The thrips ( Thrips ), belonging to the order Thysanoptera and family Thripidae are distributed worldwide, present the tropics to the polar regions depending on species. Several of them attack vegetable crops, in particular the tobacco thrips ( Thrips tabaci Lindeman) and the Californian thrips ( Frankliniella occidentalis  Pergande. The first has been rife for many years in Europe and therefore in France. supplanted by the second which has caused significant damage to many productions, especially in greenhouses, since the early 1980s (tomato, cucumber, pepper, eggplant, chrysanthemum, rosebush ). These two thrips can attack the melon. Note that they are vectors of several formidable viruses, in particular the  Tomato spotted wilt virus which affects tomatoes and various other plants. Other species of thrips attack melons, Thrips palmi Karny, Heliothrips haemorrhoidalis (Bouché) , Frankliniella spp . , Etc.
 

Silvery lesions or even patches of irregular size and shape appear on the limbus (Figure 1); they gradually necrotize and take on a beige tint. These lesions are also strewn with tiny black dots materializing the droppings of thrips . Affected leaflets tend to chlorinate and take on a dull color.

Whitish lesions may appear on the flowers (Figure 2).

Affected young fruits subsequently show more or less extensive silvery and suberized lesions (Figure 3).

 

The development cycle of thrips comprises 6 stages (figure 4): egg, two larval stages, two pronymph stages and adult (figure 6). The length of the cycle varies depending on the temperature and the host plant; for example, for F . occidentalis it fluctuates from 34 days at 15 ° C to 13 days at 30 ° C.

- Forms of conservation and / or alternative hosts : it is the larvae and adults, taking refuge in places with mild temperatures, which ensure the sustainability of these insects. For that, they settle in the plant debris, the frames of the shelters ; they sometimes bury themselves up to 8 centimeters deep. They can also hibernate on field crops, especially on various Alliums .

- Stages of development : the eggs kidney-shaped (figure 6-1) are deposited on the aerial organs, especially the leaves on tomato. Once formed, the larvae (figures 6-2 and 3) (figure 5), which are very mobile, will feed on the underside of the leaves. At the end of the second larval instar (Figure 6-3), they drop to the ground and pupate. The first pronymph stage (Figure 6-4) is characterized by the appearance of wing outlines. The clearer second stage pronymphatic (Figure 6-5) also shows larger wing outlines and long antennae curving towards the rear of the body. The adult (Figure 6-6) (Figure 6) have 2 well-developed wings pairs.

T. tabaci and F. occidentalis (Figure 6) are morphologically quite comparable. Only an observation of adults using a microscope can differentiate them. Their reproduction may be different depending on the species. It is asexual in T. tabaci, unfertilized females giving birth only to females (parthenogenesis). In F. occidentalis , it can be sexual or asexual, unfertilized females giving birth only to males while those which are fertilized give birth to females.

Thrips feed by sucking the contents of cells in the epidermis. Damaged tissue quickly necroses.

- Dispersion in crops : thrips are fairly easily dispersed in crops, passively carried away by drafts and / or actively flying. Workers can contribute to it during cropping operations. These insects can be dispersed by plants or plants of other parasitized species.

- Favorable development conditions : the nature of the plant, the temperature, the hygrometry in the crop, notably influence the development of thrips.
 

Several protection methods are recommended to control the development of thrips on melon in France:
- treat the plants before uprooting in the presence of high populations of pests (e-phy thrips , Frankliniella occidentalis ) *;

- remove and destroy plant debris and crop residues;
- disinfect the substrate reused or the soil;
- preheat the greenhouse before planting 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 ;
- install canvases insect-proof at shelter openings;
- weed the greenhouse and its surroundings;
- detect the first pests thanks to the blue sticky panels placed above the crop;
- use the auxiliary  Macrolophus caliginosus , an insect with secondary efficacy;
- consider chemical protection (r) (e-phy thrips , Frankliniella occidentalis ) *, in particular if you use auxiliaries.

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