Most wild bees hibernate as pupated larvae in their cocoons in the ground, in wood or in other protected places. Species emerging early in spring hibernate as fully developed adults in the cocoon, while species emerging later in summer will still need to finish development during spring.

Climate change is altering the timing of insect emergence after hibernation in spring or summer worldwide. These shifts can have consequences: if insects hatch too early due to higher temperatures, they may not yet find the flowers or prey they feed on. Moreover, hibernating adults burn their essential fat reserves faster under higher temperatures. This can reduce their chances of survival and reproduction.

Insects from 160 locations in Bavaria examined

What about the physical fitness of bees and wasps when temperatures change in spring, before the time of hatching? A team led by Dr. Cristina Ganuza and Professor Ingolf Steffan-Dewenter from the Biocentre at the University of Würzburg investigated this question. The researchers looked at five wild bee and wasp species that occur in Bavaria and hatch at different times of the year.

The team undertook a mammoth task for the study: it collected almost 15,000 individuals in hibernation from over 160 regions in Bavaria and then raised them at the university under controlled cold, warm and hot spring conditions to simulate different climate scenarios.

Females lose up to 34 percent of their body mass

All five species hatched earlier in warmer spring temperatures. However, populations differed according to their climatic origin: spring species from warmer regions such as Lower Franconia appeared particularly early in warm spring temperatures and retained more body mass in the following period than individuals from cooler regions such as the Bavarian Forest.

In contrast, only those individuals of late summer species that originated from cooler regions hatched earlier. Females of summer species lost body mass more rapidly under warmer conditions – in some cases up to 34 per cent.

Insects from cooler regions that fly during spring are at a disadvantage

‘Our data show that insects from cooler regions are particularly vulnerable to warm springs. They lose energy more quickly and therefore have poorer starting conditions,’ says Dr. Cristina Ganuza. The lead author of the study is a researcher at the Chair of Zoology III (Animal Ecology and Tropical Biology) at the University of Würzburg.

The results of the study have been published in the journal Functional Ecology. They are part of the LandKlif project led by Professor Steffan-Dewenter within the Bavarian Climate Research Network (bayklif).
The researchers see several open questions that need to be clarified next:

• How do additional days of extreme heat affect hatching?
• What consequences do the energy reserves of insects have on their pollination performance?
• How quickly can populations adapt to changing temperatures?