Is the Tree Weta Hemideina Crassidens an Obligate Herbivore?
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Available online at www.nznaturalsciences.org.nz Is the tree weta Hemideina crassidens an obligate herbivore? Melissa J. Griffin1, Mary Morgan-Richards and Steve A. Trewick Ecology Group, Institute of Natural Resources, Massey University, Private Bag 11-222, Palmerston North, New Zealand 1Corresponding author’s email: [email protected] (Received 29 October 2010, revised and accepted 11 February 2011) Abstract Tree weta are well known insects of New Zealand’s forest ecosystems; however, there is limited research into their general ecology. A literature review suggests they are mainly herbivores. Wellington tree weta, Hemideina crassidens, were given the choice of possible foods that they may come into contact with in the wild. Unexpectedly, leaves of Coprosma robusta were consumed the least by the weta, which suggests a more accurate label for the diet of H. crassidens may be omnivory. There are possible implications for the mixing of foods in the diet that need further investigating. Key words: diet – herbivory – Hemideina crassidens – omnivory – polyphagy. Introduction diet of one species, the mountain stone weta H. maori, using faecal analysis and Weta of the family Anostostomatidae are found plant and invertebrate fragments. large bodied, flightless, orthopterans that But for the other six Hemideina species, are well known insects in New Zealand. studies have primarily been directed at Nevertheless, detailed knowledge of their other questions. For example, while ecology in the wild is limited, even to investigating the possibility of seed the extent that information of their diet dispersal by tree weta, Duthie et al. (2006) is fragmentary. One general observation found seeds of Fuchsia excorticata in the is that tree weta (Hemideina spp.) are, faeces of wild H. crassidens and identified unusually for their family, predominantly a large variety of native fruits that they herbivores (Meads 1990; Green 2005; would consume in captivity. Trewick & Morgan–Richards 2005; In the orthopteran suborder Ensifera, Gibbs 2009; Wehi & Hicks 2010; which includes the Anostostomatidae, but see Barrett 1991). More precisely, there is a variety of insect dietary traits Hemideina appear to thrive on a diet of seen. Some taxa show extreme specialisa- leaves. Although this inference has often tion, such as the stick katydids (subfamily been repeated in the literature its basis Phasmodinae) which feed exclusively on has not been extensively tested. Wilson flowers (Rentz 1996; pp. 105). Others and Jamieson (2005) examined the show generalisation in their diets, as seen New Zealand Natural Sciences (2011) 36: 11-19. © New Zealand Natural Sciences 12 New Zealand Natural Sciences 36 (2011) GRIFFIN ET AL: Tree weta and food choice 133 in the family Tettigoniidae (Rentz 1996; plant content, whereas faeces containing ambient temperature. Each container fruit (Figure 1). Some or all of the two pp. 105). tissues from an animal diet are softer and had a hollowed flax (Phormium tenax) moths were eaten by 87.5% of the weta. We collated records on the diet of tree may more readily break down in the envi- flower stalk as a daytime roost for the Seeds were the least preferred, with only weta observed in captivity and in the wild ronment. This difference is evident when weta (Wyman 2009). The weta were two of the 32 weta consuming some part (Table 1). From this, it is apparent that large, firm plant-based droppings of tree given seven days to acclimate to this of the seeds. The seeds were not eaten plants are indeed an important compo- and giant weta are compared to the soft environment during which time they whole, rather they were gnawed and the nent of tree weta diet. Leaves generally liquid droppings of carnivorous ground were given a maintenance diet of carrot kernel inside eaten. No weta ate only dominate the food types eaten by individ- weta and tusked weta (Gibbs 1998; pers. (Wyman et al. 2010), which was removed leaves and leaves were recorded as being uals of four species of weta (H. crassidens, obs. SAT). Method bias could easily at the start of the choice experiment. eaten less often (9/32) than other food H. femorata, H. maori and H. ricta; Table result in only the plant cuticles being Leaves and fruits used were collected fresh types provided (moths + fruit + seeds = 1). However, availability of information identified and other particles from faeces from Coprosma robusta in Palmerston 55; t test P = 0.016). The proportion of is variable and almost certainly incom- being effectively ignored, so the full diet is North, a plant known to be palatable to instances of adults and juvenile eating a plete. For instance, published records for not observed. In particular, foods that are tree weta (Table 1, Pers. Obs.). The leaves combination of these foods were 82.6% one of the most well known species, the rare but perhaps nutritionally important of Coprosma robusta were approximately and 87.8% respectively. Most (18/32) Auckland tree weta H. thoracica, consists will tend to be overlooked. 85 mm x 40 mm. The fruits of C. robusta weta ate two food items, five ate three, of seed predation, cannibalism in captiv- In light of recent speculation about are elliptical and orange, approximately one ate four items and eight ate one. ity, two instances of leaf eating and one specialised interactions between weta and 8mm x 5mm and contain two seeds of a There was no difference in the average instance of fruit eating. The majority of various plants (Duthie et al. 2006; Burns size that are too large (4.2 - 6.5mm x 2 - number of different food types eaten by published observations are of captive 2006; Morgan-Richards et al. 2008) we 3mm; Webb & Simpson 2001) for tree males (2.055) and females (2.085) (t test weta and include the foods eaten under asked the more general question: does weta to ingest whole. All fruit were ripe P = 0.43). Moths and fruit were the food experimental conditions (e.g. food choice the tree weta, Hemideina crassidens show and intact when given to the weta. The types most commonly chosen by males trials). As tree weta are generally treated a pronounced choice for particular food relatively large seeds of the fruits of this and females, adults and juveniles (Figure as herbivorous and thus usually provided types? If H. crassidens is a herbivore, we species enabled a distinction to be made 1). Although there appears to be a subtle only with leaves, captive studies may not predict that under experimental condi- between the consumption of fruit pulp difference in the food choice of adults be informative about food preference. tions, individual weta will choose leaves and predation on the seeds. compared with juveniles (Figure 1), this Field observations of natural diet include more often than other food types avail- The choice experiment was run over did not result in a change in the ranking nocturnal observations of weta eating able. In contrast, if H. crassidens is poly- two consecutive nights using wild-caught of leaves over other foods, or a change in and identification of food particles in the phagous then we expect to see each weta weta. Each weta was supplied with three the number of food types eaten per weta. faeces collected in the wild. eating a number of different food types leaves, five fruit (approximately 1g) Direct field observations provide valu- when given a choice. and two freeze-killed Wiseana moths Discussion able information but their significance is (approximately 0.5g) at the start of the difficult to quantify when there are so few Methods experiment. This volume of each food Tree weta are commonly known to eat data. Similarly, determining diet from the type meant that weta consumption was vegetation (Table 1). The capacity to remains of food components in faeces also We tested whether tree weta demonstrate not limited. There were no other foods consume leaves and develop successfully has intrinsic problems as digestion differs any partiality when offered a range in the container. Weta were checked after on a purely plant diet (Morgan-Richards among food types (Fitzgerald 1976; Tre- of food types they might encounter the first night and the total of each food 2000), appears to be a specialist trait of wick 1996). Soft tissues from fruit flesh in nature: leaves, insects, fruits and type eaten was recorded on the second this genus and the sister genus Deinacrida may not be obvious in weta droppings so seeds. 32 wild Wellington tree weta, H. morning. (Pratt et al. 2008) and leads to the quantification of fruit eating may come crassidens, were collected from southern expectation that tree weta will prefer only from the presence of seeds in the North Island. These weta included males Results plant foods when given the choice. This faeces and then only when seeds are small and females, adults and juveniles. The expectation is born out of observations enough to be ingested whole. Invertebrate weta were kept individually in two-litre The experiment recorded feeding of 32 of tree weta feeding in the wild and remains in faeces are often difficult to plastic containers with lids fitted with fine Hemideina crassidens tree weta; 11 adults the consistency and content of their distinguish and identify, even to a family mesh to allow light and air circulation (6 females and 5 males) and 21 juveniles droppings. However, in our captive level (Little 1980). Large droppings that (Wyman et al. 2010). They were kept (13 females and 8 males). The most experiment we found Wellington tree persist in the environment have a high in a quiet room with natural light and frequently eaten foods were moths and weta, H. crassidens, did not exhibit a 14 New Zealand Natural Sciences 36 (2011) GRIFFIN ET AL: Tree weta and food choice 15 Scrophularaceae Hebe sp.