Cutwail - 45% of all Spam

The largest and most powerful botnet is responsible for 45% of allspam. With between 1.5 and 2 million active bots, Cutwail was perhapsthe largest botnet in history at its peak. "Cutwail's recovery to one-third of its original levels, after only afew hours, highlights the progress spammers have made since the McColoshutdown in November," said Paul Wood, MessageLabs Intelligence SeniorAnalyst, Symantec. "Spammers have learned the importance of having abackup for command and control channels."

Mega-D - 9.3%

Sounding more like a performance enhancing supplement than a botnet, this was the top botnet at the start of the year, but has been steadilydecline to the point that it is now responsible for only 9.3% of spam. However, it'sstill one of the hardest working botnets in terms of spam per bot perminute.

Xarvester - 0.2%

Should be called X-Harvester, because although it was one of the major botnets of the year, it has drastically reduced in size and output, and now  is responsible for sending out 0.2% of spam.

Donbot - 3.2%

A top 5 botnet in size and output, it's been less active in recent months sending out 3.2% of spam

Grum and Rustock - 10%

These newer (but very large) botnets are a wildcard. Combined, they'reresponsible for over 10% of spam, but their activity is hard topredict: both send spam in bursts, with Rustock often going throughperiods of zero activity.

Source: June 2009 MessageLabs Intelligence Report.