Competition in cereal aphids (Homoptera : Aphididae) on wheat plants

The cereal aphids Rhopalosiphum padi L, and Sitobion avenae (F.) coexist on the same host plant but occupy different ecological niches. R. padi arrive s first in the season and reaches its population peak before S, avenae. In addition, R padi prefers the stem and lower leaves, whereas S. avenae is fo und mostly on the ears and upper leaves of the plant. In greenhouse experim ents, the likelihood of competition between R, padi and S, avenae on wheat seedlings and tillering plants was evaluated. The reproductive rate of S. a venae on tillering plants was negatively affected by previous infestation b y R. padi, and this effect was larger when R, padi infestation started at a n earlier plant growth stage. Likewise, previous infestation by R. padi dec reased S, avenae reproduction on seedlings. In contrast, previous infestati on by S, avenae on the nag leaf of flowering plants did not affect R. padi on that plant part. When both aphid species co-occurred from the beginning, the presence of either aphid species negatively affected the reproductive rate of the other. R. padi consistently had a higher rate of population inc rease than S. avenae. The presence of the other aphid species did not affec t within-plant aphid distribution or alate aphid production. Results were s imilar on wheat seedlings and tillering plants. Finally, alate S. avenae pr eferred uninfested seedlings over R. padi-infested ones, whereas no prefere nce was observed when the experiment was performed with tillering plants.