Microclimates in herb patches can provide refuges for overheating insects

As the climate warms, it’s not the steady increase in temperature that is the only threat. With the rising heat will come wilder extremes. Gols and colleagues in the Netherlands have researched some of these variations. But their research, appearing in a forthcoming issue of the Journal of Thermal Biology, is not at a large scale. Instead, they’ve been looking at patches of plants and the differences between inside the patches and the edges. They have found that at these small scales, temperatures can vary. That will have an impact on the insects and other bugs that live in the undergrowth.

The reason for the temperature differences is the physical cover that plants provide. At the edge of a plant patch, the habitat is exposed to the wind and sun. Inside the patch, leaves buffer these effects. This buffer provides more stable conditions for wildlife. This stability could be crucial for insects that cannot regulate their body temperatures but respond to the heat around them.

Gols and colleagues examined forb (any herb that isn’t a grass) assemblages over a square metre. “This is an ecologically realistic scale at which small arthropods are exposed to potentially fluctuating temperatures,” they write. “More specifically, we monitored ambient air temperature, soil surface temperature at the edge and within a plant patch and temperature within the plant canopy. Temperatures were recorded two or three times a week from between mid-June and mid-August of 2020, in a small experimental field in the Netherlands. The period included an official heatwave according to the World Meteorological Organization, which is a period of at least five consecutive days during which the daily maximum temperature is least 5 °C higher than the average maximum temperature.”

They were able to set up the experiment by piggy-backing on another mesocosm experiment on climate change, they write. “We chose this model system as it allowed us to record temperature under controlled conditions. The plots contained several native and range-expanding forb species, representing a patchy plant community with both exposed edges and areas covered by the vegetation.”

The results showed some substantial differences between the interior and edges of patches. Soil surface temperatures peaked at 40.9 °C in the interior. At the edges, this rose to a maximum of 59.3 °C. The team found that when the air temperature rose above 22.8 °C, the soil started to get warmer than the air.

For insects, rising temperatures can be an aid until they become a significant problem. You’ll have seen insects get more active as the day gets warmer, but this activity reaches an optimum. Pass this point, and performance diminishes rapidly. “…when body temperature reaches or exceeds the relatively narrow range in which performance deteriorates quickly, insects risk to overheat and this may critically impact on insect physiology and ultimately its survival… When exposed to temperatures over 40 °C even for a short period (min), many insects are physiologically or behaviourally impaired … Moreover, in some insects the upper thermal limits on reproduction are often several degrees lower than on survival. Exposure to high temperatures at which insects can survive nevertheless can destroy their eggs and/or sperm, rendering the insects sterile…,” write Gols and colleagues.

The authors point to other recent papers on dramatic declines in insects, sensationalised as the insect apocalypse. While the losses might not be that bad yet, invertebrates are under pressure from many different directions. Heat stress will be a contributing factor in the future.

This study could add an extra dimension to campaigns like PlantLife’s #NoMowMay. The hook for No Mow May is the array of wildflowers that can appear in a lawn. They add at the end of their page, “And if you’re really keen and want to rewild all summer, try #LetItBloomJune and #KneeHighJuly“. In the future, it won’t just be the nectar the flowers provide that’s essential in the summer heat. Taller and more bountiful herbs and grass might also provide respite from the heat for insects.

In the UK, gardens provide more green space than all the national nature reserves in the country. It suggests there’s not just plenty of research to do on the thermal ecology of flowers but also on the shade the plant leaves provide.