Origin of honeydew

SEVERAL people inquired recently at the museum as to why the windows of their cars were covered with a sticky secretion. The answer was that they had parked their cars beneath sycamore trees whose leaves were covered on the underside with aphids (mostly) and scale insects, which were producing copious amounts of honeydew.

Aphids are small (24mm long) pearshaped softbodied bugs, and may be recognised by the pair of cornicles (tailpipes) on the abdomen. Scale insects are mostly small insects belonging to several families in which the females and juveniles are often covered by a scale formed from casts of skin glued together. Both belong to the order Hemiptera (true bugs) and to the suborder Sternorrhyncha, which suck sap and other plant juices through fine needlelike stylets, using a pharyngeal pump.

Like other plantsucking bugs, aphids and scale insects have a loop of the forward intestine held in permanent association with the front end of the midgut to form a filter chamber, which concentrates the highly liquid diet.

The waste, called honeydew, is conspicuous as a clear sticky liquid that rains down from the trees. It coats leaves, bark and objects beneath the plant. Honeydew is easily seen on car windscreens and lawn furniture. A black sooty mould fungus sometimes grows on the honeydew.

Throughout Dunedin, aphids are attacked by adults and larvae of ladybirds, mostly twospotted

ladybirds (Adalia bipunctata) and 11spotted ladybirds

(Coccinella 11punctata).

Lacewing larvae, called aphid lions, also control aphids.

Aphids can often be removed from shrubs and garden plants with a highpressure garden hose.

The encrustations of dry honeydew on a car windscreen can be removed by first softening it by wiping the windscreen with acetone, an organic solvent, and then scraping it off with a hardbacked razorblade.