How many times have you heard someone wishing for a plague that would stop the eucalypts' invasion of the world? Well, be careful what you wish for! A new Eucalyptus pest has arrived in North America--the redgum lerp psyllid (RLP), Glycaspis brimblecombei, first described for science in 1964 from Brisbane, Australia. Since mid-June 1998, when positive identification of RLP was made in El Monte of Los Angeles County, reports of infestations have been filed in most cities within the Los Angeles basin and neighboring Orange, San Diego, western San Bernardino, and Riverside counties. The range of this introduced sapsucker already extends as far north as San Francisco.
This psyllid pest is host-specific to eucalypts. Identification can be made by a brief examination of the leaves of eucalyptus trees, either leaves that are still on the tree or that have fallen to the ground. RLP larvae build conical "lerps" by excreting gelatinous honeydew from their posterior ends. Resembling an armored scale insect attached to the eucalyptus leaf, the lerp structure houses a larger larva beneath. A conical lerp may be three millimeters wide and two millimeters high. The larvae or nymph, colored yellow or yellow and brown, feeds and grows to adulthood under this cap of crystalline sugars. Adults are three millimeters in length, slender, and pale green with areas of orange and yellow. Winged adults fly to nearby leaves or trees where they mate, feed, and commence egg laying. Heavy infestations can cause severe leaf drop as well as black sooty mold growing on the honeydew-covered leaves.
Like other psyillids, female RLPs lay eggs especially on the succulent leaves of new shoots. RLP metamorphosis, from egg to larva to adult, can occur regularly, yielding up to four generations per year. Development slows to several months during winter but can take only a few weeks in warm temperatures. In the mild coastal climate of Westwood, one can expect all stages to be present throughout the year.
No practical method of control has yet been found. Observed predators now being tested against RLP are the Asian lady beetle (Harmonia axyridis) and Chilocorus bipustulatus. Spiders, mites, syrphid fly larvae, the lady beetles Coccinella californica and Hippodamia convergens, and the heteropteran Zelus renardii are noted predators; however, none of these have been shown to be effective biological control agent of RLP. Insecticidal sprays are impractical--the lerp mechanism shields the larvae, and due to tree size and wind drift, 95 percent of the spray becomes airborne and wasted. Endemic bio-controls and chemical controls, such as systemics, are now being studied, but it may take a year or more before research can yield solutions.
There is a growing list of Eucalyptus species for which RLP has a preference. In Australia RLP uses localized populations of red gums as its primary food source. These include E. dealbata, E. tereticornis, E. blakelyi, E. bridgesiana, and E. nitens. In California RLP is already feeding on E. camaldulensis, E. cladocalyx, E. globulus, and E. rudis, and more eucalypt species are being added to the susceptible list regularly. Our list at MEMBG includes Eucalyptus cornuta, E. robusta, E. marginata, E. botryoides, E. deglupta, E. punctata, E. grandis, and five other unidentified species of Eucalyptus.
As a note of interest, on August 18 RLP was observed on a red-cap gum, E. erythrocorys, in our raised planting bed across from the UCLA hospital emergency entrance, but by early September it was no longer inhabiting that species. With respect to our two massive specimens of rose gum, E. grandis, the smaller, more northern tree is heavily infested while the taller tree--only 125 feet away--has barely been touched. I have no explanation as to why two trees of the same species would exhibit such different levels of resistance, but it does get one thinking about plant stress levels and their cultural health.
If you observe RLP on your eucs, the following guidelines may help. Don't prune or fertilize your trees. Pruning stimulates growth and also stresses the plant. Plants respond to stress with increasing nitrogen in leaves--an effect similar to that of quick-release fertilizer with nitrogen. The is counterproductive, because increased leaf nitrogen, in turn, improves psyllid reproduction and survival. Both psyllid larvae and egg-laying females prefer nutrient-enriched leaves that develop after the application of quick-release fertilizer formulations. Drought stress also increases damage to trees from RLP, as well as from eucalyptus long-horned borers. On the other hand, excessively wet soil from irrigation or rain prevents roots from obtaining adequate oxygen, killing small roots. You can help your trees by avoiding frequent, shallow waterings that are typically used for lawns. Irrigate eucalyptus infrequently, possibly only once a month during drought, but with sufficient amounts of water to penetrate deeply into the soil. This is not a change that can be made overnight; it requires a conscientious gardener to retrain these trees roots--but healthier, less-stressed roots they will become!
Until research provides additional answers, that's the best we can do for now. So think about how you are affecting the health of your plants with your cultural practices. And let's not get too stressed.
California Plant Pest and Disease Report 18(1-2), online from numerous UC scientists at the website http://www.cnr.berkeley.edu/biocon/dahlsten/rglp/RLP, and based on our observations at UCLA.
RAND PLEWAK, Garden Manager