Gardeners in just about all areas of Orange County are noticing a new pest this summer on their citrus trees. Nurseries and garden centers throughout the county are being overrun with inquiries about this new pest, what it is and what to do about it.
The pest is the citrus leafminer (Phyllocnistis citrella) and is easy to notice on nearly any citrus tree as the larvae tunnel inside young leaves. Their white, wiggly trails within the leaves have caused panic and, in some cases, an overuse of inappropriate pesticides. In the long run some of these pesticides may do more harm than good, especially on mature trees. Grapefruit, lemon and lime seem to be the favorite hosts for the leafminer, but it feeds on all varieties of citrus and a few other related plants in the citrus family, such as Pittosporum.
Adult citrus leafminers are tiny moths about two or three millimeters long and will probably never be seen by a gardener. The larval stage is what you are seeing as they infest the new young foliage of citrus, with no interest in the older, mature leaves. The tiny larval caterpillars hatch and begin feeding immediately in nearly invisible mines under the leaf’s surface. As the larvae grows, its zigzagging feeding path become more obvious, eventually nearly occupying the entire leaf.
When the larva is mature it crawls to the edge of the leaf to make a cocoon of silk. As the silk dries the leaf curls over the developing pupa. About a week later a tiny adult moth emerges to begin the cycle again. By the next day the mated females begin to deposit eggs on the new emerging leaves. A single female can deposit approximately 50 eggs during her life.
An Asian insect, the citrus leafminer was first detected in California in Calexico, Imperial County in 2000. Its first occurrence in Orange County was a single report only a year ago. Since then it has set a record by spreading throughout every southern California county and just last month was detected as far north as Fresno County.
The citrus leafminer is the newest in a long history of exotic pests to gain a foothold in the lush warm gardens of Orange County. Over the past two decades local gardeners have seen many other exotic pests cause near panic among their plants. These pests, being new to the area, have few natural enemies and initially cause heavy damage. However, with time, their presence seems to almost disappear.
Many gardeners will recall the alarm caused in Orange County by Eugenia psyllids and then ash whiteflies in the late 80’s. The hysteria heightened in the early 90’s when Eucalyptus trees throughout the county were decimated by various species of lerp psyllids. Would Eucalyptus disappear forever from our local landscapes? Just a few years ago the ultimate challenge, the giant whitefly, tested nearly every gardeners resolve.
Where have these pests gone? We still grow Eugenia’s and Ash, Eucalyptus abound and the wooly masses of giant whitefly filaments covering the leaves of hibiscus have just about disappeared. Fortunately, two agencies, The California Department of Food and Agriculture and the University of California Cooperative Dept. of Agriculture and Natural Resources, silently work behind the scenes to bring these pests into ecological balance.
Beneficial insects, particularly tiny, stingless parasitic wasps, that attack the citrus leafminer are being evaluated and are establishing themselves in Orange County gardens. These will increase and should provide biological control in another two or three years. Gardeners will need to be patient while this occurs and should be careful not to disrupt the process with inappropriate sprays that may harm these beneficial insects.
Mature citrus can withstand heavy infestations of citrus leafminer. The loss of some new foliage is a minor problem on older trees. The loss of new leaves is a more serious problem on young trees that do not have a lot of mature foliage. Newly planted or young trees may be more seriously damaged, especially if the infestation is heavy and prolonged. Treating young trees with insecticide may be justified if it helps protect them until new leaves have grown too large for the leafminer to attack.
Most pesticides currently used home gardens are not very effective on citrus leafminer because they do not control feeding inside leaves. Many insecticides leave a toxic residue on the foliage that can do more harm than good, killing beneficial insects, resulting in additional leafminers as well as white flies, scale insects and other pests.
A new insecticide product, Spinosad, is effective on citrus leafminer. Spinosad, a naturally occurring organic compound, is produced by a bacterium and is translocated into leaves and is relatively nontoxic to beneficial insects.
Asking gardeners not to over-react when their cherished citrus trees are under attack is a difficult order, but help is on the way. With little recognition and almost no acknowledgement from gardeners, expert entomologists, especially at UC Riverside, will bring the citrus leafminer under control. Patience please.
Ron Vanderhoff is the Nursery Manager at Roger’s Gardens, Corona del Mar