INTERACTIONS BETWEEN THE BIRD CHERRY OAT APHID AND THE ENGLISH GRAIN APHID

Interactions between the bird cherry oat aphid, Rhopalosiphum padi (L.), and the English grain aphid, Sitobion avenae (F.) on Caldwell wheat were investigated under laboratory conditions. The interactive effects on feeding behavior were studied using an electronic feeding monitoring system. Intraspecific competition caused R. padi to spend more time on non-probing activities. The presence of S. avenae did not affect R. padi. However, the presence of R. padi on the plant resulted in fewer probes and shorter non-probing time for S. avenae during the observations. This apparent benefit may be due to any number of changes that occurs in plant physiology by the feeding of R. padi. Nymphal periods, longevities, and fecundities of R. padi and S. avenae on both the two-leaf and the heading stages of caged plants were observed. Nymphal periods were variable and likely to depend on the aphids\u27 condition before the experiment. R. padi responded to intraspecific competition by having a significantly shorter life and lower fecundity. When infested with 4 R. padi + 4 S. avenae per plant, S. avenae produced either no effect or a positive effect on longevity and fecundity of R. padi. When aphid densities were increased to 16 R. padi + 16 S. avenae per plant, both species suffered reduced longevity and fecundity. R. padi preferred the lower parts of the heading plant. S. avenae, however, preferred the plant heads. The presence of another species did not show significant effects on the distribution of aphids on the plants. When crowded by its own species, S. avenae spread downward on the plant to avoid competition on the heads. Some mathematical models were evaluated using the data on aphid populations. The Verhulst-Pearl population growth and the Lotka-Volterra competition models were not suitable to the observed populations. Parameter estimates varied with the infestation levels and the residual sums of squares were large. Modified models with time lag functions produced much improved results. Parameter estimates were more consistent and sums of squares were greatly reduced. The modified models had only two additional parameters. However, they may be further improved by including plant growth, immigration and emigration factors and spatial dynamics