Moth abundance declines 1961-2020 in a New Zealand montane grassland.

Recent evidence has sparked increasing global concern of widespread insect biodiversity loss. However, there are few long-term studies on insect trends, especially in the Southern Hemisphere. In a native tussock grassland at Cass, inland Canterbury, Aotearoa New Zealand, Graeme White documented a 56% decline in moth abundance from 1961 to 1989, possibly caused by the spread of the invasive grass Agrostis capillaris. To determine if this moth decline has continued since 1989, I resampled moth abundance in 2020-2021 using White’s methodology, light traps, and sites at Cass. White caught 28,505 moths of 151 species in the 1960s, and 11,608 moths of 165 species in the 1980s, while I caught 2,698 moths of 80 different species (94% of species with known biostatus were endemic). Average number of moths caught per trapping night declined significantly from 691 in the 1960s to 284 in the 1980s and 122 now. This represents declines of 57% since 1989 and 82% since 1961, with a 3.1% annual rate of decline that has not changed significantly across decades. Trends differed between macro- and micro-moths, where macro-moths increased 20% in abundance from 1961-1987 then decreased by 75% from 1987 to 2020, while micro-moths continually declined, down 82% 1961-1987 and 22% 1987-2020. One of the largest species declines was of Orocrambus vittellus (Crambidae), decreasing from 273 moths per night in the 1960s to 6 in the 1980s and 5 now. With consistent land management at both sample sites since White’s study, land use changes are an unlikely cause of this considerable moth decline. Likewise, there was no evidence of shifts in species distributions or phenology due to climate change. Similarly, there was little evidence of exotic moth invasion, although the recent invasion of Vespula wasps in the Cass area may negatively impact local moths through predation on larvae. While A. capillaris abundance has remained unchanged since 1990, a large increase in another invasive grass, Festuca rubra, may be reducing native host plants of some moth species. Nevertheless, an observed increase in native shrub cover, primarily Discaria toumatou, may benefit some moths, like Harmologa spp. (Tortricidae) which feed on D. toumatou and increased in relative abundance from 2% today. Overall, the 82% decline in this moth community shows insect declines in Aotearoa are continuing, and stresses the importance of documenting trends in abundance and identifying drivers of decline to effectively conserve native insect fauna