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Mealy bugs [ Horticulture Phytoparasitology]
Synonym: Woolly aphids

Dictionary of botanic terminology - index of names

     
  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, succulents and 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.


An infested plant of Agave

Black Sooty Mould  forms on honeydew secretion.

Ants "farm"  mealy bugs for their honeydew secretions

Cultural precautions
It is easier to keep mealy bugs out of a collection of cacti and succulents than to control them once they are in so it is a very good idea to quarantine new plants to see that they are not introducing pests. As always, examine new plants before adding them to the existing collection. Mealy bugs are visible without any additional help. Root mealy bugs also seem to prefer peat-based mixtures to soil-based composts, although not exclusively.

Manual control
When infestation is not severe it is possible to patiently picking mealy bugs off with tweezers. If there are not that many it is possible picking them off or spraying them off with a jet of warm water otherwise chemical control is unavoidable. Prune or cut the parts where infestation is severe. Pruning is a job which should be executed at sufficient distance from the collection if possible. Watch the wind direction, eggs and first generation nymphs can cover great distances when catched by a breeze!

Chemical control:
Before applying insecticides/pesticides, manual removal of the fluffy nests and most insects is advisable. It greatly increases the chance on complete elimination of the bugs.

  • Mealy Bug - It is essential that the mealy bug is killed promptly but the cotton-wool cover can repel any insecticide sprayed onto it, so often a wetting agent in the insecticide spray is required. The normal way of attacking the mealy bug is to use a contact insecticide  such as malathion (not for Crassulaceae) or systemic insecticide  usually based on a organophosphorus compound ( dimethoate is often recommended) While these can be quite effective many strains of mealy bug have built up some resistance to these and it may be necessary to try more than one type for effective control! Some insecticide such as one containing Malathion can be painted on with a fine brush, kept especially for this purpose. For large or widespread infestations, use regular applications (weekly for several weeks) A single application will often not be sufficient to eliminate all the insects and their young. Before spraying be sure that plant has been well watered a day prior to treating. This will decrease the risk of chemically burning your cactus. Good hygiene is important as mealy bugs love to hide under dead leaves or flowers or other places where you and your insecticide spray cannot reach. In a bad case, total immersion of the plant in a bucket of insecticide will get the majority of the mealy bugs including root mealy bugs.
  • Root Mealy Bug - Remove all soil and destroy it. Wash the roots thoroughly and treat (eventually immerging the whole plant) with the above mentioned insecticide, letting the roots dry after treatment and before replanting in completely fresh, sterilized soil. Always cleanse and sterilize frames and all other items used when replanting. Regular applications (weekly for several weeks) of insecticide watered into the soil are also effective, it's also possible to immerse the plant pot up to the top of the soil in a bucket of insecticide.

Other treatment:

  • Fumigant smoke:  Some fumigant smoke are effective against mealy bugs, and have the advantage of being a dry treatment, but require repeated use to be really effective.  Fumigation are particularly useful  the Spring and Autumn when it is too cold to spray or water the plants with systemic insecticide.
  • Moth ball: As a preventative measure, moth balls (paradichlorobenzene) added to the potting mix seem to discourage infestation by root mealy bug, and probably discourages other insects. However, the chemicals in the moth balls can cause damage to plastic plant pots and are best used with clay pots. 
     

 


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