Ants & Aphids

Snapped these images just beyond our front door yesterday afternoon.

What is the relationship between these two insect species?
Answer: These ants and aphids have a mutualistic relationship -- they each derive some benefit from their association: the ants obtain honeydew, a sugary secretion made by the aphids, to eat, and the aphids are protected from predators by the aggressive ants.

Why are they often found together as seen in this picture?
Answer: Some species of ants are "aphid farmers" -- they tend aphids in a way similar to human farmers tending livestock. The ants protect the aphids from predators by patrolling the area where the aphids are feeding; if they encounter other insects, they sting or bite them until they leave or fall off the branch. As the aphids feed, they suck the plant juices out of the plant leaves and stems. Eventually, the plant wilts and the juice no longer flows. The ants will then pick up the aphids and carry them to a new juicy stem where they can continue to feed and produce the honeydew that the ants eat. At night, the ants will often carry the aphids to a safe place for the night and bring them back to a plant to feed in the morning!

How might this affect your garden?
Answer: If the ants think your vegetables look juicy and tasty, they will bring their aphids into your garden to let them feed. Pretty soon, your plants will be wilted and dying. Also, the honeydew the aphids secrete is very sweet and sticky and quickly becomes covered with messy molds and mildew.

An interesting fact: For most of the year, almost the entire population of each of these insects is made up of only one sex (male/female). Which sex is it?
Answer: Female