Growth Reduction in Nonbearing Apple Trees by Woolly Apple Aphids (Homoptera: Aphididae) on Roots

Abstract

The effect of root-feeding populations of woolly apple aphids, Eriosoma lanigerum (Hausmann), on newly planted, nonbearing apple trees in an orchard environment was studied. Roots of two-thirds of the 351 ‘Red Delicious’ study trees were artificially infested with woolly apple aphids from a laboratory colony in 1986, 1 mo after planting. The artificial infestation resulted in 95% of the trees being infested (including controls), but did not produce more severe root infestations per tree than expected in natural infestations. The root infestation rating (mean = 0.35 on a scale of 0-1, SEM = 0.18) determined from destructive sampling of one-third of the orchard after three growing seasons was not correlated with population density above ground throughout the 3 yr of the study. Root feeding marginally reduced branch growth in the first and third year after infestation, crown length in the third year, and trunk diameter in the first and second years. Crown length was significantly reduced after 1 yr and trunk diameter was significantly reduced after 3 yr because of woolly apple aphid feeding on roots. Scion biomass also was significantly reduced by woolly apple aphid root feeding after 3 yr. We conclude that woolly apple aphid populations on roots have a slight, but significant, negative effect on growth of young non bearing apple trees in the orchard environment. We also conclude that, because of the lack of correlation between woolly apple aphid populations aboveground and on roots, sampling branch terminals and pruning scars yields no information on the density of woolly apple aphids on roots.