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The aphids feed by phloem feeding, taking the sap elaborated by the plants, diverting for their own profit part of the nutrient elements essential for plant growth This strategy induces damage, whose severity depends on the host’s stage of development and its level of sensitivity.

Traditionally, two categories of damage are recognized:

• direct damage linked to removal of sap and toxicity of the saliva
• indirect damage linked to the aphids’ activity on the surface of the plant and their role in virus transmission

Whether the economic losses due to aphids are caused by direct or indirect damage, it is difficult or even impossible to give an accurate assessment. They fluctuate enormously depending on weather conditions, the aphids’ population density and the variety of the crop plants grown. Some surveys give average percentages of yield loss due to the direct effect of aphids at about 10-20% in peas, 10% for wheat or 5% for potatoes. Yield reduction is very much greater when viruses produce the damage: losses of up to 85% for barley infected by yellow dwarf viruses for example.

The control strategy should be guided by the type of damage. Overall the direct damage is proportional to the population size of aphids hosted by the plant, whereas viral damage depends mainly on the number of plants infected and much less on the number of aphids per plant. Hence the core principles governing the control method will be different in each case.

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