Leafminer Tenacity By James A. Bethke and Dr. Richard A. Redak

The good guys, spray wand in hand, have finally chased awaythe deadly monsters. They have won the battle and are catching their breathwhen all of a sudden the monsters reappear, and they are off again, fightingfor their livelihoods. Like the Terminator-monster that comes back to life atleast three times before it’s completely dead, the leafminer makes itsproverbial call to the ornamental industry: “I’ll beback.” And so it is–at least, so it appears. As growers progressinto fall production, more and more reports are arising about a pest manythought 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. ornamentalindustry. Liriomyza trifolii lacks a common name but is the insect that causedthe uproar in the late 1970s and 1980s and will be the subject of thediscussion herein. Other Liriomyza species that can be significant pestsinclude the vegetable leafminer, L. sativae, and the pea leafminer, L.huidobrensis. Liriomyza sativae can be a pest of some herbaceous bedding andvegetable 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 inGeorgia. This species is a serious pest in South America, and it would behoovethe industry to catch and eliminate these infestations as they occur.

Life Cycle and Damage

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

Adult female leafminers use their egg-laying organ, theovipositor, 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 insome of the feeding punctures. When eggs hatch, the larva uses a set of mouthhooks like a pick, just like a real miner, to chop cellular tissue open so theycan feed on the fluids. The mine expands, leaving a white, meandering streakthat is visible on the leaf surface, becoming wider as the larva grows. Thenumbers of mines we’ve observed in cut chrysanthemums and gerberas havebeen staggering. Heavy infestations can also cause reduction in photosynthesis,which most likely causes subsequent reductions in yields in some ornamentalssuch as gerbera, or complete crop loss.

Monitoring for leafminers

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

Chemical Control Tactics

An effort should be made to eliminate leafminer populationsearly in the cropping cycle. Insect growth regulators may be more effectiveearly when the foliage is less dense, and when contact with young larvae is morelikely. Later in the cropping cycle, when dense foliage is present, asystemically acting material may be more efficient in reaching the insects.

Controlling leafminers, as with any other pest, should startwith the basics of integrated pest management, using everything to avoid thepest, then monitoring or scouting so that one can treat or spot treat only whennecessary. All these methods will reduce reliance on chemical control. Whenchemical controls are necessary, then a proper rotation of chemicals is thenext best method of avoiding or delaying resistance. Leafminers can complete ageneration in about 15-20 days in the greenhouse. Therefore, to avoidcontacting multiple generations of this insect with the same chemical class ormode 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 andtreat with two pesticides or two chemical classes at one time, but I do notrecommend this strategy because of the propensity of this insect to developpesticide resistance. It has been shown that insects with the propensity todevelop insecticide resistance, like the leafminer, can detoxify more than onechemical mode of action at one time. Quite simply, this means that treating aninsect 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 ofpesticides will avoid or delay pesticide resistance in the more capablespecies.

Recent Trials

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

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

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

In the field trial against highly resistant leafminers oncut mums, we did not see the same results as in the potted mum trial (SeeFigure 2, page 19). Three applications were made during the first five weeks ofthe trial. Seventy leaves were collected weekly from all treatments and thenumber of mines was recorded. Pest pressure was very high in this greenhouse,and we were attempting to provide alternative solutions for this grower. OnlyAvid sprayed weekly (see weeks 3 and 4, Figure 2) in this facility waseffective. As soon as applications ceased, the leafminer population becameunmanageable. This grower needs to review his pest management program andconsider using some of the cultural controls available to him such as steamingor 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 abilityto develop resistance to multiple modes of action by selected pesticides. Eventhe agrochemical industry has had difficulty developing new chemistries forleafminer control in the past. For example, when permethrin was fairly new, itwas considered a possible solution to leafminer resistance. However, once inuse, it lasted for no more than a few years before the leafminer developedsignificant resistance to the product. Around the mid-1980s, two products,cyromazine and abamectin, seemed to solve the problem. Later, when abamectinand cyromazine were registered for leafminer control in the mid-1980s, theproblem subsided. Resistance is rearing its ugly head again, however. Controlfailures for these chemicals and others are now being insinuated.

Because of our experience, many of us studying pestmanagement are asked what we would recommend to control specific insects pestslike the leafminer. However, because each grower’s insecticide use isunique, they will have pests with a different tolerance for pesticidesdepending on the amount used. Therefore, it is very difficult to recommendpesticides to growers. If you are concerned with the possibility of resistancebuildup, send some samples to a local scientist who can determine the level ofresistance at your particular facility. They may be able to recommendpesticides 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 helpavoid resistance buildup in the pest population. Once resistance occurs, youare practicing resistance management, and that is a very different anddifficult job.

James A. Bethke and Dr. Richard A. Redak

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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