Shading effects on susceptibility of Rosa spp. to defoliation by Popillia japonica (Coleoptera: Scarabaeidae)

Published anecdotal reports suggest that Japanese beetles Popilliajaponica Newman, feed more heavily on host plants growing in full sun than on shaded plants. We studied the effects of shading on foliar characteristics of roses (Rosa variety floribunda 'Class Act') and determined preference and fecundity of beetles provided with sun- or shade-conditioned leaves. Defoliation by P. japonica also was compared between sun-grown and experimentally shaded roses in the field. Leaves from plants grown in full sun were smaller and thicker than ones from shaded plants, with slightly higher tannins. Leaf toughness and nitrogen levels were not affected by shading, but leaves from sun-grown plants had relatively higher sugar content. Beetles consistently consumed more foliage from sun-grown plants than from shade-grown plants in laboratory preference tests. However, beetles fed continuously for 2 wk on leaves from sun- or shade-grown plants had similar fecundity. In the field, beetles fed somewhat more heavily on foliage of roses grown in full sun than on plants grown under cloth canopies that provided 73% shading. Plants grown under semitransparent canopies of spun-bonded fabric sustained intermediate damage, suggesting that one way that shading may reduce defoliation is by rendering the shaded plants less apparent or accessible to host-seeking beetles. Implications of this work for understanding feeding behavior of P. japonica, and for pest management through planting site selection, are discussed.