Glasshouse Whitefly (C) Dragonfly a leading supplier of natural pest controls in the UK
Introduction
There are about 500 different kinds of aphids in Britain. The green, black and white flies are the most common, but they also occur as brown, grey, red and yellow forms sometimes obscured by a wax or woolly coat. Aphids reproduce very quickly and populations can quickly get out of control, and although they can do a great deal of damage, it is only on certain plants that they become a serious problem. Aphids have one friend in the garden ‑ the ant. Ants feed on the honeydew secreted by the aphids, and an obvious invasion of ants is often a symptom of severe aphid attack on your plants. Ants will protect the aphids, often killing predators and even moving the aphids to fresh shoots. If this is happening in your garden, destroy the ant nest or use tree banding grease to prevent their access to the aphids. The most obvious method of control is by using a chemical spray, but other methods will be also be discussed in this Fact Sheet, which deals with the three most troublesome aphids to gardeners ‑ the white, green and black fly.
Whitefly
Whiteflies are very small insects of unmistakeable appearance. The adults are 1/20th inch long and look like small snowy white moths, the wings and body being covered by a white dust. They are usually found on the underside of the upper leaves of the host plant. If disturbed, they flutter about in large numbers. The young, sometimes called scales, can also be found on the underside of the leaves. They are very small, flattened and oval and, being almost transparent, are very difficult to see against the green of the leaf. Only the later stages of scales take on a thicker appearance and are covered with a white, waxy secretion. Both young and adults suck the sap of the host plant and spoil the foliage with honeydew, on which the familiar black sooty mould grows. Several species of whitefly are known, but the most common is the Glasshouse Whitefly.
Greenhouse Whitefly (Traleurodes vaporariorum)
This is very common on a range of glasshouse plants throughout the country. It is a most serious pest, often feeding in very large numbers on Chrysanthemums, Cinerarias, Heliotrope, Primulas, Salvias, some species of Begonia, Geraniums, Gerberas, Poinsettias, Fuchsias and ornamental peppers. It often attacks tomatoes, aubergines, melons and cucumbers. Outside it may attack a whole range of plants from French beans to Dahlias. During milder winters whitefly may survive on a range of host plants provided a suitable host survives the winter. Reinfestation can occur from this source, Infested plants lose their vigour and the foliage may become mottled with yellow, even wilting and dying in severe cases. The plants are made unsightly by honeydew and sooty moulds.
Life History
Reproduction is largely parthenogenic (unfertilized eggs hatch), the eggs being laid as long as a suitable host plant is present. Alternatively, adults will hibernate from October to May, when new plants are present. The female lays about 200 eggs in circular groups on the undersides of leaves at the rate of 8‑10 each day. The eggs are pale green when laid, but darken just before hatching. The time from laying to hatching is around 9 days at 70F, shorter at higher temperatures. Pale green larvae that hatch from the eggs are called scales. These move around the leaf in search of a suitable food site, settle down and remain in the same position until they become adult. As they mature, a waxy white coating develops. From hatching, the scale goes through four stages of pupa development to adult in 18 days. This takes considerably longer at lower temperatures. All stages of the life cycle are usually present on the host plant.
Cabbage Whitefly (Aleyrodes protetlla)
Cabbage, Brussels Sprouts and other brassicas are attacked by a different whitefly from the greenhouse species. Leaves are fouled with a sticky honeydew and sooty mould infestation, often severe on allotments, especially in southern England where various brassica crops can provide all year round breeding grounds. Although very similar in appearance to the greenhouse whitefly, this species is different as it will only live on brassica plants, and it can survive severe winter weather. Breeding takes place in spring/summer and autumn/winter. When the temperature falls, adults and pupae over winter on infested plants.
Control
Pull up and burn all severely infested plants to prevent carry over to next year's crop. Examine young plants and spray at first sign of infestation. Remember to spray undersides of leaves, short term contact killers can be used regularly. Avoid using systemic chemicals near to harvesting as this can taint the brassicas and cause illness to those eating the crop. Always read the labels and use carefully. Sprays based on pyrethrum, use only as directed and avoid use near harvesting.
Rhododendron Whitefly (Dialeivodes chittendenii)
Infestations are seen in June and July when adults congregate on the young leaves of Rhododendrons only. They do not attack Azaleas. Smooth leaved Rhododendrons are most susceptible, especially R. ''Bagshot Ruby' and R. 'Mount Everest'. This pest comes from the Himalayas and was accidentally introduced on plants in 1926. It is now well established in Southern England. Adult females lay eggs in June and July before dying. The eggs soon hatch, and the scales feed on the leaves for nearly a year before emerging as adults in May. Control is by applying a spray based on malathion, are effective.. Two or three applications at 14 days intervals are recommended during June and July.
Azalea Whitefly (Pealius azaleae)
Pale green scales infest the underside of leaves, sticky honeydew and sooty moulds accumulate on the upper leaf surfaces. Adults are present on young leaves during summer, usually confined to evergreen azaleas. This pest is also seen on Rhododendron mucronatum. This pest was first seen in Edinburgh in 1931 on imported plant material. Young scales overwinter on leaves and adults emerge in early summer, producing only one generation per year. Spray thoroughly with permethrin, pyrethrum in June and July to control adults.
Viburnum Whitefly (Aleurolrachelus jelinckii)
This pest primarily attacks Viburnum tinus, but is also seen on Arbutus unedo. First signs of attack are usually sooty moulds on the upper surfaces of the leaves, and close examination of the undersides will reveal young scales and pupae. These appear blackish with a white frill. Introduced in 1936, this species is particularly abundant in southern England. Only one generation per year is produced, and scales and pupae overwinter on the leaves of host plants. Adults emerge in June and July. The eggs are laid and the adults then die. Control by spraying with pyrethrum, in June and July to kill adults. There are other species of whitefly which can be encountered from time to time, include those which live on Lonicera (honeysuckle) and Phillyrea. Life cycles and control are basically as above.
Control Measures
Chemical Control
This is possible using a number of chemicals, but is often not successful because of immunity or resistance being built up by the whitefly. Chemicals available today kill either by contact (the spray must touch whitefly which have waxy coating) We no longer have true systemic insecticides which make the plant poisonous to the insect pests and all others feeding on the host The latter types causes serious problems so far as food crops are concerned and set periods of time must elapse between spraying and harvesting the crop. These will be clearly stated on the chemical container and must be adhered to. The killing of adults only means that repeated applications of sprays are necessary to ensure that freshly emerged adults are killed before they are able to lay eggs and so break the reproductive chain. Should any female be allowed to lay even one batch of eggs, then controlling sprays will be needed for very much longer until those emerging adults are killed. In reality, this means spraying every few days for 4‑6 weeks until no eggs or nymphs are left, and many people do not spray thoroughly enough. Different chemicals have varying degrees of effect
Selecting the Chemical
Please remember that there are some plants, which cannot be safely sprayed with certain products mentioned above. Read the labels carefully before buying any garden chemical and check that it is suitable for the plants to be treated. We have not tested all the products given in the list, but I find that products based on Permethrin is effective in my own greenhouse and garden.
Read the label thoroughly. Chemicals are available in various forms such as powder or puffer packs, aerosols, smokes or liquids. Decided which is best for the infected plant and simple for you to apply. Calculate how much you are likely to need. If it is only the odd plant, which is affected, then buy the smallest size or a ready to use container.
If buying smoke cones, it is necessary to calculate the volume of the greenhouse and pick an appropriately sized cone or cones. Under new laws, new symbols are given on some product labels indicating that special precautions are required. This is to draw your attention to a possible hazard, it does not mean that it is a new or greater hazard.
Applying the Chemical
If applying through a sprayer, make sure it is clean and useable. If in doubt, wash out and test with clean water. Read the manufacturers' instructions thoroughly. Calculate how much water and chemical is required. Always put the water into the sprayer first and add the chemical to the water. Ensure that they are thoroughly mixed together. Follow ALL the precautions recommended on the label, wearing gloves, mask etc. if necessary. Apply in a logical manner and work evenly over the whole area, avoiding spray drift, remembering to treat the undersides of leaves etc. After spraying, wash out the sprayer and wash the hands thoroughly. Where edible crops are sprayed, put up warning notices i.e. "Crop Sprayed ‑ do not eat/touch for ........ days", followed by the date, if applicable, although most modern insecticides approved for food crops in the UK have no exclusion period and are perfectly safe. Though washing before consumption is always recommended
Storage of Chemicals
Keep all garden chemicals well out of reach of children and pets, preferably in a lockable cupboard or other secure place. Keep in the original packaging and tightly closed or re‑closed. Do not store in the greenhouse. Diluted sprays should NOT be stored, and dispose of any surplus safely. Aerosol and ready‑for‑use sprays are, of course, safe to store providing the usual care is taken to keep them away from children etc.
Non-Chemical Control
There are alternative methods to control aphids which do not involve the use of chemicals, but they are often time-consuming and sometimes less effective.
Hygiene - Ensure that only clean, healthy plants are brought into the garden and greenhouse. Avoid leaving plants about out of season where the pests may over winter. Keep weeds under control, as these often act as alternative hosts. Clean and spray greenhouses against over wintering adults and eggs.
Physical Control - In cases of slight infestation, the physical killing of pests or the burning of infected leaves is worth trying. However, this is not practised once the pest is established and on a large scale.
Natural Control - Nature provides her own controls, such as hard winters, lack of suitable hosts, predators etc. Low temperatures during summer will slow down breeding cycles. Aphid infestations will eventually succumb to predators, parasites and diseases, but because the aphids increase, so rapidly the damage is often done before the natural enemies take control. Before spraying, check that it is really necessary. Check to see that the aphids are still moving and that not just the shed skins of aphids are left on the leaves. If they are still active, encourage their natural enemies including ladybirds, hoverfly larvae, lacewings, flower bugs, predatory midges and parasitic wasps. Spiders and birds are also effective predators, but they will turn to other sources of food when the aphids run out. In fruit orchards, even earwigs are important as aphid predators. Encouraging birds, especially tits, into the garden in winter, will help to control aphids as they will also eat aphids, which are over wintering on trees. Early flowering plants such as Convolvulus tricolor and Hypericum calycinum will provide nectar and pollen for hover flies.
Migrating aphids are often attracted to single, isolated plants. Mixed plantings are less liable to attack than neat rows. Mulching vegetables with aluminium foil may confuse the aphids and prevent them from landing. Aphids prefer soft young shoots and excessive use of high nitrogen fertilizers should be avoided.
Biological Control - It is possible to obtain supplies of a parasitic wasp that destroys the nymph stage of the life cycle and gives a cheap and chemical free method of control. However, there are a number of serious drawbacks and are summed up as follows:
a) The wasp must always have a supply of whitefly to lay her eggs on. This may mean bringing in whitefly as food !
b) Wasps die or escape, and therefore it is necessary to keep reintroducing wasps.
c) The wasp originates from South America and will not survive in temperatures below 56F (13C). Therefore, in winter, control in all but the warmest greenhouses is not practical.
d) The wasps are easily killed by the use of chemicals applied to control other pests and diseases.
In commercial greenhouses it is quite practical to overcome the drawbacks and have effective control, but for the amateur, however, the problems are rather more difficult to solve. However, for those gardeners wishing to try this method of control, supplies of the wasp are available from various online biological control suppliers.
RHS List of Biological Control Suppliers Links to RHS website
PDF of List of Biological Suppliers View a PDF version here
Alternative Sprays - A potassium salt soap is available as a liquid to be sprayed onto the infected plants. Savona (manufactured by Koppert) is non-persistent and therefore will not give 100% kill. It is however safe to humans and wildlife (including bees and ladybirds). Soft soap for use as a wetting agent in combination with other organic sprays or as an insecticide in its own right can also be obtained from suppliers in the UK such as Harrod Horticultural. Amazon.co.uk offer a wide range of insecticidal soap sprays and also offer Neem Oil. This is widely used in countries such as New Zealand Australia and India, but there seems some doubt as to how the oil is processed, meaning some formulations make actually be unsafe to use in the UK under current legislation. However, it can be applied as a shampoo for humans and dogs in this country, some clarification is urgently needed. Neem oil is available from many UK suppliers, these include Neem Oil UK, The Natural Gardener, Soapy water can also be used as a spray to dislodge the insects, but there is some doubt as to the efficacy of this method for heavy infestations, but is not strictly legal under the Food and Environment Protection Act since any chemical used in the garden MUST carry a government number indicating that it has been tested and approved.
Other Natural Sprays
The following may be legal to use
Vegetable oil spray
Diatomaceous Earth is made from a sedimentary rock created by fossilized algae (diatoms), and which is a rather abundant resource (diatomaceous earth is said to make up 26 percent of the earth's crust by weight)
Garlic sprays a number of commercial sprays are now based on specific extracts from Garlic and are highly effective.
Chile Pepper spray Similar to garlic spray, Chile pepper spray is a great homemade natural insect repellent that can be used for a variety of different pests. Chile spray can be made from either fresh hot peppers or Chile pepper powder.
Tomato spray Tomato plants are part of the nightshade family, and as such, contain alkaloids such as the aptly named "tomatine," which can effectively control aphids and other insects.
All in one spray To make it, purée one bulb of garlic and one small onion, add one teaspoon of cayenne pepper powder and let steep for an hour. Strain the mixture and add one tablespoon of liquid soap and mix well. To apply this homemade insecticide, spray it full-strength onto both the upper surface of the leaves, and the undersides, and store the remainder in the refrigerator for up to a week if desired.
Flying Insect Traps - These sticky cards can be suspended in the greenhouse where they will attract insects, including the whitefly. These are particularly effective if placed in position in the greenhouse BEFORE any aphids are noticed. Hang cards just above the top leaves of the plants to be protected. These are now widely available.
Greenfly
This is the commonest and most serious of all rose pests and of many other garden plants. The aphids suck the sap from their leaves or young stems, often crippling the growth. Infested leaves may cockle or curl up completely when badly attacked, making the efficient application of sprays difficult. Greenfly can spread virus diseases, and the honeydew created by the insects is soon covered by a black fungus (sooty mould). Like whitefly, greenfly multiply rapidly (up to 60 offspring per week from a single aphid). There are numerous species of greenfly, some of which produce live young only produce males and subsequent laying of eggs in autumn to over winter on host plants.
Control
Greenfly can be controlled in much the same way as whitefly, using contact and/or systemic chemical sprays. There are several preparations on the market specifically for the eradication of greenfly/blackfly.
Blackfly
This species of aphid is distinguished from the familiar greenfly by its black colour. It is also known as the Black Bean Aphid because it so frequently attacks young broad beans. Chrysanthemums, dahlias, runner beans, spinach, beetroot, turnips and rhubarb can also be affected. The blackfly suck the sap from the plants, crippling them and sometimes even killing the plants. They also spread virus diseases.
Control
Once again, the methods used to control green and whiteflies will also kill blackfly, but when spraying food crops be certain to check that you are using a suitable chemical and do not forget to allow the recommended period to elapse before harvesting the crop. Follow instructions given earlier in Fact Sheet for methods of control. Attacks on broad beans can often be prevented by pinching out the tips of the plants, as soon as the first beans begin to form.
Approved Insecticides List
The Royal Horticultural Society offers an up-to-date list of insecticides approved for home or amateur use. All insecticides on the list should not require special clothing or certification to use them. However, you should always follow the manufacturer's instructions in full to prevent issues to our or our plants' health or damage to our environment.
Here is the link to the RHS List of approved chemicals on their website
Here is the RHS List of approved chemicals as at 2nd April 2022 in PDF format, and you can download it
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The information given in this Fact Sheet is provided in good faith. It is however of necessity general information and advice on the topic. Howard Drury will not be under any liability in respect of the provision of such advice and information, and you are strongly advised to seek independent advice on any particular gardening problems or queries you may have, preferably from experts who can (when appropriate) inspect the problem before providing advice.
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JHD//02/04/2022
