"The warmer, the more interesting for the insects"

"Only as an entomologist are you so crazy that you look forward to a dangerous heat wave because you can get something positive out of it," wrote entomologist Thomas Hörren, 32.

"The warmer, the more interesting for the insects"

"Only as an entomologist are you so crazy that you look forward to a dangerous heat wave because you can get something positive out of it," wrote entomologist Thomas Hörren, 32. So he packed traps, a net and a lamp to take advantage of the special situation. Why?

WORLD: Where did you go to collect and watch beetles?

Thomas Hörren: I'm in a larger forest complex south of Karlsruhe.

WORLD: And why in this heat?

Hörren: Very many insects avoid the heat in summer and are not active during the day. They then swarm in the evening hours and into the night. The higher the temperature, the longer they swarm. Temperature is a limiting factor: If it cools down to, for example, 16 degrees and there is dew, then nothing will fly anymore. On tropical nights when the air is as warm as possible, even the very small organisms can fly.

WORLD: Does that apply to all species?

Hörren: On these nights you will mainly find beetle species that live in small standing bodies of water. They need these swarm flights to find new waters. Especially in summer there is a risk of drying out.

WORLD: What is the reason for the swarming?

Hörren: In many species, these are pure mating flights. Some species wait for the hot summer days before even swarming. Various beetles actively migrate, sometimes hundreds of kilometers, and open up new habitats. Especially at these high temperatures one finds species that do not fit the habitats at all. They must have covered tens of kilometers in one night.

WORLD: How does it work with the light?

Hörren: The lighting system is operated with a 12 V battery and uses light spectra in the UV range. On the one hand, there are active light traps. The beetles fly against a window pane behind which UV light shines. They fall down and end up in a sample. This can also be used to quantify, they are preserved in alcohol and evaluated in the laboratory. On the other hand, we use the "lighthouse", which is a net-like structure with a lamp in the middle. The animals land on the net and can be observed. You have a colorful smorgasbord of insects in the light, which are very differently active.

WORLD: What did you find?

Hörren: When a glowing evening takes place in such heat, almost 300 beetle species are detected. That is much. You get an insight in one evening that rarely works with other methods. Types of beetles that live underground and develop on fungi such as truffles have emerged. For example, the horned dung beetle (Bolboceras armiger) or the truffle beetle of the Leiodes genus. They only fly very briefly to reproduce and are rarely found by other methods.

WORLD: Are there temperature limits at which swarming stops?

Listen: Not with us. Days with 40 degrees Celsius and nights with 30 degrees are exceptions, even in the traditionally hot extreme southwest of Germany - also within the past decades. Therefore one cannot say much. We only observe this increased swarming effect.

WORLD: What changes with increasing warming?

Hörren: Some habitats suffer enormously from the heat and extremes. Temporary small bodies of water carry much smaller amounts of water or none at all for several years. Also alluvial water. The species have fewer habitats where they can reproduce. In the case of peatlands, it is now known that they are more severely affected by climate change. The alpine insects also have a problem when they are in glacial areas and need moisture and seasonal snow. These species are currently migrating upwards.

WORLD: What about the majority?

Hörren: With most insect species we have the trend: the warmer, the more interesting for them. The open-land insects depend on a certain number of hours of sunshine. you benefit. We also have a north-south divide, there are far fewer species in Schleswig-Holstein than in Baden-Württemberg. And in many areas, change is underway.

WORLD: Are there species from the south?

Listen: Yes. On the one hand, species migrate from the Mediterranean region. The Kaiserstuhl is considered to be an absolute warm region, where a species appears almost every year that has never before been found in Germany, such as the strawberry star beetle (Stelidota geminata) from South and Central America. And the southern species continue to migrate north, extreme where heat disperses, along the Rhine plain and in the big cities on the river. This has increased significantly due to the warm years.

WORLD: If you're out half the night, how are you doing?

Listen: I'm a bit exhausted. The hot nights are not easy to plan ahead, it interferes with the regular work. I still find it very exciting. I spend most of the day cool in the office. Yesterday it went until three o'clock, so I sleep a little longer in the morning.

WORLD: What about mosquito bites?

Hörren: I use a wide variety of sprays for short-term protection against mosquitoes, as I get bitten quite a lot and react strongly. But that's only a small help, on nights like this I'm really mauled by mosquitoes. But that is part of it.

Thomas Hörren, 32, is an entomologist and coleopterologist. He is a member of the Krefeld Entomological Association. There he was co-author of the long-term insect study in 2017, which caused a sensation worldwide. As Totholz.thomas he informs about his work.