Lisbon just released 30,000 mosquitoes. On purpose.

On Thursday 24 September, thirty thousand sterile male mosquitoes were released in the Lisbon parish of Alvalade, in the first phase of an experimental project to reduce invasive Asian tiger mosquitoes. Nobody needed to panic. Male mosquitoes don't bite.

It sounds like a prank. It is actually one of the cleverer ideas in insect control, with roots in 1950s livestock pests and backing from, of all places, the International Atomic Energy Agency. It is also being slightly oversold in the headlines. Here is what is really going on.

Only the females bite

Both sexes feed on nectar and plant sugars. Only females take blood, because they need the protein to develop eggs.

A male mosquito, sterile or not, has no interest in you.

Afonso Moreira, the public health doctor involved, told the Lusa news agency that the released mosquitoes don't bite, precisely because they are males.

The target is Aedes albopictus, the Asian tiger mosquito: a small, striped, daytime biter that has spread through much of southern Europe. Lisbon is focusing on it because it can transmit viruses including dengue, Zika and yellow fever. Alvalade was chosen for its vegetation and courtyards, which can give the mosquito favorable conditions to develop.

How sterility becomes a control method

The sterile insect technique (SIT) was pioneered by American entomologists Edward Knipling and Raymond Bushland against the New World screwworm, a livestock parasite. The principle has barely changed. Rear huge numbers of insects, sterilize the males (usually with a low dose of radiation), and release them. When wild females mate with sterile males, their eggs don't hatch. Repeat, and the population shrinks.

Researchers like it for mosquitoes partly because it targets a single species and leaves nothing in the environment once releases stop (Oliva et al., 2021). The bees, butterflies and native insects in Alvalade's gardens are untouched.

The Lisbon project is funded by the International Atomic Energy Agency and involves the Spanish company Tragsa, alongside Portugal's Directorate-General for Health, the national health institute INSA and Lisbon City Council. Tragsa's involvement matters, because it ran one of Europe's longest mosquito SIT pilots, in Valencia.

What Lisbon is actually testing

Some coverage described this as a "final trial". It isn't. It was the last release of phase one, and the main aim of this phase is to understand how the released mosquitoes behave — particularly how well they disperse and survive in the intervention area.

That is not a technicality. Sterile males are only useful if they reach wild females. Programs in Italy, France, Spain and Switzerland commonly plan around dispersal distances of 100 to 200 meters, so Lisbon needs its own local numbers before a real test makes sense.

That real test comes later. If phase one results are positive, a comparative study between release areas and control areas is expected in May. For now, nobody knows whether this will reduce Lisbon's tiger mosquitoes.

What Europe has found so far

The European evidence is promising, but it is young and mostly comes from small pilot areas.

Valencia, Spain. From 2018 to 2020, more than 15 million sterile males were released across 80 hectares at two pilot sites (Tur et al., 2021). The program was reported as effective in reducing tiger mosquito populations, and researchers also studied the problem of wild mosquitoes migrating into treated towns from surrounding areas (Tur et al., 2023).

Greece. Weekly releases over 22 weeks in 2019 gradually cut egg density by up to 78%, and the reduction held at 70 to 78% for four weeks after releases ended (Balatsos et al.).

Ticino, Switzerland. A 2023 pilot recorded a 57% reduction in egg abundance and a 66% fall in adult females within the intervention area.

Guangzhou, China. The most dramatic result used a different recipe. Millions of males carrying an artificial Wolbachia infection were released, and were also irradiated to stop any accidentally released females from reproducing. The combined approach achieved near elimination of field populations of Aedes albopictus (Zheng et al., 2019), though other researchers have published critiques of the method (Moretti and Calvitti, 2021).

Two caveats matter. The same Nature paper notes that radiation-based SIT has suppressed several insect pests, but its effect on mosquito control has been limited. And these studies measure eggs and adult mosquitoes, not illness.

Fewer mosquitoes in one district is an entomological result, not proof of wider public health impact.

The least glamorous tool still matters most

Perhaps the most useful line in the Lisbon coverage came from Moreira himself. He stressed that releasing sterile mosquitoes is only one control measure, and that breeding sites still need eliminating — particularly small accumulations of water in public and private spaces.

Tiger mosquitoes need remarkably little water to breed: a plant-pot saucer, a blocked gutter, a forgotten bucket, a child's toy left out after rain. Emptying them once a week is free, requires no radiation, and works in every city on the continent.

Why this story is worth watching

What makes Lisbon's experiment interesting isn't the shock value. It's the thinking behind it: understanding one species' biology so precisely that you can turn its own mating habits against it, while leaving everything else in the garden alone. Whether it works in Alvalade will be known next year. Either way, it is a reminder that the smartest insect control often starts with knowing the insect.

The Wild Bunch Naturals Journal follows mosquitoes, plants, repellents and the changing science of outdoor life.