Shoo, little wasps! – Insect friendly mowing

I didn't think much of it when I agreed to take a closer look at the box full of test tubes. What I couldn't have guessed was what would develop from this: a visit to the Black Forest, a roadside experiment and a common goal: to save insects from death by lawn mower.

What’s in the box?

My supervisor brought a box to a meeting, which was supposed to be about my doctoral thesis. "These are from an experiment with an insect-friendly mower. That's a super exciting project," he said, "and it would be a great fit for your work!" Well, I thought, looking through a few test tubes and doing a bit of statistics should be quick. That was before I had a closer look at the box and its contents. In retrospect, I'm glad I agreed to do it because, thanks to that box, I learnt a lot about mowing equipment and its effects on insects.

Insect-friendly mowing – Is that even possible?

An insect and spider-friendly mower? When the company MULAG Fahrzeugwerk Heinz Wössner GmbH u. Co. KG in the Black Forest set itself the goal of building such a tool, it was clear that it had to be demonstrably effective and be able to protect the small inhabitants of lawns and meadows alike. Therefore, the University of Tübingen with Prof Dr Oliver Betz and the University of Hohenheim with Prof Dr Johannes Steidle were brought on board. In an initial trial, they were to test how the insect- friendly device performed in comparison to conventional flail mowers. A flail mower is a mowing device that cuts and shreds the grass at the same time and is often used on roadsides. It should also be tested to what extent a lorry tarpaulin placed in front of the mowing head helps to scare away insects before they get under the mower. This procedure originally comes from bird conservation and is intended to scare away breeding birds during hay harvesting so that they can flee from the approaching mower in good time. In this study, the tarpaulin was tested on insects for the first time. This is where I came in, as I wanted to take a closer look at the smallest of the sampled insects: the parasitoid wasps.

Mowing impacts insects

Mowing has serious consequences for most insects and other “creepy-crawlies”, as between 60 and 88% of the animals are killed when mowing with a lawnmower. In addition, the survivors are often deprived of food by the mowing of plants. In the case of roadsides, however, mowing cannot be avoided for safety reasons. One approach is therefore to make mowing as insect-friendly as possible.

Unfortunately, the effects of mowing have not yet been investigated for parasitoid wasps – which are my area of expertise. They are significantly smaller than their black and yellow striped relatives, are incredibly species-rich and lay their eggs on or in their hosts, which are usually other insects or spiders. The host dies during the development of the wasp larva, which means that parasitoid wasps fulfil an important role in the ecosystem: They keep their host populations in check so that they do not get out of hand.

Insect-friendly mower and tarpaulin tested

For the experiment, strips along the roadside were either mowed with a conventional mowing head or the insect-friendly mowing head, or left untreated as a control. The strips were then sampled by setting up three 1 m³ cages per strip and vacuuming them with a special “insect vacuum cleaner”. The insects were stored in sample tubes with alcohol. These sample tubes were in the mystery-box that my supervisor had given me. In a second experiment, we tested the lorry tarpaulin suspended in front of the mower, by pulling it over more grass verges at the side of the road. As we only wanted to test the influence of the tarpaulin, we did not mow in this trial. Afterwards, I counted the parastoids caught in both experiments with the help of students and identified them down to the family. In both experiments, we only counted the intact insects left on the patches of grass after mowing or “treatment” with the tarpaulin compared to the control.

Only partially better for parasitoids

The conventional mowing head had a serious impact on the parasitoid wasps. In comparison, the mowing head labelled as insect-friendly was actually gentler, but only for chalcid wasps, which made up a large proportion of the wasps counted. Our hopes were therefore pinned on the simple tarpaulin. However, even that only had a limited positive effect on the small wasps. This shows that small parasitoid wasps are only partially protected by the insect-friendly mower and the lorry tarpaulin during mowing. It is therefore all the more important not to mow everything at once, but to always leave strips in alternating places. For our insects, it is even best to mow only twice a year, once in summer between June and July, and once in autumn starting from mid-September. The clippings should then be removed.