Various species of Eucalyptus have been popular in California for at least the past century. Some have been useful for screens, windbreaks or shade trees. Others have been plantation grown for wood pulp, railroad ties or cut foliage. Most Eucalyptus grow well on their own and will even naturalize. They are often planted where other trees will not grow or outside of landscaped areas. Their performance has always been very reliable because they were not susceptible to disease or insect infestation. Unfortunately, their carefree lifestyle is not as secure as it once was.

In the last few years, several of their natural pathogens from Australia have mysteriously appeared in California. What is worse is that the new pests did not arrive with their natural predators. They are proliferating at an alarming rate with nothing to stop them. Eucalyptus trees in California are now much more susceptible to infestation than they would be back in Australia.

The effectiveness of chemical control has not yet been determined for most of these newly arrived insects. Even if chemicals are found to be effective, many trees are too large to be sprayed thoroughly. Large groves of trees would be even more difficult. Systemic insecticides are currently being tested on some of these new pathogens. Biological agents will likely be the most effective control. Research continues on finding, producing and releasing the parasites and predators associated with each species of pest in their natural habitats.

The main pests are the eucalyptus longhorn borer, eucalyptus gall wasp, various leaf eating beetles and at least six species of psyllids, especially the lerp psyllid. Each pest attacks specific species of eucalyptus. Some are limited to only a few potential hosts; but others are not so discriminating. For example, the eucalyptus longhorn borer will infest any one of at least six species. Although some eucalyptus are not attacked by any of these pests, many may be attacked by more than one.

Because there is no practical treatment for most of these new pathogens, prevention of infestation is important. Stressed trees are most susceptible. It is therefore important to keep trees healthy. Eucalyptus which normally do well without irrigation might appreciate occasional deep irrigation through summer. Damage to roots should be avoided. When choosing new trees, resistant species should be considered.

If the species of the eucalyptus in the landscape is known, it is easy to determine which insects may be a problem and what symptoms to watch for. Identification of potential pests is also useful when trying to protect trees. For example, redgum lerp psyllid is attracted to fresh new growth. Therefore, fertilizing should not be done because it would promote such growth, increasing the likelihood of infestation.

Before I forget, I should say that it is important when researching gardening concerns online, to limit the geographical area from which information is retrieved. Otherwise, there are far too many variables to consider. (I was recently reminded of this by a client in Altoona Pennsylvania who wanted to know how to get more syrup out of his sugar maple. He should probably ask someone who knows, like Aunt Jemima.)