The small wriggling creatures in a rain barrel, bird bath, blocked gutter or garden pond are almost certainly mosquito larvae ('wrigglers'), with comma-shaped pupae ('tumblers') among them. In summer the whole cycle from egg to biting adult can take a week or less. The fix is to take the water away or treat it. Empty and refill bird baths weekly, and screen or seal rain barrels. Where water has to stay, float a Bti product such as Mosquito Dunks: the label rate is one dunk per 100 sq ft of surface, broken down to a quarter dunk for 1–5 sq ft such as a bird bath, and it works for about 30 days. Bti kills only mosquito and related fly larvae, and the label allows it in fish habitats and water gardens. For bites, repellents and disease, follow CDC advice.
Mosquito larvae hanging from the water surface by their breathing tubes, with a comma-shaped pupa. Photo: pending.
Larvae are slender, usually black or brown, and hang from the surface film to breathe. Culex and Aedes larvae breathe through a distinct tube (siphon) at the tail; Anopheles larvae have none. They feed on microorganisms in the water and grow fast in warm weather through four instars. The newly hatched ones are almost invisible to the naked eye.
Pupae are dark, comma-shaped and also breathe at the surface; they tumble end over end through the water. UKY notes that larvae and pupae both dive when the light changes, so approach without casting a shadow. They are easy to see in clear, shallow water over a light bottom and hard to see in dark water full of debris or plants. Culex females lay floating rafts of 100 or more eggs on the water. Aedes lay eggs just above the waterline on damp container walls, and these hatch only when the water rises, sometimes months or years later.
Larvae do nothing to plants. They matter because each container of still water is a nursery for biting adults, and a small one produces a lot of them. UKY warns that a neglected bird bath, swimming pool or clogged rain gutter can produce hundreds of new mosquitoes in just a few days. UC IPM singles out rain barrels and landscape drainage as sources that can affect a whole neighbourhood. Some adults stay near where they hatched, while others fly several miles. Clearing your own water cuts the local supply but may not end the bites. For mosquito-borne disease and personal protection, the CDC is the authority.
The whole life cycle happens in still water except the adult stage, which is why larval control works better than anything aimed at the adults. How long it takes varies with species and water temperature. In summer, UC IPM puts egg to adult at a week or less. Eggs laid on the water surface usually hatch within a couple of days. Aedes eggs laid on the dry sides of containers can wait months or years for a flood, so a container can breed again as soon as it refills. The pupa does not feed and Bti cannot kill it, so pupae already present when you treat will still emerge.
Larvae need still or slow water, not a plant. The garden sources are rain barrels, bird baths, plant saucers, buckets, tarps, old tyres, tree holes, clogged gutters, unused pools and ornamental ponds. Moving water helps: UKY notes that mosquitoes prefer quiet, non-flowing water, so a pump or fountain in a water feature reduces breeding. UC IPM also advises removing excess vegetation from ponds, and notes that fish work best where there are no dense stands of emergent plants.
There is no tolerance level for a garden container: UC IPM's rule is that anything holding water for more than a few days should be drained, filled with soil or cement, or treated. For a pond or water feature that has to stay, the choice is between moving the water, stocking fish where that is legal and appropriate, and treating it with Bti.
Once a week in warm weather, walk round and look into everything that holds water, including saucers, gutters, tarps and the inside of rain barrel lids. Approach without casting a shadow, and dip a white cup or long-handled dipper quickly into the water to catch larvae and pupae. In a pond, check the margins and among the plants.
Bti is the control to buy. It is a bacterium whose toxin kills mosquito larvae once they eat it, and UC IPM says it kills only mosquitoes and the larvae of a few related flies. Mosquito Dunks are slow-release rings that float and release Bti for about 30 days. Mosquito Bits are granules that act within 24 hours and last up to 14 days. The fish most often suggested, the western mosquitofish Gambusia affinis, comes with a serious warning. It is native only to the Mississippi River basin and the Gulf Slope drainages from Mobile Bay into Mexico. Introduced elsewhere, it has contributed to the elimination or decline of endangered and threatened fish in the West, and in California it preys heavily on native newt larvae and treefrog tadpoles. USGS reports that critical reviews of the evidence have not supported the view that it is particularly effective at reducing mosquitoes or mosquito-borne disease. UC IPM says it works best in man-made water that does not connect with natural waters, that it must never be released into streams, ponds or lakes (in California, where it is not native, that is illegal), and that you can get it from your local vector control district. Dragonfly and damselfly naiads eat mosquito larvae in ponds — about 40 a day each in container experiments, by a 2023 meta-analysis — but no source measured how far they reduce biting in a garden, so a dragonfly pond is a bonus, not a control.
Taking the water away beats treating it. What cannot be emptied can usually be screened, sealed or kept moving.
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
UC IPM Pest Notes: Mosquitoes, UC ANR Pub. 7451 (Walton & Eldridge; updated 2020)
Univ. of Kentucky ENTFACT-005, Mosquitoes: Practical Advice for Homeowners (Potter et al.; rev. 4/26)
EPA-accepted label, Summit B.T.I. Briquets / Mosquito Dunks, EPA Reg. 6218-47 (Mar 2008)
EPA-accepted label, Summit Mosquito Bits, EPA Reg. 6218-73 (Jan 2011)
USGS Nonindigenous Aquatic Species, Gambusia affinis fact sheet (Nico et al.; rev. May 2025)
UF/IFAS Featured Creatures EENY-355, Dragonflies and Damselflies (Rhodes & Bybee)
Priyadarshana & Slade 2023, meta-analysis of odonate predation on mosquitoes, J Anim Ecol 92:1589–1600
US CDC, Mosquitoes — prevention and mosquito-borne disease (pointer only)
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
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