Leafminer in tomato: damage and management

The tomato (Solanum lycopersicum L.) is one of the most important vegetables in Brazil and the world, generating billions of dollars annually. The main pest of the crop currently is Tuta absoluta or tomato moth. However, there are sucking insects such as the whitefly (Bemisia tabaci) and thrips that cause losses mainly by transmitting viral diseases. In tomato cultivation, the so-called leafminer flies may also occur Liriomyza huidobrensis (Diptera: Agromyzidae), L. sativae, L. trifolii, and L. brassicae. Among these L. sativae stands out as an economically important pest in tomato crops.

Liriomyza huidobrensis was considered an important pest in South America until the 1970s. However, with the outbreaks of tomato leafminer (Tuta absoluta) and use of insecticides to control the tomato moth, the population of L. huidobrensis low. However, exposure to these insecticides led to the evolution of resistance. In addition, the use of insecticides reduced the population of natural enemies of leafminer larvae, thus causing outbreaks of secondary pests. Worldwide, the species Liriomyza huidobrensis stands out for being an economically important pest in crops. The reduction in productivity can reach up to 15%. This is because the miners promote a reduction in the photosynthetic area of ​​the plants, therefore reducing the production of photoassimilates, which compromises productivity.

The damage caused by leafminers greatly affects tomato plants, with 18% of the leaf area affected by mines representing a reduction in photosynthesis of up to 60%. Symptoms of infestation range from the formation of mines to the appearance of rounded, white punctures caused by the feeding and deposition of eggs.

The leafminer fly is a polyphagous pest, and is reported as an important pest in several crops such as melons (Cucumis melo L.), pumpkin (Curcubita maxima L.), cucumber (Cucumis sativus L.) potato (Solanum tuberosum L.) and several weeds, which makes pest control difficult in the agroecosystem.

The leafminer fly is a secondary pest. However, it has increasingly assumed the role of a key pest due to population outbreaks, due to the selection of populations resistant to chemical insecticides, which makes pest management difficult.

Pest characteristics

Adults are about 2 mm long, with a grayish to black coloration with yellow spots. The eggs are small, 1,0 mm long and 0,2 mm wide, and grayish white or translucent yellowish in color. The eggs are laid inside the leaf tissue or epidermis of the fruits, which facilitates tissue infestation.

After three days, the eggs hatch, giving rise to larvae that feed on the tissue of the leaves and fruits. The larvae are small (2 to 3 mm) and yellowish in color. They are found feeding inside the leaf tissue, leaving galleries through the leaf, and are easy to recognize in the field. The larval stage lasts 5 to 7 days and they pupate on the surface of the leaves, in the soil or inside the leaves. The infestation occurs from the seedling stage, and can be observed in all stages of plant development.

Sampling, level of control and economic damage

The occurrence of leafminers can be detected by observing adults in the cultivation area, as well as by the presence of mines on the leaf surface. For monitoring, plastic trays with water and detergent can be used under the foliage of plants distributed randomly in the cultivation area. The larvae abandon the mines to pupate in the soil, thus falling from the leaves and being collected in the trays. The use of yellow sticky traps is another simplified form of monitoring, indicating the occurrence of the pest in the area.

Sampling should be carried out weekly, by inspecting the leaves of the upper third of the plant, observing the presence of holes in the epidermis, or mines. In plots of up to 10 ha, 73 sampling points are recommended, for an assertive control positioning. For tomato plants, the action level occurs when 25% of the leaves evaluated show signs of the pest. The level of economic damage for L. huidobrensis is 3,24 larvae per leaf sampled.

Control placement

To control leafminers in tomatoes, measures that exist in integrated pest management (IPM) must be adopted, which advocates the combined practice of several management tactics. The aim is to reduce the pest population to acceptable levels. Management techniques present in IPM include: biological control, cultural control, environmental manipulation, pheromones and chemical control.

The main management tactic used for leafminers is chemical control, which is mainly aimed at the larval stage of the pest. However, this control tactic is difficult due to the biology of the pest, which has a short cycle, high fecundity, small size and a pupal stage in the soil, as well as the habit of the immature phase that feeds on the inside of the leaf. The pest's feeding habit provides it with protection inside the leaves, requiring the use of systemic and highly toxic insecticides. Systemic insecticides with translaminar properties are more effective in controlling leafminers. The use of these products can compromise several aspects, such as the sustainability of the agroecosystem, selection of resistant populations, causing an increase in production costs and compromising the crop's production chain.

Currently we have about 59 commercial insecticides among the insecticides used for the control of Liriomyza sp. in tomato plants (click here to see which ones), the main active ingredients used being: acephate, chlorpyrifos, deltamethrin, spinetoram, abamectin, milbemycin, cartap hydrochloride, cyromazine and cyantraniliprole. Cyromazine has a residual effect of 20 days, followed by abamectin and spinosad. Of these, abamectin is the most specific, with less effect on natural enemies.

However, the high reproductive potential, short life cycle and frequent applications of insecticides have facilitated the selection of resistant populations, influencing pest management.

There are numerous natural enemies capable of regulating leafminer fly species in the field, including parasitoids. Among the predators, species of lacewings, earwigs, wasps, ants and beetles feed on leafminer fly larvae and/or pupae.

Considering the diversity of natural enemies, as well as their importance as pest control agents, L. huidobrensis It is necessary to adopt management strategies that preserve this beneficial entomofauna in the field, mainly by adopting insecticides that are more selective for these natural enemies.

The cost of control will depend on the level of pest infestation, the number of applications made with the same product during the tomato cycle, and the value of the product.

The management of leafminer flies in tomato crops should be directed towards diversifying control tactics, as recommended by IPM, seeking to reduce selection pressure and increase the efficiency of available tools. Advocating the adoption of IPM tactics is of utmost importance to reduce the losses caused by the pest, in addition to reducing outbreaks, which directly impacts costs.

By Franciely da Silva Ponce e Claudia Ap. de Lima Toledo (FCA/UNESP)

Article published in issue 138 of Revista Cultivar Hortaliças e Frutas