Variety-based control

Genetic control aims to offer farmers aphid-resistant or aphid-tolerant varieties, thus eliminating the need for insecticides to control their proliferation in crops. Although the process of creating the resistant varieties is relatively long, once launched, it proves fruitful. The history of genetic control against the cotton-melon aphid Aphis gossypii, which attacks cucurbits in particular, illustrates this process and its dynamics.

Melon varieties tested for aphid resistance (photo credit: Pascale Mistral INRAE)

The first research into resistance in melon dates back to the mid-20th century, and gained momentum in the 1960s-1970s, with work carried out by American and French teams. Strong resistance was observed in melon genotypes from India and Korea. Genetic analysis revealed that the resistance has a simple genetic determinism: one gene largely explained the level of resistance. Two independent breeding programs were then launched to transfer this resistance to Charentais-type varieties in France and Western Shipper-type varieties in the United States. Margot became the first melon variety to be declared resistant in 1987, and was included in the official French catalog of varieties.

Charentais-type varieties in France and Western Shipper-type varieties of melon (Photo credits: International standards for fruit and vegetables, Melons, OECD 2014)

This registration guarantees farmers that resistance has been rigorously evaluated in the variety, while encouraging private companies to introduce resistance in the hybrids they market. By 2016, 110 new Charentais-type varieties had been declared resistant to aphids when they were included in the official catalog of varieties. The commercial success of some of these varieties has resulted in almost 80% of melon crops in south-east France since 2000 are carrying this resistance, leading to a sharp reduction in the use of aphicides. The use of resistant varieties has spread to other French production zones, with the appearance of hybrids adapted to these regions and registered in the catalog. Today, 100% of Charentais melon varieties proposed for inclusion in the catalog are aphid-resistant. The success of these varieties has encouraged private companies to introduce aphid resistance in other types of variety, notably Galia, Canari, Piel de Sapo and Honey Dew varieties grown in Spain and Italy, for example. As a result, the deployment of resistant varieties has become much more widespread.

Piel de Sapo, Honey Dew, Piel de Sapo, Canari melons (Photo credits: International standards for fruit and vegetables, Melons, OECD 2014)

However, the widespread use of resistant varieties runs the risk of selecting aphid populations capable of developing on them. Indeed, populations of Aphis gossypii capable of developing on resistant melon varieties have been observed. Resistance is controlled by the Vat gene (see Plant resistance to aphids to reduce virus epidemics), which confers a variable level of resistance depending on the Aphis gossypii clone. The composition of clones colonizing melon crops has evolved since the 2000s. Early communication from research to breeders raised awareness and led them to broaden the spectrum of resistance action in certain varieties offered in the catalog: variety development has been very reactive. The proposed resistances are controlled by a complex genetic system; they reduce the populations of each clone less drastically, but are effective in crops. What now remains to be done is to develop the criteria for registering resistance to take full account of its spectrum of efficacy, and to take into account partial-effect resistances.