Sempervivum leaf miner was first recorded damaging house leeks in Britain in 2008. It is likely to spread rapidly and cause serious damage to house leeks. It originates from elsewhere in Europe.
What is sempervivum leaf miner?
Sempervivum leaf miner is a pest of Sempervivum species. The damage is caused by the larval stage of a fly belonging to the hoverfly family.
You may see the following symptoms:
- The outer leaves on affected house leek rosettes become greyish or brownish-green where the internal tissues have been eaten.
- Damaged leaves go limp before drying up.
- If a larva has left a leaf to move to another, or to pupate in the soil, there will be an exit hole near the base of the leaves.
- Damage is mainly seen in mid-May to June and again in August to September. Small rosettes may be killed.
Over-watering or poor drainage can cause similar symptoms to sempervivum leaf miner, however closer examination should reveal the presence of plump whitish maggots, up to 8-10mm (about 3/8in) long, inside some of the damaged leaves.
Non chemical control
Remove damaged leaves and larvae when you see them.
Leaf-mining insects are difficult to control as the pest is concealed inside the leaves.
Systemic insecticides are absorbed into plant tissues and should control the larvae, but because of the thick cuticle on Sempervivum leaves, uptake may be poor if applied as a foliar spray.
A better means of control would be to use a systemic vine weevil pesticide such as Provado Vine Weevil Killer 2 (thiacloprid) or Scotts Bug Clear Ultra Vine Weevil Killer (acetamiprid). These are watered into the potting compost, where they are absorbed by the roots and taken up into the foliage. In addition to protection against leaf miner, these pesticides will also protect against vine weevil grubs.
When the leaf miner has been a problem, treat container-grown plants in mid-May and early August.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
The adult fly is 8-10mm (approx 3/8in) long and is black with short whitish hairs on the thorax and abdomen. Eggs are laid on the foliage of house leeks in late spring to early summer, with a second generation in late summer.
After hatching, the larvae bore into the outer leaves and eat out the internal tissues. Each larva will tunnel several leaves before it is fully fed and ready to pupate in the soil.
Adults of the second generation emerge in July to August and further larval feeding occurs in August to September. This pest overwintes as pupae in the soil.