Neonicotinoid Insecticide Imidacloprid Causes Outbreaks of Spider Mites on Elm Trees in Urban Landscapes

Abstract

BackgroundAttempts to eradicate alien arthropods often require pesticide applications. An effort to remove an alien beetle from Central Park in New York City, USA, resulted in widespread treatments of trees with the neonicotinoid insecticide imidacloprid. Imidacloprid's systemic activity and mode of entry via roots or trunk injections reduce risk of environmental contamination and limit exposure of non-target organisms to pesticide residues. However, unexpected outbreaks of a formerly innocuous herbivore, Tetranychus schoenei (Acari: Tetranychidae), followed imidacloprid applications to elms in Central Park. This undesirable outcome necessitated an assessment of imidacloprid's impact on communities of arthropods, its effects on predators, and enhancement of the performance of T. schoenei.Methodology/Principal FindingsBy sampling arthropods in elm canopies over three years in two locations, we document changes in the structure of communities following applications of imidacloprid. Differences in community structure were mostly attributable to increases in the abundance of T. schoenei on elms treated with imidacloprid. In laboratory experiments, predators of T. schoenei were poisoned through ingestion of prey exposed to imidacloprid. Imidacloprid's proclivity to elevate fecundity of T. schoenei also contributed to their elevated densities on treated elms.Conclusions/SignificanceThis is the first study to report the effects of pesticide applications on the arthropod communities in urban landscapes and demonstrate that imidacloprid increases spider mite fecundity through a plant-mediated mechanism. Laboratory experiments provide evidence that imidacloprid debilitates insect predators of spider mites suggesting that relaxation of top-down regulation combined with enhanced reproduction promoted a non-target herbivore to pest status. With global commerce accelerating the incidence of arthropod invasions, prophylactic applications of pesticides play a major role in eradication attempts. Widespread use of neonicotinoid insecticides, however, can disrupt ecosystems tipping the ecological balance in favor of herbivores and creating pest outbreaks.

Citrus rootstocks influence the population densities of pest mites

“…Also, higher densities of T. mexicanus were observed in February 2012 and January 2013 following previous applications of imidacloprid. Indeed, imidacloprid is associated with population increase of tetranychid mites (SCLAR et al, 1998;SZCZEPANIEC et al, 2011). Lower densities of T. mexicanus over the evaluation period may be also related to pesticide sprayings.…”

Incorporation of Biorational Insecticides with Neonicotinoids to Combat Resurgence ofTetranychus urticae(Prostigmata: Tetranychidae) on Rose

“…Imidacloprid-treated T. urticae produced 10 to 26% more eggs during the first 12 d of adult life and 19 to 23% more during adulthood compared with a water-only treatment (James & Price 2002). Szczepaniec et al (2011) reported that the fecundity of Tetranychus schoenei McGregor (Prostigmata: Tetranychidae) that consumed leaves from treated elms increased by nearly 40% compared with females feeding on untreated foliage. This effect, however, was only present when spider mites consumed leaves from treated elms.…”

Indirect Effects of Pesticides on Natural Enemies

“…However, this may not be the case as systemic insecticides may exhibit indirect effects on natural enemies via several mechanisms including elimination of prey, contamination of floral parts by the active ingredient, consumption of the active ingredient while ingesting plant fluids, and contamination of prey ingesting either lethal or sub-lethal concentrations of the active ingredient [52][53][54]. Systemic insecticides, when applied to the soil or growing medium, may have minimal direct effects on aboveground natural enemies (both parasitoids and predators); however, they may indirectly influence natural enemies if mortality of prey populations is high (>90%).…”