You may have noticed many gum trees in the Shoalhaven appear sick or even dying. Gary Daly explains that they have been attacked by lerps and that an unbalanced ecosystem could be the reason why.
There has been a recent change in the appearance of some gum trees in the Shoalhaven. Some trees look sick and partially defoliated. In particular the Illawarra Blue Gum or Bangalay Eucalyptus botryoides/saligna is affected. Often remnant trees beside roads look sick but in some situations such as the Kangaroo Valley side of Cambewarra Mountain whole blocks of forest are affected.
These trees are sick because of sap sucking insects called psyllids which produce sugary shells called lerps. The term lerp is an Aboriginal word. The psyllids hide under the lerp. When psyllids pierce and suck the leaf they remove a portion of the plant's chlorophyll. The degree of infestation is most evident when the leaves of infected trees are wet and the leaves look brown.
Lerps are eaten by a wide range of animals such as possums and birds. Recently there has been an expansion in their population in the Nowra area. In some place the degree of infestation is such that trees may die.
Expansions and contractions of populations of animals is a natural process. however, the presence of lerps can also indicate that the ecosystem is out of balance. This can be seen in the northern Illawarra around Stanwell Park and Coledale where Blackbutts E. Pilularis beside Lawrence Hargraves Drive have a larger degree of lerp infestation than those occurring some distance away from the road. In this case, the road/exhaust fumes are probably related to the level of infestation.
The presence of roads is not the cause of lerp infestation in the Shoalhaven.
When whole blocks of regenerating forest (such as the case in Kangaroo Valley)
look sick then exhaust fumes can be ruled out. The reason for the lerp expansion
is motivated by other causes. Let us hope that the infestation is temporary
and the population brought back into a more normal level. If this does not
occur and there are other stresses brought upon the trees (such as drought)
then they may succumb and die.
Gary Daly