Use of cluster photographs to estimate overwintering monarch butterfly, Danaus plexippus Linnaeus, 1758 (Lepidoptera: Danaidae), populations at selected California overwinter sites

Abstract. Conducting a population estimate of overwintering monarch butterflies, Danaus plexippus Linnaeus, 1758, at California winter sites traditionally involves many early morning hours of using binoculars to count the insects as they cluster in tight aggregations among the high foliage of trees. This study investigates the effectiveness of counting the butterflies by using photographs of specifically selected clusters. Counts based on photographs of ten clusters from each of three overwintering sites were compared to counts of the same clusters using binoculars. The data were analyzed using mixed model analysis of variance (ANOVA). The results show that the photograph-based counts were equivalent to the binocular counts within ±10%.