Small ermine moth adults have white wings with black markings, they have a wingspan of approximately 20mm and belong to the family Yponomeutidae. The creamy white, black marked larvae reach 20mm in length and feed gregariously under the cover of a dense silk webbing. They are difficult to identify as adults but each has a different host range  There are several species that are often found in gardens, these include:

• The common small ermine, Yponomeuta padella. Found on hawthorn, blackthorn and wild cherry, but not bird cherry (Prunus padus)
• Bird cherry small ermine, Y. evonymella.  Found on bird cherry, Prunus padus. This species can cause complete defoliation and spectacular webbing during the later spring and early summer
• Euonymus small ermines, Y. cagnagella and Y. plumbella. Found on Euonymus
• Willow small ermine Y. rorrella. Found on willow, Salix
• Sedum small ermine Y. sedella. Found on Sedum
• Apple ermine Y. malinellus. Found on ornamental and fruiting apple

Most small ermine moths emerge as adults in July or August, and soon mate an lay eggs. Larvae hatch from late August and overwinter whilst still small. They begin feeding again in spring and are fully grown by June. They can cause extensive defoliation - this and the silk webbing produced whilst feeding make can cause alarm. Plants however, often recover from any damage. The caterpillars pupate within the webbing.

The sedum small ermine can have two generations a year with active caterpillars in June-July and September-October.

These caterpillars should not be confused with the webbing caused by box tree caterpillar, cotoneaster webbers, brown tail moth  or oak processionary moth.

Defoliation is most severe on small trees and plants and the webbing can cause alarm, however it should not affect the long term health or vigour of host plants and where possible can be tolerated.

Non-pesticide control

Check susceptible plants frequently from spring onwards so action can be taken before a damaging population has developed. 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.
Treatment is usually only possible and worthwhile on small trees and shrubs.
Defoliation is most severe on small trees and plants and the webbing can cause alarm, however it should not affect the long term health or vigour of host plants and where possible can be tolerated.

Non-pesticide control

• Where possible tolerate populations of these caterpillars, as butterflies and moths are an important part of the garden ecosystem and damage to plants is usually only short term
• Encourage predators and other natural enemies in the garden such as birds, hedgehogs and ground beetles
• Check plants regularly from spring for the presence of larvae and remove by hand where practical. The gregarious nature of these caterpillars means it is sometimes possible prune out affected shoots.

Pesticide control

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

• Organic contact insecticides containing natural pyrethrins (e.g. Bug Clear Ultra 2, Neudorff Bug Free Bug and Larvae Killer). Several applications of these short persistence products may be necessary to give good control
• More persistent contact 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

Follow label instructions when using pesticides. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number applications, spray interval and harvest interval. 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 (pdf document)