Managing European Elm Scale in the Period of Neonicotinoid Insecticide Resistance

Abstract Background The European elm scale (EES), Gossyparia spuria (Modeer)(Hemiptera: Eriococcidae), has been a marked pest on American elm ( Ulmus americana ) in western states since its invasion. Tactics to control this insect pest have been largely based on chemical controls; first insecticidal sprays and then systemic applications, but in recent years insecticide resistant scales have become an apparent problem. Methods This paper (1) outlines how insecticide resistance was likely established in Colorado, (2) documents neonicotinoid resistance in this plant parasite by showing scale insects feeding on trees with high levels of imidacloprid insecticides, and (3) explores alternative control options that will be integral to maintaining American elms as part of the urban landscape in western states: i.e., acephate, e.g., ACE-jet (Arborjet, Woburn, MA, USA) and Lepitect (Rainbow Ecoscience, Minnetonka, MN, USA); azadirachtin, e.g., AzaGuard ® (BioSafe Systems, LLC, East Hartford, CT, USA) and AzaSol (Arborjet, Woburn, MA, USA); buprofezin with and without horticultural oil, e.g., Talus (SePRO Corporation, Carmel, IN, USA); and pyriproxyfen with and without horticultural oil, e.g., Distance ® (Valent Professional Products, Walnut Creek, CA, USA). Results Based on our findings, the current recommendation for control of neonicotinoid resistant EES is using the insect growth regulator pyriproxyfen (e.g., Distance), which is applied as a spray. In addition to pesticides, we found several natural insect enemies that attacked the EES in Colorado. We captured 11 species of wasps that parasitize the EES from emergence cages. Through cultivar resistance experiments, we have also identified several elm varieties that show promise in reducing EES damage, suggesting their suitability for planting in urban landscapes. Conclusion This study investigated alternative chemical control treatments, documented biological control agents present in the area, and screened for cultivars with scale resistance, all of which need to be considered to maintain American elms with longstanding EES infestations successfully.