Scale insects ....

 I recently discovered that the orchid on my desk is infested with these tiny scale insects Coccus hesperidium. Each one looks like a miniature tortoise, about 3-4mm. long and tightly attached to leaves.

The tell-tale symptom was the sticky secretion on my desk under the leaves - scale insects suck sap, like aphids, and excrete honeydew.

When I took a close look it was easy to see how the infestation had built up so rapidly. Under the heat of the microscope lamp these infants, known as 'crawlers', emerged from under their mother. The females reproduce parthenogenetically, producing about 1000 nymphs during their three month life span and sheltering the young under their shield.

When you flip a scale insect over you can see the hollow cavity which acts as a nursery for the nymphs. Each is less than 0.25mm long

Nymphs are flattened and are very active, but as they begin to develop their broad shield they settle in one spot to feed.

Here's an older individual, where you can see the shield beginning to grow outwards around the insect. The waxy shield makes these insects impervious to most insecticide sprays - so they are difficult to control. Suffocating them under horticultural oil sprays or picking them off laboriously with a paintbrush dipped in alcohol are really the only effective treatments for infected plants.

This nymph, which is about 3mm. long, has settled to feed. It's probably about a month old.

After two months they reach maturity and begin to reproduce, and then their shield grows darker as they age.

Scale insects infest a very wide range of plant hosts. You can read more about them by clicking here.