Palatability of British trees to insects: constitutive and induced defences

Undamaged foliage of sixteen species of broadleaved trees was assessed for background (constitutive) palatability using larvae of Spodoptera littoralis (Lepidoptera) in laboratory bioassays. Palatability (dry weight consumed in 48 h) varied significantly between species and exhibited a four-fold range. Leaves of fifteen species were damaged artificially in the field and offered with control (undamaged) leaves to Spodoptera after 48 h and two weeks on the tree. Leaves adjacent to the damaged ones were similarly tested. Ten species exhibited significant (P<0.02) wound induced declines in palatability; damaged and adjacent foliage was involved. Although there was no significant relationship between the trees' constitutive palatability and the number of invertebrate herbivore species they support, this previously-demonstrated relationship closely approached significance when the species showing wound-induced effects were excluded from the regression. These results are discussed within the limitations of laboratory bioassays and the possible field consequences are briefly considered.