Entomologist says horseflies are having a good year in Estonia
This summer's lack of rainfall has given people a welcome break from mosquitoes. But horseflies, which depend on larger bodies of water, are doing quite well, says entomologist Urmas Tartes.
Blood‑sucking insects often appear where walkers don't expect them, and horseflies leaving thick forests may search for prey even above asphalt roads. "It seems horseflies are doing pretty well. I usually meet them on forest paths, but yesterday I went to Tartu's Lõunakeskus shopping centre and right there in the parking lot, in bright daylight, a nice grey‑banded specimen came to suck blood," Tartes said.
Dry period affects mosquitoes
He noted that the perceived number of insects strongly depends on people's habits and expectations. A long dry period emptied forest floors of mosquitoes, and people quickly got used to a mosquito‑free start to summer. "We probably notice horseflies more this year because we've become used to a situation without mosquitoes. Their attacks feel unexpected," Tartes added.
The difference in abundance between the two groups comes directly from their larval habitats. Mosquitoes rely on temporary puddles formed by snowmelt and heavy rain — and the dry early summer left these breeding sites bone‑dry.
Other blood‑sucking insects grow in more stable conditions and survive drought more easily. In the thick mud of lakes and ponds, their larvae develop into strong, capable fliers, and even harsh weather does not stop their life cycle. "Horseflies live in permanent water bodies that don't dry out completely. Their larvae can move and follow the water as levels change," Tartes explained.
Air temperature also determines insect activity. Both groups prefer humid air, but cool weather keeps them hidden. Around 10°C, all flying insects stop flying. As the air warms, mosquitoes appear first.
When temperatures reach around 20°C, the heat energises larger insects. "For horseflies, the rule of thumb is about 20 degrees or warmer. When mosquitoes stop flying, horseflies begin — that's their gradation," Tartes said.
Only female insects hunt blood
Horseflies attack with a much more robust mouthpart than mosquitoes, which explains why their bite is far more painful. A mosquito pierces the skin with a thin needle‑like structure, causing minimal tissue damage. A horsefly tears an open wound into the skin using jaws located in its snout. "They don't have a proboscis but a snout with jaws. With those, the horsefly bites a hole into the skin. To eat the blood seeping from the wound, it has to lick and suck it," Tartes said.
Only female insects hunt blood — they need it for egg development. Males avoid mammals entirely and feed on plant juices and nectar, making them invisible to people in the forest.
Estonian horseflies fall into three groups: Chrysops, Hybomitra, and Tabanus. Walkers often encounter yellow‑winged Chrysops species, while darker Hybomitra horseflies approach silently and land unnoticed.
Different flight styles make it difficult to repel all insects at once. Specialists recommend wearing light‑coloured clothing, as dark tones attract heat‑loving insects.
The most intimidating are the large, loud horseflies approaching with a heavy buzz. Their size reveals their true preferences: they rarely attack humans. "The biggest ones are called cattle horseflies, almost the size of a child's thumb. As the name suggests, they prefer large animals, and in my experience they rarely attack people," Tartes reassured.
--
Editor: Jaan-Juhan Oidermaa, Argo Ideon
Source: ERR interview by Sten Teppan and Marju Himma
