Aphid damage on Poaceae

Three species of aphids are frequent on wheat, barley and maize: the oat-bird cherry aphid (Rhopalosiphum padi), the English grain aphid (Sitobion avenae) and to a lesser extent the rose-grain aphid (Metopolophium dirhodum). The former is particularly abundant on maize at the end of summer and beginning of autumn, and on early-sown winter cereals (beginning of October). The other two are more active mainly in spring on barley and especially wheat where Sitobion avenae can decrease the yield by 10% and more when it proliferates from the beginning of ear formation to grain filling. At the end of autumn, colonies of the corn-leaf aphid (Rhopalosiphum maidis)  are also frequent on barley regrowth, then on young barley seedlings, although they do not appear to cause significant damage on the latter.

In maize, when growth has finished, leaves and panicles can be seen covered with colonies of Rhopalosiphum padi which excrete honeydew on which sooty moulds develop. The damage results from reduced photosynthesis which can lead to failure of grain formation.

These aphids can also develop on Poaceae for fodder and forage and show a degree of host specifity: hence Rhopalosiphum padi is found mainly  on ryegrass, Sitobion avenae on Dactylis  (cocksfoot grasses) and Metopolophium dirhodum on Festuca (fescue). A fourth species, Metopolophium festucae, is quite frequent on leaves of a greater range of Poaceae. On the collar and roots underground aphid species occur such as Anoecia spp. and Tetraneura spp.

Rhopalosiphum padi, Sitobion avenae and Metopolophium dirhodum are the principal vectors of barley yellow dwarf virus (BYDV and/or CYDV). These viruses are transmitted  by the persistent mode: at the end of summer, the aphids (especially Rhopalosiphum padi, which are the most abundant), have acquired the viruses on regrowth of straw cereals, maize and Poaceae for fodder, and transmit them to barley and wheat seedlings that have managed to sprout before the end of the aphids’ flight-time (beginning of November in the northern half of France). On winter cereals symptoms start to show in February. They are particularly severe in barley (death of plantlets, yellowing of leaves, abnormal tillering) and amplify in spring (poor stem growth, weak ear formation, small seeds). Spring barley is extremely sensitive to any contamination in spring, passed on mainly by Metopolophium dirhodum and Sitobion avenae.

In wheat the symptoms are milder (adult plants slightly smaller and reddening of the last leaf) than in barley. However, yield reduction, which can reach several tens of quintals per hectare, is in the same order of magnitude in the two growing plots where the sowing date was exactly the same. In maize BYDV is transmitted in spring by aphids coming from infected cereals and grasses grown for fodder. The infection is often asymptomatic, yet reddening sometimes appears at the edges of the leaf blades. Any yield losses appear to be rare and small. Rhopalosiphum padi, Sitobion avenae and also numerous species not dependent on Poaceae, transmit maize dwarf mosaic virus (MDMV) by the non-persistent mode. That disease is quite frequent in the Mediterranean area. The fodder species therefore act as reservoirs for viral diseases transmitted by many species of aphids. 

The barley yellow dwarf viruses are capable of infecting all Poaceae grown for fodder and forage. Symptoms are seen only rarely, except in ryegrass (reddening or yellowing of older leaves). The cocksfoot streak virus (CSV) is also passed on by the aphids of Poaceae through le non-persistent mode. 

Insecticide dressings only involve barley and wheat, on autumn seedbeds against vectors of BYDV, and only wheat in spring against Sitobion avenae. In the first situation, seed coating with a systemic neonicotinoid insecticide can help avoid the spread of the viruses in the field. Foliar treatments can also be applied using a pyrethrinoid at the plantlet stage. There is a decision support tool that helps in deciding what treatment to use. It enables the operator to conduct the appropriate treatments based on a simulation of aphid populations from the point of ear formation. In the case of Sitobion avenae in spring, only foliar treatments are possible. They are not necessary if less than half the ears are infested at ear-formation stage.