Note 20: A note on the Rain tree bug: Ptyelus grossus by Moira FitzPatrick

This note describes the bug Ptyelus grossus which gives rise to the phenomenon of "rain trees".

Small, spittle-like masses of white foam known as cuckoo-spit may be seen on the twigs and leaves of a number of different plants. These shelter the immature plant bugs known as spittle-bugs. This froth is produced by fluid bubbled from the anus, providing the nymphs with protection from predators and preventing them from drying out. Along each side of the abdomen of the bug a membranous flap folds under the belly. These flaps meet, enclosing a space between themselves and the underside of the abdomen. This chamber contains the spiracles through which the bug breathes and is opened and shut by a valve situated near the anus. Glands that extrude a waxy secretion are located near this valve and consequently the insects’ excrement is mixed with this secretion.

By flexing its tail in and out, the bug forces both air and liquid through the valve, thus producing the innumerable small bubbles that form its foamy home. These bubbles do not burst for some time because they are strengthened by the waxy secretion and as they are being constantly renewed by the bug the foam provides a cool shelter for the insect as long as it is feeding.

The spittle bug, Ptyelus grossus, is common in Zimbabwe and occurs in large numbers on the rain tree Philenoptera violacea but are also found on other trees like Tipuana tipu and Rauvolfia caffra. The spittle bug feeds on the sap of the plant by piercing the bark of the tree with their stylets (sucking mouthparts) and sucking the sap at great speed. The plant sap is a weak solution of sugars and salts and the insect has to consume a great deal in order to obtain sufficient nourishment, so they eject almost pure water equally fast. This drips from the tree in sufficient quantities to form pools on the ground below and infested trees have acquired the name "rain tree".

Eventually when it is fully grown, the bug leaves the foam and casts its skin for the last time. The wing-buds on its back expand into a pair of stiff wings which it holds roof-like over its back. They are small insects only about 6-8 mm in length.

This article reproduced on this website by kind permission of Dr Moira FitzPatrick

Copyright: Mike Bingham, Annette Willemen, Bart Wursten, Petra Ballings and Mark Hyde, 2011-17

Bingham, M.G., Willemen, A., Wursten, B.T., Ballings, P. and Hyde, M.A. (2017). Flora of Zambia: Note 20: A note on the Rain tree bug: Ptyelus grossus., retrieved 30 March 2017

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