What are rhododendron leafhopper and bud blast?

There are several species of leafhopper that can be encountered by gardeners, they are sap sucking true bugs.

Rhododendron leafhopper only feeds on rhododendron. It is active on rhododendrons from late spring to autumn but is most noticeable when the brightly coloured bluish-green adults are present in late July-October.

Bud blast spoils developing flower buds, it is a fungal infection thought to be associated with the activities of the leafhopper.

Symptoms

The nymphs of rhododendron leafhopper are creamy white wingless insects that live on the underside of rhododendron leaves. The adults are 8-9mm long and have pale yellow heads with a bluish green thorax. The wings are folded back along the body and are bluish-green with two orange stripes. The adults often rest on the upper surface of the foliage in sunny weather but readily leap off when disturbed. White cast skins shed by the nymphs may be seen attached to the underside of leaves.

The feeding activities of the adults and nymphs have no obvious adverse effect on the appearance of the foliage or the plant's vigour. There would not be a problem if the females did not make egg incisions in next year's flower buds in late summer-autumn. A fungal disease, known as bud blast (Seifertia azalea), may infect the buds through the egg wounds, although some recent research has placed doubt on this assumption. Infected flower buds turn brown and die.

Control

There are no easy control measures for rhododendron leafhopper. However, flower buds infected with bud blast can be picked off and disposed of to reduce the amount of fungal spores being released in the vicinity of the plants. This may only be feasible on smaller plants.

When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.

Non-pesticide control

• Rhododendron leafhopper does not affect the growth or vigour of plants and so can be tolerated
• Flower buds infected with bud blast can be picked off and disposed of to reduce the amount of fungal spores being released in the vicinity
• Encourage predators and other natural enemies of suckers, in the garden, such as birds, ladybirds, wasps and ground beetles.

Pesticide control

• There are no effective fungicides for bud blast
• Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of leafhopper. These pesticides have a very short persistence and so may require reapplication to keep leafhopper numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults 
• Plant invigorators combine nutrients to stimulate plant growth with surfactants or fatty acids that have a physical mode of action (e.g. Ecofective Bug Control, RHS Bug and Mildew Control, SB Plant Invigorator and Westland Resolva Natural Power Bug & Mildew). These are not considered organic
• More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
• The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
• Several applications may be needed during late summer-autumn to deal with adult leafhoppers flying in from nearby gardens or from Rhododendron ponticum growing in woods

Follow label instructions when using pesticides. 
Plants in flower should not be sprayed due to the danger to bees and other pollinating insects.
Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener.

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Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biology

Rhododendron leafhopper has one generation a year and it overwinters as eggs that are laid during late summer-autumn in developing flower buds. It is during the egg-laying period that flower buds probably become infected with bud blast. The eggs hatch in late April-May and the wingless nymphs feed by sucking sap from the underside of the leaves. The nymphs reach the adult stage in late July-August.