Greenhouse Pests and Diseases


It is very difficult to talk about pests without sounding pessimistic but the fact remains that you will be very lucky if sooner or later you are not invaded by at least one or two of the following. The secret is to examine plants regularly and spot the pests before they start to multiply to plague proportions. Learn to recognise which ones you are dealing with in order to be able to pick the correct chemical to control them from the armoury available on the shelves. None of us likes using chemicals so it is vital to follow the directions given with the product. This is essential for your safety, the safety of other members of the family and pets, the environment and not least the poor plant. Be particularly careful to get the dilution rates right. In order to make the chemical as effective as possible pay great attention to the directions that tell you how often and at what intervals to repeat the spray. This is relevant to the period between laying and hatching out of the pest eggs. If you have sprayed, which kills most of the adults, it is necessary to follow this up again and probably again to kill any more that have hatched out subsequently. If these are allowed to develop and lay their own eggs you are really back to square one.

For those intent on an organic basis for their garden including the greenhouse there is also advice for biological control which for a mixed collection of plants is not as straightforward as it may seem.


Aphids are a great nuisance but are relatively easy to kill. Often the first indication of their presence, sucking the sap usually on the young leaves and shoots of the plant, is stickiness and white skins on the leaves below. It is usually what the layman would call ‘greenfly’ which attack greenhouse plants although blackfly sometimes join in. The most peculiar aphid I have ever seen is a strange greyish waxy looking effort which attacks the new shoots of Eucalyptus species. The nice thing about controlling aphids is that there are specific aphicides which only kill aphids and are thus more in harmony with an organic way of thinking. A thorough spray should be 100 per cent effective, but do make sure the undersides of the leaves are well covered by spray.

Whitefly are extremely difficult to get rid of as they seem to be immune to so many of the chemicals we use. They also breed so much faster in warmer temperatures which makes it difficult to pinpoint a correct spraying interval accurately. They are seen as small white flies dancing around the plants and settling under leaves to suck the sap. Their eggs and young stages or scales can be seen stuck to the undersides of leaves. When spraying against these in a large greenhouse I used to wear bright yellow plastic protective clothing. Whilst this might have repelled everybody else it had the opposite effect on whitefly which loved the colour and followed me around in clouds despite the lethal spray. Thus I was not surprised to find that a non toxic bright yellow greasy stick was being sold to attract and trap whitefly. Unfortunately it only seems to work when your plants are clean and the odd marauding fly comes along. Otherwise one has to shake the plants in order for the whitefly to fly up and notice their favourite colour.

Red Spider Mite

Red Spider Mite is a horrible pest as it, like whitefly, has become immune to so many of our chemicals. It is also very difficult to see until a lot of damage has been done to the plants they attack. Signs to look out for are very fine yellow speckling of the leaf surface followed by leaves appearing brittle and scorched looking. In advanced stages of infection webbing appears on the leaves and stems by which time the plant is probably not worth saving. The mites themselves are not the easily visible, bright red mites one often sees running around on walls and paths individually. They are only just visible and rather than red seem to be a pale brown colour with two dark spots. Mites and their eggs can be seen by using a magnifying glass and tend to cluster on the undersides of leaves. They prefer hot dry conditions so keeping plants humid and damped down, particularly under their leaves, will help ward them off especially during summer. My only advice with spraying is that more than with any other pest it is vital to spray several times at the correct interval.

Tarsonemid Mite

Tarsonemid Mite is a pest I do not really want to tell you about as it is virtually impossible to control with the chemicals available. Fortunately it rarely occurs but it is best to burn infected plants and start again. The pest is completely invisible to the naked eye. They live and feed in the growing point of the plant which, damaged, leads to distorted growth. Leaves and often flowers are twisted and have bronze russet-like scars. Plants likely to be affected include members of the family Gesneriaceae such as Streptocarpus and Gloxinia, Cyclamen, Celosia and Aphelandra. Symptoms can sometimes be mistaken for spray damage which can have the same effect of distorting the growing points. If you suspect infection find someone with a microscope and inspect dissected shoots tips for nasty little glistening mites.

Mealy Bug

Mealy Bug are frequently mistaken for woolly aphid by those who are more used to growing fruit trees than greenhouse plants. The small greyish-pink flat bugs secrete around themselves a mealy or wool-like protective covering. Sometimes the bugs are easily seen beneath the wool or crawling around at various different sizes. Sometimes the wool conceals batches of pinky-orange eggs. A large variety of plants are affected including Hoya, Stephanotis, bulbs, cacti, Codiaeum and Passiflora. Small infestations can be controlled to an extent by dabbing at the pest with a brush soaked in methylated spirits. When using sprays it is important to blast the mealy bug rather than spray at them as the chemical has got to penetrate the mealy wool to do any good.

Root Mealy Bug

Root Mealy Bug are similar but occur feeding on the roots. If a plant becomes sick looking for no apparent reason knock it out of its pot and you may see the tell tale signs of wooliness on the roots and clinging to the sides of the pot. The treatment is to mix up a bucket of chemical at spray strength and submerge the whole root ball into the solution. Alternatively, drench the compost with the solution. Streptocarpus and cacti are often affected.

Scale Insects

Scale Insects are a similar nuisance to mealy bug and are often treated by the same chemicals. Look for clusters of yellow or brown scales on the stems or leaves of the plants, especially along the veins. They vary in size but the largest ones are often hard and conceal eggs. Smaller ones tend to be very soft when you scrape them off. These can also be treated by using methylated spirits. Any spray used against them also has to be applied with force to penetrate the scale and get at the pest lurking beneath. Orchids have a type of scale which is white and looks more like mealy bug at first glance. Plants they like include Citrus, Platycerium ferns, Coffee plants and Ficus especially F. benjamina.


Thrips in my experience are not an everyday pest of the greenhouse although those in the know say they are on the increase. Damage is seen as flecking and silvering of leaves, buds and flowers. The yellowish-brown long thin larvae feed off the plant but pupate in the soil and hatch out as little flies sometimes called ‘thunder flies’ which have characteristic thin fringed wings. Most likely to be infected are Begonia, Chrysanthemum, Cineraria and Zantedeschia.

Leaf Hoppers

Leaf Hoppers are really only a problem with the Primula family, in particular P. malacoides, P. obconica and P. kewensis. It is difficult to spot the pests as they move around and a plant showing the characteristic coarse whitish mottling on the leaf surface may not still harbour the pest. Occasionally the whitish dead skins of the insects can be seen which is why they are referred to as ghost flies. Adults are pale and jump when disturbed. Control is difficult as the sprays do not always find the adult.

Leaf Miners

These are not usually a serious pest but disfigure the leaves of plants they feed on. First signs of infection are the white spots made by the female fly as she lays her eggs in the leaves of plants such as Chrysanthemum and Cineraria. These eggs hatch into small larvae which mine between leaf tissues, eating as they go. The effect is to leave white squiggles in the leaf. Eventually these pupate inside the leaf. With small attacks it is possible to effect control by squashing the larvae and pupae which can easily be seen as bumps in the leaf. This is actually quite satisfying. However, for more widespread attacks chemical controls have to be resorted to.

Tortrix moth caterpillars

Tortrix moth caterpillars are not a common greenhouse pest but where they are a nuisance they can be most frustrating. I once had a lovely collection of begonias to look after and often, just as they were coming into flower, I would find flower buds and leaves rolled up with webbing which if unravelled showed a small greenish-yellow caterpillar inside. On exposure the caterpillar would always wriggle rapidly backwards and get lost somewhere on the floor robbing me of the satisfaction of stamping on it. Squashing the caterpillars is really the most effective method of control as it is difficult to get spray on to them. I never found a systemic insecticide which proved effective and although smokes would have some effect it is sometimes unwise to use them in very mixed collections.

Sciarid Flies/Fungus Gnats

These are the bane of those who like to sow pots of seed early, cover them with polythene and then, instead of good germination, get a nasty film of algae and moss on the surface of the pot followed by small opaque little maggots which turn into small black flies. Of course these do not restrict their attack to pots of seedlings but will live in wet compost at the surface of any pot. For the most part these larvae will feed on rotting organic matter in the soil or in dead plant matter, for example a cactus rotting as a result of overwatering in winter. However, they will sometimes feed on plant roots causing damage. The control is to spray with malathion over pot surfaces, capillary matting and wherever the adults are flying.

Vine Weevil is an annoying pest because you rarely know your plant is under attack until it collapses and dies. Carry out a post mortem and you will find the potting compost infected with creamy-coloured larvae with brown heads which have eaten away at the roots of the plants and any tubers they can get their teeth into. Think carefully about any other plants that might be infected and take the precaution of drenching the soil with a spray strength solution of a product containing HCH. The adult weevils are small black beetles, characterised by their projecting ‘snout’ and elbowed antennae. Should you find one of these do not hesitate to drop it on the floor and tread on it. For a small beetle they make a loud crunch. Plants liable to be attacked include Cyclamen and Kalanchoe blossfeldiana.


Slugs are mostly associated with being a pest of gardens but not greenhouses.

Unfortunately this is far from the truth. They are worse the larger the greenhouse and like to lurk under pots by day, emerging at night to eat lacy holes in plant leaves. It is particularly annoying to find a tray of perfectly germinated seedlings decimated with a tell-tale slimy trail giving away the culprit. The best answer is to scatter slug pellets over the greenhouse staging except of course in cases where cats might find them, or if you are lucky enough to have a large greenhouse where birds like to hunt for food. Rather than poison your allies use sprays which will only kill slugs and snails.


Woodlice are an occasional problem so I am told. I rather like them, probably because my aunt used to call them Didemydods and because they roll into balls like small armadillos. I can appreciate, though, the frustration of a greenhouse grower who finds seedlings eaten off at soil level and roots, stems and leaves chewed leaving irregular holes. Feeding is mostly carried out at night with the pests hiding in crowds during the day under bricks, old seed trays, clay pots and other debris to be found in greenhouses in need of a good clean out. Most of the time they feed on decaying matter but if they do present a problem try pouring boiling water where they congregate during the day. Dusting with an HCH product should work. Be very careful that other animals do not pick this up.

Biological Control

This sounds like the answer to everything but is not necessarily as straight-forward. If you only have one major pest problem in your greenhouse, which usually means you grow only one type of plant, then it can be easy.

Taking whitefly as an example the parasitic wasp Encarsia formosa can be introduced and becomes most effective providing the temperature is over 21°F (70°F). The wasp lays her eggs in the scales of the whitefly which then turn black. The adult Encarsia emerges through the top. Clearly, having introduced Encarsia or any other parasite or predator the use of insecticides must be very restricted if used at all. To control red spider mite a larger predatory mite called Phytoseiulus persimilis is used. These also work better at higher temperatures to such an unfortunate extent that sometimes they outgrow their food supply, die out and have to be reintroduced when necessary. They also become confused with their prey as they are not much bigger and are redder than the spider mites. However, they move very fast and are a different shape. The first time I saw the mealy bug predator I nearly went mad because I thought I had discovered a monster strain of the mealy bug itself. This is in fact a type of ladybird which looks like a giant mealy bug. Again, high temperatures are needed for them to work well. There are similarly cunning controls for most of the major pests.

Summing up biological control it is a good idea but new predators will have to be reintroduced at the beginning of each season. Keep a close eye on the pest situation and as soon as they appear send off for the relevant control. Remember to introduce the control here and there all over the greenhouse so they do not have to travel too far to reach all the pests. Occasionally, if the integration of natural control has gone wrong and pests are increasingly unchecked, it may be necessary to use chemicals (though not systemic ones) for a thorough clean up and then reintroduce the predators. Also do not expect instant control overnight after they have been introduced. They are only small and will need time to get to work.

26. June 2013 by admin
Categories: Tips and Advice | Comments Off on Greenhouse Pests and Diseases


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