Mealy bugs

Dummer. ゛☀
09-24
These are a 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". These 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 where the females are nesting up inside the 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 invasive ants even though they are not intrinsically harmful to succulent 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. Small numbers can be removed carefully with a pin, but it is hard to spot them all.
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. Watering with Imidacloprid (Provado Vine Weevil Killer) seems to be very effective against mealy bugs and has not so far shown any sign of toxicity to a wide range of succulent plants.
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, but it is getting hard to find effective types of smoke cone in the UK.

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