Behavioural ecology of Australian gall thrips (Insecta, Thysanoptera)

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Abstract
Collections, observations and experiments were used to investigate the behavioural ecology of gall thrips (Thysanoptera: Phlaeothripidae) in New South Wales and Queensland, Australia. Data are presented on aspects of gall morphology, male and female morphology, behaviour, life cycles, and sex ratios for six gall-forming species, five species of inquilines (invaders that do not form galls), and one genus that uses secretions to glue phyllodes (modified petioles) together. During gall foundation on phyllodes of Acacia pendula, females of Kladothrips rugosus Froggatt fight other females to the death, males fight other males to the death, and one female or one adult of each sex remains alive within the gall to breed. In addition to the live adults, half of newly found galls contained dead bodies of from one to 13 males and females, which were apparently killed in fights. In Oncothrips tepperi Karny, which forms galls on Acacia oswaldii, single females found new galls, and females engage in lethal fighting during the period from gall initiation until closure. First-generation broods of Oncothrips tepperi comprise winged males and winged females with a sex ratio of about 1:6 female biased. Each gall also contains several wing-reduced adults with larger forelegs than winged adults, which probably either fight among themselves or defend their siblings from gall invaders. Adults of Oncothrips tepperi breed in the galls for two or more generations, whereas in all of the other species studied on Acacia only one generation occurs in a gall. Females of the inquiline Koptothrips flavicornis invade young, first-generation galls of Oncothrips tepperi, kill the foundling female, and produce their own offspring. Single females of Oncothrips antennatus (Moulton) form galls on Acacia aneura, and apparently do not fight. Winged females of Oncothrips antennatus have smaller forelegs, relative to their body size, than do winged females of Oncothrips tepperi. In Onychothrips arotrum Mound and Onychothrips tepperi (Uzel), single females from galls on Acacia aneura. In each species, females engage in lethal fighting during gall initiation. The sex ratio of Onychothrips arotrum broods is about 1:18 female biased. The inquiline Csirothrips watsoni Mound invades galls of Onychothrips arotrum, apparently after most or all of the offspring have left, and breeds inside. Females of this inquiline will kill live Onychothrips arotrum inside the galls, and they apparently plug gall entrances with cast O. arotrum exuviae. Iotatubothrips crozieri Mound and Crespi is involved in the formation of large, woody galls on stems of Casuarina, perhaps in association with a microorganism. They breed in these galls for many generations. Galls contain several to thousands of individuals, and the adult sex ratio is about 1:4 female biased. Adults of the inquilines Thaumatothrips froggatti and Phallothrips houstoni Mound and Crespi invade these galls, kill the Iotatubothrips adults, and breed therein. The Iotatubothrips occasionally attempt, ineffectually, to fight off the invaders, and they apparently make partitions within the gall to protect themselves from takeover. Adults of Lichanothrips spp. glue phyllodes of Acacia harpophylla together using eggs and patches of secretion, and they breed in the resultant narrow space. Xanothrips xantes Mound breed in these spaces after the Lichanothrips have left. Fighting in Kladothrips rugosus, Oncothrips tepperi and Onychothrips tepperi involves two adults rearing up head to head, sparring with their enlarged, armed forelegs raised, and attempting to grasp and hold their opponent with their forelegs and drive their sharply pointed fore-tarsal teeth into their opponent's body. Females of Onychothrips arotrum also grasp, stab and kill with their forelegs, but they have not been observed to rear up head to head. Three of the inquiline species, Csirothrips watsoni, Thaumatothrips froggatti and Phallothrips houstoni, kill the original gall inhabitants by extending their forelegs directly in front of their bodies, tilting their heads back, remaining in this position for variable periods of time, and sharply pulling their armed fore tibiae towards the fore femora when their victim is near. Inquilinism in gall thrips may have evolved from lethal intraspecific fighting. Certain aspects of behaviour and morphology in Australian gall thrips, such as high prevalence of lethal fighting, gall ‘plugging’ in Csirothrips watsoni, attempted gall defence and apparent formation of partitions in Iotatubothrips crozieri, and the presence of wing-reduced adults in Oncothrips tepperi, indicate that these species exhibit some of the most complex behaviour thus far discovered in Thysanoptera.