Honeydew is the sticky, sugary substance that coats leaves, cars, and pavement beneath certain trees throughout the growing season. This residue is often mistaken for tree sap leaking out, but it is actually the waste product excreted by sap-sucking insects. Understanding the source and the life cycle of these pests is the only way to know when this phenomenon will finally cease for the year.
Identifying the Source of Honeydew
The sticky coating is a byproduct of insects feeding on the tree’s vascular system, specifically the phloem sap. This sap transports sugars and nutrients throughout the plant, but it is extremely rich in carbohydrates and low in protein. To extract the limited protein they need, small insects must consume a massive amount of the sugary sap.
The primary culprits are tiny plant-feeding insects such as aphids, soft scale insects, and mealybugs. Once the insect pierces the phloem duct, the high pressure of the sap forces a large volume of sugar-water through its digestive system. The excess sugar and water are then excreted as a clear, sticky droplet, which falls as honeydew onto surfaces below. Common affected trees include maple, linden, oak, and fruit trees.
The Seasonal Timing of Production
The cessation of honeydew production is directly tied to the end of the insect’s feeding cycle, which is triggered by environmental changes. Honeydew typically begins to appear in late spring or early summer, with peak production often occurring during the warmest months when insect populations are booming. The sticky rain will not stop until the insect population is significantly reduced or goes into a dormant phase.
This natural decline usually happens in late summer and early fall, particularly in temperate climates. As daylight hours shorten and temperatures begin to drop consistently, the insects’ life cycle shifts. Aphids, for example, will transition from producing live young to laying overwintering eggs on the host plant, effectively ending the current season’s feeding and honeydew production.
Consistent frosts or prolonged cool weather below freezing are the final environmental signals that completely halt insect activity. The pests either die off, seek shelter, or enter a dormant stage until the following spring. Honeydew production may become sporadic in early fall, but it stops entirely once the cold weather settles in.
Addressing the Sooty Mold Aftermath
Once the honeydew has fallen, it creates a secondary problem known as sooty mold. This is a non-parasitic fungus that feeds exclusively on the sugary residue left by the insects. The mold appears as a dark, black coating, giving leaves, branches, and surfaces a dirty or “sooty” look.
While sooty mold does not directly infect the plant tissue, a heavy coating can indirectly cause damage. By covering the leaf surface, the black layer blocks sunlight, impeding photosynthesis. This reduction in energy production can stunt growth or cause leaves to drop prematurely. The mold is primarily an aesthetic issue on hard surfaces, but it persists until the honeydew is removed.
Management and Cleanup Strategies
The most effective way to stop the honeydew from falling is to manage the insect population responsible for its creation. For trees, this often involves the application of horticultural oils or insecticidal soaps, which are low-toxicity options that smother the soft-bodied pests. Targeting the pest is the only method that will stop the flow of honeydew mid-season.
For surface cleanup, the residue is water-soluble, making removal straightforward. Honeydew and the resulting sooty mold can be washed off cars, outdoor furniture, and patios using a solution of mild dish soap and water. It is important to remove the sticky material quickly from vehicle finishes, as prolonged exposure can etch the paint. Applying the soapy solution and rinsing with a strong jet of water is often enough to dissolve the sugar.