Tirathaba mundella is a pyralid moth whose larvae infest oil palm inflorescences, causing bunch failure. This study quantified sex-specific infestation patterns and larval stage compositions of T. mundella on peat-grown palms. Over 20 weeks, 882 inflorescences (441 females; 441 males) were harvested from the Jobenar Raya Estate in Sarawak, Malaysia. Larvae were dislodged, counted, and assigned to five instars. Infestation incidence was greater in female inflorescences (27.9%; 123/441) than in male inflorescences (8.2%; 36/441), yielding a risk ratio of 3.42 (95% CI: 2.41–4.83). Among infested inflorescences (n = 159), multinomial logistic regression revealed a strong shift in stage structure by inflorescence sex (likelihood ratio χ² = 1578.9, df = 4, p < 0.001). Females were enriched for later instars (Stage 4–5), whereas males were dominated by early instars (Stage 1–2). These results indicate that female inflorescences serve as the principal developmental sites that sustain advanced stages, whereas male inflorescences may act as transient sites for early-stage establishment. Surveillance and control should prioritise female inflorescences and incorporate male inflorescences into sanitary pruning. Future work should determine whether larvae migrate between inflorescence types or whether dual-site oviposition with differential survival underlies the observed pattern, thereby improving the timing and efficiency of integrated pest management strategies.