Submitted to: SciTopic
Publication Type: Other
Publication Acceptance Date: June 21, 2010
Publication Date: June 21, 2010
Citation: Rayamajhi, M.B., Pratt, P.D., Center, T.D. 2010. Herbivorous insects and plant pathogen work together to suppress an invasive tree performance. SciTopic. Interpretive Summary: Exotic, invasive plants continue to plague managed as well as natural ecosystems. Land managers rely heavily on cultural, mechanical, or herbicidal tools (singly or in combination) to control weedy invasive plants. While these tools are effective, managers and consumers are increasingly opting for more environmentally friendly tools wherever and whenever possible. One approach is the use of biological control tactics that employ natural enemies (pathogens and insects) of the invasive weed to exert negative effects on target plants’ performance in terms of survival, growth, and reproduction. There are a few published examples that document the impact of combined use of insects and plant pathogens in invasive tree control. One of such cases is the biological control program targeting melaleuca in the greater Everglades region of southern Florida, USA. Two insects, a leaf-feeding weevil and a sap-sucking psyllid, were introduced in 1997 and 2002, respectively, and play an important role in the biological control of melaleuca trees. In addition, a foliar (succulent leaf and twig attacking) rust fungus that is believed to have been introduced into Florida through the ornamental trade, also attacks melaleuca and further weakens the tree through twig defoliation and dieback. Combined deployment (instead of individual deployment) of these insects and fungal pathogen enhanced defoliation and increased melaleuca mortality rates, promoted light penetration to the forest-floor, and facilitated the recruitment of more desirable vegetation. Recent studies have demonstrated an approximately 4-fold increase in plant species diversity and abundance in melaleuca monoculture forests following the impact of these insects and pathogens on melaleuca trees. Such combined deployment of these insects and rust fungus has also been beneficial in controlling regrowth from cut stumps of melaleuca tree and play important role in reducing the use of herbicides. In conclusion, the integrated use of herbivorous insects and plant pathogens in classical biological control is rarely occurring but when it occurs may result in greater levels of control than when deployed individually.
Technical Abstract: Exotic, invasive plants continue to plague managed as well as natural ecosystems. Land managers rely heavily on cultural, mechanical, or herbicidal tools (singly or in combination) to control weedy invasive plants. While these tools are effective, managers and consumers are increasingly opting for more environmentally friendly tools wherever and whenever possible. One approach is the use of biological control tactics that employ natural enemies (pathogens and insects) of the invasive weed to exert negative effects on target plant’s performance, including reduced survival, growth, and reproduction. Because many exotic weeds leave their natural enemies behind when introduced to the new range, biological control seeks to reestablish this relationship by intentionally introducing select natural enemies that will suppress the target weed but not affect other plant species. The long term role of herbivore damage on the population dynamics of weedy plants, however, is often subtle and successful biological control only becomes apparent when severe damage is accompanied by reductions in density or inhibition of further spread of the targeted weed. Identifying which suite of potential natural enemies will maximize control with minimal competition has been a subject of vigorous debate. Under field conditions, for instance, specialist herbivorous insects and phytopathogens may compete for the same resources (individual plants from the same population) so it is logical to question the advisability of combining various agents in an invasive plant management program. Various reports have documented the plant-mediated interactions or competition between introduced pathogens and insects that simultaneously exploit a common host plant. There are a few published examples, however, that document the use of natural enemies to control invasive trees. One of such cases is the biological control program targeting Melaleuca quinquenervia (melaleuca) in the greater Everglades region of southern Florida, USA. Two insects, Oxyops vitiosa (weevil) and Boreioglycaspis melaleucae (psyllid), were introduced in 1997 and 2002, respectively, and they play an important role in the control of the exotic tree. In addition, a rust fungus (Puccinia psidii) that is believed to have been introduced into Florida through the ornamental trade, also attacks melaleuca and further weakens the tree through twig defoliation and dieback. Together, these natural enemies defoliate melaleuca trees, increase their rate of mortality, promote light penetration to the forest-floor, and facilitate the recruitment of more desirable vegetation. Recent studies in Florida have demonstrated ca 4-fold increase in plant species diversity and abundance following attack of melaleuca by natural enemies. Some invasive trees tend to coppice or re-grow vigorously if stumps are not treated with herbicides immediately after felling. Approximately 96% of felled melaleuca trees, for instance, coppiced vigorously within 1 year suggesting that mechanical removal alone can exacerbate the problem if not carried out in conjunction with the application of herbicides. Herbicide treated stumps tend to coppice when cut-surfaces are 1) not sprayed immediately after felling, 2) not treated over the entire cut surface or 3) washed by rain before the herbicide is absorbed through the cut surface. There are a few alternatives to the use of herbicides for the control of coppicing weedy species, which involves the use of bioherbicides developed from one or more native fungal strains. The fungus Chondrostereum purpureum, for instance, can be applied to cut stumps for the suppression of regrowth through the inundative application of mycelial and/or spore inocula on freshly cut surfaces of felled Alnus, Betula, Populus, Prunus, and Ulex species in forests and natural other natural areas. Published examples of the simultaneous use of classical (obligate, self dispersing and propagating) biological control agents, especially insects and pathogens, in regrowth suppression are rare. We designed an experiment to assess the efficacy of integrating the melaleuca biological control insects, O. vitiosa and B. melaleucae with the rust fungus P. psidii for the suppression of melaleuca regrowth. Though the sole deployment of insects or rust fungus had a markedly negative impact on plant performance, their combination caused higher levels of coppice damage and mortality compared to the control treatments. The impact of insects and rust fungus in combination acted synergistically on overall coppice damage while additively on stem mortality, and reduction in height, branching, leaf biomass and total biomass of coppices. The enhanced efficacy may be attributed to resource partitioning among guilds of agents. Gravid females of O. vitiosa, for instance, prefer to lay eggs on P. psidii-free leaf tissues and hence avoid fungus infected tissues whereas, B. melaleucae indiscriminately lays eggs regardless of the presence of weevils or rust (personal observation). In this example, B. melaleucae and P. psidii showed a greater ability to simultaneously attack the same leaf tissues compared to O. vitiosa and P. psidii but both combinations had significant negative impact on melaleuca tree mortality and reproductive potential. In conclusion, the integrated use of herbivorous insects and plant pathogens in classical biological control is rarely occurring but when it occurs may yield greater levels of control than when employed individually.