[Objectives]  To determine the timing of occurrence of adults of Monolepta hieroglyphica in cornfields in Qiqihar and provide a scientific basis for forecasting and developing an IPM strategy for this pest. [Methods]  The emergence dynamics and body size (length and width) of M. hieroglyphica adults were systemically measured in cornfields by covering fixed sites with nets in from 2017 to 2019. [Results]  The first adults were found in early to mid-July with the peak of emergence occurring from mid-July to early August, The number of emerged adults significantly decreased after mid-August, with few emerging from late-August to early September. No newly emerged adults were observed after mid-September and all had disappeared by early October. The adult post-emergence period was 61 d to 74 d, and the number of adults per square meter ranged from 12.0 to 97.8. Adults were smaller in terms of length and width at the beginning of emergence, were of maximum size during the peak of emergence, and then of a similar size to that observed during the initial period of emergence. Temperature had a significant influence on initial and peak emergence, and the duration of emergence. High temperatures during May-July were beneficial to adult emergence. More rainfall from June to August decreased the number of adults that emerged. [Conclusion]  The first adult M. hieroglyphica emerged in cornfields in Qiqihar in early to mid-July, and the majority in early to mid-August. The number of emerged adults deceased significantly after mid-August, with few emerging from late August to early September. No newly emerged adults were observed in late September. The length and width of emerged adults was related to the timing of emergence, being greatest during the peak of emergence. Temperature and rainfall affected the emergence time and the total number of adults that emerged.