The black fly (Simulium erythrocephalum) is not an invasive species in Spain; it is an native insect of Iberian ecosystems. In recent years, however, its populations have surged, and the regions of Madrid, Catalonia, Valencia, Murcia, Aragon, and Andalusia report that these flies now behave like a persistent problem for both people and animals.
In Madrid alone, health services recorded more than 300,000 people treated for bites from 2015 to 2019. Health authorities note that more than 20,000 patients are affected in Zaragoza each year, a figure that stood at under 4,500 a decade ago. The bite of the black fly can be especially intense, more accurately described as a skin puncture that leaves a bruise lasting several days to a week. Residents in affected towns have often sought emergency care for large swelling, dermatitis, and severe itching.
Pets and farm animals are not spared. Species such as sheep, cattle, horses, and goats show particular vulnerability, and dogs and other pets are also at risk. The insects congregate in large swarms around livestock, harassing and biting persistently, pushing farmers to adjust grazing times to minimize exposure. Vets have seen increased workloads during several summers due to this nuisance.
The black fly is very small, typically 3 to 6 mm, and resembles a tiny wasp in appearance. Observers suggest that inadequate riverbed cleaning has contributed to the surge, with residues from products used during the long coronavirus lockdown era having been trapped in water channels. This fly lays larvae on vegetation that touches fresh water, and the failure to clean these channels properly has helped its multiplication. High temperatures also accelerate hatch rates, while a decline in natural predators such as swallows, swifts, and bats reduces natural control. Some experts advocate protecting beneficial insect species as part of long-term management strategies to curb this nuisance.
The bite is often stealthy; the insect lands briefly and minimizes contact with the skin, making it easy to miss. It injects an anesthetic and an anticoagulant to keep blood flowing, so the sting may not be noticed until later. When a bite is finally recognized, it is usually inflamed with a red center, and the surrounding skin may be itchy and swollen for days. The bite is typically painful and can leave a marked red spot that persists for some time. In many cases, applying cold to the area and seeking medical advice when signs of allergy appear is recommended. According to the Spanish Society of Emergency Medicine (SEMES), the bite is painful but does not carry a risk of disease transmission.
As temperatures rise, reports of black fly activity increase in Madrid, especially in areas near the capital’s periphery. Outdoor activities in certain zones can become challenging due to the high presence of these insects. In 2020, the City Council conducted a major fumigation along the Manzanares River to curb the problem, but the issue has reemerged in the current year.
Similar trends are observed in Catalonia. The Ebro Environmental Policy Consortium (Copate) initiated the first large-scale treatment against the black fly plague in February of the current year, targeting the lower stretches of the Ebro river. This action has shown effectiveness following last year’s efforts. The organization manages a budget of 1.3 million euros to combat black flies and mosquitoes, with half funded by the Generalitat and the remainder shared by the Tarragona Provincial Council and the Delta de l’Ebre municipalities.
1º How can I prevent stinging?
– Wear long, light clothing
– Limit time spent near rivers or reservoirs
– Install mosquito nets at home
– Eliminate standing water to reduce breeding sites
– Use appropriate flying insect repellents
2. What are the symptoms?
Severe pain at the bite site
– Central red mark and possible bleeding
– Inflammation of the surrounding area
– Itching and swelling or blotching around the bite
3º What is the treatment?
– Clean and disinfect the bite area with an antiseptic
– Apply cold to reduce swelling
– Refrain from scratching to avoid worsening the itch
– Use topical products to soothe the bite
4. Consult a health center or your specialist
SEMES guidance on black fly bites can be found on SEMES materials . Readers may also be interested in the broader topic of climate-related pest impacts on crops and ecosystems.