Understanding Pest Activity After Rain

Why rain changes pest behaviour

Many common pests live in places that flood: drains and sewers, soil, leaf litter, mulch and low-lying ground. Heavy rain forces them to move to higher, drier ground — and a house is often the nearest option.

Rain also raises humidity, which suits insects that lose water easily, such as cockroaches and silverfish, and it creates still water for mosquitoes to breed in. Warm rain has the biggest effect, because insects are already active and develop quickly. A summer storm in Brisbane or Sydney produces far more pest activity than the same rainfall on a cold winter night in Melbourne.

Rain is also a signal. Many insects time breeding and dispersal to wet weather: winged ants and termites swarm on warm, humid evenings after storms, mosquito eggs laid on dry ground hatch when flooded, and lawn pests such as armyworm often build up after rain in late summer.

What to expect, and when

During and within hours

• Large cockroaches leaving flooded drains and gardens, appearing near doors, drains and weep holes. See cockroaches after rain.
• Ants moving nests and trailing indoors to benches and bathrooms. See why you suddenly have ants.
• Huntsman and other hunting spiders coming inside to shelter.

Warm, humid days after

• Winged ants (and sometimes termites) swarming, often around lights in the evening.
• Funnel-web spider males wandering in parts of NSW and South East Queensland. See funnel-web spiders.
• Fire ant mounds becoming more visible in lawns and open areas in South East Queensland. See how to identify fire ants.

One to two weeks later

• Mosquito numbers rise as larvae in containers, gutters, low-lying land and saltmarsh mature. See mosquitoes after rain.
• Flies build up where rain has soaked rubbish, compost or spoiled food.

Several weeks later

• In late summer and autumn, armyworm caterpillars can appear in lawns. See how to know if you have armyworm.

Once things dry out

• Most activity settles. Cockroaches that keep appearing after conditions dry may be established in the roof void, sub-floor or walls.

Why some pests come inside and others don’t

Pests that come indoors after rain are mostly looking for dry shelter, not food. That has two practical consequences.

First, sealing the routes they use — gaps under doors, dry drain traps, weep holes without covers, gaps around pipes — is more effective than spraying. A cockroach displaced from a flooded drain will take the first gap it finds; if there isn’t one, it shelters somewhere else.

Second, most of these visitors leave or die off once conditions dry out. A few large cockroaches or a huntsman in the week after a storm is normal in subtropical Australia and doesn’t mean the house has an infestation. It is the pests that stay — or that appear in large numbers well after the rain — that point to a problem worth investigating.

Winged ants or termites?

After storms, people often find swarms of winged insects and worry about termites. Ants have a narrow waist, elbowed antennae and front wings longer than the hind wings. Termites have a thick waist, straight bead-like antennae and four wings of equal length. If you find termite swarmers indoors or piles of discarded wings, arrange an inspection by a licensed timber pest inspector; see our termite basics.

Where mosquitoes breed after rain

Backyard mosquitoes need surprisingly little water. After rain, check:

• pot-plant saucers, buckets, watering cans and children’s toys;
• blocked roof gutters and box gutters, which hold water out of sight;
• tarps, boat covers, wheelbarrows and bins without lids;
• old tyres, tins and anything else lying in the yard;
• bromeliads and other plants that hold water in their leaves (flush them weekly);
• rainwater tank inlets and overflows, which should have intact insect-proof mesh;
• low spots in the lawn and blocked drains where water pools for more than a few days.

Tip out, cover, flush or fix each of these within a few days of rain. Mosquito larvae can complete their development in about a week in warm weather. See reducing mosquitoes around your home.

Before the wet season

• Clear gutters and downpipes so water drains away from the house.
• Check door seals, sweeps and fly screens.
• Move mulch and garden beds back from walls and weep holes.
• Remove or cover containers, tarps, toys and tyres that could hold water.
• Screen rainwater tanks and check the mesh on overflow outlets.
• Seal gaps around pipes and cables where ants trailed in previously.
• Fit weep-hole covers and make sure every floor waste has an intact grate.
• Store firewood, timber and boxes off the ground so they don’t become shelter for displaced pests.

After a storm: a simple plan

Day 1

• Check doors, weep holes and floor wastes where cockroaches and ants come in, and top up water in rarely used drains.
• Relocate spiders outside; wear gloves when clearing debris and shake out shoes left outdoors.

Days 2–3

• Walk the yard and tip out anything holding water.
• Wipe up ant trails with detergent and seal the entry point.
• Clear debris from gutters and drains.

Week 1–2

• Use repellent outdoors at dawn and dusk as mosquito numbers rise.
• Check lawns for brown patches and birds feeding, especially in late summer and autumn.

After that

• If cockroaches, ants or other pests are still appearing regularly once things are dry, look for an established population and consider an inspection.

After a flood

Floods are different from ordinary storms. Water may have entered the house, sewage and debris may be spread across the property, and food may have spoiled. These conditions bring their own pest and health issues.

• Flies breed quickly in spoiled food, wet rubbish and flood debris. Bag and remove spoiled food and debris as soon as you can.
• Mosquitoes breed in pooled floodwater and debris. Health departments often issue alerts about mosquito-borne disease after floods.
• Rodents displaced by floodwater may move into buildings and sheds.
• Leptospirosis, a bacterial infection spread through water or soil contaminated with animal urine, is a known risk after floods. Wear boots and gloves during clean-up, cover cuts, and wash your hands well.

Follow your state health department’s advice on flood clean-up, and see a doctor if you develop a fever after flood exposure. See are mosquitoes dangerous in Australia.

Lawns and gardens after rain

Wet weather changes what happens outside, too. In late summer and autumn, armyworm caterpillars can appear in large numbers after rain and strip a lawn in days; brown patches that spread quickly, with birds feeding on the lawn, are a common early sign. See how to know if you have armyworm.

Curl grubs, the larvae of scarab beetles, also feed on roots in moist soil, and birds digging up the lawn are often searching for them. See what curl grubs are.

Slugs and snails become active after rain. In Queensland and NSW they can carry rat lungworm, so wash garden produce well and supervise children and dogs around them.

Overwatered or poorly drained garden beds against the house keep the soil damp for longer, which suits ants, cockroaches and termites. After the wet season, check that beds and mulch aren’t built up against walls or covering weep holes.

An indoor check after heavy rain

• Look under sinks and around the laundry for new leaks and damp, which attract cockroaches and silverfish.
• Check ceilings for stains that show a roof leak; damp roof timber suits pests and should be repaired.
• Run water into spare drains and floor wastes.
• Wipe up ant trails with detergent and water, which removes the scent trail, and find where they are entering.
• Store food in sealed containers until ant and cockroach activity settles.

Protecting your family from bites

Mosquito numbers usually peak one to two weeks after heavy rain, and in Queensland some mosquitoes can carry diseases such as Ross River virus. Removing breeding sites around your home helps, but personal protection matters most during the peak:

• Use an insect repellent containing an active ingredient such as DEET, picaridin or oil of lemon eucalyptus, and follow the label, particularly for children.
• Wear long, loose, light-coloured clothing outdoors at dawn and dusk.
• Keep screens on windows and doors in good repair, and use a mosquito net for babies’ cots and prams.
• See a doctor if you develop fever, joint pain or a rash after mosquito bites.

See are mosquitoes dangerous in Australia.

When to worry

Most post-rain pest activity settles within a week or two. It is worth a closer look if:

• cockroaches keep appearing well after the weather dries, or you see small cockroaches and nymphs;
• ants are trailing from inside walls, power points or window frames rather than from outside;
• you find termite swarmers indoors, piles of wings, or mud tubes;
• mosquitoes stay heavy around the house despite tipping out water, which suggests breeding you haven’t found yet, such as a blocked gutter or tank;
• you hear rodents in the ceiling.

For local detail, see pests after rain in Brisbane.

Answers related to this guide

• Why Do Cockroaches Appear After Rain?
• Why Do Mosquitoes Increase After Rain?
• Why Do I Suddenly Have Ants?
• Why Are Mosquitoes So Bad in Brisbane After Rain?
• Are Mosquitoes Dangerous in Australia?
• How Do I Know If I Have Armyworm?

Local information

• Pests After Rain in Brisbane

Pests in this guide