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Pests and diseases

06 Oct 2005

This page: Introduction - Insect pests - Diseases - Weeds

 

Introduction

Houseleeks
Introduction
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References

Sempervivum species
Sempervivum cultivars

Jovibarba species
Jovibarba cultivars

Rosularia

Fernwood Nursery

 

The name "Sempervivum", literally translated, means "always alive" and this is a reference to the hardiness of most species. It is very difficult to kill them accidentally. Rosettes left dry and without soil for several months, will usually survive and grow rapidly as soon as suitable conditions are provided.

There are, however, a number of pests that will attack houseleeks and, although their effects are not often lethal, they can spoil the appearance of the plant and reduce its growth rate. The most common pests are described below.

Insect pests

Vine Weevils (Otiorrhyncus sulcatus)

The larvae of vine weevils feed on the roots of houseleeks (and other plants) mainly during the autumn, winter and spring. If not checked, they eventually eat up through the stem of the rosette until all that remains is a few detached leaves.
Vine weevil larvae

The larvae seem to prefer the large, smooth leafed varieties to the hairy types and as the roots are eaten the plant becomes detached from the soil and lies on the soil surface. The larvae can often be found actually in the base of the plant but if not, they will be found in the soil below. The simplest treatment is to check each plant in the early spring and re-pot any that have lost their roots. The larvae can then be found and destroyed.

For chemical control, a recently introduced insecticide called imidacloprid, is claimed to give long-lasting protection against vine weevils and other pests but we have not tried it ourselves. It is available under the trade name of Provado.

A biological control method is available and sold under the name of Nemasys. This is a culture of parasitic nematode worms that attack and kill the vine weevil larvae. Nemasys is expensive and best used for crops grown under glass although outdoor treatment is now possible and may be worth trying if vine weevils are a problem.

Adult vine weevils are about 10mm long and a greyish black colour. There are no males and the females are mainly nocturnal so not often seen. Adult females may sometimes be found crawling on the ground around the plants where they are probably laying eggs!

There is a Vine Weevil Advice Centre at http://www.vine.weevil.org.uk

Root Mealy Bugs


Root mealy bug infestation
Plants infested with root mealy bugs grow very slowly or even stop growing altogether if the infestation is heavy. Examination of the roots of an infested plant will reveal the presence of a bluish white, mould-like material that is secreted by the bugs as they feed.

Mealy bugs are more often found on potted plants than on those in open ground but they are not usually discovered until the plant is re-potted. Once mealy bugs have been detected it is best to remove all infested roots from the plant, then re-pot it in fresh compost.

Pots which have held infested plants should be thoroughly cleaned and disinfected before being used again. If mealy bug is a severe problem it may be worth watering all plants with a systemic insecticide once or twice during the growing season.

Aphids

Occasionally rosettes will be found infested with greenfly or blackfly although populations of predators such as ladybirds will reduce this problem. If necessary, plants can be sprayed with a suitable insecticide. Infected plants may be marked for a while but as new leaves grow they soon recover.

 

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Diseases

  It is rare to see houseleeks suffering from bacterial, virus or fungus infections. Under damp growing conditions the occasional rosette will rot away but this does not usually spread to other plants. It is best to destroy any unhealthy looking plants, and to make sure that the compost used is well drained. Normally no other treatment is necessary.

 

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Weeds

 

Most houseleeks should be kept outdoors even when grown in pots, so weeds can be a problem. It is best to remove weed seedlings as soon as they are seen and before they develop large roots or start to shade the plants.

One common weed is Pearlwort. This has fine grass-like leaves and small green or white flowers. Seedlings can easily be uprooted but they are often so numerous that re-potting the plant in fresh compost is the easiest treatment.

During the winter and spring the growth of moss can be a problem, especially among the rosettes of smaller varieties. It is probably easiest to remove moss by hand when the plant is re-potted.

 

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