Larval food inhibitory activity against Paenibacillus larvae growth in colonies with and without a propolis envelope, and challenged or not with Paenibacillus larvae. Antimicrobial activity of larval food (mean ± s.e.m.) for samples collected in: (a) Asymptomatic period in August, and (b) Symptomatic period in September were measured as percent optical density (OD600) relative to untreated controls (N = 20 replicate wells per colony and five colonies per treatment). Low P. larvae growth (y-axis) indicates higher inhibition activity of larval food. Significant difference among groups was determined by ANOVA with colony treated as a random variable (α = 0.05).