Mast seeding and Lyme disease.

We wish to comment on two issues relating to a prediction raised in the article by Randolph1 about our work2. Randolph argues that because ‘low acorn production and moderate mouse densities are the norm [our] data do not explain the cause of the severe acorn failures and contingent crash in mouse populations that might initiate [gypsy] moth outbreaks’. We made no attempt to explain masting periodicity, but autumnal mast production does predict white-footed mouse (Peromyscus leucopus) density the following summer3–5. Moderate mouse densities are indeed the norm. In most summers, densities are at or above c. 10 mice ha21, which is sufficient to keep gypsy moth (Lymantria dispar) populations low via pupal predation4–6. However, mouse populations do periodically collapse to below 10 ha21 for one to two summers3,4. Evidence indicates that this is when moth populations, with their high fecundity, can rise to levels mice cannot suppress2,4,5. Mouse population collapses tend to occur if major masting events, which result in very high mouse densities, are followed by mast failure and food limitation. These events occur with a frequency similar to moth outbreaks3,4. Randolph challenges our conclusion that whitefooted mice play the major role in infecting Ixodes scapularis ticks with Borrelia burgdorferi. However, in the eastern United States, P. leucopus is both the most abundant host for ticks and by far the most competent reservoir for B. burgdorferi7. Furthermore, Mather and Ginsberg8 demonstrated that the prevalence of spirochete infection in nymphal I. scapularis ticks strongly correlates with white-footed mouse density in the previous year. In our forests, acorns markedly affect mouse density, the density of host-seeking larval ticks and the number of ticks attached to mice2. Therefore, we conclude that acorn production should allow epidemiologists to predict Lyme disease risk well in advance, here in the USA9. The European situation, in which several tick hosts are competent reservoirs, will probably make predicting Lyme disease risk more difficult. Nevertheless, we hope that epidemiologists will test the power of acorn production as a predictor of Lyme disease risk in European oak forests.