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Zelkova gall-bamboo root aphidOn this page: Identification & Distribution Biology & Ecology Gall maintenance Natural enemies Other aphids on the same host Damage & Control
Identification & Distribution
In East Asia the fundatrices of Paracolopha morrisoni induce galls on its primary host Japanese zelkova (Zelkova serrata, see first picture below). The gall is a club-shaped outgrowth from the upper surface of the leaf lamina between veins, with a narrow stem and a rough swollen apex. Paracolopha morrisoni alatae emerge from their galls and emigrate to bamboos where they colonise the roots (see second picture below). The emigrant alate has the head and thorax dark brown, the antennae and legs are brown, and the abdomen is wholly membranous. The antennae (see third picture below) are usually 6-segmented (sometimes 5-), with antennal segment III relatively short: being 0.53-0.86 times the length of IV, V and VI combined. There are secondary rhinaria on segments III-VI. The apical rostral segment is 0.71-0.84 times as long as the second hind tarsal segment, and bears 5-8 hairs. Siphunculi are absent. The body length of adult Paracolopha morrisoni alate is 1.35-2.36 mm.
First image above copyright Anonymous under a Creative Commons Attribution-Share Alike 3.0 Unported Licence.
Paracolopha morrisoni apterae on roots of bamboo (not pictured) are globular, pale yellow and with greyish legs and antennae. They are completely covered with white filamentous wax. They have very small siphunculi, one-segmented tarsi and long, stout claws. The body length of adult apterae is 1.5-1.8 mm.
The primary host of Paracolopha morrisoni, which bears the galls, is Japanese zelkova (=keyaki , Zelkova serrata). In Japan and south China Paracolopha morrisoni host alternates from keyaki to the roots of various bamboo (Sasa species). In the USA it has been found as anholocyclic populations on the roots of several other genera of bamboos (Arundinaria, Phyllostachys and Pleioblastus, but apparently not Bambusa). Paracolopha morrisoni has been present on bamboo roots in nurseries in England since at least 1998, and has also been recorded in Denmark, Netherlands, Germany, France and the Czech Republic.
Biology & Ecology
Gall maintenance (sh*t happens)
Aphids can produce remarkably large amounts of honeydew, as most car owners know. Honeydew being, in-effect dilute sugar water, these aphid faeces promote mould and attract unwelcome visitors such as syrphids, predatory ants and wasps. Aphids producing copious quantities of honeydew can drown in their own excrement, or become hopelessly sticky as it it dries.
Aphids have several ways to cope with honedew. They may ignore it, or simply drop it, kick it away, produce melizitose to render the honeydew ant-attractive, or use wax. As melizitose and wax cost energy to make, aphids economize where possible and seldom produce both. Managing their waste is an especial problem for gall-inhabiting aphids - particularly where galling is extensive. Doing nothing may suffice for a briefly occupied gall, but kicking it away within a closed crowded space does not help - nor does attracting ants if the advantages of a closed gall are to be retained.
For some time it was unclear how aphids living in completely closed galls could survive until the galls mature and finally form an exit for emigration. (Wax, it was assumed, made aphids less palatable or acted as camouflage by covering the aphids and sometimes their immediate surroundings.)
Pike et al. (2002) showed closed gall dwellers produce a waxy environment which, being hydrophobic (=water repellent), causes honeydew to form globules (or 'honeydew marbles') rather than covering the aphids or the gall interior. Since mouldy galls also damage the host, some plants cover the gall's inner surface with dense trichomes to enhance water repellency. The picture below shows the honeydew marbles formed by Pemphigus bursarius inside their closed galls. For more on 'honeydew marbles', see Pemphigus populi.
Kutsukake et al. (2019) found that some species living in closed galls have a different solution to the problem. Their initial studies were on the social aphid, Nipponaphis monzeni, a species in the Hormaphidini which forms completely closed galls on the isu tree (Distylium racemosum). The galls are extremely long-lasting, taking some 2.5 years to mature, and containing a large number of insects (over 2,000 aphids in mature galls). Despite the large colony size, Kutsukake et al. found no honeydew droplets accumulating within the galls, but only some powdery wax and cast-off skins. The honeydew was somehow being removed from the inner cavity of the closed galls. Further studies revealed a novel gall-cleaning mechanism: the gall inner surface was itself absorbing and removing the liquid waste through the plant vascular system.
It seems this plant-mediated water-absorbing property has evolved at least three times independently, with Paracolopha morrisoni in the Eriosomatini being one of the species which uses it. Aside from reducing the risk of mould growth, this strategy allows aphids to produce less wax and their hostplant to recover its sugars. Given it benefits both parties this solution is presumably uncommon because its evolution requires a close, prolonged, relationship between an aphid species and host.
Sunose (1980) observed in Japan that galls of Paracolopha morrisoni were pecked by tree-sparrows (Passer montanus) and the aphids consumed.
The sparrows preferred trees crowded with galls to less crowded ones, but also preferred lightly galled to heavily galled leaves. The reason for the latter preference was apparently because the sparrow found it difficult to grasp a single gall with its beak if the leaf was crowded with other galls. The level of predation on trees with moderate or high density of galls was fairly low at 10-16%.
By late June an increasing number of galls had opened to release the alate aphids, and the number of galls being pecked by the birds decreased. By 1 July the percentage of the galls that had opened was 54.1%, and after 2 July the pecking was hardly seen. By 5 July, all the galls had opened leaving only a small number of anthocorid bugs and some dead aphids.
Another unusual and fascinating natural enemy of Paracolopha morrisoni is the larva of the flea weevil Orchestes (=Rhynchaenus) hustachei (see Orchestes larva feeding on Paracolopha morrisoni). Yamazaki & Sugiura (2001) describe how the eggs of the weevil are laid into the gall, and hatched larvae live in the galls. Each parasitized gall was found to contain only one weevil larva. At three of the six study sites predation by weevil larvae was the greatest mortality factor. Tomishawa & Akimoto (2004) found that, as well as predating the inhabitants of Paracolopha morrisoni galls on Zelkova, the larvae are also found in the aphid galls of four Tetraneura species on the David elm (Ulmus davidiana) in China and Japan. There is evidence that this is leading to host race formation between the weevil populations using the different galls.
Other aphids on the same host
Paracolopha morrisoni has been recorded from just 1 species of zelkova ( Zelkova serrata), and possibly also from Celtis sinensis.
Note: In Britain Paracolopha morrisoni is only recorded from bamboo roots, its secondary host.
Paracolopha morrisoni has been recorded from the roots of 1 Arundinaria species (Arundinaria gigantae), 1 Pseudosasa species (Pseudosasa japonica), 5 Phyllostachys species (Phyllostachys aurea, Phyllostachys aureosulcata, Phyllostachys bambusoides, Phyllostachys nigra, Phyllostachys vivax), 1 Pleioblastus species (Pleioblastus fortunatus) and 3 Sasa species (Sasa ramosa, Sasa senanensis, Sasa veitchii).
Damage and control