Tree weta ecology

Tree Weta (Hemideina) Ecology

There are 7 species within the genus Hemideina (tree weta). Because many tree weta species are common and widespread they have been used extensively in studies of ecology and evolution. Tree weta are common in forests and suburban gardens throughout most of New Zealand. They are arboreal nocturnal orthoptera that hide in hollow tree branches during the day and feed on leaves, flowers, fruit and insects at night.

Diet
We now know that tree weta are not much like mice (Griffin et al. 2011). Although weta are nocturnal and produce large poos (frass), they are much slower than mice and have quite a different impact on the forests in which they live. Weta have a slower rate of reproduction than mice and as cold blooded animals (ectotherms) they have a much lower metabolic rate. Thus weta eat less and grow more slowly than small mammals can. Tree weta mostly eat leaves (Wehi & Hicks 2010) rather than the mouse diet of seeds and insects, although tree weta do scavenge dead invertebrates and eat some fruit and seeds. Some tree seeds pass through weta intact and can germinate (Duthie et al. 2006), but the majority of seeds eaten by weta are digested (Wyman et al 2010).

Hiding
Tree weta hide in holes in hollow tree branches during the day where they are safe from birds and mammals than might like to eat them. Many native birds eat weta (e.g. kiwi, robin, tomtit, ruru) and many introduced mammals also like weta for lunch (e.g. hedgehogs, stoats, possums, mice, rats, cats). During the day while the weta hide in refuge holes (roosts) they are safe, but at night they have to come out to feed. When attacked tree weta raise their spiny back legs above their bodies, looking large and prickly, to try and escape being eaten. When the weta brings its legs down again, the inside of their femur, where there are many small pegs, rubs against the stridulatory ridges on their abdomen. This rubbing of pegs on ridges makes a loud rasping or scratching sound as a defence. Tree weta also stridulate to each other at night by moving their abdomen up and down across the femora pegs (Field & Jarman 2001).

Mating
Weta adults like to live together in the summer and autumn but not during the winter (Wehi et al 2012). During the summer 10 or 12 adult tree weta can be found squeezed together into the same refuge hole. Usually there is just one or two male tree weta with many females (the harem). Males have elongated mandibles that they use in combat with other males over access to large tree holes where many adult females hide together during the day. In Hemideina maori the male weta with the largest heads are found hiding together with the largest harems, so it is thought these large-jawed males have won fights for the right to mate with many females (Gwynne & Jamison 1998). In H. crassidens large-jawed males also win fights and mate with many females (Kelly 2006).

Female tree weta lay their eggs into the soil by inserting their ovipositer into the ground. The eggs hatch about 8 months later and the nymphs look like tiny versions of their parents.

References

Duthie C, Gibbs G, Burns KC 2006. Seed dispersal by weta. Science 311 (5767): 1575.

Field LH, Jarman TH 2001. Mating behaviour. In: Field JH ed The Biology of Wetas, King Crickets and their Allies. CABI Publishing, Wallingford. Pp. 317–332.

Griffin MJ, Trewick SA, Wehi PM, Morgan-Richards M. 2011. Exploring the concept of niche convergence in a land without rodents: the case of weta as small mammals. New Zealand Journal of Ecology 35(3) 302-307. Available on-line at: http://www.newzealandecology.org/nzje/.

Griffin MJ, Morgan-Richards M, Trewick SA. 2011. Are tree weta (Orthoptera, Anostostomatidae, Hemideina) obligate herbivores? New Zealand Natural Sciences. 36: 11-19.

Gwynne D & Jamison I. 1998. Sexual selection and sexual dimorphism in a harem-polygynous insect, the alpine weta (Hemideina maori, Orthoptera Stenopelmatidae). Ethology Ecology & Evolution 10: 393-402, 1998

Kelly CD 2006. The relationship between resource control, association with females and male weapon size in a male-dominance insect. Ethology 112: 362–369

Trewick, S. A. and Morgan Richards, M. 1995. On the distribution of tree weta in the North Island, New Zealand. Journal of the Royal Society of New Zealand, 25(4): 1-9.

Wehi, P.M. & Hicks B.J. 2010. Isotopic fractionation in a large herbivorous insect, the Auckland tree weta. Journal of Insect Physiology 56: 1877 – 18882.

Wehi PM. Jorgensen M, Morgan-Richards M. (in revision) Sex and season dependent behaviour of a flightless forest insect the Auckland tree weta. NZ Journal Ecology.

Wyman TE, Trewick SA, Morgan-Richards M, and Noble ADL. 2010. Mutualism or opportunism? Tree fuchsia (Fuchsia excorticata) and tree weta (Hemideina) interactions. Austral Ecology. DOI: 10.1111/j.1442-9993.2010.02146.x.