Impact of Larinus latus Infestation on the Growth and Reproductive Performance of Silybum marianum under Field Conditions in Egypt

Larinus latus is one of the most important phytophagous insects associated with Silybum marianum weed (milk thistle). A comprehensive field survey was conducted over two consecutive winter growing seasons (2023/2024 and 2024/2025) starting in January for both seasons in El-Beheira and Kafr El-Sheikh Governorates, Egypt. The study aimed to assess the biological impact of L. latus infestation on key reproductive traits of S. marianum, including the number and fresh weight of insect-infested inflorescences and their seeds, across multiple field locations (18 sites in each governorate). The main findings showed that insect infestation of S. marianum inflorescences caused a marked reduction in fresh weight and resulted in the production of fewer and lighter seeds compared with uninfected inflorescences in both seasons. Moreover, insect infestation significantly inhibited the reproductive capacity of S. marianum, suggesting a regulatory role in limiting its growth. This study underscores the ecological importance of weed–insect interactions and supports the potential for incorporating insect-induced effects into integrated and sustainable management strategies for S. marianum weed.