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Mutualisms or interspecific interactions involving net mutual benefits, are an important component of ecological theory, although effectively demonstrating mutualism is notoriously difficult. Among two New Zealand endemics, a slightly elevated germination rate of Fuchsia excorticata (Onagraceae) seeds after passage through tree weta (Orthoptera: Anostostomatidae) compared with seeds manually extracted from fruit, led to the proposal that a mutualistic relationship exists between this plant and animal. An improved germination rate, or any other single trait, however, does not alone constitute evidence for mutualism; the relative costs and benefits of numerous components of the interaction need to be accounted for. We considered the costs and benefits to F. excorticata of the putative seed dispersal mutualism with tree weta. Tree weta provided with F. excorticata fruits destroyed 78% of the seeds they consumed, did not move fruit; and faeces containing seeds were deposited near their roost holes (which are naturally in trees). The seeds remaining after fruit consumption and those that are ingested but survive gut passage are unlikely to be deposited in suitable habitat for seedling survival. Plant food preferences of captive tree weta assessed using pairwise leaf choice tests showed that the leaves of F. excorticata were the least preferred of six commonly encountered plants. In addition, we found that tree weta did not show a preference for F. excorticata fruit over a standard leafy diet, indicating they are unlikely to be actively seeking fruit in preference to other sources of food. These observations indicate that any interaction between tree weta and F. excorticata is likely to be opportunistic rather than mutualistic, and highlight the difficulty of characterizing such interactions. [ABSTRACT FROM AUTHOR]