Prevention · Rainy season
Rainy season pests: what to expect and when
Every May we get the same call: “overnight the place filled up with ants” or “scorpions showed up that I had never seen before.” They didn't all hatch at once. What happens is that water floods the spaces underground and pushes them out, and the nearest dry shelter is usually your house. Knowing what moves at each point in the year lets you get ahead of it instead of reacting.
- The first heavy rains push ants, scorpions and centipedes out of the ground and indoors.
- Mosquitoes don't come out with the rain but 7 to 10 days later: that's how long the breeding cycle takes.
- The July veranillo (the short dry spell in July) brings a burst of fly and cockroach activity from heat combined with moisture.
- With the heavy rains at the end of the season, rodents come up from sewers and storm drains.
May and June: what comes up from the ground
The first heavy rains saturate the ground. Everything that lived in the top few centimeters of soil moves up and indoors.
- Ants: trails show up where there never were any. Colonies move their brood to dry spots: under the floor, inside hollow walls, behind the baseboard.
- Scorpions: this is when they are reported most. They look for dark, dry places and turn up in shoes, behind picture frames and in clothes left on the floor.
- Centipedes and pill bugs: more annoying than dangerous, but they're a good indicator: if they show up, humidity is high and there's probably an entry point at floor level.
- Winged ants and termites: mating flights take off with the first downpours. A swarm of winged insects inside the house deserves attention: it can point to a colony nearby. See how to identify termites.
What to do now: check the seals on doors at floor level, cover the drain in the service bathroom when it's not in use, and get anything stored in the garage and storage room up off the floor.
The mosquito lag
There's a common mix-up: people expect mosquitoes the day it rains. In fact the downpour only fills up containers; what comes next is development, which takes about a week to ten days in the heat here.
In other words: today's mosquitoes come from last week's breeding sites. That's good news, because it means there's a window to act. After every heavy downpour it pays to do a ten-minute walk around: gutters, plant saucers, tires, buckets, lids, tarps with folds, bottles in the yard, the dog's water bowl. Anything that holds water for a few days.
The key point is that most breeding sites are inside the property, not in the neighbor's empty lot. See the details in the weekly breeding site check.
The veranillo: heat with built-up moisture
The July break combines the worst of both worlds: temperatures rise, but the moisture stays in the structures. This is the window of peak insect activity.
- Cockroaches: heat speeds up their cycle. A population that was under control can explode in two or three weeks.
- Flies: the egg to adult cycle gets shorter with heat. In sodas (small Costa Rican eateries) and restaurants this is when it shows the most; see how to break their cycle.
- Mold and moisture: walls that got wet in June and didn't dry out feed insects that live on mold. See how to eliminate sources of moisture.
September and October: what comes in from below
The rainfall peak at the end of the season overwhelms sewers and storm drains. The rodents living there move out and look for higher ground: they get in through poorly sealed pipes, ducts and drains.
- Put metal grates on yard and utility sink drains.
- Check where pipes pass through the wall: that gap around the pipe is a classic entry point.
- If you hear movement in the ceiling, check whether it's a rodent or something else: see the signs of rats and mice.
A preventive application goes much further done in April, before the rains start, than in June when the house is already occupied. If you had a problem last rainy season, book before the next one begins.
Does the problem come back every rainy season?
A perimeter treatment before the rains costs less than dealing with it once the house is already occupied.
References: