The Oak Processionary Moth caterpillar's defence mechanism is the production of a toxin (thaumetioieun) that is held in the hairs on it's body. The small brown hairs that cover their bodies can cause a number of problems, most commonly severe skin rashes but also respiratory problems and conjunctivitis. Health problems can occur even where there is no direct contact with the caterpillars as their hairs break off easily and can be easily transported in the wind. Spent nests should be treated with extreme care as they are often full with hairs from shed skins.
Animals are subject to the same risks as humans but consequences are often exacerbated as hairs can easily become trapped in their fur. Horses are particularly sensitive too as they may try to sniff or eat the caterpillars or their nests. Signs of this include excess salivation, swollen tongues, conjunctivitis, gagging, respiratory distress and mouth inflammation.
Forestry workers, arborists and arboricultural consultants are at the greatest risk of exposure. These workers must take particular care to protect themselves when working with or close to oak trees with OPM nests and caterpillars. Workers must be vigilant and employers must have an adequate occupational health monitoring system in place.
Oak processionary moth can cause a serious impact on tree health. Large populations of OPM caterpillars have the ability to completely strip the leaves from an oak tree. Loss of leaves reduces the trees ability to photosynthesise thus reducing the energy the tree has to grow. This leaves trees in a weakened state making them more susceptible to other pests and disease.
If OPM were to become widely established in Britain, it could pose a serious threat to native oak trees, particularly if repeated cycles of defoliation occur. In addition, it is likely that tree/ woodland management would decrease as risks to arborists and woodland managers would be increased. This would reduce the value of woodlands and trees as habitats and could increase risk to humans where trees cannot be effectively managed when defective.