Larval alarm pheromones as a potential control for invasive cane toads (Bufo marinus) in tropical Australia

Abstract

Novel approaches to control invasive species are urgently needed. Cane toads (Bufo marinus) are large, highly toxic anurans that are spreading rapidly through tropical Australia. Injured toad larvae produce an alarm pheromone that elicits rapid avoidance by conspecifics but not by frog larvae. Experiments in outdoor ponds show that repeated exposure to the pheromone reduced toad tadpole survival rates (by [50%) and body mass at metamorphosis (by 20%). The alarm pheromone did not induce tadpoles to seek shelter, but accelerated ontogenetic differentiation. Perhaps reflecting mortality of weaker individuals during larval life, growth rates post-metamorphosis were higher in animals emerging from the pheromone exposure treatment than from the control treatment. Nonetheless, body size differentials established at metamorphosis persisted through the first 8 days of post-metamorphic life. We will need substantial additional research before evaluating whether the alarm pheromone provides a way to reduce cane toad recruitment in nature, but our field trials are encouraging in this respect.

Embryonic exposure to conspecific chemicals suppresses cane toad growth and survival

“…Future work could usefully explore these alternatives, as well as assessing the consistency of this response across pond conditions, sibships and the like. Unlike other toad pheromones [18,35], these effects reduce toad viability in the long term only after brief exposure. Thus, they have great potential as a species-specific pheromonal control for invasive cane toads.…”

Larger Body Size at Metamorphosis Enhances Survival, Growth and Performance of Young Cane Toads (Rhinella marina)

“…Smaller body size also renders a young cane toad more vulnerable to desiccation [51], [52], predation by carnivorous ants [55], cannibalism [54], and mortality after lungworm infection [53]. Thus, for this species, we can confidently conclude that (a) toad body size at metamorphosis can be reduced by multiple biotic and abiotic characteristics of the larval environment [43], [44], [45], [46], (b) that any reduction in body size at metamorphosis is likely to impose subsequent fitness costs, and (c) the strength of any link between metamorph mass and subsequent survival and growth rates will vary through time and space, at least partly because of variation in the availability of environmental resources.…”

“…Size reduction could be achieved by pheromonal means [45], [46], by concentrating toad oviposition in a smaller number of ponds and thus increasing larval densities [42], by increasing parasite uptake [53], or by encouraging native frogs to spawn in the same ponds as the toads, thereby intensifying larval competition [43], [44]. Controlling the metamorph stage can be an effective method to decrease population growth rates in invasive bullfrogs [96].…”

“…This could be done by increasing larval competition with conspecifics (through concentrating oviposition in a smaller number of sites, thus increasing larval density: [42]), or heterospecifics (by encouraging other taxa of anurans to oviposit in the same ponds as used by the target species: [43], [44]. Alternatively, we could use pheromonal cues (from conspecifics, heterospecific anurans, or predators) to induce reaction norms that result in reduced size at metamorphosis [45], [46]. Such manipulations rely upon the assumption that reducing mean body size at metamorphosis will reduce subsequent recruitment of that cohort.…”

“…Toad invasion has had major ecological impacts on native predators, stimulating a search for novel methods of toad control [49]. Both in the laboratory and the field, experiments have shown that body size at metamorphosis in cane toads can be reduced by exposure to alarm cues from injured conspecifics [45], [50] and by the presence of tadpoles of native frog species [43]. In other work, smaller body size in immediately post-metamorphic toads has been shown to increase the individual’s vulnerability to desiccation [51], [52], to mortality caused by lungworm infection [53], and to predation by cannibalistic conspecifics [54] and predatory ants [55].…”