Cane toads, Bufo marinus, are among the iconic poster children of invasive species, in part thanks to Mark Lewis's 1988 Australian documentary, ‘Cane Toads: An Unnatural History’, and subsequent follow-up, ‘Cane Toads: The Conquest’. Unfortunately, the conquest in his latest video refers to what the cane toads have achieved, not the Australians' efforts to control them, despite the impressive research program of Rick Shine and his colleagues at the University of Sidney, among others. In this issue's featured paper, Pizzatto & Shine (2012) show that a lungworm parasite of the invasive cane toad, apparently brought to Australia along with the toad, can infect native green tree frogs, Litoria caerulea, with no evident detrimental effect on the frogs. These frogs can maintain the infection for up to 120 days postexposure, and infective lungworm larvae in the native frogs' feces can then infect cane toads, substantially reducing their stamina. This leads to the intriguing suggestion that biological control of the cane toad could be undertaken by deliberately infecting these native frogs, which could then disseminate the lungworm to the cane toads, as a sort of ‘Typhoid Mary’ – an asymptomatic carrier of the pathogen that, while remaining healthy, spreads disease to others.
The authors are not ready to advocate this approach in this system, ‘except in areas of high conservation value where toads are likely to have major impacts’, primarily on the grounds of possible undetected effects on green tree frog viability and secondarily on animal welfare grounds – although if, in fact, the parasite has no effect on green tree frogs, it is not clear how infecting them is imposing suffering. A more important reason to avoid the ‘Typhoid Mary’ approach in this case would seem to be because a close relative of the green tree frog, the splendid tree frog (Litoria splendida), has been found to be very sensitive to this lungworm parasite, at least in laboratory trials (Pizzatto & Shine, 2011a). I wonder if the likelihood of spread to L. splendida by deliberately infected L. caerulea would actually be greater than spread to cane toads, considering the two native frogs are congeners.
The concept is certainly novel, and we need novel ways to control invasive pests. However, this approach, while using a native species to spread the parasite, ultimately depends on the use of an exotic organism. Although it is true that many parasites and parasitoid insects are quite host specific, unfortunately, this is not always the case. A parasitoid fly introduced as a biological control agent for gypsy moth and other pest Lepidoptera was subsequently found to impact populations of several native giant silk moth species in North America (Boettner, Elkinton & Boettner, 2000). Other examples of negative effects of biological control agents are cited by Howarth (1991), Simberloff & Stiling (1996) and Barratt et al. (2010). Current regulations emphasize the need for host specificity of any introduction, and concern for non-target impacts is more and more a concern in the regulatory environment (Hajek, McManus & Júnior, 2007; Barratt et al., 2010). The cane toad is also an iconic poster child for biological control gone wrong, and in our efforts to control it, we must ensure that things are not made worse – if that is possible.
Typhoid Mary unknowingly spread the bacteria that cause typhoid fever to other members of her own species, as a rather unsanitary cook to a whole series of human families. A possible approach in the cane toad system might be to take heavily infected cane toads and release them at the invasion front (where the lungworm is often absent), or otherwise enhance the level of lungworm parasites, if this can be done in a way that would not significantly increase the toad population – perhaps by releasing small infected metamorphs that would likely be cannibalized by larger cane toads (Pizzatto & Shine, 2011b).
So far, efforts to identify a truly cane toad-specific pathogen for use as a biological control agent have been unsuccessful, as have been efforts to induce autoimmunity and disrupt metamorphosis (Pallister et al., 2011). Alarm pheromones and translocation of parasitic lungworms to the invasion front may provide new tools in the management of this damaging pest species (Saunders et al., 2010). Unfortunately, at present, a truly effective and safe ‘integrated toad management’ program still seems elusive.