Why are the leaves on my lemon tree sticky?

If you are anything like me then your precious lemon tree is one of the prized possessions of the garden, and as such gets more than its fair share of close attention. Any change in leaf colour, or sign of insect damage receives detailed inspection to determine the cause so it is hardly surprising that sticky leaves will start alarm bells ringing.

Sticky leaves are a sign that there are aphids either infesting your lemon tree or a plant whose canopy is above the lemon tree. To make sure that the stickiness isn't cause by children's ice cream fingers check the new growth for signs of aphids and ants.

Why are the leaves on my lemon tree sticky? 
Aphids passively feed themselves on the sap of phloem vessels. The phloem is the living tissue that carries organic nutrients, in particular sucrose, to all parts of the plant where needed. The aphid stabs the phloem vessel through the leaf using specialised mouthparts. The sap, which is under high pressure, is forced into the aphid's food canal.

The sap pressure is so great that the aphids ingest more than they can digest and the excess is released from the terminations of their alimentary canals.

This excess drops onto the any leaves below which over time become sticky as the sap dries out leaving a sticky, sugary substance. This is known as honeydew and is the substance that is making your lemon leaves sticky. The honeydew if often covered by a black fungus known as sooty mould. Although the sooty mould looks rather worrying it does not directly feed from the plant, instead it receives its nutrition directly from the honeydew.

This honeydew is often 'farmed' by colonies of ants. The ants provide protection from predators and in return eat the energy-rich honeydew that the aphids release. The ants 'milk' the aphids by stroking them with their antennae.

This mutualistic relationship is even closer in some farming ant species as they are known to gather and store aphid eggs in their nests over the winter. Come the spring the ants carry the newly hatched aphids back to the plants ensuring a food source for the following year.

Dawidi, Johannesburg file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

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