Red Scale, Rooidopluis (Afr.)
Although unsightly, this species of scale insect is harmless to the health of Horse Chestnut Trees. They're soft-bodied true bugs that suck the sap from Horse Chestnut Trees, producing honeydew as a result. Honeydew is a top resource for things like fungi, so it's often likely to see black, sooty moulds near infested areas. This can be problematic in damp, urban areas with higher temperatures. These scale insects were only discovered in the UK in the 60s after being transported from Southern Europe by accident.
Won't impact tree health.
They look unsightly and may attract mould.
Adults: Chestnut coloured and disk-shaped insects which are located on the trunks and woody stems, often surrounded by a fluffy waxy substance. They are most evident in autumn. They're immobile so don't move. They're only 5mm in diameter. Nymphs: Small, white insects which may be seen crawling about the plant round the time of June. Eggs: You can find these hidden in the 'fluffy wax' found around the insects.
Oval, red-brown disks on trunks. Fluffy wax on trunks. May reduce vigour on smaller plants. Black sooty mould. Clear sticky honeydew. Ant infestations.
Asia, the UK and Europe
Older, more established plants can tolerate infestations. Regularly tending to plants and practising good housekeeping in the garden will give plants a better chance at survival. Infested branches may be pruned and removed and disposed of sensibly. Lighter infestations may be rubbed or picked off with hands. Ladybirds, hoverflies, lacewing and earwigs are great natural enemies that feast on scale insects. Letting parts of the garden 'grow wild' paired with an abundance of pollinator-friendly plants you can attract the latter into your garden. Insecticidal soaps can be advantageous if applied regularly over several intervals. If ants are also present at the site, these may be providing some protection to the scale. These can be controlled using a product such as Tanglefoot Pest Barrier. This will hopefully present a barrier for the ants that are attempting to reach the scale. Assess the damage in relation to plant size and the time of the season. Sometimes it's not worth paying a fortune if your plant is likely to recover! Nature will take care of infestations over the years if you are prepared to wait.
As they possess a hard waxy layer, they are difficult to kill with pesticides. It's not advised to treat infested trees because they don't reduce the overall vigour of the tree. Treating large plants like trees is often impractical and ineffective.