  Automated social media accounts, known as bots, have been shown to spread
disinformation and manipulate online discussions. We study the behavior of
retweet bots on Twitter during the first impeachment of U.S. President Donald
Trump. We collect over 67.7 million impeachment related tweets from 3.6 million
users, along with their 53.6 million edge follower network. We find although
bots represent 1% of all users, they generate over 31% of all impeachment
related tweets. We also find bots share more disinformation, but use less toxic
language than other users. Among supporters of the Qanon conspiracy theory, a
popular disinformation campaign, bots have a prevalence near 10%. The follower
network of Qanon supporters exhibits a hierarchical structure, with bots acting
as central hubs surrounded by isolated humans. We quantify bot impact using the
generalized harmonic influence centrality measure. We find there are a greater
number of pro-Trump bots, but on a per bot basis, anti-Trump and pro-Trump bots
have similar impact, while Qanon bots have less impact. This lower impact is
due to the homophily of the Qanon follower network, suggesting this
disinformation is spread mostly within online echo-chambers.
