Sublethal exposure to the lowest field-realistic doses of imidacloprid disrupts colony performance in the Neotropical bumblebee Bombus pauloensis.

<title>Abstract</title> <p> <italic>Bombus pauloensis</italic> is a notable South American bumblebee species due to its ecological significance as a pollinator of both natural ecosystems and agricultural crops. Imidacloprid is among the most widely used active ingredients for pest management across various crops in the region. Numerous studies have documented its lethal and sublethal effects on bumblebees, primarily focusing on species from temperate zones. The aim of this study was to evaluate the impact of low, field-realistic doses of Imidacloprid on colony growth and performance in <italic>B. pauloensis</italic> . Colonies tested for treatments and control (n = 6 each), were reared from wild-caught queens. Two low-realistic field concentrations (0.1 ng/mL and 0.01 ng/mL) were tested. These dilutions were mixed into a 1:1 (w/v) sucrose solution and administered chronically over a 70-day period to colonies. Colonies were monitored twice a week to record the number of egg cells, larval cells, pupae, and adults. In addition, colonies were CT-scanned once monthly to quantify colony growth. Worker size was estimated using intertegular distance (ITD) as a proxy. Results indicate that exposure to low, realistic doses of Imidacloprid adversely affects colony growth and development, as well as worker size, highlighting the vulnerability of <italic>B. pauloensis</italic> to this insecticide. </p>