British Cactus and Succulent Society

Highlands & Islands Branch

Biological Control

Mealy Bugs ©Tony Mace
Scale insects ©Tony Mace


Biological methods are self-regulating to some extent and there are at least three approaches. One can introduce a predator, a parasitoid, or a pathogen. It should be said that the best time to introduce any of these measures is early in the outbreak, otherwise the countermeasure may not develop enough advantage to overcome the outbreak. Secondly, these measures take time, and there is a delay, so they are not immediate solutions. The easiest choice is the predator. In most cases these are tiny wasps which operate by laying an egg inside the body of their prey. They do this by inserting an ovipositor which is an extremely thin 'hypodermic-type' needle based in their tails. The egg hatches and the emerging grub then eats the host alive, pupates inside it, and eventually emerges through a hole it makes in the dead body. An astonishing piece of research from Rothamstead Experimental Station in Harpenden, has found that parasitoid wasps can detect the 'smelly feet' of Ladybirds. Otherwise, their larvae developing inside an aphid might be eaten as well as the aphid.

It is worth saying that these parasitic wasps are harmless to animals, including humans, as they do not have a sting! Another useful servant in the war on insect pests is the Common Wasp (Vespula vulgaris) which will pick an aphid, or any other small insect, off a plant, and take it back to feed its larvae in the wasp-nest. The Common Wasp is incredibly effective in this way and it is reckoned that a fair-sized wasp colony (up to 20,000) may collect 10,000 small insects per day. By the way, all wasps are meat-eaters!

For many years I have practiced positive conservation by not persecuting wasps. I leave them alone as long as they do have a nest right under my nose, so to speak. They will not sting unless provoked. Above all, do not harm the biggish wasp (probably a queen) which wanders about in Spring inspecting your property, but chase it if you think it is going to initiate a colony too close by. In the season it is even sensible to put some sugary substance such as fruit, jam, and the like, to attract wasps. If you have a 'pest' in your greenhouse, the wasp will spot it before you do!! So encourage them. Some people provide nesting places for that fantastic superfamily, the 'digger wasps'. They dig a hole, then find a fat caterpillar, place it in the hole and lay an egg on it, and then seal up the hole. Eventually, an adult wasp pupates, digs its way out, and the cycle starts all over again.

Parasitic Wasps and Beetles

Aphidius spp
This is a species of tiny black parasitic wasps which give their attention to aphids. These slender 2mm predators lay their eggs inside an immature aphid. They do this by inserting an ovi-positor into the body of the aphid and so deposit an egg. The egg hatches and the larva eats the aphid alive, pupates, and emerges through a hole it cuts in the body of the then dead aphid. The parasitised aphids can be recognised by their dark swollen bodies. Supplied as adults wasps or parasitised aphids. See also Appendix for wasps and Aphidoletes for aphids

Cryptolaemus montrouzieri
Here we have a beetle from Australia. It is a dark-brown/black ladybird about 4mm in length with an orange head and tail. The larvae is white and about 10mm long and looks like a mealy bug. Adults and larval ladybirds both eat adult mealy bugs and their eggs. When mealy bugs are scarce they will go for scale insects, and will even eat each other when nothing else is available. Another behavioural habit of Ladybirds, it is said, is that they prefer higher-growing plants, rather than low ones of about 1 foot (30cm).The bad news is that they are not efficient on cacti, and no good at all on root mealy bugs. They are supplied as adult beetles which will fly away if not contrained by a guaze cage.See also Leptomastix for Mealy Bugs

Delphastus pusilis
A tiny black beetle (Ladybird) which attacks white-fly adults, nymphs and eggs. However, being a winged, it needs to be constrained by a guaze cage. Furthermore, it requires a temperature range of 59-70degreesF(15-21C). Supplied as adult beetles.
See also Cryptolaemus (above) and Encarsia (below).

Encarsia formosa
A 1mm black and yellow wasp which lays its eggs in the nymphs of whitefly. For best results it needs good light and warmth. Success in an application is indicated by whitefly corpses which turn black. For serious infestations it is recommended that both Encarsia Formosa and Delphastus pusilis be applied. Encarsia is supplied as parasitised whitefly scales.

Leptomastix dactylopii
A parasitic wasp which only attacks certain species of mealy bugs (). It does in the usual way of laying an egg in the bug. This wasp is also sensitive to environment in that it needs plenty of sunlight and a temperature above 25degrees C (77F). Also, this wasp works well in conbination with Cryptolaemus montrouzieri

Metaphycus helvolus
A 2mm wasp which parasitizes scale insects. Eggs are laid inside the victim but only when these are young. Supplied as adults and release must not be delayed. Needs two or three day-time hours at a temperature of 72degrees F (22C).

Mites and Midges

Amblyseius cucumeris and Amblyseius mackenziei
These tiny 1mm mites and their flattish, pear-shaped bodies are brown. They attack the nymph stage of the thrip and can survive on other insects when their is no more prey. Available in a bran-mix can which can be hung on plants allowing the mites to emerge. A humid environment is beneficial but no direct sunlight.

Aphidoletes aphidomyza
It is the larvae of these 2mm slender midges which feed on aphids whilst the midge itself feeds on the honeydew secreted by the aphids. The larvae are easily recognised as orange-red maggots about 3mm in length. They are delivered as pupae but only between April and September because they are sensitive to day-length and will become dormant as day-length shortens.

Hypoaspis miles
Another 1mm long soil-dwelling mite which feeds on the larvae of sciarid flies, where both adult and larval mites feed on the larvae of sciarid flies (mushroom flies). It seems that Hypoaspis is most useful on plants up to a height of 1 foot(30cm) because Ladybirds prefer to fly to the top ofhigher plants. They are available in a peat/vermiculite mix which should be added to potting composts. See also Steinernema for sciarids

Phytoseiulus persimilis
A very efficient, very small predatory mite which attacks red spider mites at all stages of their development. These predators, themselves, breed very rapidly and are claimed to eat their way through hundreds of red spider mites in their life-time. Physeiulus adults look rather like their prey in autumn colours, being an orange-red, whilst the immature mites are pink with oval bodies. Adults have long legs and pear-shaped bodies. Sensitive to temperature (need around 68 degrees F (20C), and what might be regarded as high humidity, so damping-down and spraying will help.

Galendromus occidentalis
One of the few parasitoids with a polular name, the Western Predatory Mite is smaller than P. persimilis and develops best at cooler temperatures, but it tolerates a wide range of relative humidities (40-80%). It has the capability of regulating spider mite populations at lower densities and for longer periods of time than P. persimilis, although it will not control spider mite populations as quickly as P. persimilis can. It can also survive long periods without prey. Several different strains are commercially available, including non-diapausing strains that allow control of spider mites through the winter, when days are short, and strains resistant to a number of organophosphate insecticides.


Heterohabditis megidis

This an effective remedy for vine weevil problems. It is a microscopic eelworm which attacks the larvae of weevils. Applied as a water-based solution and needs a soil temperature above 54 degrees F (23C).

Phasmahabditis hermaphrodita
Perhaps not applicable to cacti, but possibly succulents, this microscopic nematode is applied to slugs to which it transmits a fatal disease. They need a moist soil so that they can move about in the water film between soil particles. They are supplied in a clay-mix.

Steinernema species
The same type as above and it too is effective against weevils and also sciarid flies..

Bacillus thuringiensis
Not a worm but a bacterium and the only one I could find so I have not set up a Pathogen group. The bacillus is only applicable to leafy plants which caterpillars will eat. It is not selective and will kill most caterpillars including ones we like such as Red Admirals and Peacocks if their food-plants are watered/sprayed. It is supplied as a toxic protein powder which contains bacterial spores. Caterpillars eat leaves watered with a water-mix and die of septicaemia. Could not find what happens if birds or animals eat these caterpillars.

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