Host development offers new insight into insect-plant interactions

Abstract

maturity cause a concomitant shift in herbivore species. Both adult and larval beetles (Chrysomela confluens) are found primarily on young trees, while aphids (Pemphigus betae) are restricted to older hosts. A less appreciated but equally important corollary of Feeny’s study was the demonstration that a developmental change in the plant can affect food quality for the herbivore. In the winter moth case, tannins and leaf toughness increased as the season progressed while available proteins and water content decreased. These temporal changes in host quality have been well documented at several levels. Although seasonal or phenological changes are undoubtedly the most widely studied, long-term developmental changes may occur as trees mature6a7. More subtly, plants may actually respond to herbivores by fabricating defenses only when attackedarg. A recent report by Greene10 documented an even more fascinating twist: a geometrid caterpillar, Nemoria arizonaria, responds to differing tannin levels in its diet equivalent to those found in its early or late season feeding sites - by developing into different morphs. Low tannin levels induce a caterpillar that mimics the oak catkins fed upon in the spring; higher tannin levels typical of those found in late season leaves induce a morph that mimics twigs. Narrowleaf cottonwood is in many ways an ideal system to test ideas on the importance of plant development to herbivores. It forms extensive clonal patches of trees sharing an interconnecting root system. Because cottonwood tends to reproduce by sucker growth, each such patch may include 20-40 trees of identical genotype but varying ages. Furthermore, simple measures such as diameter at breast height (dbh) provide a reliable index of plant age or developmental stage. The herbivores chosen for the study encompass the range of leaffeeding strategies. Pemphigus betae is a leaf-galling aphid with a complex life cycle. Overwintering eggs are laid by immigrating sexuales on the bark of cottonwood branches. In the spring these hatch, giving rise to a generation of wingless stem mothers which form galls on developing leaves. The stem mother and her progeny develop within these galls, feeding on phloem sap. Successful galls produce a generation of spring alates which migrate to herbaceous summer hosts. Aphids on cottonwood, therefore, require both safe overwintering sites for eggs and suitable young leaf tissue for the spring generation.