Mealybugs

Mealybugs is the common name for some among the most destructive plant pests, they are scale-like insect coated with a powdery mealy waxy secretion; that feed on plant juices and are destructive especially for greenhouse ornamentals, succulentsand fruit trees.

Mealy bugs (or woolly aphid) are certainly the worst and more common insect that attack cactus and succulents, They can live on the plant or on the roots in the soil and are capable of very rapidly killing large specimens. Mealy bugs belongs - like other scale insect - to the Coccoidea superfamily and reproduce very rapidly laying their eggs underneath a cotton-like elliptical covering so they can consequently attain large numbers and also quickly acquire resistance to pesticides. They are small (about 1-3mm) and have a characteristic loose, hairy and waxy cover used to build their nests (depending on the species) and retain well-developed legs and thus remain mobile, even as adults, unlike most other scale insects.
This means that they can easily spread and infect neighbor plants!
They return over and over again. In addition many different species are found on cacti  (mostly the citrus mealy bug = Pseudococcus (Planococcus) citri) Mealy bugs are polyphagous and are known to inject toxins into the epidermis and spread pathogens and viruses. Plants like Asclepiads are known to be easily infested with root mealybug and by the time you have noticed the infestation it is too late. Root mealybug attack the roots just below the level of the soil, especially where the root and the stem meet.

Symptoms:

• No parts are of the plant are immune to their attack. The infested plant appears covered with small insects, distinguished by a grey/white cotton-wool type spot or covering, and so difficult to see among the spines of cacti, mostly hidden at the base of the plant at soil level, or accumulate to feed on the tender tissues at or near the growing point.
• Nests appear like a tuft of small waxy filaments (fluff like).
• Plant surface are covered with sticky colourless drops, better known as honeydew.
• Frequently a  sooty Mould (black mould) forms on honeydew secretion on the plant surfaces.
• Infested plants will stop growing, take on a sickly appearance with distorted stem and grow point deformation and start to shrivel.
• A crushed mealy bug leaves a characteristic red stain.
• Leaves are reduced in size, discoloured, wilted and easily drop prematurely.
• Regular visit of ants. Ants breed and protect  mealy bugs for their honeydew secretions and may help to spread them through the collection, so to discourage any invading ants even though they are not harmful to the plants.
• Weakened plants often succumb to fungi and rot.
• A particular species of mealy bug attacks the roots of cacti. This form will be seen as white patches on the roots when repotting a plant. If a plant is unaccountably sick and not growing, take it out of its pot and examine the roots. Sometimes also hidden at the outer side of the pots, at the underside of the border.
     

Some people manage to control and get rid of them but in other collections they seem to last forever. In fact their waxy and woolly covering make difficult for contact insecticides to penetrate to the insect. They seems also able to lay dormant on inert material for considerable periods of time breaking out when conditions become favourable.