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Leaf miners

Several flies miners polyphagous ( leafminers ) (Figure 1) are likely to attack vegetable crops, and sometimes salads that supports pretty much the presence of these pests. These insects are classified in the order of the Diptera and the Agromyzidae family. The main species of leafminers found in Europe are the tomato leafminer ( Liriomyza bryoniae Kaltenbach, often wrongly called " Liriomyza strigata " [Meigen]), the American leafminer ( Liriomyza trifolii Burgess), the South American leafminer ( Liriomyza huidobrensis Blanchard and Liriomyza strigata Meigen), and the horticultural leafminer ( Chomatomyia horticola Goureau). Several of them are classified as quarantine pests.
  • Nature of damage
Numerous chlorotic nutritional punctures are first observed on the limbus; they can be very numerous and are carried out by the females with their ovipositor (figure 2). Mines (figure 3) subsequently appear on the leaves. The most affected leaves, bearing many larvae per leaf, may turn yellow and dry out. The photosynthetic activity of plants, their growth and yields can thus be greatly reduced during an infestation. Controlling the populations of these pests is often problematic because of their possible resistance to several insecticides, the latter also eliminating useful fauna (Hymenoptera parasitoids).
  • Biology
Leafminers have 6 stages of development during their cycle (figure 4): egg, 3 larval stages, pupa and adult.

- Forms of conservation and / or alternative hosts : few adults are observed during the winter due to the entry into diapause of the pupae. Since these leafminers are polyphagous, they can multiply and be preserved on many cultivated alternative hosts (tomato, cucumber, lettuce, melon, pepper, celery, beans, potato, chrysanthemum, gerbera), as well as on weeds present. in or outside the culture.

- Stages of development : the eggs (figure 4-1), cream-colored and oval in shape (those of Liriomyza bryoniae measure 0.12 x 0.27 mm), are deposited in the tissues during the bite punctures with the using the female ovipositor. A female can produce several hundred eggs which subsequently hatch and give birth to larvae transparent and 0.5 mm long (Figure 4-2) (Figure 5). These dig galleries in the leaflets that the presence of black excrement makes clearly visible. The 2.5 mm long, third instar white larvae pierce the blade, leave the leaflets, drop into plastic folds or onto the ground, and bury shallowly. Subsequently, they transform into pupae (Figure 4-3) barrel-shaped and whose color varies as they age from yellow to dark brown (Figure 6), blackish pupae are often the ones that are parasitized. The pupation of Chromatomyia horticola takes place in the leaf: the puparium, covered by the epidermis of the leaf, is then visible in the form of a small protuberance at the end of the larval gallery. The adults (Figure 4-4) are small flies 2-3 mm in length, yellow and black ( Liriomyza spp., Figure 7) or blackish gray (C hromatomyia horticola ). The adult females, present on the upper surface of the limbus, perforate the epidermis thanks to the real auger that is their ovipositor, suck the vegetable juice (nutritional bite) and deposit their eggs (egg-laying bites). Note that the males, devoid of augers, also take advantage of the feeding bites to feed themselves.

The duration of their cycle varies according to the temperature; for Liriomyza bryoniae for example, it is 41 days at 15 ° C and passes to 17 days at 25 ° C, the values ​​for the other species remain the same, with the exception, however, of L. trifolii more or less which would be more sensitive to low temperatures. The lifespans of females at these temperatures are 14 and 7 days, respectively.

- Dispersion in the crop : the adults easily fly in the greenhouse, or even from greenhouse to greenhouse, and thus disperse in the crop (s). Newly contaminated plants (carriers of eggs or very young mines) can also contribute to the spread of these insects.

- Favorable development conditions : the evolution of the population levels of these insects is rather influenced by high light intensities, certain rather vigorous host plants, high humidity (80-90%) in particular.

Leafminers are generally heavily parasitized, mainly by the Hymenoptera chacidiens. Insecticide treatments, sometimes unjustified, are very damaging to this useful entomofauna, and are often responsible for the outbreaks observed.
  • Protection methods
Several protection methods are recommended to control the development of leafminers on crops in France:
- remove and destroy plant debris and crop residues;
- preheat the greenhouse before placing the plants and carry out an insecticide or acaricide treatment ( link e-phy );
- check the sanitary quality of the plants before and during their introduction into the shelter;
- produce the plants in an shelter insect-proof  ;
- weed the greenhouse and its surroundings;
- detect the first pests thanks to the yellow sticky panels placed above the crop as soon as the plants are introduced;
- consider chemical protection * (r) ( e-phy-Laitue ; e-phy-Scarole-frisée ), in particular if you use auxiliaries.

(r): resistance to insecticides or acaricides is known in these pests.

* Chemical control : As the number of pesticides available for a given use is constantly changing, we advise you to always confirm your choice by consulting the e-phy site of the Ministry of Agriculture and Fisheries which is an online catalog plant protection products and their uses, fertilizers and growing media approved in France. This also applies to all biological products based on microorganisms or natural substances.
Last change : 04/27/21
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