Aphids are small insects that tend to stay in one place drinking plant sap, excreting honeydew, and minding their own business. Ants are constantly on the move, eat everything, and are highly aggressive to almost all insects except members of their own hill and aphids. At first glance, this would seem like an incongruous relationship because aphids are rather delicate, defenseless, slow moving insects that would be easy prey for a foraging ant. Close observation of the relationship soon sheds light on how the ants' tolerance of aphids has developed.
The Relationship Between Aphids and Ants
The key to the relationship between aphids and ants is in the way aphids eat. Being restricted to feeding on plant sap, aphids find it necessary to eliminate large amounts of sugar as honeydew to obtain enough amino acids from the sap to build proteins. The sugar is eliminated as a moderately concentrated solution that ants and other organisms find irresistible. Since one law of nature is “Don't bite the hand that feeds you,” ants that feed on honeydew not only do not attack aphids, but will often attack their predators to protect nearby aphid colonies from those predators.
Aphids line up along plant stems and on buds, insert their styli into the plant's phloem (sap) cells, and pump sap into their guts. Since the sap is rich in sugars but not in amino acids, the aphids must expel the sugars or become overloaded with sugars and become comatose. The easiest way to expel sugar is to pass it unabsorbed through the digestive tract. This unabsorbed sugar makes honeydew sweet.
Ants have an extremely varied diet. They eat seeds, fruits, insects, and almost all organic matter except plant stems and leaves. With a rich supply of amino acids, when ants find a supply of sugar, they collect as much of it as possible to be stored until winter. Field ants are one case of this. Worker ants collect honeydew from aphids, bring it back to the nest, and transfer this honeydew to specialized honeypot ants. Some honeypots store so much sugar water that they swell up to the size of small grapes and are incapable of movement. These ants then provide sugars to their nest mates through the winter when ready supplies of sugars are in short supply.
How Ants Gather Honeydew
Aphids that take part in the ant-aphid symbiosis have large projections on their abdomens called cornicles. The posterior parts of aphids look quite similar to ant heads that have pointed chins, round heads, and two antennae. Ants communicate by stroking the head and antennae of their nest mates with their own antennae. It is a simple step to stroke the abdomen of an aphid (that looks much like an ant's head) in a similar manner. When stroked, the aphid releases a drop of honeydew just as another ant might release a drop of food it is carrying, the ant swallows the honeydew, and carries it back to the nest after consuming several dozen droplets.
How Ants Assist Aphids
Ants tend their herds of aphids providing many types of benefits for them:
- Aphids have delicate bodies that are defenseless. Ants have strong jaws, poison glands, and can rapidly mobilize large numbers of defenders through the release of pheromones. When ants find aphid predators (such as ladybugs or spiders) in the colony, they immediately attack these predators, biting at their legs, heads, bodies, and wings in an attempt to discourage them. Thus, ants are usually found in aphid colonies at all times.
- Most aphids are wingless and walk slowly from place to place. Their top speed is about two inches a minute. Because of this, it would take an aphid an hour or more to move down a branch and up another to find a supply of tender young shoots. Ants often pick up the aphids and carry them to new shoots in less than two minutes. Thus, the ants assist aphids in dispersal to new feeding grounds. Ants sometimes even move aphids from one type of plant to another when either the first species stops producing new growth or the second species of plant begins to flower or grow.
- Most aphids die in the winter as temperatures drop below freezing. Some ants carry their cows into their underground chambers where they place the aphids on tender subterranean roots on which the aphids feed during the cold months. The ants then carry mature aphids back to surface plants after temperatures warm up again.
The symbiosis between ants and aphids is more complex than can be presented in this article. More information about this lifestyle can be found by clicking here.
 |
