The papaya rust mite Calacarus brionesae (Acari: Eriophyidae) and the powdery mildew Oidium caricae (Erysiphales) are two major foliar pests of papaya grown in Hawaii. Both organisms are host specific and show irruptive population growth: they can quickly cover the entire surface of a leaf, and their distributions often overlap. Informal observations made by researchers suggested some sort of positive association between the mite and the fungus. A positive association could be generated if the mites disseminate spores of the pathogen, or if the mites achieve protection from predators and bad weather by residing in a powdery mildew colony. In contrast, there are also reasons to expect a negative association between mites and powdery mildew: these two organisms may compete for space or resources. We conducted a 11-month field study to address the following questions: 1) do the rust mite and powdery mildew interact directly on the papaya foliage?; 2) does one organism benefit or suffer from the presence of the other?; 3) what are the individual impacts of rust mites and powdery mildew on papaya growth and reproduction?; and 4) are their combined impacts on papaya performance additive or synergistic? The manipulative experiment comprised four treatments: 1) untreated control; 2) selective suppression of rust mites; 3) selective suppression of powdery mildew; and 4) suppression of both pests. We found no evidence for any direct interaction between the two organisms. Acting individually, the mites and the fungal disease each produced substantial decreases in leaf longevity (F=17.06, P<0.0001), the total number of leaves present per tree canopy (F=73.16, P<0.0001), and fruit yield (F=11.03, P<0.0001). The study also revealed that the combined impacts of the pests on fruit yield were additive. The significance of these results for pest management and theoretical models of plant compensation are discussed.
Species 1: Acari Eriophyidae Calacarus brionesae (papaya rust mite, papaya leaf edgeroller mite)
Species 2: Fungi Erysiphales Oidium caricae (papaya powdery mildew)
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA