Long-term sexual allocation in herbivore resistant and susceptible pinyon pine (Pinus edulis)

Chronic herbivory by the stem-boring moth (Dioryctria albovittella) alters the sexual expression of a monoecious tree, pinyon pine (Pinus edulis) by reducing female function and increasing male function. Observations and long-term moth removal experiments show that 55% of susceptible trees can lose all female function. Moth herbivory has little effect on male function in young trees, but has an important effect on older trees, where moth-susceptible trees produced 1.5 times more pollen than moth-resistant trees. Susceptible trees were 6.5 times more likely to exhibit male-only function than either resistant trees or susceptible trees that have had their moths experimentally removed. This herbivore-induced sex change is caused primarily by differential moth attack and the resulting mortality of the shoots that bear female reproductive structures. Moth attack rates were positively correlated with individual stem biomass (female stems >non-reproductive stems >male stems). Moth attack also increased conelet abortion on unattacked shoots, indicating that moths indirectly reduce female function. Moth-induced altering of sexual function is also expressed at the population level. Male function is relatively greater in stands with high moth densities on stressful soils than in stands with few moths. Under certain conditions, sexual selection theory suggests that the negative effects on female function could be overcome with greater investment in male function. Because susceptible trees produce large amounts of pollen and are more abundant than moth-resistant trees, frequency-dependent selection may counteract selection against susceptible genotypes. Plant-herbivore interaction studies typically examine negative impacts of herbivory on female function, but not positive effects on male function. Here we demonstrate that herbivory may have important effects on the evolutionary ecology of pinyon by both promoting male function and depressing female function.