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New, Rapidly Spreading Insect Threatens State's Eucalyptus

New, Rapidly Spreading Insect Threatens State's Eucalyptus

(July 29, 1999)

One of California's most common landscape trees - already under attack by nearly a dozen different insects from its native Australia - is facing perhaps its greatest challenge in an eighth-inch pale green, plant-juice sucking bug called the redgum lerp ps yllid.

And, there is little that homeowners and landscape managers can do to protect their eucalyptus trees until scientists identify and study the psyllid's natural enemies in hopes of establishing a permanent, non-pesticide "biological control," according to M ark Hoddle, a Cooperative Extension entomologist at the University of California, Riverside.

Of the dozen known eucalyptus pests now in California, the redgum lerp psyllid has been among the fastest to spread. Discovered just over a year ago in El Monte, it is now commonly found in eucalyptus landscapes from San Diego to the Bay Area.

Both immature and adult psyllids feed by sucking plant juices out of the leaves of eucalyptus, causing serious leaf drop which weakens the trees and makes them more susceptible to other pests, such as the longhorned borer beetle, Hoddle said. "It's easier for the longhorned borer to colonize a stressed, weak tree," he said.

Extensive leaf fall can also pose a fire hazard. The insects excrete a sticky, watery waste product called honeydew onto leaves of eucalyptus trees, leading to the formation of a blackish sooty mold that further hastens defoliation. Honeydew can also stain sidewalks and cars parked beneath infested trees.

Psyllid infestations can be readily identified by the characteristic protective cover - called a "lerp" - which the nymph or immature stage of the psyllid forms as it is feeding on leaves. A lerp resembles a small white, round cap that grows to about the size of a lentil or larger. Lerps are made by feeding nymphs from crystallized honeydew. In a serious infestation, eucalyptus leaves can contain several dozen lerps.

The nymph protected beneath each lerp is yellow or brown and looks like a wingless aphid. Adults are light green with orange and yellow spots. They hold their wings like a roof over the backside of their abdomen.

Infested trees typically respond by sprouting new growth - even as the trees are being defoliated, Hoddle said. Often, the new growth is subsequently infested, he said.

"This insect has tons of food. It's got great weather and nothing is stressing it out. It's got a great life in California, and we need to disrupt this great lifestyle by finding and releasing safe natural enemies to feed on the psyllid," said Hoddle.

Currently, the insect attacks only redgum varieties of eucalyptus, but entomologists fear that populations of the psyllid will get so high that they will begin to feed on other varieties as well, Hoddle said.

Present in California are several natural enemies of the redgum lerp psyllid, including two different ladybug beetles, but it is not known if they can control the psyllid without additional reinforcements. Scientists plan to collect and identify natural e nemies from the psyllid's native Australia in hopes of establishing a non-pesticide biological control in California. But, it will be at least 12 to 18 months before such a management program can begin, Hoddle said. Natural enemies located in Australia need to be cleared through quarantine and deemed safe for release before they can be mass reared and distributed.University of California entomologists have recently coordinated their research efforts.

The biological control effort is being led by UC Berkeley Professor Donald Dahlsten. UCR entomologists are helping to locate study sites to monitor psyllid numbers before and after natural enemy releases begin to document the impact of pest suppression efforts. They are also conducting limited pesticide trials and developing pesticide application methods to minimize the impact on beneficial insects.

In the meantime, homeowners and landscape managers should do all they can to keep their eucalyptus trees healthy and robust. Hoddle said that means irrigating trees slowly and very deeply during the dry summer and fall seasons. The best method, he said, is to water trees once a month with a dripping hose for two to three days. Hoses should be placed underneath the outer edge of a tree's canopy, above the underground "feeder roots" that take up moisture.

"We want to get the water to penetrate into the soil a long ways," he said. It is a good idea to avoid frequent, shallow watering such as is common for lawns.

Hoddle also advises to avoid fertilizing eucalyptus, as the additional nitrogen taken up by the trees could aid reproduction of the psyllid. Experts are also advising that infested trees not be pruned. Pruning additionally stresses trees, especially over the summer months, and psyllids will readily invest young foliage that grows following pruning.

Likewise, he said, it is not a good idea to use pesticides, which may harm existing natural enemies that prey on the insect. Lerps covering nymphs also protect them from the effects of pesticides. And, it is difficult to spray large landscape trees without causing a spray drift, especially in residential areas.

The redgum lerp psyllid is one of about a dozen insects currently infesting eucalyptus in California. Others include the Australian tortoise beetle, which chews on eucalyptus leaves, and the eucalyptus longhorned borer, which has been under study at UCR for several years.

The University of California, Riverside ( is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment has exceeded 21,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual statewide economic impact of more than $1 billion.

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