Leafminer Tenacity

August 22, 2002 - 08:49

Just when you thought they were defeated for good, leafminers are back in this fall?s crops, with a vengeance!

The good guys, spray wand in hand, have finally chased away
the deadly monsters. They have won the battle and are catching their breath
when all of a sudden the monsters reappear, and they are off again, fighting
for their livelihoods. Like the Terminator-monster that comes back to life at
least three times before it's completely dead, the leafminer makes its
proverbial call to the ornamental industry: "I'll be
back." And so it is--at least, so it appears. As growers progress
into fall production, more and more reports are arising about a pest many
thought was long since vanquished--the leafminer.

There are 23 species of economically important leafminers.
However, only a few are of interest as major pests to the U.S. ornamental
industry. Liriomyza trifolii lacks a common name but is the insect that caused
the uproar in the late 1970s and 1980s and will be the subject of the
discussion herein. Other Liriomyza species that can be significant pests
include the vegetable leafminer, L. sativae, and the pea leafminer, L.
huidobrensis. Liriomyza sativae can be a pest of some herbaceous bedding and
vegetable pony packs. Liriomyza huidobrensis can be a pest in asters,
gypsophila and some bedding plants, including many vegetables. Recently, Dr.
Ron Oetting investigated an infestation of L. huidobrensis in chrysanthemums in
Georgia. This species is a serious pest in South America, and it would behoove
the industry to catch and eliminate these infestations as they occur.

Life Cycle and Damage

Serpentine leafminers are small (ca. 1/16-inch) bright
yellow and black flies. They can cycle through several generations in a year in
the greenhouse, with lower numbers seen during the winter months. Eggs hatch in
about 3-5 days. The larval stage takes approximately 4-7 days to develop to the
pupal stage, and the pupa takes approximately 11 days to emerge as an adult. In
all, they require about 15-20 days to develop from egg to adult, depending on
temperature.

Adult female leafminers use their egg-laying organ, the
ovipositor, to puncture the leaf surface, damaging some cells just beneath.
They then back over the hole and feed on the exposed fluids. Eggs are laid in
some of the feeding punctures. When eggs hatch, the larva uses a set of mouth
hooks like a pick, just like a real miner, to chop cellular tissue open so they
can feed on the fluids. The mine expands, leaving a white, meandering streak
that is visible on the leaf surface, becoming wider as the larva grows. The
numbers of mines we've observed in cut chrysanthemums and gerberas have
been staggering. Heavy infestations can also cause reduction in photosynthesis,
which most likely causes subsequent reductions in yields in some ornamentals
such as gerbera, or complete crop loss.

Monitoring for leafminers

Yellow sticky cards are an effective method of monitoring
for leafminer adults. One trap per 10,000 square feet of growing area is
recommended. Traps are optimally placed a few inches above the plant canopy and
moved up as the plants grow. Traps should be examined weekly. Records can be
used to identify trends in pest pressure on different cultivars, record
immigration pressure from specific directions and confirm results from
treatment applications. Recognizing hotspots and treating them early may reduce
the overall need for pesticide applications.

Chemical Control Tactics

An effort should be made to eliminate leafminer populations
early in the cropping cycle. Insect growth regulators may be more effective
early when the foliage is less dense, and when contact with young larvae is more
likely. Later in the cropping cycle, when dense foliage is present, a
systemically acting material may be more efficient in reaching the insects.

Controlling leafminers, as with any other pest, should start
with the basics of integrated pest management, using everything to avoid the
pest, then monitoring or scouting so that one can treat or spot treat only when
necessary. All these methods will reduce reliance on chemical control. When
chemical controls are necessary, then a proper rotation of chemicals is the
next best method of avoiding or delaying resistance. Leafminers can complete a
generation in about 15-20 days in the greenhouse. Therefore, to avoid
contacting multiple generations of this insect with the same chemical class or
mode of action, rotation to a new chemical class should occur every 1-2 months,
to avoid resistance buildup.

Many growers would prefer to kill both stages at once and
treat with two pesticides or two chemical classes at one time, but I do not
recommend this strategy because of the propensity of this insect to develop
pesticide resistance. It has been shown that insects with the propensity to
develop insecticide resistance, like the leafminer, can detoxify more than one
chemical mode of action at one time. Quite simply, this means that treating an
insect population with several modes of action at one time can, in time,
produce what seems to be a super bug. The proper, labeled use and rotation of
pesticides will avoid or delay pesticide resistance in the more capable
species.

Recent Trials

Figures 1 and 2, below, are summaries of recent trials
we?ve conducted against leafminers. Some of the pesticides are registered
for use on the intended target and others are experimental. We do not always
use labeled rates in our trials because these trials are for experimental
purposes, to increase our knowledge about the products and their capabilities.
Labels constantly change; therefore, it is always the pesticide
applicator?s responsibility, by law, to read and follow all current label
directions for the specific pesticide being used.

The following trials were conducted on potted and cut
chrysanthemums. Potted mums were infested with leafminer larvae by exposing
them for four hours to an existing colony of adults. After four days, eggs hatched
and a pre-treatment count of all mines was recorded. A single treatment
application was applied on the same day as the pre-treatment count. Spray
treatments were applied to run off using a backpack sprayer at approximately 25
psi. The following day, when the treatments had dried, plants were tipped over
trays filled with sand. Larvae emerged from leaves and pupated in the sand
where they were collected and counted. In addition, emerging adults were
counted two weeks later when all adults had emerged.

All treatments performed very well against leafminers
infesting potted mums (See Figure 1, page 19). The recommended rate of Azatin
XL and the three rates of Gwn-1535 (Gowan Co.) did not have an appreciable
effect against leafminer larvae. However, they were extremely effective in
eliminating any emerging adults. Our experiment showed that when Flagship is
sprayed at 4 oz., it is not as effective as when it was applied as a soil
drench. Only one larva was able to complete development in the drenched plants.
Flagship?s ability to translocate throughout the plant and affect
leafminer larvae needs further study.

In the field trial against highly resistant leafminers on
cut mums, we did not see the same results as in the potted mum trial (See
Figure 2, page 19). Three applications were made during the first five weeks of
the trial. Seventy leaves were collected weekly from all treatments and the
number of mines was recorded. Pest pressure was very high in this greenhouse,
and we were attempting to provide alternative solutions for this grower. Only
Avid sprayed weekly (see weeks 3 and 4, Figure 2) in this facility was
effective. As soon as applications ceased, the leafminer population became
unmanageable. This grower needs to review his pest management program and
consider using some of the cultural controls available to him such as steaming
or sterilizing the soil between crops, exclusion screens, etc.

Pesticide resistance in the leafminer

Why can the leafminer be so difficult to control using pesticides?
The serpentine leafminer, like some other insect pests, has an innate ability
to develop resistance to multiple modes of action by selected pesticides. Even
the agrochemical industry has had difficulty developing new chemistries for
leafminer control in the past. For example, when permethrin was fairly new, it
was considered a possible solution to leafminer resistance. However, once in
use, it lasted for no more than a few years before the leafminer developed
significant resistance to the product. Around the mid-1980s, two products,
cyromazine and abamectin, seemed to solve the problem. Later, when abamectin
and cyromazine were registered for leafminer control in the mid-1980s, the
problem subsided. Resistance is rearing its ugly head again, however. Control
failures for these chemicals and others are now being insinuated.

Because of our experience, many of us studying pest
management are asked what we would recommend to control specific insects pests
like the leafminer. However, because each grower's insecticide use is
unique, they will have pests with a different tolerance for pesticides
depending on the amount used. Therefore, it is very difficult to recommend
pesticides to growers. If you are concerned with the possibility of resistance
buildup, send some samples to a local scientist who can determine the level of
resistance at your particular facility. They may be able to recommend
pesticides that can help manage the resistant pest.

A better tactic would be to practice resistance avoidance.
Exclusion, monitoring, spot treating and rotating chemical class will help
avoid resistance buildup in the pest population. Once resistance occurs, you
are practicing resistance management, and that is a very different and
difficult job.

About The Author

James A. Bethke is a research associate and Richard A. Redak is professor of entomology in the Department of Entomology at the University of California-Riverside. They can be reached by phone at (909) 787-4733 or E-mail at bethke@citrus.ucr.edu.

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