual uptake of water allows time for the larvae to return to the surface and be funneled into the intake hose. Water is pumped into a bucket which is easily inspected for the presence of immatures. This approach is useful for determining presence of larvae but has not been standardized to estimate total number of larvae in a dry well. A similar device was described by Livdahl and Willey (1991).

Construction of sampling pump

Materials

• Stainless Worm Gear Hose Clamp, 5/16" - 7/8"
• Hopkins FloTool 10803 Transfer Pump or Similar
• Silicone Foldable Funnel with a 5/8” OD at the output, 3 ¼” OD at the Input, and an overall extended length of 3 7/8”.
• 5/16" ID 7/16" OD Clear Vinyl Tubing Bucket with volume markings and a minimum capacity of roughly 2 liters of water

Pump assembly

1. Cut vinyl tubing to length needed to reach the sampling environment. Having a length of roughly 15 – 20 feet in length will be adequate to reach most water levels from the surface. Having a longer length of tube available that can be switched out may be helpful in reaching particularly deep-water sources.
2. Cut vinyl tubing to length needed to reach from the pump to the collection bucket.
3. Attach tubing to the hand pump in the corresponding positions depending on water flow using worm gear hose clamps (Figure 5).
4. Attach collapsible funnel to the end of the collection tubing using a worm gear hose clamps.

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Pump use 

1. Push flexible funnel through manhole cover slots and lower vinyl tubing to the surface of the water. Ensure that the funnel is just at surface level.
2. Place output hose in collection bucket.
3. Use pump handle to draw water out of the dry well and into the collection bucket.
4. Collect roughly 2 liters of water in collection bucket.
5. Remove input tubing and funnel from the dry well.
6. Inspect the water for larvae and pupae. Transfer all immatures to a collection tube for further analysis. Dump water or keep collected water depending on growth regulation eclosion testing.

Larval identification

Based on our research in southeastern Maricopa County (see below), the primary mosquito species found in urban drywells was Culex quinquefasciatus, although Aedes aegypti were also found occasionally. With practice, the larvae of the two species are easy to distinguish:

Culex quinquefasciatus larvae

Culex quinquefasciatus are characterized by a long thin siphon (breathing structure on the tail of the larva; Figure 7A, arrow). Under a microscope, the larvae also have multiples lines of comb scales on the 8th abdominal
segment next to the siphon (Figure. 7B).

Aedes aegypti larvae

Aedes aegypti larvae are characterized by a short, thick siphon (Figure. 7C, arrow). Larvae are highly reactive. Under a microscope, the larvae also have a single curved line of comb scales on the 8th abdominal segment next to the siphon (Figure. 7D).

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Larvicides can be applied to drainage features when clogging or larvae are observed. Long-lasting larvicide briquettes may also be applied proactively in anticipation of clogging and mosquito activity later in the season. Table 1 shows a list of different larvicide products available both for professional mosquito control and for residents with home infestations of mosquito larvae. For residents, the safest options are Bti-based products available in dunks or other easy-to-use formulations. Always follow the label instructions.

Larvicide field trials

We conducted a field trial of two different longlasting larvicide products in 12 drywells with a history of mosquito larval infestations in Chandler, Arizona during the summers of 2017 to 2020 (Ph.D. dissertation by Williamson D). The treatments included the Altosid 150-day briquette and the FourStar 150-day Bti briquette. Label application rates for FourStar are one briquette for up to 100 sq. ft. of surface area with an additional briquette for each additional 100 sq. ft. regardless of depth. Application rate should be doubled in water with lots of organic matter or high populations of larvae. For Altosid, recommended application rates are one briquette per 100 sq. ft of surface up to two ft of depth and an additional briquette for each additional two feet of water depth. In our field trials, all treated drywells received two briquettes of either FourStar or Altosid. Each of the 12 dry wells were checked for immature mosquitoes weekly between July and October. Altosid is an insect growth regulator and thus does not directly kill mosquito larvae but instead prevents adult mosquitoes from emerging. Therefore, all immature mosquitoes collected from dry wells treated with Altosid were collected and maintained under laboratory conditions monitored daily for adult emergence. Trials in 2017 and 2018 suggested that neither of the briquettes were effective, possibly due to fluctuating volumes of water, sedimentation and the tendency of larvae to stay near the water surface as contrast to the larvicide briquette sinking to the bottom of the drywell. It should be noted, however, that immature larvae from Altosid-treated drywells might have recovered after being transferred to the laboratory. Those immatures were kept in the water that they were collected from, but they were now separated from the Altosid briquette that continued to supply the water source with the growth regulator.

To address these possibilities a floating device was developed to maintain the released larvicides close to the surface (Figure 8). In 2019 and 2020 the study was conducted using the floating device that could be inserted into the drywell without the removal of the manhole cover to suspend the larvicide briquettes. In assays with the improved delivery system, both the Altosid and FourStar briquettes reduced the probability of finding viable larvae in the dry wells, but the Altosid was much more effective.

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Due to manufacturing concerns about the FourStar product, the trial was repeated on the 12 dry wells in 2020 using VectoMax FG granules which contains 2.7% Ls and 4.5% Bti as active ingredients and a dose of 115 g was applied instead of the FourStar briquette. VectoMax FG granules are designed to float, but are loose, so to keep them from being washed away the briquette netting was replaced with a floating muslin bag. Field trials in 2020 demonstrated that both larvicides reduced the probability of finding viable larvae in the dry wells, but the Altosid was again much more effective (Figure 9).

Major findings

• Locating all drywells in an area was challenging. Many drywells were not visible from the street.
• Most of the larvae in drywells were Culex quinquefasciatus. Aedes aegypti larvae were only occasionally found in that habitat, usually in dry wells with water levels close to the top of the well (see Figure 3B).
• The long-lasting sinking briquettes were not effective when placed at the bottom of the drywells, likely due to the large volumes of water and the tendency of larvae to remain near the water surface. Briquettes provided better larval control when applied in floating nets.
• Altosid briquettes applied in floating nets were effective at preventing adult emergence.
• The VectoMax FG granules were more effective than the Fourstar Bti briquettes, possibly due to problems with the formulation. NOTE: The formulation of FourStar Bti has been changed since 2020.

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Centers for Disease Control and Prevention (CDC. 2024. Life Cycle of Culex Mosquitoes. https://www.cdc.gov/mosquitoes/about/life-cycle-of-culex-mosquitoes.html.

Edwards EC, Harter T, Fogg GE, Washburn B, Hamad H. 2016. Assessing the effectiveness of drywells as tools for stormwater management and aquifer recharge and their groundwater contamination potential. Journal of Hydrology. Volume 539, Pages 539-553. https://doi. org/10.1016/j.jhydrol.2016.05.059

Livdahl T, Willey M. 1991. An efficient, inexpensive and fun-to-use contraption for sampling mosquito larvae. J Am Mosq Control Assoc. vol. 7, no. 3, www.biodiversitylibrary.org/content/part/JAMCA/JAMCA_V07_N3_P496-498.pdf.

Maricopa County. 2014. Stormwater Management Facilities Operation and Maintenance Manual (template).

Moser SK, Barnard M, Frantz RM, Spencer JA, Rodarte KA, Crooker IK, Bartlow AW, Romero-Severson E, Manore CA. 2023. Scoping review of Culex mosquito life history trait heterogeneity in response to temperature. Parasit Vectors. 16(1):200. doi: 10.1186/s13071-023-05792-3. PMID: 37316915; PMCID: PMC10265793.

New Jersey Stormwater Best Management Practices Manual. 2004. Chapter 9.3: Standard for Dry Wells. Page 9.3, 1-7.

Suman DS, Tikar SN, Mendki MJ, Sukumaran D, Agrawal OP, Parashar BD, Prakash S. 2011. Variations in life tables of geographically isolated strains of the mosquito Culex quinquefasciatus. Med Vet Entomol., 25(3), 276-288

Walker KR, Joy TK, Ellers-Kirk C, Ramberg FB. 2011. Human and environmental factors affecting Aedes aegypti distribution in an arid urban environment. J Am Mosq Control Assoc. 27(2):135-41. doi: 10.2987/10- 6078.1. PMID: 21805845.

Williamson D, Jiang H, Karanikola V, Young S, Wissler C, Townsend J, Damian D, Will J, Degain B, Dutilleul P, Carrière Y, Walker KR. 2025. Interannual changes in the association between land use, abundance of Culex quinquefasciatus and Culex tarsalis (Diptera: Culicidae), and occurrence of arboviruses in Maricopa County, Arizona. J Med Entomol. 2:tjaf080. doi: 10.1093/jme/ tjaf080.