Adaptive significance of egg size in longhorn beetles: Species‐specific responses to larval starvation and host tree species

Optimal reproductive strategy theory predicts that egg size, a key reproductive trait, positively correlates with offspring performance and, ultimately, parental fitness. However, this relationship is not universal and is more frequently observed in environments with adverse larval conditions. Egg size may be particularly critical in xylophagous insects, as woodborers face unique challenges associated with the hardness, chemical recalcitrance and low nutritive value of wood. In this study, we investigated the influence of egg size on larval performance, settling success, larval moulting likelihood and early larval survival in two sympatric, oak‐dwelling longhorn beetles, Cerambyx cerdo L. and Cerambyx welensii (Küster) through controlled experiments in a wood‐boring context, where larvae were also subjected to increasing durations of starvation from 1 to 4 weeks. Larger eggs resulted in higher survival times in starved larvae of both species. Moulting likelihood was unaffected by egg size, but it was higher in C. cerdo than in C. welensii , despite the former species laying smaller eggs. In starved C. welensii larvae, both survival time and moulting likelihood were strongly influenced by the interaction between egg size and the parental host tree (holm oak, cork oak or Pyrenean oak). Larval settling success increased with egg size in C. welensii under moderate starvation (2 weeks), but not under short (1 week) or prolonged starvation (3–4 weeks), whereas in C. cerdo , larval settling was unaffected by egg size. Our results demonstrate that larger egg size provides an adaptive advantage in these woodborers by enhancing the survival chance of starved larvae and potentially improving settling success. However, the magnitude and direction of these benefits vary between longhorn species and parental host oak.