Research Focus Biological pest control in beetle agriculture Duur K. Aanen 1 , Bernard Slippers 2 and Michael J. Wingfield 2 1 Department of Plant Sciences, Laboratory of Genetics, Wageningen University and Research Center, 6703BD, Wageningen, The Netherlands 2 Department of Genetics, Forestry and Agricultural Biotechnology Institute, University of Pretoria, 0002, Pretoria, South Africa Bark beetles are among the most destructive tree pests on the planet. Their symbiosis with fungi has con- sequently been studied extensively for more than a century. A recent study has identified actinomycete bacteria that are associated with the southern pine beetle and produce specific antibiotics against an antagonist of the beetles’ mutualistic fungus. In addition to highlighting the ecological complexity of bark-beetle– microbial symbioses, this work reveals a potential source of novel antibiotics. The complexity of mutualistic interactions ‘...I will be an enemy to your enemies and will oppose those who oppose you.’ Exodus 23:22 The traditional view of mutualism is one of a recipro- cally beneficial interaction between two species (Box 1). However, it is becoming increasingly evident that the distinction between mutualistic and parasitic inter- actions is far from clear and, also, that the nature of these relationships can vary for a single interaction. Moreover, although the net result of a mutualistic inter- action must by definition be positive for the interacting species, these species usually also pay a price. For example, gut bacteria usually provide benefits to their host and, therefore, must be categorized as mutualistic, but the benefits come at a cost for the host. This explains why swine farmers add antibiotics to livestock feed (i.e. to suppress their gut flora). The symbiosis between fungus-growing beetles and fungi illustrates clearly that the distinction between mutualistic and parasitic interactions can be context de- pendent. A large group of wood-inhabiting beetles live in a mutualistic symbiosis with various fungi, which provide nutrition to their larvae. In the well-studied southern pine beetle, the main mutualistic fungus is a species of the genus Entomocorticium, but an additional symbiont, Ophiostoma minus, has a context-dependent effect [1]: it can assist adult beetles in depleting tree resources during the early part of an attack, which is beneficial for the host at that stage; however, at a later stage, it competes with the more beneficial Entomocorticium sp. A, which makes it a parasite of the mutualism [1]. This example illustrates yet another complication of the traditional view of mutualisms: mutualistic inter- actions are not usually restricted to two species. A recent paper by Scott et al. [2] has identified an additional bacterial symbiont in the previously mentioned symbio- sis between southern pine beetles and their associated fungi. The bacterium belongs to the actinomycete genus Streptomyces, a group well known to humans (and other organisms [3]) because it is an important source of antibiotics. The Streptomyces sp. isolated by Scott et al. [2] shows antibiosis towards O. minus, the fungus which is – at least sometimes – a parasite of the mutu- alism. This bacterium appears to be another mutualist in this symbiosis because it can protect the mutualistic fungus against the parasite. Protecting your mutualists In mutualistic host–symbiont interactions, hosts employ various mechanisms to protect their symbionts against parasites. One form is simply physical protection (for example, by providing a protective growth environment, which can even be the host body or cell itself, in endo- symbioses). Another possible form of protection can be found in the domestication of secondary symbionts that are harmful to parasites and thereby protect the primary symbiont against parasites. The first example of such a protective symbiont was discovered by Currie et al. [4] in the well-studied mutualism between attine ants and fungi. These ants were found to carry specific bacteria (actinomycete bacteria of the family Pseudonocardia- ceae) that produce antibiotics targeted at a specialized parasitic fungus (genus Escovopsis) of this fungus-grow- ing insect mutualism [4,5]. It has been suggested that other insect–fungus mutualisms are also likely to have additional symbionts or even consortia of multiple sym- bionts and that this might lie at the heart of the success that has stabilized such interactions for more than 50 million years [6]. Scott et al. [2] have provided intriguing evidence that fungus-growing beetles carry bacterial symbionts to pro- tect their mutualistic fungi against antagonistic fungi. First, they have shown that one of the two isolated bac- terial morphotypes specifically inhibits the antagonist O. minus, and much less so the mutualistic fungus. Second, the authors have isolated and identified the chemical (which they have named mycangimycin [7]) that causes this specific antibiosis. Third, the bacterium seems to be specialized to this mutualism: it has been isolated from most beetle individuals tested (92 of 110) and from five of ten tested mycangia, which are specialized structures for the vertical transmission of the mutualistic fungus. Fourth, using scanning electron microscopy techniques, abundant filamentous growth of the bacterium could be shown in natural galleries. Update Corresponding author: Aanen, D.K. (duur.aanen@wur.nl). 179