British Cactus and Succulent Society

Highlands & Islands Branch

Plant Management

 

Lacewing - voracious feeder on aphids (courtesy Wikipedia.org)

 

Ladybird larva eating Mealy Bugs
(courtesy Wikipedia.org)

Lacewing over-wintering house
http://www.greengardener.co.uk

Ladybird overwintering house
http://www.greengardener.co.uk

 

An Overview of Insect Pests

This section contains descriptions of most of the pests found in cacti and succulent collections. They are listed below as Mealy Bugs, Root Mealy Bugs, Red Spider Mites, and Other Insects Pests -

Mealy Bugs

The three worst pests, most growers will agree, are mealy bugs (sometimes called the woolly aphid), root mealy bugs, and red spider mites; and there are others of course. Mealy bugs look like small woodlice and vary in size from some which are quite visible to others which require an eye-glass as the illustration shows. Also seen near them are white woolly-like tufts are their nests. It is said that mealy bugs are flightless and that new infestations must come from other plants. Apparently, the nymphal stage mealy bugs drop off plants and crawl to new hosts. Diatomaceous Earth (see Appendix) claims to be effective against mealy bug as well as many other pests. Spraying and/or watering with an insecticidal soap (see Appendix), or a contact and systemic insecticide, based on an organophosphorus compound, may work. Unfortunately, some mealy bugs can, or may already have developed an immunity to such chemicals. One spraying or watering is seldom enough as the eggs will survive and another infestation will appear in a few weeks. So, spray again, and possibly yet again at 3 week intervals.

Small infestations can be treated by washing away the bugs with a jet of water, or even by physically removing them with tweezers. Some growers (me for one) use methylated spirits on a small water-colour-type paint brush Many useful older remedies are no longer available because manufacturers cannot afford the expensive testing required by regulations. The same goes for some old-fashioned methods such as one given by Tony and Suzanne Mace in their admirable book 'Cactus and Succulents', which by the way, is excellent on pests and diseases. They give an old gardeners' remedy which was 'to accumulate cigarette butts in water: the resulting solution then used to kill the bugs' They go on to say that home-made remedies like that are now illegal in some countries, and anyway can bring alternative problems such as tobacco mosaic virus.

A parting shot - apply biological and chemical methods with care by following manufacturers' instructions in every way. For example, read the labels on packages and bottles carefully!!



Root Mealy Bugs

White grubs of the Rhizoeccus spp, about 2mm long, which secrete a white waxy powder. This can be seen coating not only themselves, but also roots and soil when repotting. Although particularly troublesome for cacti and succulents because of rather dry composts, they also infect African violets, ferns, pelargoniums, and fuchsias, and therefore may migrate from any of them. They are difficult to irradicate, even by chemical means, and are therefore a good candidate for biological control.



Red Spider Mites

These tiny, eight-legged sap-feeding mites are less than 1mm in length, and an eye-glass (X5-X10) may be necessary to see them. They are not spiders even though they have eight legs and are Arachnids. The entomologists tell us they have four pairs of two legs. Yellowish-green in colour, with two black markings, but may be orange-red in autumn. The spherical eggs are red, and the fertilised females hibernate in winter in sheltered places.

The two-spotted, or greenhouse mite is a world-wide pest on practically all crops, both indoors and out. With a favourable temperature (about 80F) and high humidity (over 90%) the egg-to-adult stages takes about 8 days. In least favourable conditions (10-20C) it takes about 28 days. Another odd feature of a mite population is that there are usually three times more females than males. A female will lay about 100 eggs in a life-time, and they over-winter as fertilised females. This means they can get well ahead of any predators and parasitoids before they arrive.

That explains why they can even be troublesome outdoors in this country in warm, dry summers. There are several species the most common being the 2-spotted, or greenhouse variety we know, Tetranychus urticae. Whether the Fruit Red Spider Mites or the Conifer Red Spider Mites attack cacti and suculents is not yet known, nor is it known whether they cross-breed.

What can be done to avoid, or at least minimise, the possibility of red spider mite trouble. Firstly, they like dry hot conditions so a measure of humidity will not be to their liking. Spraying at intervals in the greenhouse will help, as will putting plants outside in suitable weather so that the formidable forces of nature can apply, particularly hover-flies and our friend the wasp. Thirdly, encourage the wasp family to patrol your greenhouse by leaving them alone, and even putting out bait to attract them.


Other Insect Pests

Some other insect pests which may be candidates for biological control are -

Sciarid Flies
The Bradysia spp, generally known as Mushroom flies, or Fungus gnats, are particularly troublesome for cacti seedlings and can "wipe out a complete sowing in no time at all" says John Pilbeam. They breed in peat-based composts where their tiny black-headed, white larvae eat the roots of plants, particularly young roots. The adult fly often flies close to one's face, probably attracted by carbon dioxide in our breath. I use a simple trap of a small dish, particularly a yellow one, with some water in it, in which they seem to drown.

Scale Insects
These small, brown limpet-like insects come in at least three varieties. The common one is the 'softscale' or Coccus hesperidum. Next is the 'mussel scale' or Lepidosaphes ulmi, and thirdly the 'horse chestnut scale' or Pulvinaria regalis.

Western Flower Thrips
This is a new pest on cacti and succulents. It is a fast moving, lemon-yellow to dusky-brown insect -

 
 
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