Abstract
Systemic insecticides such as neonicotinoids are widely used in seed coating practices for pest control in many crops, e.g., corn. Their success is due to their ability to protect the whole plant, from the roots to the upper leaves, but their use at high amounts is causing possible adverse effects on non-target animals exposed to contaminated pollen, nectar, leaves, and dust emitted during sowing. In 2018, the European Union banned some neonicotinoids and fipronil as seed coating insecticides in open fields. Consequently, the methylcarbamate methiocarb and less-toxic neonicotinoids, e.g., thiacloprid, have been authorized and largely used as alternative pesticides for corn seed coating. Here, an analytical protocol based on QuEChERS extraction/purification procedure and analysis by liquid chromatography-mass spectrometry has been optimized for the identification and the quantification of methiocarb, thiamethoxam, thiacloprid, and their metabolites in guttation drops, the xylem fluid excreted at leaf margins, and in leaves of corn plants grown from coated seeds. Although methiocarb is a non-systemic pesticide, we unexpectedly found high concentrations of its metabolites in both guttations and leaves, whereas methiocarb itself was below detection limits in most of the samples. The methiocarb main metabolite, methiocarb sulfoxide, was found at a mean concentration of 0.61 ± 1.12 µg mL−1 in guttation drops and 4.4 ± 2.1 µg g−1 in leaves. Conversely, parent compounds of neonicotinoids (thiamethoxam, thiacloprid) are systemically distributed in corn seedlings. This result raises safety concerns given that methiocarb sulfoxide is more toxic than the parent compound for some non-target species.
Highlights
Methiocarb and its metabolites in corn plants grown in pots and in open field Twenty-one guttation samples were collected from plants treated with methiocarb
Four methiocarb metabolites were identified in guttation drops: methiocarb sulfoxide (0.61 ± 1.12 μg mL−1), methiocarb sulfoxide phenol (0.54 ± 0.42 μg mL−1), methiocarb sulfoxide hydroxy (0.068 ± 0.138 μg mL−1), and methiocarb sulfone phenol (0.018 ± 0.025 μg mL−1), while methiocarb phenol and methiocarb sulfone were below the MDL in all samples (Table 1)
The approach of measuring the presence of the active ingredients and their metabolites both in guttations and leaves highlighted that guttation analysis can be an effective and innovative tool for the study of systemic properties of the insecticides, as well as their metabolic pathways, in plants grown from coated seeds
Summary
Large-scale worldwide use of neonicotinoid insecticides in recent decades (Bonmatin et al 2015; Giorio et al 2017) was found to impact the health of honeybees and wild pollinators (Tapparo et al 2012, 2013; Wood and Goulson 2017; Pisa et al 2017), aquatic species (Gibbons et al 2015; Pisa et al 2015), birds, and mammals (Gibbons et al 2015) including humans (Ichikawa et al 2019). Despite being introduced as a safe and more environmentally friendly approach, the seed coating application of these systemic insecticides did not prevent their release into soil, ground and surface waters, in guttation drops, and into the air (through dust emitted during sowing) (Tapparo et al 2012; Bonmatin et al 2015; Giorio et al 2017; Hitaj et al 2020). While thiacloprid is a systemic insecticide, methiocarb is considered a non-systemic insecticide and bird repellent (EFSA 2018) For these insecticides, information on their residue levels within the plant, and on the presence and the environmental fate of their metabolites, is scarcely available (Böhme et al 2018; Erban et al 2019; Martinello et al 2019; Lentola et al 2020). Samples collected from plants treated with the well-known systemic pesticides thiacloprid and thiamethoxam were analyzed to compare residue levels in both leaves and guttation drops
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
