Leaf Toughness in Quercus agrifolia and Its Effects on Tissue Selection by First Instars of Phryganidia californica (Lepidoptera: Dioptidae) and Bucculatrix albertiella (Lepidoptera: Lyonetiidae)

Abstract

Causes of toughness in mature leaves of coast live oak, Quercus agrifolia Nee, were identified and quantified, their effects on early instars of the California oakworm ( Phryganidia californica Packard) were determined, and the strategies used by the larvae for successful feeding on tough leaves were compared with those used by an oak leaf skeletonizer ( Bucculatrix albertiella Braun). Histological methods showed that tissue thickness, bundlesheath extensions, areoles, and lignin contributed to leaf toughness. At the abaxial surface, the extensions were significantly larger, areole areas significantly smaller, and lignins were concentrated in the lower epidermal tissues. The abaxial surface and adaxial areas near the primary vein were significantly harder than other leaf areas measured with a penetrometer, and this was correlated with structural differences. P. californica larvae confined to the tougher abaxial surface were unable to reach the palisade cells because of the insects' head capsule and mandibular morphology. These larvae weighed significantly less than adaxial feeders. On the adaxial surface, larvae fed near the marginal vein and preferred the palisade tissue. The gape and dimensions of the mandibles restricted the method and depth of feeding within an areole. In contrast, B. albertiella was not affected by most of the components of leaf toughness; the morphology of the head capsule permitted larvae to feed from the tougher surface.