Ecology in a Microscopic Scale on Rose Apple Tree [蒲桃葉下的微觀生態]

(Original article written for the Newsletter of Hong Kong Gardening Society)

The weather is becoming hot recently while the life of citizens is still under the impact of the Covid-19 pandemic.  More people are coming out for some fresh air nearby their home for a jogging or just a sit under tree shade.

Round and dense canopy

Not all trees provide shade comfortable enough for cooling, however.  Rose Apple (Syzygium jambos, 蒲桃) is within my favorite list for shade and cooling due to its thick and dense canopy.  It is a medium-sized tree with well-developed branching system that allows it to be trained to either shrubby or within certain height – a character that suggests the species to be a good choice for limited space such as on a podium or in a small garden.

Deformed leaves

With its low branching property in general, one may easily discover abnormality, if any, on the leaves of a Rose Apple tree.  It is not uncommon to spot reddish and curled leaves especially on the lower branches.  If you get a closer look dimple-like depressions may be seen on the underside of a deformed leaf.  The more these dimples are present the more reddish or ugly the leaves would look like.  If a tree is affected seriously it may look very unhealthy too.  It is nearly not possible to know by naked eyes what has happened to the trees, but in fact hustle and bustle on the underside of the leaves can be seen if you put them under a hand lens or a microscope.

  

Pupa under a 'lid' (opened liberally)

Lacewing larvae

Juvenile of Syzygium Leaf Psyllid

Adult of a Syzygium Leaf Psyllid and
covered pupa

Such deformed leaves are actually infested by Syzygium Leaf Psyllid, which is a tiny insect from the Order Hemiptera (members including many sap-sucking insects such as cicadas, aphids, scale insects, mealy bugs).  There are many species of psyllid but they are mostly sucking type.  Their eggs were laid on the undersize of the leaves.  After hatching the nymphs then become quite stationary and start sucking on the leaf surface.  The affected area then swells outwards to the upper side of the leaf (and turn red gradually) and the nymphs at the trough on the underside then start secreting wax to cover themselves inside the dimples, in which the insect pupates.  When there is an outbreak, other predatory insects such as lacewing larva then comes to draw psyllid’s body fluid in turn.  A food chain can be observed on a single leaf – with lacewing larva, mummy psyllids or more!  Improving the air flow and removing affected leaves sometimes help controlling the pest to Rose Apple.

Flower of Syzygium jambos

Though it is a tree closely related to Wax Apple (or Java Apple) with its fruits also edible, Rose Apple is rarely seen in local fruit store.  If you are not a hungry fruit-hunter, you are recommended to enjoy the pale-yellowish blossom recently (April-May) and then leave the fruits for bats and birds (they also helps sowing the seeds of this somewhat-naturalized tree).  The flowers of Rose Apple are not small but in fact it is the anthers (not the petals) which attract our sight and perhaps, the honeybees as well!