Developing an economic injury level for Asiatic garden beetle ( <i>Maladera formosae</i> ) larvae in commercial mint production systems of the Midwest

The Asiatic garden beetle (AGB) Maladera formosae (Brenske) has become a damaging pest of commercial mint in northern Indiana. The larvae feed on plant roots, causing stunting and plant death when damage is severe. However, the seasonal relationship between AGB larval density and above-ground mint biomass (foliage and stems) is unclear, leaving farmers without fundamental knowledge necessary to implement sound IPM strategies. We assessed AGB larval density and above-ground biomass in 3 mint fields during the spring and fall of 2021 and 2022 to establish a damage boundary and develop an economic injury level (EIL) based on currently available management tactics. A soil-quadrat excavation method was used to estimate AGB larval density and above-ground biomass in 15 replicate 0.25 m2 sampling areas in each field. AGB larval densities varied annually and seasonally and although fall and spring AGB larval densities were correlated, the highest densities within each larval cohort were observed during fall. Accordingly, a significant negative relationship between larval density and above-ground biomass was only observed in the fall, when mean larval densities surpassed 15 larvae/0.25 m2. Moreover, our damage probability model estimated 50% probability of reduced above-ground biomass at larval densities of 15/0.25 m2. A conservative EIL of 19.4 to 23.3 AGB larvae/0.25 m2 was estimated based on use of chemical or biological control strategies available to farmers. Results indicate that field estimation of AGB larval densities can support management decisions in mint production, and estimates of larval density based on summer/fall populations are most likely to predict economic damage.