Tomato Leafminer

Introduction:

Regarding this pest, what is emphasized more than anything else is the use of physical or biological control methods to combat the pest. Why is such emphasis placed on these methods for the tomato leafminer? Is chemical control ineffective? Are physical methods such as pheromone traps sufficient to combat this pest? Do non-chemical methods have sufficient efficacy to combat this serious threat? It appears that the use of a combination of chemical, physical, and other methods within integrated management programs can be key in controlling this pest. Below, some of the most important morphological and biological characteristics, as well as the main control methods for this moth in other countries, are mentioned.
This moth, known scientifically as Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae), is considered one of the major pests of tomato and potato worldwide, prevalent in South American and Mediterranean countries. The high reproductive capacity of this pest has made it one of the most resistant insects to pesticides. Each female insect can lay up to 300 eggs, thus producing 10-12 generations annually. On tomato plants, this pest attacks all parts of the plant, although larvae prefer to feed on apical buds, young leaves, flowers, and green fruits. The damage caused by this pest can completely destroy 100% of the crop. While the origin of this pest appears to be Latin America, its geographical distribution has now spread to Europe, the Middle East, and Iran. Given its destructive potential, it has quickly emerged as a key pest.

As mentioned, the high reproductive capacity and short life cycle have increased this insect's ability to resist pesticides. Resistance occurs, especially when pesticides are not rotated. Currently, resistant populations have been reported in Latin American countries; these regions have been exposed to repeated and improper use of chemical pesticides for decades, and consequently, the selective pressure of these chemicals has led to the growth and spread of resistant populations to various groups of insecticides. Since it is likely that the new wave of the pest's spread to European and Middle Eastern countries originated from Latin American populations, the likelihood of resistance in Middle Eastern and Iranian tomato leafminer populations is high. Therefore, it is crucial to first evaluate the efficacy of insecticides on this pest's populations in different geographical regions. Following these preliminary trials, recommendations can be provided for the use of insecticides in an Integrated Management Program or for resistance management of this pest.

Damage Method:
This pest damages tomato plants at all growth stages, and damage is possible at all larval stages due to this insect's feeding. Identifying damage from the tomato leafminer on the plant is usually easy; on leaves, larvae feed on the mesophyll tissue, and their feeding paths appear as irregular spots, with the black feces of the larvae visible within the mines. These spots may later necrotize. Larvae may also create tunnels in the stems, which can alter the overall development of the plants. As soon as fruits appear on the plants, they are exposed to this pest's infestation. The mines caused by this pest inside the fruit can serve as entry points for secondary pathogens, leading to fruit rot and decay. The damage caused by this insect to the tomato crop is significant in both quantitative and qualitative terms, and if left uncontrolled, can reach 100% of the crop.
On potatoes, only the aerial parts of the plant are attacked, and larvae do not damage the tubers.

Biology
T. absoluta belongs to the microlepidoptera group, and adult moths are observed with a brown-silver color and a length of 5-7 mm. The entire life cycle is completed in 24 to 40 days (except for winter months when the life cycle exceeds 60 days). The minimum temperature required for the biological activity of this pest is 9°C. After mating, female insects lay small, yellowish-cream colored eggs with a length of 0.35 mm. Larvae emerging from these eggs appear in light yellow or green colors and are 0.5 mm in length. Upon completion of the larval stage, a distinct dark green band appears on the dorsal surface of the larvae up to the capsule head. Four larval stages are observed during the life of this insect. Larvae do not enter diapause as long as food is available. Pupation occurs in the soil, on the leaf surface, or within larval tunnels. If pupation does not occur inside the soil, it is usually covered with a silk cocoon. Overwintering is observed in the form of eggs, pupae, or adult moths depending on environmental conditions.
Methods for Controlling Tomato Leafminer
Sustainable and efficient management of this pest is based on integrating preventive methods with chemical and non-chemical control methods.
Physical and agricultural methods include:
- Observing crop rotation
- Using pest-free seedlings
- Applying yellow sticky traps before transplanting
- Monitoring the pest using delta pheromone traps
- Deep plowing of the soil at an interval of two growing seasons and covering the soil with plastic mulch or performing solarization
- Observing a 6-week interval for replanting in infested land
- Greenhouse sealing with high-quality nets (9×6 cm²)
- Regular inspection of the crop to identify the first signs of damage
- Using water and oil traps (at a rate of 20-40 traps/hectare) for mass trapping
- Collection and destruction of infested crop products and plant waste
- Weed control (especially weeds of the Solanaceae family) to prevent pest population increase on alternative plant hosts
Biological control methods for tomato leafminer include:
- Establishing populations of biological control agents such as the predatory bug Nesidiocoris tenuis, Pseudapanteles dignus, and Trichogramma pretiosum in areas of pest infestation
- Use of the entomopathogenic fungus Metarhizium anisopliae and the bacterium Bacillus thuringiensis

In studies, the role of the parasitoid wasp *T. pretiosum* has been mentioned, which has the ability to parasitize the eggs of this pest. The parasitism rate of the eggs of this moth by the aforementioned parasitoid has been reported to be 28%.
Additionally, references have been made to the existence of a wasp from the Braconidae family, known as *P. dignus*, as a parasitoid of tomato leafminer larvae, which has a parasitism capability rate of 30%.
Furthermore, the activity of the fungus *M. anisopliae* on the eggs of this pest has been studied, which has been described as promising.
The bacterium *B. thuringiensis* is mentioned as a pathogen of tomato leafminer larvae.

Pheromone traps
Among the tools used in the integrated management of this pest are pheromone traps. By using pheromone traps, it is possible to monitor and be aware of potential pest outbreaks, and the density and abundance of the pest can be estimated. In these types of traps, the female sex pheromone is used to attract male moths. Male moths are trapped and captured as soon as they enter the trap due to contact with its sticky inner surface.

The use of pheromone traps in mass trapping of tomato leafminer moths has also been reported to be satisfactory. According to existing reports, each trap catches an average of 1200 male moths per night. Pheromone traps should be used before the emergence of the first generation and their application should continue until the end of the season to be effective.
Two types of traps can be used for this pest. Funnel traps and Delta traps. In case of high pest population density or the presence of dust in the air (which most western and southern provinces of Iran face), funnel traps show better performance compared to Delta traps.
It is worth mentioning that 1 to 2 traps are needed per hectare. The trap should be installed near the highest point of the plant and at approximately one meter above ground level.

Chemical Control
What should be known about the chemical control of tomato leafminer:

- Application of local and regional economic thresholds for starting insecticide applications
- Selection of insecticides based on local efficacy and selective action
- Rotation of insecticide applications based on mode of action
- Use only insecticides registered for T. absoluta
- Adherence to the instructions on the pesticide label
- It should be noted that, chemical control of this pest is difficult due to the larvae being inside the plant's mines, and the success of chemical control depends on the proper use of appropriate insecticides within a single growing season. According to available reports, the insecticides Indoxacarb and Chlorpyrifos are among the effective pesticides against this pest. However, due to the points mentioned about the high resistance power of this moth to pesticides, resistance management principles must be considered when applying insecticides.

Resistance Management
As mentioned, resistance to various insecticides is observed with high intensity in populations of tomato leafminer. In Latin America, severe and widespread resistance has been reported in field populations of this pest to organophosphates (Group 1B), synthetic pyrethroids (Group 3), and benzoylurea (Group 15). However, resistance to new insecticide groups has probably also spread, as resistant populations from Latin America have migrated to Europe, North Africa, and the Middle East. Therefore, what is of primary importance is the preparation of a sensitivity profile (map) of different T. absoluta populations by local experts, to enable local and indigenous recommendations for pesticide use in the fight against this pest.
In general, the best recommendation for the efficacy of existing pesticides emphasizes the integration of all possible tools in the management of this pest, and the use of insecticides is only permitted when necessary, based on economic thresholds, while observing rotation in the use of different types of chemical pesticides (based on mode of action).
In explaining the rotation of insecticide use, it should be said that in this approach, repeated spraying with one group of insecticides against successive populations of a pest is avoided. A profile known as A treatment windows Mo is used for this purpose.
Below is a treatment window defined for a 30-day period, based on the minimum life cycle of one generation of T. absoluta.
Once the first 30-day grouping window is completed, if re-application of an insecticide is required based on the economic threshold, a different and effective group must be selected for use in the next 30 days (second grouping window). Similarly, a third grouping window is used based on another different group, and so on.
The proposed program aims to reduce the occurrence of resistance to a pesticide group by ensuring that the re-application of insecticides belonging to that chemical group is avoided for at least 60 days after the end of the window. This time period is predicted based on the likelihood of a longer pest life cycle during the agricultural season.
This strategy requires at least three chemical groups of insecticides, but in theory, more chemical groups that are locally registered and effective against T. absoluta can be added to this table.

In conclusion, and summarizing the points presented in this article, it must be stated that, first and foremost, the development of a management program for the prevention, monitoring, and control of pests in tomato fields and greenhouses is vital. Within the framework of this program, preventing the entry of the tomato leafminer into the field or greenhouse is of paramount importance. The use of the physical and agricultural methods mentioned earlier reduces the likelihood of an outbreak of the pest population. Additionally, implementing a biological control program, considering the potential of the affected area, is essential, although the success rate of this program will not exceed 30%. Therefore, the most fundamental and reliable method for controlling the population of tomato leafminer in case of appearance in the area is the optimal and rational use of pesticides. According to reports, the insecticide Indoxacarb has been reported to be effective against this insect. It is recommended that the aforementioned insecticide be used with proper rotation, and to prevent resistance development in the moth population, use Scheme Number 1. The use of other insecticides against this insect should only be carried out after conducting sensitivity tests, and if resistance is reported in a population, that area's chemical control program should be removed.

Sources:
- Anonymous, Tomato Leafminer/ A Pest that has Infested Tomato Farms. IRNA News Agency, 16/5/2011
- Sarosh, M. J., Kalantar Hormozdi, F. & Rajabi, M. Z., 2010. Tomato leafminer Tuta absoluta (Meyrick, 1917) (Insecta: Lepidoptera: Gelechiidae)

-Anonymus, ?. The Tomato Leafminer, Tuta absoluta. Recommendations for Sustainable and Effective Resistance Management. www.irac-online.org

-Anonymus, 2005. Data sheets on quarantine pests Fiches informatives sur les organismes de quarantaine, Tuta absoluta. Bulletin OEPP/EPPO,35, 434–435.
-http://www.koppert.com/print/pests/butterfly-and-moth/tuta-absoluta