You hardly see them anymore, insects. Logical, because it’s winter. But they are there. They hibernated, as an egg, as a larvae, as a pupa or as an adult individual. Each species has developed its own success strategy. With the rest period they prevent a dangerous period. There is little or no food, because much of nature goes into hibernation. And at rest you don’t run the risk of freezing if the temperature drops below zero.
But the insects are still alive. They simply use much less energy when they rest. Going into hibernation, staying dormant, and coming out again is a complicated process. All kinds of chemical processes occur that cannot be stopped or reversed overnight. The timing of when you go into hibernation and when you come out again is vitally important. Only individuals who time it well and have built up sufficient energy reserves for the rest period survive. The times largely depend on the length of the day and the temperature. And this last factor has changed spectacularly in recent years due to climate change. This year is also truly exceptional. The following figure shows the trend of the average daily temperature in 2023 (orange line) and the normal one fifty years ago (blue line), the temperature to which the insects that naturally live in our country have adapted. This year will most likely be the warmest year ever and the graph shows that the average temperature this year was (much) higher than normal.
Daily body weight loss of 125% more than normal
Very high temperatures make it more difficult for insects to determine when to go into hibernation and when to come out again. However, one of the biggest problems with (very) high temperatures in autumn, winter and early spring is that the energy consumption of insects during hibernation is much higher than in colder conditions. They are cold-blooded animals so their body temperature is the same as room temperature. The higher it is, the faster the energy burns. And since they cannot absorb energy, they use up their energy reserves much faster. Several scientific studies show that a higher temperature during the rest period leads to greater mortality during this period. Surviving individuals emerge from the resting period in a weaker condition than if the temperature during the resting period were normal.
In a recent study, a laboratory examined how the pupal weight of small white veins (Pieris napi) during rest changes as the temperature increases. At a temperature of 2°C a doll loses 0.023% of its body weight per day. At 15°C the body weight loss is 0.091%, at 20°C it increases to 0.135% body weight loss per day. In recent days the average daily temperature has been around 9.3°C. This is much higher than the 2.1°C normally recorded at the end of December. This means that nowadays daily weight loss is 125% (!) higher than normal and on average 50% every day since the beginning of autumn.
Less defense against pathogens
A decrease in energy reserves has all sorts of consequences, including susceptibility to pathogens. Insects have several options to defend themselves from pathogens. One of the ways is to encapsulate foreign particles. All types of enzymes and sufficient fat reserves play a role in this. In the bumblebee field (Forest bombs) it was shown that larger queens at a higher winter temperature (7 to 10 °C) had more fat remaining at the end of the winter and their defense was better than smaller queens. At lower temperatures (0 to 3°C) no difference was found between larger and smaller queens. From the earth bumblebee (A land bomb) it was already known that survival during hibernation is partly determined by weight. Queens that weighed less than 0.6 gram before dormancy did not survive the winter, regardless of how long they remained dormant. If a warm winter follows a spring and summer in which food was difficult to find, the effect on insect survival during the winter will be even greater because insects begin hibernation with fewer food reserves.
Contribution to the spectacular decline of insects?
The fall was the second warmest on record. The insects that entered hibernation from September onwards had to deal with an average temperature that was about three degrees higher than the normal temperature of fifty years ago. On Boxing Day the Butterfly Foundation published the message Butterfly Year 2023: even fewer butterflies. Since the 1990s the number of butterflies registered in the Butterfly Monitoring Network has continuously decreased. In the same period there is a clear increase in autumn and winter temperatures. From 6.9 °C in the nineties to 8.6 °C in recent years.
Considering the strong influence of insect dormancy temperature on insect body weight, viability and survival, climate change may play an even greater role in the spectacular insect decline observed in recent decades than previously thought. A 2017 study published in PLosOne showed that insect biomass decreased by 70% in 27 years. A study published in September this year in the scientific journal Nature concluded that meteorological variables explain this observed change in insect biomass. Above all, the temperature of the previous winter appeared to be decisive. The higher the winter temperature, the lower the insect biomass in the following year. According to the authors, higher winter temperatures can lead to greater exposure to low temperatures (due to earlier exit from hibernation), dehydration and increased predation. There is no discussion of a possible effect due to increased energy consumption. It therefore seems important to take a closer look at changes in insect energy metabolism during warm winters. If high winter temperatures truly weaken hibernating insects, then the extremely high temperatures today and in recent months do not bode well for insect populations in the coming year.
Text: Arnold van Vliet, Wageningen University and Research
Photo: Willem van Kruijsbergen, Saxifraga (main photo: small veined white); Arnold van Vliet, KNMI data
2023-12-27 17:47:03
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