Leafminer Tenacity

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.