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Wisteria is prone to scale insects, however, it’s important to know that there are many different types of scale insects, and it’s the specific Wisteria scale insect that can cause serious problems if they are left untreated. In small numbers, they aren’t usually a huge issue and can be easily maintained with the removal of them by hand.
They are easier to identify in comparison to other scale insects because they are usually much larger than other scale insects (at around 10mm wide) and have blackish-brown scales. Other scale insects are usually much smaller, usually under 5mm, and light brown in colour.
We have written a more detailed guide on how to recognise the Wisteria scale and what you can do about them should you be unfortunate enough to find your Wisteria getting attacked by them.
What are Wisteria scale insects?
Scale insects come in many varieties (species) and they range in size from less than 1mm to over 10mm in diameter, with the majority of them being under 5mm. Most of these insects are found on outdoor plants, especially ornamental plants and fruit trees.
As already mentioned, Wisteria scale insects are larger than most other species. This insect grows up to 10mm in diameter and has a soft body covered by a hard semi-spherical shell when it becomes an adult. Wisteria scale insects are found, as the name suggests, predominately on Wisteria plants.
How can you recognise them?
Adult scale insects appear on plants in late spring or early summer. A brownish-black shell covers the insect and its eggs usually have a white dusting on them. Younger scale insects are smaller and are pale brown with pinkish-white bits on them that look like sugar crystals. This is another way it may be easier to try and correctly identify them when they are much smaller.
What do they do?
Wisteria scale insects gather in masses on the stems of your Wisteria, where they often develop heavy infestations and you suddenly notice the mass of brown/black scales. They suck the sap from the leaves, leaving them deformed and weakened. The growth of the entire plant suffers, the leaves become yellow and may wilt and drop off. Even the branches may die back in serious infestations.
Wisteria scale insects may excrete honeydew onto the upper surface of the leaves. This is a sticky sweet substance that attracts ants and aphids who feast on it, so this is usually around the time when most people will notice them. The honeydew also encourages a fungus called sooty black mould which may also develop into a problem.
What do I do?
First step, non-chemical
First of all, remain vigilant and regularly examine your Wisteria for these blackish-brown limpet-like structures. A few here and there are tolerable, to both you and your plant but ideally, remove them anyway. This doesn’t, however, stop the new ones from hatching but it does reduce the overall population so it’s good practice. Ladybirds/ladybugs eat scale insects, so encourage these into the area where your Wisteria is planted. You can, believe it or not, even buy ladybird larvae online to get your own populations established.
As for pesticide control, try a plant oil wash in winter when the plants are fully dormant. This gets the scale nymphs that are overwintering.
Deal with heavy attacks in the late spring and early summer. This is when the scales are newly hatched and most vulnerable to sprays because the adult shells seem to provide them with some protection from sprays.
Use an organic spray such as natural pyrethrum. You need to respray this type of pesticide regularly as it has a short working life. A more persistent insecticide containing the organic compound acetamiprid is available for heavy infestations of Wisteria scale insects.
It’s worth noting that even when you succeed in killing the scale insects, they can stay firmly attached to the plant and not drop off. You will know if they are all dead when any new plant growth doesn’t have the brown insect bodies on it.
Not got scale insects? I also have a guide on pests and diseases as well as a guide that covers other Wisteria problems that you may find helpful. If your Wisteria isn’t blooming, we have a guide to cover that too.