Feeding responses of adapted and non-adapted insects to the defensive properties of Baccharis halimifolia L. (Compositae).

Trirhabda bacharidis (Weber) (Chrysomelidae), a univoltine, monophagous beetle is the dominant herbivore on Baccharis halimifolia (Compositae), a woody, perennial shrub that leafs out in early spring and retains its leaves into November. Available plant biomass increases during the season but T. bacharidis feeds only during spring and early summer. During the remainder of the growing season, there are no major herbivores feeding on B. halimifolia. B. halimifolia leaves increase in toughness and thickness and decrease in moisture and nutrients as the season progresses. In feeding preference tests, T. bacharidis larvae preferred young leaves over leaves of older age classes. When reared on leaves of different ages, larvae fed on young leaves, weighed more, pupated earlier and had greater survivorship. T. bacharidis larvae showed no significant feeding preference for similarly tough B. halimifolia leaves painted with differing concentrations of late season acetone leaf extract. Elm leaves painted with the same leaf extracts were avoided by larvae of the non-adapted specialist Pyrrhalta luteola (Mueller) (Chrysomelidae).These results suggest that the adapted specialist, T. bacharidis, is not deterred by the B. halimifolia acetone soluble secondary chemical which increases in amount over the season. Decreasing leaf nitrogen (perhaps in concert with increasing leaf toughness) seems to be the primary factor that dissuades its feeding. However, acetone soluble secondary chemicals in the leaves of B. halimifolia may be effective in preventing herbivory from non-adapted insects.