Pest Control

The key to having problem free plants is the constant attention to the hygiene of your plants!

Pests can be a problem all the year round indoors, and from early June outdoors, so one should remain vigilant at all times.

Newly purchased or gifted plants should be kept away from your other plants for a minimum of 14 days, to allow any pest/s to manifest itself rather than risk any imported contamination to them.

During the autumn/winter months, the greenhouse should be completely cleaned and sterilised to destroy any hidden insects or eggs.

Once this is done, ensure that your plants are scrupulously clean before they are placed back inside.

To prevent any pest becoming immune to an insecticide, use different brands of insecticide at each application.

This practice is becoming increasingly difficult due to the limited availabilty of products to the amateur gardener!

Manufacturers of insecticides generally give full details on the packaging of the pests that the contents will control and the mixing instructions, so without exception always adhere to these instructions.

They may also make reference of when to spray, if not, spraying is best done in the evening, preferably on a calm night for outdoor spraying and with the vents closed for indoor, opening them again the following morning to rid the greenhouse of fumes.

Common Pests and Treatment


There are many types of aphids, the most common being greenfly.

Its eggs are normally laid on the underside of the leaves and on the tips of young shoots.

After hatching the young greenfly suck the sap from the plant, causing distortion of the foliage.

As the greenfly grow they shed their skins leaving the white membranes on the surface of the leaves and compost, it is quite common for these cast off skins to be mistaken for whitefly.

The honeydew they produce attracts a black sooty mould that is harmless to the plant but it can look unsightly.

Aphids are known to swarm in great numbers during warm weather descending on to the plants, causing a major infestation.


This can be achieved by using one of the several brands of insecticide available.

Varying the chemical at each spray will help to avoid any resistance.

A little washing up liquid in the water will, allow the insecticide to adhere to the plant better and may also assist with the removal of sooty mould.

For those who don’t want to use chemicals, the introduction of natural predators, such as ladybirds, is quite effective.


Capsid bugs, Leafhopper and Thrips:

These are mainly found on outdoor plants and only occasionally indoors.

Their appearance and lifecycle are similar, therefore it is possible to cover them under one heading.

They are quite small and are generally found in hedgerows and shrubs.

When feeding they inject a poison into the tissue of the plant causing some distortion to the plant and it may even completely stop the growing tip causing the plant to grow blind.


Spray with a suitable systemic insecticide at 10 to 14 day intervals, starting as soon the new shoots appear.

Alternate the brand of insecticide regularly, and continue until the middle of July or until the crown bud appears.

(It would seem the bud is not as attractive as the succulent growing tip).


Froghopper (Cuckoo Spit):

Froghoppers do not do much damage to plants, but the froth in which it lives can be unsightly.

The insect is very small and yellowish in colour.



Wash away the froth by pressure spraying with a suitable systemic insecticide.


Red Spider Mite:

Red spider is difficult to see with the naked eye, and its presence is very often undetected until irreversible damage has been done.

To successfully combat this mite, early recognition of the problem is required.

The symptoms are a very slight silvery mottling on the upper surface of the leaves, and a bronze colouring to the underside.

With the aid of a magnifying glass one can see the mites.

The next stage is leaf fall resulting in complete defoliation.

Normally the tallest plants in the greenhouse are affected first, the cause of this is, the air at the top of the greenhouse is drier and warmer the very conditions red spider mites thrive on.


To avoid infestation of red spider mite, maintain high humidity, cool temperature and good ventilation.

In the event this does not control them, prompt action should be taken to prevent infestations reaching the web stage.

Generally, insecticides have limited effect any suitable ones (mixed to the manufacturer's instructions) should be used alternately.

Smoke fumigation can help.

There are some biological controls available to the amateur grower, but these should not be used in conjunction with chemicals.

The foliar feed Maxicrop can discourage red spider to settling on plants when used at the recommended strength.


Vine weevil

It is the maggot/ larvae of the vine weevil which causes the worst damage to plants, by feeding on he roots of the plant.

They generally remain undetected until the mutilation done, causes the plant to topple over.

Early indications of the presence of vine weevil are stunted growth, i.e. smaller leaves on one plant compared to other plants of the same specie.

Looseness in the compost and semicircular bites out of the edges of the leaves can indicate that the adult beetle has been present, and has probably laid its eggs in the compost.


Control can be very difficult, using nematodes as a biological control is possibly the best method available to the amateur grower.

Apply the predator to the compost during July/August and follow the suppliers instructions, taking care to keep the temperature above 10°C (50°F) for as long as possible to enable the nematodes to do their work.

Another method of control is a non-setting glue smeared around the inside edge of the pot, thus entrapping the adults before they lay their eggs, remove the trapped weevils from the glue daily.

Composts containing an insecticide that assists in mite control by killing off the soil borne larvae are now available.


Western flower thrip:

A comparatively new pest to the United Kingdom, the western flower thrip causes damage to the flower of the plant.

It is a very small thin insect it is easily found in the flower head, but not so easily found prior to flowering.


Pick off all open flowers and destroy them.

Spray the plants with a suitable insecticide at regular intervals. (Follow the manufacturer’s instructions).



The small white, moth-like pest must never be allowed to breed unchecked in a greenhouse for more than a week, ignore this rule at your peril, as whitefly will then become almost impossible to eliminate.


Constant vigilance is the key.

Place yellow sticky traps in convenient places around you plants, these will indicate the presence of whitefly.

Any found on the underside of the leaves should be squashed between the finger and thumb and any scales (eggs) present should be given the same treatment.

Insecticides have limited effect on whitefly.

As a preventative measure use several suitable sprays with different base chemicals to the manufacturer's instructions.

Biological control is available to the amateur grower and may be of some assistance.


Critical times:

Week 3: Red spider mite can be a problem indoors.

Week 5: Keep a look out for Aphids that appear as the greenhouse temperature rises.

Week 13: Soft plant growth will encourage insect pests, so be vigilant, check under foliage, control any insect infestations promptly with a suitable spray.

Week 27: Aphids, Red spider mites and White fly are especially prevalent at this time of the year.



There are three types of man-made chemical insecticides for aphid control.

Contact insecticides:

These are applied as spray or dust and it kills only the aphids it touches.

The natural plant extract insecticides such as derris and horticultural soft soap are examples of tried and tested contact insecticides.

Tar oil winter wash:

This is used only on dormant, deciduous (leafless), shrubs and trees, and kills both hibernating aphids and their eggs, which are hidden in cracks in the bark.

Systemic insecticides:

These are taken up by the plant and kill aphids that suck the sap as well as those directly sprayed.

They can even reach aphids hidden in curled or tightly packed leaves.


Garden insecticides, both organic and artificial, are considered safe for plants, pets, people and the environment providing you follow the rules.

The law requires you to read and follow the instructions on the label.

Check that the insecticide used is recommended for aphids, and that it is safe for your plants before you start spraying.

Apply before damaging populations build up.

Examine plants frequently, especially during March to October.

Protect beneficial insects by spraying in the evening and treating fruit trees only before and after flowering.

Spray only where there are pests.

All-over spraying is dangerous to wildlife, and can cause pollution and wastes money.

Plant Treatments:

Apple: Apply a winter wash on dormant trees, and systemic insecticide after flowering.


Broad bean: Pinch out growing tips after five flower clusters have formed to deter blackfly, and spray with contact insecticide if plants become affected.


Cabbage: Rub off small infestations,and or spray under leaves with contact insecticide.


Cherry blackfly: use systemic insecticide to reach aphid rolled within leaves.


Indoor plants: Wipe off smooth-leaved plants, and or insert systemic insecticide sticks into compost.

Lettuce root aphids: drench soil with suitable insecticide.

Lupins: Spray with contact insecticide as soon as aphid is seen, or destroy plant to prevent spread.

Nasturtium: Rub off, or pinch out infested tips, and or spray with systemic insecticide.

Rose: Soap based insecticides are quite effective, and or use systemic insecticide on heavy attacks.

Viburnum: Tar oil winter wash on deciduous types, or use a systemic insecticide in severe cases.

Garden Chemicals and Pesticides

Government legislation has resulted in many chemical treatments that were once available to the amateur gardener being taken off the market.

Some products that are still available may require a users licence.

For this reason the writer has refrained from naming any particular chemical treatments.

Here are a few addresses where you might be able to resolve your problem/s;


Chemicals: using them in gardens.

Health and Safety Executive

Common sense gardening

Just Green

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