Hydrangeas are susceptible to a variety of garden pests such as aphids, spider mites, whiteflies, caterpillars, snails and more. Some of these pests leave behind white spots, white webbing, whitish slime or white eggs on the underside of hydrangea leaves. Looking for these symptoms helps you determine what pest you have so you can treat it.
White spots on the underside of a hydrangea leaf are a sign that an insect has been draining the leaf of its nutrients. The circles may appear yellowish-white in colour, may eventually turn bronze or brown and become holes. These white spots often signs of spider mites, a small spider-like creature or leafminers, small flies that in larval stage burrow tunnels through the leaves. Large splotches of raised white spots could be an infestation of scales, insects resembling bumps or irregular white growths on the leaves and stems of a plant.
White webbing on the under side of a hydrangea leaf is a sign that a pest infestation. The webbing is most likely the work of a web-spinning spider mite species, such as the two-spotted spider mite or the Pacific spider mite. The mites are difficult to see with the naked eye but when magnified resemble a tiny spider with eight legs and a roundish body. The webbing could signify a colony has taken residence and hundreds of mites could be living on one leaf.
A shiny silvery or whitish slime may be due to snails or slugs. These members of the Molluska phylum also chew holes leaves. The damage can be confused with that caused by caterpillars. A silver or whitish shiny slime trail across your leaves indicates the problem comes from snails or slugs. Some insects, such as aphids, deposit s slimy, sticky substance called honeydew. However, this white or clear sticky residue usually turns black in places due to fungus growing on top of the honeydew.
Insects may not cause significant damage right away. Instead, they will start in small numbers and begin colonising by laying eggs on the underside of hydrangea leaves. These white or grey egg clusters should be visible to the naked eye. They could indicate spider mites have infested the plant, but they could also be a sign of whiteflies. These small flies are white as adults and leave mottled spots or holes and honeydew on the hydrangea as well.