WILLOW ERMINE PHENOMENON

This week, while working on-site in Oxfordshire and undertaking a walkover survey for nesting birds, I encountered a remarkable natural phenomenon. A row of willow trees showed extensive and unusual defoliation, with dense, ghostly silken webbing draped across their canopies.

As you can see from these photographs, the Willow trees were covered in 

 ghostly silken webbing

On closer inspection, the cause became clear: the trees were hosting thousands of Willow Ermine moth caterpillars (Yponomeuta rorrella). The conspicuous webbing concealed vast numbers of caterpillars within almost every tree, creating a striking and somewhat unsettling visual effect.

Although alarming at first glance, infestations of ermine moth caterpillars are generally harmless in the long term. The caterpillars hatch in late spring and feed on leaves while constructing protective communal webs. By early to mid-summer, typically June or July, they pupate and emerge as adult moths.

Adult moths lay their eggs on host trees in late autumn. These eggs overwinter and hatch the following spring, which can lead to a sudden and dramatic surge in feeding activity. Despite the heavy defoliation that can occur, willow trees usually recover well once the caterpillars disperse and feeding ceases.

Mild winters are often cited as a contributing factor to increased ermine moth populations, as higher overwinter survival rates allow more caterpillars to emerge and feed in spring. Such conditions can result in the large-scale webbing events observed this year.

Willow Ermine moth

I will continue to monitor this site over the coming months to observe how the willow trees respond and recover following the infestation. Events such as this serve as a powerful reminder of the dynamic interactions between insects and their host plants, as well as how seasonal conditions can influence population outbreaks.