Possible Clues:

• Weed oddly pernicious -- it can damage crop
• Itsy-bitsy, nearly repulsive insect
• Pest's not great, not nice, not finished
• (Said patronisingly) 'You naughty pest!'
• Small beetle with a long snout
• One that's a bore?
• Type of beetle that feeds of stored grain etc
• Just a tiny six-fifty for grub