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MEALY BUGSLike most plants grown under intensive conditions cacti and succulents can be attacked by a variety of pests. Undoubtedly the worst and most persistent problem is the woolly aphid or mealy bug. Whilst most other pests may damage plants this one is capable of very rapidly killing large specimens. There are several reasons for it being particularly dangerous. It breeds very rapidly and can consequently attain large numbers and also quickly acquire resistance to pesticides. It seems to be able to lay dormant on inert material for considerable periods of time breaking out when conditions become favourable. Its waxy and woolly covering can make it difficult for contact insecticides to penetrate to the insect. In addition there are many different species of mealy bugs which have slightly different characteristics and habits which further compounds the problem.
The normal way of attacking the mealy bug is to use a systemic insecticide usually based on a organophosphorus compound. 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. This has been made somewhat more difficult by the fact that some of the most effective types are no longer available on the market as it has not been economic to undertake the more expensive testing which is now required for market approval. Because of this wide variability of what is available in different markets it is not possible to make a recommendation for specific chemicals, it is necessary to check what is available locally.
With some plants it is possible to wash away the bugs in a jet of water perhaps with some wetting agent. With small infections it may be possible to squash or otherwise physically remove the bugs. Some growers use methylated spirits or other forms of alcohol to dab on the insects which removes their waxing coatings and definitely kills them. However there are dangers in damaging the plants themselves with the alcohol.
An old gardeners remedy was to accumulate cigarette butts in water and the resulting nicotine solution was used to kill the bugs. Such home made remedies are now illegal in many countries and also carry dangers of transmission of Tobacco Mosaic Virus to some susceptible plants.
One favoured alternative to the use of Insecticides is the use of biological control methods. These are often used successfully on the commercial scale but it is not easy using the technique on small personal collections. Several different predators can be purchased from commercial sources for mealy bugs, including lacewings and species of Cryptolaemus which is related to the ladybird insects. Both will also help to control other aphid related pests such as scale, greenfly and whitefly. If you have a particularly bad attack of mealy bugs they may well be worth considering.
The drawbacks include:
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. If the bugs are found wash off all the soil and bugs in a jet of water, allow to dry and report in fresh clean soil.
RED SPIDER MITESRed spider mite is on the whole less of a problem than mealy bugs. Only the minority of cacti and succulents are susceptible. In the cacti Rebutias, Lobivias and Coryphanthas are most commonly attacked but it will occasionally also go for Melocactus, Sulcorebutia, some Mamillarias and some of the smaller mexican globular cacti such as Lophophora, Turbinicarpus and Pelecyphora. These creatures are not actually spiders (do not confuse them with the much larger red sand spiders often seen on dry stone walls, which are quite harmless to plants), but actually a mite. The worst feature is that the mites are very small such that, unless you have particularly good eyesight, the damage is the first thing that you may see. This usually appears as brown scarring on the younger growth. Persistent use of appropriate chemicals can kill the pests (again you need to check what is available at your local garden centre). The pest is usually encouraged by hot dry conditions and lack of adequate ventilation. We have found that a more humid atmosphere on its own is insufficient to prevent recurrence but if this combined with the maximum ventilation the problem is quite rare.
It is recommended that you be particularly vigilant for this pest if you have a nearby hedge of Chamaecyparis leylandii which can harbour the pest over winter.
Some of the Mesembryanthemaceae, for example Faucarias, are also prone to red spider mite attack. We believe this may be a different species as the mites appear black rather than red as in the case of the ones found on cacti. However, the damage and recommended treatment are the same.
Certain species of caudiciform succulents are also very prone to similar mite attack on the leaves of their annual vines. Again this is certainly not the same as the cactus species and may be different again from those attacking the Mesembs.
SCALE INSECTSA relatively uncommon pest which is most often seen on Agaves and Opuntias. Direct spraying with insecticide is not very effective because the insects have a hard impenetrable coating. A systemic insecticide which the pests will suck from the plants can be more effective. Sometimes hand removal can be fairly easy and effective.
WESTERN FLOWER THRIPSThis is a new pest to cacti and succulents. These are small fast moving insects which are often seen in the flowers. They can cause distortion of the flowers and lack of fertility but are unlikely to harm the plants. They can best be controlled with blue sticky traps.
SLUGS AND SNAILSThese can be a problem particularly on the more fleshy succulents. Greenhouse hygiene is important. Location of the offenders is more successful at night when they are active. In extreme cases it may be necessary to use slug bait.
ANTSAnts do not actually damage the plants themselves but may introduce mealy bugs or other aphids. In extreme cases nest building activity may upset roots. If necessary a powdered insecticide may be used.
LEAF CUTTER BEESThese have been an annoying pest in our greenhouses for many years. They excavate tunnels in pots to lay their eggs in carefully constructed leaf lined cells. This can cause two problems. Firstly the leaves can initiate rotting of the plant roots and secondly if a large tap root gets in the way the bees are quite capable of boring a large hole straight though it. The remedy is to repot the plants removing the leaf lined cells as soon as this activity is spotted. They are particular fond of hanging baskets which provide an attractive warm dry spot for this egg laying activity.
SCIARID FLIES(Often locally called Mushroom flies or Fungus Gnats)
It is the grubs of these tiny flies which can cause problems. They are rarely a danger to larger plants but can do serious damage to very young seedlings. The problem is almost exclusively one of peat based composts. Yellow sticky insecticidal strips are very effective trapping the adult flies and breaking the breeding cycle.
EELWORMS OR NEMATODESThis pest seems largely confined to relatively warm areas and we have fortunately never seen it. The creatures cause large cyst like growths on the roots which then cause stunting of the plants. Normal insecticides are not very effective. The usually recommended treatment is to put the plant in water at 50C for 15-20 minutes. There are effective chemical treatments which are only available to commercial growers.
TORTRIX MOTH CATERPILLARSThese are an occasional problem with leafy succulents or mesembs. If some leaves appear stuck together with a cocoon, prise them open and you may find a small grub eating the leaves. Hand removal is adequate control.
VINE WEEVILSVine weevils can attack a limited number of succulent genera. Echeverias are the most susceptible and occasionally Aeoniums. The first signs are usually collapse of the stem. If this is hollow with grubs inside, then the stem needs to be cut back to a sound pot and the plant treated as a cutting and planted in clean fresh soil. It may be prudent to incinerate the infected plant material.
GREEN FLYGreen fly is not usually a problem on cacti and succulents. The one area we have experienced it is on the flower spikes of Aloes and Haworthias. The usual chemical sprays are effective or the flower spikes simply removed and destroyed.
WHITE FLYWhite fly also leaves most cacti and succulents alone but we have occasionally had troublesome outbreaks on leafy caudiciform succulents or succulent pelargoniums. Yellow sticky insecticidal strips help control it. The problem will eventually correct itself during the dormant period when the plant has no leaves and is being kept relatively cool.
RottingThe main diseases of cacti and succulent plants are due to various fungal and bacterial rots.
In general healthy, growing plants are quite able to resist infections with these. However they commonly occur as a secondary effect of other problems such as attack by insects pests, physical damage leaving exposed plant tissue or incorrect growing conditions. The single biggest cause is probably fungal rots entering via dead roots caused by poor root aeration.
The best way of avoiding these problems is to provide conditions which prevent their development, i.e., a healthy pest free environment. If they occur and are spotted early it may be possible to save the plant by cutting away all the diseased tissue with a clean knife. In particular look for discoloured vascular tissue which may be red or brown and penetrate some way into otherwise healthy tissue. The knife should be cleaned with alcohol to prevent spreading fungal spores. It may be beneficial to dust the cut surfaces with flowers of sulphur.
Fungicidal chemicals can be used to give some protection but this should only be tried as a last result since they are not effective against the whole range of different rot producing fungi. Young cactus and succulent seedlings are particularly prone to 'damping off' which is a fungal attack. This can be partially controlled by copper fungicides. The other frequently mentioned chemical for this purpose, "chinosol", is suspected of causing some damage to the seedlings and is not recommended.
Deficiency diseasesThe other group of diseases affecting succulent plants are caused by soil deficiencies of various minerals. This may not simply be that the soil does not contain the required elements but that they are not available due to the soil become too alkaline from build-up of salts from minerals contained in the water which is used for the plants. It is useful to know how alkaline your local water is, and if necessary take steps to correct this by adding suitable acid. Unless you are a chemist and have the necessary knowledge it is inadvisable to use strong mineral acids for this purpose. It is possible to use citric acid or acetic acid (vinegar) but my favourite remedy is potassium dihydrogen phosphate which also supplies useful elements. If your local rainwater is sufficiently clean then this is the best way to avoid this problem.
It is more likely that deficiency diseases will shown up in peat based rather than soil based composts. If a plants looks chlorotic or refuses to grow properly it is worth trying fresh compost to see if this solves the problem. If locally available soils are know to have particular deficiency problems it may be worth adding appropriate supplements.