Two whitefly ( whiteflies ) rather polyphagous are damaging in France on melon: Trialeurodes vaporariorum (Westwood) and Bemisia tabaci (Gennadius). This last species is represented (in certain populations) by a particularly formidable biotype-B, also known under the name of B. argentifolii (Bellows & Perring), which develops on more than 900 hosts. These whiteflies belong to the order Hemiptera and the family Aleyrodidae. T. vaporariorum , also called “ white greenhouse fly ”, originates from Central America and has been rampant for many years in Europe, particularly under shelters. B. tabaci , the tobacco whitefly , described for the first time in Greece, has been emerging for several decades in several regions of the world (since 1988 in France). It has become problematic everywhere for at least three reasons:
- its high population levels are responsible for significant damage;
- its vectorial potential allows it to transmit numerous and formidable viruses, more than 111;
- the resistance of some of its populations to one or more insecticides calls into question the effectiveness of chemical control.

 

As with aphids, the numerous bites and food sucks caused by whiteflies present on the foliage (figure 1) cause a slowdown in plant development.

Honeydew is also produced in large quantities; it is subsequently colonized by Fumagine covering the surface of the aerial organs of the melon and soiling them, in particular the fruits (figure 2) making them unfit for marketing.

 

These 2 whiteflies have 3 developmental stages that take place on the underside of melon leaflets: egg, 4 larval stages, and adult. The duration of the complete cycle (figure 3) varies according to the temperature. It fluctuates for T. vaporariorum from less than 20 days at 27 ° C to more than 40 days at 14 ° C (more than 50 days for B. tabaci ).

- Forms of conservation and / or alternative hosts : these insects do not have a stage suitable for the winter phase. They only survive if their hosts do not die. Note that the eggs can be subjected to temperatures below 0 ° C for several days. These whiteflies are easily maintained on many cultivated hosts, but also on various weeds, which should therefore be removed carefully.

- Developmental stages : the eggs (figures 2-1 and 2) (figures 4 and 5) are mainly deposited on the underside of the leaflets at the apex. White in color, they are oval and have a diameter of 0.25 mm. In the days following the laying, they turn dark. Between 7 and 10 days later, the hatch larvae (figures 2-3) (figures 6 and 7); these, oval and flat, measure 0.3 mm and have well-developed antennae and legs. Losing these later, they are motionless and feed with their rostrum. The second instar larvae are flattened, transparent and measure 0.37 mm. The third and fourth instar larvae are quite comparable (Figures 2-4 and 5), but with respective lengths of 0.51 and 0.73 mm. In the last larval stage, the insect secretes wax. It is when its red eyes appear that it is referred to as a puparium (Figure 2-6) (Figure 8). Subsequently, the whitefly develops and takes on a white tint. The adults (figure 4-7) (figures 9 and 10) have 2 pairs of wings, their size different according to the sex: 1.1 mm for females and 0.9 mm for males. The body and wings are coated with a characteristic white waxy powder. Slight morphological criteria (which can be consulted in the table below) make it possible to differentiate the two insects. The larvae and adults, often present on the underside of the blade, feed through their rostrum which acts as a suction pump. The excess sugar in the sap is released in the form of honeydew, especially by the large larvae.
 

 

 

 

 

 

 

 

 

 

 

 

 

- Dispersion in the crop : concentrated in a few plants at the start of an infestation, the adults easily fly in the greenhouse and disperse as the size of the plants and temperatures increase. The wide distribution of plants of horticultural species in particular has largely contributed to the dissemination of B. tabaci throughout the world.

- Favorable development conditions : these insects appreciate mild temperatures and summer conditions in shelters. Their lifespan is between 10 and 20 days on tomato; of course, it fluctuates according to the temperatures. B. tabaci , which does not survive temperatures below 0 ° C, has higher thermal requirements than T. vaporariorum .

 

 

Several protection methods are recommended to control the development of whiteflies on melons in France:
- treat the plants before uprooting in the presence of high populations of pests;
- 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 yellow sticky panels placed above the crop as soon as the plants are introduced;
- use auxiliaries such as the insects Encarsia formosa, Eretmocerus eremicus (these two insects are especially effective on Trialeurodes vaporariorum, Eretmocerus mundus (mainly effective on Bemisia tabaci ) and Macrolophus caliginosus and the fungi Paecilomyces fumosoroseus and Verticillium lecanii (note that its effectiveness may vary from strain to strain);
- consider chemical protection (r) (e-phy whiteflies ,  Trialeurodes vaporariorum and Bemisia tabaci ) *, in particular if you use auxiliaries.

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