You might have noticed if you looked at the Hairy Vetch bloom up close that there were aphids crawling on my hands... Some of our weeds, including Vetch and Henbit, harbor aphids. Our alleyway is full of Hairy Vetch and there were ants all over the place, and later we discovered aphids. Ants and Aphids have a weird mutualistic relationship see here: http://rspb.royalsocietypublishing.org/content/274/1629/3127
We are organic gardeners, we don't use any chemicals or unnatural fertilizers. A lot of the times when we have a pest show up in our garden we hand-pick them off of our plants (which is upsetting and gross to be perfectly honest) but some pests, like aphids and fire ants, are too small for that to be practical. So we use neem, oil soap spray, or diatomaceous earth. Diatomaceous earth, also known as DE, is the fossilized remains of marine phytoplankton. It looks like an off-white powder but it is actually hundreds of thousands of sharp little shards. These microscopic shards can't hurt humans or animals, but they can wreak havoc garden pests. DE is certified safe for organic gardening... which has its secret, brutal bug massacring points. Ugh.
Here is a picture of me applying the d.e. powder in an attempt to save our young tomato plants and our lettuce from the aphid infestation that has already begun. Pest control is my least favorite part of gardening!