Extended winters entail long-term costs for insect offspring reared in an overwinter burrow

Abstract

Winter imposes an ecological challenge to animals living in colder climates, especially if these adverse conditions coincide with reproduction and offspring rearing. To overcome this challenge, some insects burrow in the soil to protect adults, larvae, or eggs from negative effects of winter. However, whether this protection is effective against any long-term consequences of changes in winter duration is unclear. Here, we investigated the long-term effects of winter length variation on eggs of the European earwig Forficula auricularia. In this insect, females construct and maintain a burrow between late autumn and spring, in which they provide extensive forms of care to their eggs and then juveniles. We experimentally maintained earwig females under two winter durations of either four or six weeks and examined the resulting effects in terms of 1) hatching date, 2) developmental time of juveniles until adulthood, 3) adult mass at emergence, and 4) investment of adult offspring females in three key immune parameters: hemocyte concentration, phenoloxidase, and prophenoloxidase activities. Because earwigs' resistance against pathogens relies on their social environment, effects of winter length on immunity were tested on females exposed to different social environments: with familiar conspecifics, unfamiliar conspecifics, or in isolation. Our results reveal that after the winter treatments, eggs reared in short winters hatched earlier and the emerging juveniles reached adulthood faster than juveniles from eggs exposed to long winters. We also showed that prophenoloxidase was 30% higher in females from the long compared to short winter treatment, regardless of social environment. Finally, we found that hemocyte counts where twice as high in short compared to long winter females, but only with unfamiliar conspecifics. Overall, our study reveals that maintaining and caring for eggs in a burrow does not prevent the costs associated with increased winter duration.

Variations in seasonal (not mean) temperatures drive rapid adaptations to novel environments at a continent scale

“…Similarly, cold Februaries might slow down the development of first‐clutch eggs and thus extend the corresponding period of egg care. This, in turn, might inhibit females’ physiological transformation to produce a second clutch (Vancassel , Gingras and Tourneur , Tourneur , Körner et al ). However, our results were obtained under common garden conditions, which reveals that the observed effects of the variation in seasonal temperature on egg‐laying dates are not a plastic response to their current environment, but are due either to the environment experienced during their early life development (i.e., before field sampling), or to an inherited basis that possibly emerged through canalization (Nylin and Gotthard , Van Buskrik and Steiner ).…”

“…Females then burrow in the ground from mid‐fall to early winter and build a nest, where they lay their first clutch of eggs. After egg laying, females stop their foraging activity and provide extensive forms of egg care until hatching (Gingras and Tourneur , Boos et al , Koch and Meunier , Thesing et al , Diehl and Meunier , Körner et al ). The eggs of this first clutch hatch in spring and mothers remain with their newly hatched larvae for several weeks, during which mothers provide larvae with multiple forms of care (Gingras and Tourneur , Kölliker et al , Kramer et al ) and larvae exhibit forms of sibling cooperation (Falk et al , Kramer et al , Kramer and Meunier , Körner et al ).…”

“…During their transport and outdoor maintenance, containers received an ad libitum amount of carrots and pollen as a food source for earwigs, and were supplied with water by means of a cotton pad regularly soaked in water. This setup allowed earwigs to perform noncontrolled mating and to live in groups, which is similar to their natural living conditions (Weiss et al , Sandrin et al , Kohlmeier et al , Körner et al ).…”

“…This is because the severity and duration of changes in the temperatures experienced by an individual during its entire or a specific period of its life cycle can alter a great number of its life‐history traits and ultimately modify its own fitness. For instance, extended winter durations or exposure to warmer environmental conditions can shape the size and time of first reproduction (Fretwell , Altizer et al ); the duration of fetal development and levels of immune activity (Körner et al , Zhang et al ); as well as the mobility, morphology, and metabolism of individuals (Adamczewski et al , Danks , Polidori et al ).…”

Friend or foe? The apparent benefits of gregarine (Apicomplexa: Sporozoa) infection in the European earwig

“…Our measurement of total-PO (i.e. the sum of prophenoloxidase and phenoloxidase) activity followed a standard protocol (Vogelweith et al, 2017a;Körner et al, 2018), in which each field-sampled individual was first CO2-anaesthetized, then pricked with a clean needle between the seventh and eighth dorsal tergite and 1-2 µl of their hemolymph were finally extracted through the wound using a glass capillary. The extracted hemolymph was immediately mixed with a Cacodylate buffer solution (0.01 M sodium cacodylate, 0.005 M CaCl2; pH 6.5) in an Eppendorf tube placed on ice and then maintained at -20°C.…”