Aphid honeydew is a clear, sticky substance often found coating plant leaves and stems. This sweet residue indicates an aphid infestation, making surfaces feel tacky to the touch, resembling a sugary syrup.
How Aphids Create Honeydew
Aphids produce honeydew as a byproduct of their feeding habits, which involve consuming large amounts of plant sap. These insects possess specialized mouthparts, known as stylets, which they insert into the plant’s vascular system to reach the phloem. Phloem sap is rich in sugars, primarily sucrose, but contains relatively low concentrations of nitrogenous compounds like amino acids, which aphids require for growth and reproduction.
To acquire sufficient amino acids, aphids ingest a large volume of sap. Their digestive systems rapidly process this liquid, efficiently absorbing the limited amino acids. Excess sugars and water, not needed for their metabolic processes, are then excreted from the aphid’s posterior end as honeydew, leading to its constant presence on infested plants.
The Impact of Honeydew on Plants
The presence of honeydew on plant surfaces can directly interfere with a plant’s normal physiological functions. The sticky layer can block the stomata, small pores on the leaf surface that regulate gas exchange and transpiration. This physical obstruction can impede the plant’s ability to “breathe” and release water vapor, potentially stressing the plant.
A more significant consequence of honeydew accumulation is the growth of sooty mold, fungi that thrive on sugary excretions. These fungi do not directly infect plant tissue but form a black, powdery layer on the honeydew. As the sooty mold spreads, it creates a dark coating that reduces sunlight reaching the plant’s leaves. This reduction in light inhibits photosynthesis, the process by which plants convert light energy into chemical energy, leading to stunted growth, reduced vigor, and a general decline in plant health.
Honeydew as an Insect Attractant
Honeydew serves as a food source for various insects, especially ants, which have a mutualistic relationship with aphids. Ants are highly attracted to the sugar-rich honeydew and actively protect aphid colonies for a continuous supply. They “tend” aphids, gently stroking them with their antennae to stimulate honeydew excretion, a behavior often referred to as “milking.”
In return for this sugary reward, ants defend aphids from natural predators such as ladybugs, lacewing larvae, and parasitic wasps. This protection reduces aphid mortality and allows their populations to flourish. Other insects, including wasps, bees, and flies, are also drawn to honeydew as an easily accessible source of carbohydrates, utilizing it for energy.