For the first time outbreak mechanism of bird-cherry ermine moth Yponomeuta evonymella (L.) is described. Under the influence of weather factor combinations, in which key role play the winter drought, at the bird-cherry trees can be disturbed development of a new generation of absorbing roots. This cause the appearance of filiform absorbing root deficiency in bird-cherry stands that continues 4 years. Bird-cherry ermine moth caterpillars which feed on the host plant with deficiency of filiform absorbing roots have increased rate of survivorship that cause increase of pest abundance. Bird-cherry stands with deficiency of foliform absorbing roots are becoming outbreak foci of the pest. Formation of ermine moth outbreak foci takes place in a very short period of time and is connected with disturbance of the very initial development stages of absorbing roots. Average datum of bird-cherry ermine moth outbreak foci formation in the south of Sverdlovsk region is 30 April. Bird-cherry ermine moth have in all probability the same outbreak mechanism in all parts of its areal.