Mites ( mites ), which are part, like insects, of the phylum arthropods, belong to the class of Arachnids (spiders ). Adults have four pairs of legs, unlike insects which only have three. Several families of mites include plant pest species, the best known is that of Tetranychydae ; these species are commonly referred to as yellow, red or green “mites” or even “spiders”. Note that a few species attack tomatoes, including Tetranychus urticae (Koch) which is the most widely reported mite on this crop. Also called “tetranic weaver” ( spider mite ) because of the webs it forms on plants, it is cosmopolitan, common and very polyphagous since it has been reported on nearly 2,000 plant species. It can be responsible for significant damage, even lightning infestations, especially under shelters, this on many ornamental and vegetable crops. Resistance to acaricides has been repeatedly reported in this mite. Other species of Tetranychidae phytophagous , as well as predatory mites Phytoseiidae , can occasionally be found on tomatoes.
 

Tiny chlorotic spots more or less scattered on the lamina of tomato leaflets (Figures 1 and 2). The latter gradually turns yellow and takes on a dull color . In severe attacks, some leaves may turn yellow, wilt and dry out. Delicate silky webs are visible within the plant cover (figure 3).
 

The weaver spider mite goes through 5 stages of development (figure 4): egg, larva, protonymph, deutonymph and adult. The duration of a cycle varies depending on the temperature: about 7 days at 30 ° C and 36 days at 15 ° C.

- Forms of conservation and / or alternative hosts : it is the females that have entered diapause that ensure the overwintering of this mite. This diapause occurs as soon as the temperature or food drops.

- Stages of development : the eggs (figure 5) are laid mainly on the underside of the leaflets. They are round, tiny (0.14 mm in diameter), translucent at first, then become opaque, and eventually turn yellow as they approach hatching. The larvae (figure 6), which have 3 pairs of legs, are initially very pale, then take on a greenish tint. They have two red eye spots as well as two dark spots in the middle of their body. The larger protonymphs (Figures 6 and 7) have four pairs of legs and vary in color from light green to dark green; they also have two more contrasting spots on their body. The deutonymphs are larger than the protonymphs but of the same color. It is at the adult stage (figures 9 and 10) that we can distinguish males from females: the female, oval, measures 0.5 to 0.6 mm in length. Its color is variable (orange, light yellow, light green to dark green, even red). The male (Figure 6-6) is more active, smaller and narrower than the female. It also has a variable color (light yellow to orange, dark yellow to brown). On tomato, T. urticae is frequently reddish.

Larvae, nymphs and adults, often present on the underside of the leaf blade, feed by biting and then sucking on the contents of plant cells. These bites, which are sometimes very numerous, are the cause of the symptoms observed.

- Dispersion in the crop : the mites present on a plant can fall on the ground and reach other plants or do so through the culture threads. They are also transported by workers, equipment and tools, or even disseminated by plants.

- Favorable development conditions : pruned, smoked plants, and therefore more growing, are more favorable to the development of mites. Likewise, hot and dry periods are favorable to them. All the factors that are detrimental to the auxiliaries (too low temperatures, insecticide applications, etc.) contribute to the spread of T. urticae .
 

Several protection methods are recommended to control the development of mites on tomatoes in France:
- treat the plants before uprooting in the presence of high populations of pests;
- leach with water and treat the walls of shelters , posts, concrete walkways with an insecticide or a contact acaricide;
- disinfect the equipment used in the greenhouse (drip system, boxes );
- 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;
- weed the greenhouse and its surroundings;
- use auxiliaries such as the insects Feltiella acarisuga and Macrolophus caliginosus (note that the latter has secondary efficacy) and the mite Phytoseiulus persimilis ;
- consider chemical protection (r) ( e-phy site ) *, in particular if you use auxiliaries.

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

Last change : 07/08/21