Euonymus Scale (Homoptera: Diaspididae) Effects on Plant Growth and Leaf Abscission and Implications for Differential Site Selection by Male and Female Scales

Abstract

Effects of infestation by the euonymus scale, Unaspis euonymi (Comstock) on leaf abscission and growth of the woody plant Euonymus fortunei (Turcz) Rand.-Mazz. were measured in the field during 1985 and 1986. During the first summer, leaf abscission by infested plants significantly increased in August and September, coinciding with development of the second generation of scale insects. Infested plants suffered relatively greater winter injury than did uninfested plants, so that by spring 1986, they supported significantly lower stem and leaf weights. Reduction in root weight of infested plants was not evident until July of the second growing season. Proportionately more mortality of leaf tissue than of stem tissue occurred during the winter, so that the probability of plant tissue bearing over wintering females surviving until spring was nearly three times higher for stems than for leaves. We suggest that the characteristic distribution of U. euonymi with males mainly on leaves and longer-lived females on stems reflects the relatively greater risk to females of mortality from leaf abscission and from winter injury to leaves.