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Home > Types of Plants > Cacti and other succulents > Pests Plague Cacti and Succulents
Pests Plague Cacti and Succulents
Pests  Cacti From time to time most collections of cacti and succulents develop problems, related to either pests and diseases or to incorrect cultural conditions. If well cared for, succulent plants should grow well and remain healthy, but this may not be easy when growing plants in a climate which is different from that in their native habitat.

Pests and diseases strike large collections in greenhouses and small windowsill collections indiscriminately, irrespective of the experience of the grower. However, a few precautions can reduce the frequency with which the problem occurs. Most pests can be controlled by appropriate treatments, although it is probably fair to say that these problems are never really eliminated from large collections.

A number of pests, diseases and cultural problems are described below with some ideas for control measures. Whatever you use, read and carefully follow the instructions supplied with the product and avoid personal contact with insecticides and other chemicals.

Quarantine

When new plants are acquired, from whatever source, it is a good idea to keep them separate from the rest of the collection for a few weeks so that obvious pests can be spotted. This allows time for eggs of pests to hatch and the progeny dealt with. A good way to do this is to maintain a "quarantine" window ledge separate from other plants. Many growers repot newly aquired plants into their favourite growing medium, and this is a good occasion to examine the general condition of the roots and check for pests such as root mealy bugs.

It may seem over-cautious, but many people like to treat their new plant with systemic insecticide while re-potting. This doubtless helps to avoid introduction of new pests into the collection. Repotting with "sterilised" compost which has been heated sufficiently to kill insects, larvae and eggs is a good idea.

Pest control measures

Many common pests can be controlled by use of systemic insecticides, contact insecticides, insecticidal soaps and, in some cases, natural predators. Systemic insecticides are very effective as they are absorbed by the plant, making its sap poisonous to the pests. However, they are also toxic to people and absorbed through the skin in the same way. Dimethoate is an effective ingredient of a systemic insecticide if you can find it, but unfortunately it has generally been withdrawn from the market, presumably because of worries about potential harm to people. Plants can be watered with a systemic insecticide a few times during the growing season as a preventative measure.

Contact insecticides such as Malathion can also be effective, but only at the time of application and all parts of the plant must be covered. Unfortunately, Malathion is toxic to Crassulaceae and some other succulents.

A range of insecticidal soaps are also available, and some people swear by spraying with diluted washing-up liquid (a few drops in a litre), which at least is fairly harmless.

It is worth noting that repeated use of insecticides can select for resistant insects among any survivors. It is not yet clear whether resistance will develop to the new insecticidal soaps. This can be avoided by ensuring that treatments are as thorough as possible, so there are no survivors and by using more than one insecticide in rotation.

Biological controls are available for some pests but are incompatible with insecticides. Use one or the other. It is difficult to obtain a predator/prey balance that allows long-term protection in a small collection, and I would question the efficacy of biological controls in a small glasshouse, but it may be worth experimenting with these if you dislike using insecticides.

Mealy bugs

Mealy bugsA very common pest of cacti and succulents, and potentially a huge topic ! There are many species of mealy bugs, but these insects are all small and hard to identify by amateur growers. Their host-plant range and individual sensitivity to control measures are poorly characterised anyway. There are probably several species of mealy bug going around collections in the UK and elsewhere. From time to time one certainly sees mealy bugs which "look different".

The insects are small and grey or light brown and so difficult to see among the spines of cacti. Their general appearance is reminiscent of tiny woodlice about 2-3 mm long. A squashed mealy bug often leaves a characteristic red stain: the cochineal insect, from which a food colourant is made, is a type of mealy bug. Recently, a species that leaves a green stain has appeared in the UK. Mealy bugs often accumulate to feed on the tender tissues at or near the growing point. Very often, when nesting, they hide around the base of succulent plants, just below soil level or under the old dried leaves of Mesembs such as Lithops.

The first sign of a problem is often small balls of white fluff on the plant, on cactus spines or around the base or under the rim of pots. These are actually where the females are nesting up inside the white fluff and producing young, which may be either born live or produced from eggs. There may also be some sugary honeydew produced by feeding mealy bugs, which can encourage black mould. Ants "farm" mealy bugs for their honeydew secretions and may help to spread them through the collection, so it is a good idea to discourage any invading ants even though they are not intrinsically harmful to the plants.

Control of mealy bugs: If there are only numbers of mealy bugs to be dealt with, dabbing a little methylated spirit (industrial alcohol, denatured alcohol) will kill them. Some people also spray their plants with methylated spirit diluted at least 1:3 with water. If you try this, remember that the fumes are potentially toxic and flammable and the liquid could harm the epidermis of delicate plants.

For large or widespread infestations, use regular applications (weekly for several weeks) of insecticidal sprays (read the label to find pests controlled, use and precautions). Wash off as many of the mealy bugs as possible with a high pressure water jet from a sprayer, and treat the plant with a contact insecticide such as malathion (not for Crassulaceae) or a systemic insecticide containing dimethoate, taking precautions to keep the insecticide off your skin and avoiding inhaling the spray. Adding a drop of washing-up liquid may help the insecticide to wet waxy surfaces and penetrate into all crevices. A single application will often not be sufficient to eliminate all the insects and their young. In a bad case, total immersion of the plant in a bucket of insecticide with a couple of drops of washing-up liquid to help wetting of the soil, will get the majority of the mealy bugs including root mealy bugs. The plant will need to be carefully dried out after a soaking and is at risk of rotting where damage from mealy bugs has occured below soil level, so although effective, this can be a "kill or cure" method.

Some fumigant smoke cones are also effective against mealy bugs, and have the advantage of being a dry treatment, but require repeated use to be really effective. Give the cone a good shake before igniting to reduce the risk of poor burning, place on a non-flammable surface and retire promptly after lighting the blue touch-paper fuse, before smoke emission begins. I like to do a preventative fumigation in the Spring and Autumn when it is too cold to spray or water the plants with systemic insecticide.

Biological control of mealy bugs: Introduce the predator Cryptolaemus montrouzeri, which requires temperatures of at least 70 degree F (21 degreeC). It is difficult to obtain a predator/prey balance that allows long-term protection in a small collection.
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