Rhododendron cicada: How to prevent black buds


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Many gardeners are wondering why their rhododendron carries so many black, dead flower buds. The cause of this problem is not a frost damage but a fungal disease. It is transmitted by a pest that actually looks harmless: the rhododendron cicada. These cicadas are not originally native pests. They originate from North America and were first introduced to the United Kingdom around 1930 and later from there to Central Europe.

Suction damage only low

Actually, the rhododendron cicada (Graphocephala fennahi) does not cause any significant damage to the evergreen flowering shrubs: the pests, which are only eight millimeters in size, feed on the sap of the leaves, but are less common than, for example, aphids. Therefore, the leaves of the rhododendrons do not cripple, but show at best a slight light green mottling because of some sucked leaf cells.

Rhododendron cicadas transmit a fungus

The problem is a fungus (Pycnostysanus azaleae), which the females transmit when laying eggs: they cut small cracks in the flower buds and lay in each case only one egg. In the resulting wounds, the pathogen nests and leads to the so-called bud tan: He covers the dying buds until next spring with a lawn of small, brown-black hairs. These are the fruiting bodies of the fungus called Pycnostysanus azaleae.

larva

Rhododendron cicadas lay their eggs on the underside of the leaves

Larvae of the rhododendron cicada

The larvae hatch from the end of April and collect on the undersides of the leaves along the main artery. In the course of their development, they shed several times until they are fully mature at the end of June / beginning of July. During the moulting they leave the old, whitish covers on the leaves. For this reason, examine the undersides of your rhododendron regularly at the latest from May, paying attention to the stripped casings and the small, green-yellowed larvae.

Combat rhododendron cicada

Direct control of the harmful fungus with fungicides (fungicides) is not possible. Therefore, only the indirect remains, by fighting the Rhododendronzikade in good time with suitable preparations. Since the adult animals are very resistant to insect repellents, you should proceed against the insect as early as possible in the larval stage, so by the end of May at the latest. Suitable biological agents are, for example, "spruzit pest-free" or "pest-free neem". Only if these are not successful, you should resort to chemical insecticides such as "pest-free Careo" or "pest-free Provado" in case of heavy infestation. The adult animals as well as the larvae are very nimble and jump away from all sides in case of disturbances. Therefore, use the early hours of the morning to combat, because then the pests of the nocturnal cold are still immobile.

Do not forget the undersides of leaves

Rhododendron cicada

The female of the strikingly colored, about five millimeters large rhododendron cicada lays in the summer from one egg per flower bud and infects them with the bud tan fungus

Mainly treat the underside of the plants thoroughly and enrich the spray solution with chemical preparations with a few drops of dishwashing detergent. This lowers the surface tension of the water and promotes wetting of the leathery leaves. At the latest next spring before hatching of the new generation you should also break out all affected flower buds and dispose of them with the household waste. Often, yellow tablets are also recommended for control. Although they can curb the infestation somewhat, but bring no lasting success. In addition, stick on the sticky glue paper and many useful insects.

Susceptible rhododendron varieties

Particularly prone to cicadas are rhododendron varieties from the group of large-flowered hybrids and some wild species. The Yakushimanum hybrids as well as the Japanese and summer green azaleas, however, are hardly affected.

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