Aphids are not entirely defenseless against predators, although their defense is a rather passive one. At the end of the abdomen, on the top surface, you can find a pair of thin tubes sticking up. These are called “cornicles”, and they are unique to aphids and help us to separate aphids from some related plant parasites. The cornicles are the openings from which all that sticky honeydew comes, and the honeydew may serve as a defensive weapon. If the aphid feels threatened by some nearby predator it begins to emit pheromones from the cornicles that alert nearby aphids to this threat, and then exudes a large drop of honeydew that, it hopes, will repel the predator. This may be due to the honeydew getting into the mouth of the predator and gumming things up, or it may be distasteful to that predator. The result to us, though, is the rain of sticky droplets coming down from infested plants and landing on surfaces below, often creating an unsightly mess. Cars parked beneath infested trees get a layer of honeydew on the windows and paint, causing other materials to stick to the surface or making it difficult to see out the front window. Sidewalks and playground equipment become sticky and less desirable to walk, sit, or play on.
Honeydew dripping onto leaves below the aphids acquires a shiny appearance that detracts from the aesthetics of the plant. Large accumulations become very attractive to sugar-feeding insects, and wasps or bees may swarm over the plant to gather this nutritious food. Ants are extremely fond of honeydew, and recognize the source of the material. It has been observed that ants will protect aphids from predatory insects that would feed on them, may move aphids from one part
of the plant to another to ensure a continual supply of the honeydew, and may “tickle” the aphids with their antennae to stimulate the aphid to produce that treasured drop of nectar. This “milking” of the aphids has led to a nickname for aphids as “ant cows”. Some kinds of ants will even take aphid eggs into their colonies in the fall to provide the eggs with a protected location, bringing them or the newly hatched aphids back out in the spring to place them on plants. This incredible relationship between ants and aphids is referred to as a form of “symbiosis”, in which both organisms benefit, and the presence of honeydew-producing aphids can clearly encourage the presence of ants and the size of their population on a property. This symbiotic relationship between ants and aphids has a few more fascinating twists to it. A few species of aphids apparently cannot exude honeydew without the stimulation by ants, adding to the reliance between the two kinds of insects. If aphid populations get too heavy on a plant, at least in the eyes of the ants, the worker ants may gather a few aphids and provide them as food for their own larvae, back in that ant colony. Ants not only chase away potential predators of the aphids, but have been seen removing the eggs of those predators from the plants as well. Controlling the ants helps to control the aphids, and this is one form of integrated pest management of aphid problems.
The feeding of aphids on plants has some interesting similarities to the feeding of blood sucking insects on humans. First, it is done with a “proboscis”, a mouth not unlike a hollow straw, which is inserted into the food host and fluids are sucked back out. The proboscis of an aphid is very thin and not particularly strong, so to be able to push this through the harder cells of the leaf and to reach the fluid-carrying phloem layer inside they coat their proboscis with a liquid that hardens to create a sheath around it. It then takes anywhere from a half hour to 24 hours for the process of insertion to take place, and when the aphid finally is getting that meal of plant fluids their saliva may help to keep the liquids flowing and the puncture site to not seal over. The diet of the aphid must consist of more than just sugars in the fluids from the plants. They also need nitrogen and amino acids, and plant juices typically have higher levels of the sugars than the other ingredients. To account for this the aphid has special filters in its digestive system that allow them to excrete the excess water and sugars, and this is what results in those copious amounts of honeydew.
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