MODELING GYPSY-MOTH VIRUS LEAF CHEMISTRY INTERACTIONS - IMPLICATIONS OF PLANT-QUALITY FOR PEST AND PATHOGEN DYNAMICS

1. The decline phase in the population dynamics of the gypsy moth, Lymantria dispar L. (Lepidoptera: Lymantriidae) is associated both with nuclear polyhedrosis virus epizootics and greater phenolics levels in preferred hosts such as red oak. Recent studies have shown that hydrolysable tannins in red oak reduce both gypsy moth (GM) fecundity and virus susceptibility. 2. We modified the Anderson & May (1981) analytical model of invertebrate-pathogen dynamics to explore this complex interaction over a range of tannin levels, gypsy moth mortality from non-virus causes, and virus life span held constant for 100 gypsy moth generations. 3. The most realistic gypsy moth dynamics required moderate to high levels of tannin (30-45%), low non-virus mortality (3-0 per GM per year), and long-lived virus (1 year). For simulations with low non-virus mortality and long-lived virus, the ratio of highest to lowest population density increased from 10-fold at 10% tannin to over 1000-fold at 40% tannin, then stabilized with little or no fluctuation at 50% tannin. 4. The shift to stability at 50% tannin results from the lower sensitivity of fecundity than virus to hydrolysable tannins and a consequent decrease in the difference between virus pathogenicity and the gypsy moth's intrinsic rate of increase at high tannin levels. 5. The dynamics generated agree well with predictions of the original Anderson & May (1981) model that stable limit cycles are promoted by: (a) longer-lived virus and (b) high pathogenicity relative to the intrinsic rate of increase. Overall, our simulations suggest that the net effect of moderate constant tannin levels over multiple generations will be destabilizing compared with low and high tannin levels.