Seasonal Phenology and Developmental Potential of Halyomorpha halys Under Temperate Continental Conditions

The seasonal development and overwintering success of Halyomorpha halys remain insufficiently resolved in temperate continental regions of southeastern Europe, where climatic conditions may permit more than one generation per year. We monitored seasonal development, thermal accumulation, overwintering survival, and pheromone-trap captures of H. halys during 2024 and 2025 in continental Croatia. Experimental populations were maintained under unheated semi-outdoor conditions and followed from overwintered adults through oviposition, all five nymphal instars, and adult emergence. Degree-days were calculated above a lower developmental threshold of 12.2 °C. Two complete egg-to-adult generations developed in both years. The observed phenological interval from the first egg masses to first adult emergence was 56 days in 2024 and 42 days in 2025 for G1, and 49 and 53 days, respectively, for G2. Thermal accumulation during immature development ranged from 468.5 to 562.7 DD12.2. First-generation adults emerged in early July in both years, whereas second-generation adults appeared on 12 September 2024 and 23 September 2025. Developmental stages overlapped during summer and early autumn. Overwintering survival under the two anthropogenic refuge conditions was low, reaching 9.4% under heated and 7.8% under unheated indoor conditions. Pheromone-trap catches showed marked seasonal variation but were not correlated with temperature, relative humidity, or precipitation during corresponding trapping intervals. These results demonstrate the developmental potential for two generations under temperate continental conditions in Croatia and provide phenological benchmarks for stage-specific monitoring. Monitoring should begin with spring adult activity and be intensified during May–June and again from late July onwards as successive generations develop.