Submitted to: Biological Control
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/3/2005
Publication Date: 11/1/2005
Citation: Rayamajhi, M.B., Van, T.K., Pratt, P.D., Center, T.D. 2005. Interactive association between Puccinia psidii and Oxyops vitosa, two introduced natural enemies of Melaleuca quinquenervia in Florida. Biological Control. 37:56-67. Interpretive Summary: A leaf-rust fungus and a weevil are two established biological control organisms of invasive melaleuca trees in south Florida. Both organisms attack healthy melaleuca leaves to survive and produce offspring and cause a significant damage to the trees. Thus, interactions between these two organisms are unavoidable. We studied their interaction and discovered that the adult weevils prefer to feed and lay eggs on healthy leaves where the hatchlings have better survive rate. Rust infected leaves contained less nitrogen and therefore are less nutritive. This slightly negative interaction between rust fungus and weevil may force these organisms to look for additional leaves and cause more damage to the trees. It may be noted that branch-tip damage due to weevil feeding causes bud to be released and grow even in off-season and produce new leaves. These new leaf-tissues can support weevil and rust population year around.
Technical Abstract: The neotropical rust fungus Puccinia psidii G. Wint. (Basidiomycetes, Uredinales) and the Australian weevil Oxyops vitiosa Pascoe are two well established biological control agents of the invasive paperbark tree Melaleuca quinquenervia (Cav.) Blake in south Florida. Both agents preferably attack expanding healthy leaves at branch tips as food and place for breeding to sustain their populations. Thus, interactions between these two organisms are unavoidable. We conducted studies to assess these interactions and the overall impact on each others performance as biological control agents. In dual-choice tests, adults consumed more rust-free (P=0.0558) than rust-diseased tissues. In no-choice tests more feeding activities were detected on rust-free leaves but the difference was not significant (P=0.1375). In dual-choice tests, weevils laid larger proportions of eggs on rust-free than on rust-diseased leaves (P=0.0001), but the proportions of eggs hatched were similar in both types of leaves (P=0.5825). In no-choice tests in Petri-dish, larval mortality increased with increased rust-pustule coverage on leaves. A large proportion of larvae died during early developmental stages (1st and 2nd instar) when fed on leaf tissues with 50% or more rust-pustule coverage. Leaf tissue analyses showed decreased nitrogen content when rust-pustule coverage on leaf surface increased. Overall C:N ratio was significantly higher (P=0.0347) on leaves that had 50% or more rust-pustule coverage on diseased leaves. Adult weevils vectored a large number of uredospores that initiated a significant rust infection (P=0.0001) on rust-free plants. Periodic feeding on melaleuca tips by weevils induced greater amount of axillary growth (P=0.0776) which may facilitate sustained development of rust inoculum and larval populations of weevil by providing suitable substrate year around.