Why Are My Citrus Leaves Stippled and Pale?

A fine, dusty stippling of pale dots across the upper surface of citrus leaves, often accompanied by a dull bronzed or silvery sheen and fine silk webbing on the undersides, is the classic signature of a spider mite infestation. Two-spotted spider mite (Tetranychus urticae) is the species most commonly responsible on indoor and glasshouse lemons. These tiny arachnids — barely 0.5 mm long and almost invisible to the naked eye — pierce individual plant cells and extract the contents, leaving empty cells that scatter light differently and give the leaf its characteristic mottled appearance.

Confirming the Diagnosis

Hold a white sheet of paper under a suspect leaf and tap the leaf sharply. Look closely at the paper — if you see tiny dots moving, those are spider mites. A hand lens or magnifying glass makes identification much easier. You will also find the mites themselves on the undersides of leaves, particularly along the midrib, along with clusters of tiny spherical eggs and shed skins. Fine silk webbing, initially just in leaf junctions, becomes more extensive as the colony grows and can eventually tent entire shoot tips.

Why Hot, Dry Conditions Drive Outbreaks

Spider mites thrive when temperatures exceed 25 °C and humidity is low — exactly the conditions found near a south-facing conservatory window in summer, or next to a radiator in winter. Their life cycle compresses to eight to ten days in these conditions, meaning a small population doubles rapidly. Conversely, they struggle to reproduce below 15 °C and are killed by prolonged contact with water. If your lemon is in a very dry spot, moving it somewhere with more ambient humidity is the single most effective cultural control you can make.

Washing and Physical Control

A forceful spray of water — targeted at the undersides of leaves — dislodges mites and their eggs from the foliage without any chemistry. Do this outdoors on a warm day and repeat every three to four days. For a container tree, wiping each leaf individually with a damp cloth is tedious but thorough and highly effective at reducing population numbers quickly. Follow up with a miticide or acaricide spray once the numbers are reduced, as physical removal alone rarely eliminates the infestation.

Miticide Treatments

Not all general insecticides are effective against spider mites because mites are arachnids, not insects, and many products target insect-specific physiological pathways. Look specifically for products labelled as miticides or acaricides, or broad-spectrum options that explicitly list Tetranychus urticae on the label. Horticultural oil and sulphur-based fungicide/miticides are both effective and have low mammalian toxicity. Rotate between at least two different modes of action to prevent resistance building up, especially if the colony has been treated before.

Increasing Humidity and Biological Controls

Stand the pot on a tray of damp gravel to raise the humidity immediately around the foliage. Mist the undersides of the leaves with plain water daily during the warmest part of the year. The predatory mite Phytoseiulus persimilis is commercially available and devastatingly effective against two-spotted spider mite — it eats only mites and their eggs. Introduce it when the temperature is above 18 °C and daytime humidity is above 60%. It will work through the canopy methodically and die off naturally once the prey population is exhausted.