B₀ is the mean contact rate; ε_B, the magnitude of seasonal variation in contact. Computed curves, as follows: SN - saddle node bifurcations; PD - period-doublings; BC - boundary crises; IC - interior crises. Regions I, II and III refer to coexistance between ρ = 1/3 attractors and the main period doubling sequence (modulations of the annual cycle): I - ρ = 1/3 attractors coexist with the main sequence; III - ρ = 1/3 attractors part of the main sequence; II - some 1/3 attractors part of the main sequence; some, not.