Biological features of the development of the corn earworm in Ukraine are insufficiently studied. Therefore, the research conducted in 2017–2019 were the first attempt to describe in detail the phenology of the pest in the maize agrocenosis of the Left Bank Forest-Steppe of Ukraine. It was found, that the emergence duration of the corn earworm imagoes during the years of research lasted 46–52 days. The first butterflies on maize appeared in the first and third decades of June. The mass emergence of the phytophagous occurred in the second-third decade of July (the bloom period of maize). The seasonal dynamics of corn earworm imago in maize significantly depended on both the weather conditions of the growing seasons and the phases of crop development. The duration of development of the stages of eggs, caterpillars and pupae in 2017–2019 was 5.3, 31 and 20 days, at the sum of effective temperatures of 38.5, 243.6 and 147.0 °C, accordingly. In maize crops in the Left-Bank Forest-Steppe of Ukraine corn earworm developed during one generation, and the second generation was optional. Corn plants are damaged only by the first generation of phytophagous. For the development of the first generation of phytophagous, the sum of effective temperatures must be 590–610 °C at the development threshold of +15.0 °C, the duration of development is on average 61.3 days, at the average daily air temperature +21.6 °C.

Goal. Determine the damage to modern corn hybrids by the caterpillars of the cotton bollworm, develop a point scale for assessing the damage of kernels on the cob, and calculate the direct losses of grain yield. Methods. Field. The studies were carried in the research fields of the Cherkasy Research Station of Bioresources of the NSC «Institute of Agriculture of NAAS» 2017—2019. The cultivation of corn was carried out in accordance with the cultivation technology recommended for the Forest-Steppe zone of Ukraine, Cherkasy region. Plot size — 56 m2, in four replicates. Placement of plots — randomized. The number of damaged corncobs of cotton scoop was determined per hectare. To calculate the number of damaged kernels per corncobs, each damaged corn cobs was opened and the number of eaten and damaged kernels was counted. To calculate the direct yield losses for each corn hybrid, the total number of kernels eaten and damaged by the caterpillars of the cotton bollworm (per hectare) was calculated, which was multiplied by the mass of 100 kernels, respectively. Results. The degree of harmfulness of cotton bollworm caterpillars on maize hybrids of different maturity was investigated. In 2017—2019 The corncobs are most damaged in hybrids of the mid-maturing group — 58.4%, less — in early-maturing hybrids — 41.1%, and in the middle-early ones — 47.8%. For the first time, a nine-point scale was developed for assessing the defeat of caryopses on the cob by Helicoverpa armigera Hbn. caterpillars direct losses of grain yield are calculated. It was found that significant yield losses were observed in all mid-season maize hybrids (0.185—0.280 t/ha). Conclusions. In the Left-Bank Forest-Steppe of Ukraine, the cotton bollworm turned out to be a dangerous pest of the generative organs of corn. Therefore, the selection of resistant hybrids to attack by caterpillars of the cotton bollworm is a key element in the protection of this crop. And the nine-point scale developed by us allows us to assess the degree of damage to caryopses by the caterpillars of Helicoverpa armigera Hbn.