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SignificanceInvasive social insects are among the most damaging of invasive organisms and have proved universally intractable to biological control. Despite this, populations of some invasive social insects collapse from unknown causes. We report long-term studies demonstrating that infection by a microsporidian pathogen causes populations of a globally significant invasive ant to collapse to local extinction, providing a mechanistic understanding of a pervasive phenomenon in biological invasions: the collapse of established populations from endogenous factors. We apply this knowledge and successfully eliminate two large, introduced populations of these ants. More broadly, microsporidian pathogens should be evaluated for control of other supercolonial invasive social insects. Diagnosing the cause of unanticipated population collapse in invasive organisms can lead to applied solutions.

Although the ecological effects of invasions often become obvious soon after introduced species become established, more gradual effects may take years to manifest and can thus require long-term data for quantification. We analyzed an 8-year record of stable isotope data on Argentine ants ( Linepithema humile ) from southern California to infer how the trophic position of this widespread invasive species changes over time as native ant species are displaced. We couple this longitudinal analysis with a biregional comparison of stable isotope data (δ 15 N) on ants from Argentina (native range) and California (introduced range) to quantify ( i ) how the trophic position of L. humile differs between native and introduced populations, and ( ii ) how relative trophic position as estimated by δ 15 N values of Argentine ants compare with those of other ants at the same site. Both long-term and biregional comparisons indicate that the Argentine ant's relative trophic position is reduced at sites with a longer history of occupation. Over the course of 8 years, the relative trophic position of L. humile remained high at the leading edge of an invasion front but declined, on average, behind the front as native ants disappeared. Relative to native populations, where L. humile is among the most carnivorous of ants, Argentine ants from California occupied lower trophic positions. These results support the hypothesis that Argentine ants shift their diet after establishment as a result of resource depletion and increasing reliance on plant-based resources, especially honeydew-producing Hemiptera. Our results demonstrate the value of long-term and biregional data in uncovering ecological effects of invasions.