Gardeners in just about all areas of Orange County
were 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 about 50 eggs during her life.
An Asian insect, the citrus leafminer was first
detected in California in Calexico 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 '80s. The hysteria
heightened in the early '90s 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 gardener's resolve.
Where have these pests gone? We still grow eugenias
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 used in 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 overreact when their
cherished citrus trees are under attack is a
difficult order, but help is on the way. With little
recognition and almost no acknowledgment 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 in Corona del Mar.
ASK RON
I just moved to the area and enjoy your column and
advice. Where can I get a competent soil analysis?
Michael
Costa Mesa
Three local laboratories are outstanding, one right
here in Orange County.
Soil & Plant Laboratory, 1594 N. Main St., Orange,
(714) 282-8777, www.soilandplantlaboratory.com.
Wallace Laboratories, 365 Coral Circle, El Segundo,
(310) 615-0116, www.bettersoils.com.
Timberleaf Soil Testing, 39648 Old Spring
Road, Murrieta, (951) 677-7510. Timberleaf
specializes in organic principles and organic
fertilization recommendations.
ASK RON your toughest gardening
questions, and the expert nursery staff at Roger's
Gardens will come up with an answer. Please include
your name, phone number and city, and limit queries
to 30 words or fewer. E-mail stumpthegardener@
rogersgardens.com, or send to Plant Talk at Roger's
Gardens, 2301 San Joaquin Hills Road, Corona del
Mar, CA 92625.
