Occurrence of Z- and E-strain Ostrinia nubilalis in Sweden shortly after first detection of the Z-strain

Introduction: The European corn borer (ECB), Ostrinia nubilalis (Hubner) (Lepidoptera: Crambidae), is a major pest insect in maize. New cultivars and climate change have allowed range expansion of the crop northwards in Europe, and a similar range expansion of associated pests has been predicted. ECB exists as two distinct pheromone strains using different blends of (E)-11-tetradecenyl acetate and (Z)-11-tetradecenyl acetate for sexual communication; in addition, the Z-strain feeds almost exclusively on maize. In Sweden, the maize cropping area has increased tenfold during the last two decades, increasing the potential for spread of the Z-strain, which was recorded for the first time in the country in 2010.Aim of the study/methods: In this study, we performed trapping experiments with strain-specific lures in maize fields in southern Sweden during 2013-2022 to monitor the presence and relative abundance of the two ECB strains and evaluate the potential increase of the Z-strain. Trapping was also performed at least 5 km away from maize fields. Molecular markers were used to verify the strain identity of trapped males and larvae that fed on maize.Results: Catch data revealed that both pheromone strains were present in the major maize cultivation areas of Sweden, as predicted by recent modelling studies. Our data indicate that the flight period of ECB in southern Sweden lasts from the middle of June to late August, with peak flight in July. Catches of both strains were low at all trapping sites and throughout the study period and decreased further during the years following the dry summer of 2018. No Z-strain males were trapped far away from maize fields.Conclusion: Based on our results, and the low levels of ECB-related damage observed in local maize fields, the Z-strain is currently not imposing a threat to Swedish maize growers.

Automated summary

This study conducted trapping experiments in maize fields in southern Sweden using strain-specific lures to monitor the presence and abundance of two pheromone strains of European corn borer (ECB) and assess the potential increase of the Z-strain. The results indicate that both strains are present in the major maize cultivation areas of Sweden and the flight period of ECB in southern Sweden lasts from June to August, with peak flight in July. However, the catches of both strains were low throughout the study period and decreased further after the dry summer of 2018. No Z-strain males were trapped far away from maize fields.