Thrips are a group of insects that feed on a great variety of materials; there are a number of species that attack cyclamen, making them Enemy Nº 1. They must be fought right from the first stages.
Chemical means of control can be problematical, in that plant treatment may not reach all the individual thrips; or some may be resistant.
Moreover, chemical treatments should be sparingly used, or there may be toxic effects on the plant later.
‘Blue’ sticky traps can be used, so as to keep a check on numbers and intervene before the problem gets out of hand.
Two treatments a week may be needed during the summer period to keep the numbers down.
In the Mediterranean region the application of choice is a damp spraying during the hot season.
Extra Low Volume spraying are to be kept for the flowering period.
Biological control methods may be used also.
Thrips belong to the order Thysanoptera (those that empty plant cells), of the family Thripidae; they are present all over the world. In glasshouses, the species mainly encountered are:
Thrips attack a great variety of foods, and have been found on many glasshouse flower species (140). The species most often affected in France are chrysanthemums, St.Paulia, cyclamen, roses, pelargonium lilies and gerbera.
Thrips’ life cycle has 6 stages:
For all species, the time taken for development depends on temperature. With some simplification we may say that within a certain temperature range (not too hot or too cold) development time varies inversely with temperature. The Californian thrips, for instance, takes 15 days to complete the cycle at 26ºC (79ºF) but 44 days at 20ºC (68ºF).
Above 35ºC (95ºF) the cycle stops entirely.
The eggs are laid individually in the parenchyma cells of young leaves, petals or tender stalks. A female lays between 60 and 100 eggs in the course of her life.
After incubation the larvae appear. They look fairly similar to the adults.
The body is rounded, oblong. They are wingless, and move about by means of their pairs of legs.
The two larval stages feed copiously.
At the end of the larval stage (between 8 and 15 days) they pupate.
The pupae are immobile, often found in dark places, such as in the soil where the larvae fall to pupate. 2 to 5 days later the adults emerge.
The adults are light yellow to brown, depending on the species of thrips. The body is flattish and from 1 to 2 mm long, again depending on species. In general the males are lighter in colour and slightly smaller than the females.
They have 3 pairs of legs and also 2 pairs of wings folded on their backs. These are narrow and pointed, with long hairs. With these bristles they can fly in gliding fashion for (according to species) some seconds or some hours. The females have an ovipostor.
Though it is hard to identify the main species correctly in the field, even with the aid of a magnifying glass, we list here some of the characteristic features of each.
The tobacco thrips is found world-wide, except in polar regions. It lives on cotton, tobacco, rape, leek and onion in the open; in the glasshouse it attacks the cucumber family, tomatoes and a number of leguminous and ornamental species. Historically it was one of the earliest greenhouse pests.
Reproduction is generally asexual, males being rare. The eggs are cream in colour.
The larva measures 0.6 mm, is cream-coloured with a large head and bright red eyes. The second larval stage measures 0.8 mm and is light yellow to yellow-green.
The adult is grey to chestnut, with two pairs of wings folded on its back.The female is from 1 to 1.3 mm long.This species reproduces only parthenogenetically. Development takes 16 days at 25ºC (77ºF), less at higher temperatures. In glasshouses there may be from 2 to 15 generations in a year.
This thrips invades each part of the plant. It carries the TSWV (Tomato Spotted Wilt Virus), which causes bronze mosaic disease in tomatoes.
A pest of many crops: cotton, strawberries, onions, potatoes, oranges, legumes and glasshouse ornamentals. The Californian thrips causes a great deal of damage. It lives on all the plant’s upper parts, and damages especially the growing organs such as shoot tips and flower buds. It is less at home on leaves, though, since it is light-shy; on leaves, it prefers the under side. It will eat many sorts of material, and is hard to eradicate.
This thrips also carries virus diseases. It transmits TSWV (Tomato Spotted Wilt Virus), which is damaging in hot climates, and also INSV.
The Californian thrips is an American import. It showed up first in chrysanthemum cuttings. Its first appearance in Europe was in Germany in 1984; by 1986 the pest had got to the Paris region, Touraine, the Vaucluse and the Var, and nowadays it is reported from all over France and other European countries (Netherlands, Great Britain, Denmark, Greece, Spain, Italy, Sweden,Norway...).
Populations of Thrips Frankliniella occidentalis have as many males as females. They reproduce both sexually and asexually.
The larvae are an orange yellow. The adults can be told apart from Thrips tabaci by the number of segments on the antennae (7 for T. tabaci; 8 for Frankliniella). These adults are light yellow to brown, and 0.9 mm (males) or 1.2 mm (females). The females are also lighter in colour and more hairy. Under glass there may be 5 to 7 generations in a year.
The rose thrips is an occasional pest of cyclamen which is not hard to combat. It affects Europe only. It feeds mainly on flowers and does considerable damage to peppers growing under glass in May and June. On ornamental plants the larvae are generally present between May and September. As early as November the female goes into hibernation, in litter and on the ground. At the end of the winter she reappears and lays, from February on. Males and females are in equal numbers: the male is yellow to chestnut, the female chestnut. The adults are 1.2 to 1.6mm long. The larvae are white to pale yellow. Even in a heated greenhouse the females still have to hibernate from November until the start of spring.
The adult is around 1 to 2 mm long. It moves little. The body, dark brown, is narrow and flattish. The end of the abdomen is somewhat orange. The larva is 1mm long, and yellow/brown.
The female reproduces by parthenogenesis: there are no males. In heated glasshouses, development is continuous; the optimal temperature is between 20ºC ( 68ºF) and 28ºC ( 82ºF). Signs of attack appear between June and October, and many generations can overlap (9 to 15 a year under glass).
This is found on cyclamen as well. The adult is generally a pale yellow. It is very much like the other thrips.
Thrips shun the light and prefer soft plant material. They are to be found mostly in the warm parts of the glasshouse, in flower buds and growth buds which they can reach because they are small; they are also often to be found in young leaves that are not yet unfurled near the plant bulb.
Thrips lesions on row of leaves
In many cases the damage becomes apparent only when it is too late to take effective measures. Damage depends on the part of the plant and its stage of development.
Only larvae and adults feed at all; and they feed, not from the sap, but on the cell contents of young, growing organs.
In feeding, they lacerate the cells of the parenchyma with their distinctive mandible, and inject their saliva into the cell.
As a result, the cell contents begin to decompose (lysis), and the thrips then absorb the contents.
Emptied of their contents, the cells fill with air, lose their colouring and dry out.
With cyclamen, these attacks generally reveal themselves by various symptoms:
The larvae have another bad feature. Those of Thrips tabaci and Thrips Frankliniella occidentalis can, unlike the adults, transmit a virus, TSWV (Tomato Spotted Wilt Virus). When larvae attack a plant which is infected with this virus, they may themselves become carriers of it; the virus is absorbed through the insect’s salivary pump, passes through the gut wall, and is then carried in the haemolymph until it reaches the salivary glands. After a few days’ latency, the larva is capable of passing on the virus: and will be similarly infectious after turning into an adult. The virus cannot, on the other hand, be transmitted to the eggs.
Thrips attack on young leaves
Infestation often originates with the bringing of “carrier” plants into the glasshouse. Adult thrips can also enter from outside, through the various openings of the glasshouse (door, ventilation openings). They do fly, but being so light they are also naturally spread far and wide by the wind. They can also spread themselves around by jumping.
Once infestation has taken place, the individual thrips that manage to evade detection and overwinter in the warm parts of the glasshouse or nearby (in cracks in the ground, under plant debris) will reappear once there is new growth.
Flower damage caused by thrips
Biological control is carried out with the help of the thrips’ natural predators. The ones most often used are mites of the Phytoseidae family in the order Acarina: species of Amblyseius.
These predators develop through:
A fertilised female lays around fifty eggs over 20 days. The eggs are laid on the underside of leaves. The larvae which hatch from them do not feed.
The two nymph stages, on the other hand, are highly mobile and seek food actively.
The adults are feeders also.
Each stage consumes one thrips larva per day.
Adults and nymphs have four pairs of legs (the larvae have three), and are a pinkish beige in the case of A. cucumeris and reddish beige in the case of A. barkeri. Development takes 6 to 9 days at 25ºC (78ºF), depending on the kind and amount of prey available and the moisture content.
The mites can reproduce in the glasshouse provided wide-spectrum insecticides are not used. They are present from March to October; during winter they are dormant.
A. degenerans would also appear to have some effect against thrips. This mite does not become dormant.
The mites pierce their prey (thrips larvae) and empty their contents. Amblyseius cucumeris attacks the first larval stage. Amblyseius cucumeris also feeds on pollen, which has the advantage that they can be released preventively, before the presence of thrips has been confirmed.
Success of these measures depends on the amount of food the predators have available (when hungry, they eat more), on the size of the larvae, on the species of predator and on environmental conditions.
For the operation to succeed there needs to be a great number of these helpers: 100 to 200 individuals need to be introduced per square metre every 8 to 15 days.
There are a number of commercial versions sold by Biobest, Ciba Bunting and Koppert.
Other predatory mites are effective too: Hypoaspis miles is able to attack thrips larvae provided they are on the ground. The commercial versions are:
Bugs belong to the order Hemiptera, sub-order Heteroptera. The ones that attack glasshouse thrips are in the family of Anthocoridae, in particular the genera Anthocoris and Orius. These are feeders with a varied diet (thrips, aphids, mites...).
Bugs at all stages of development (there are 7, of which 5 are nymphal) attack the adult, nymph, and larval thrips and their eggs. They catch their prey with their forelegs and empty them.
Orius bugs are varied feeders (thrips, aphids, mites). In France there are 9 species.
Orius have been introduced since 1991 against Thrips Frankliniella occidentalis; the Orius eggs are placed in the plant tissues. The nymphs are of different colours at various stages, but at every stage the red eyes can be clearly seen.
The adults are generally chestnut brown, even black, with whitish-grey patches.
At 20ºC (68ºF) development takes around 20 days.
Orius recognise their prey by touch. Since they can move from place to place and fly they can seek out new colonies of prey.
Commercial products based on Orius are:
In the case of Anthocoris nemorum the adult is 4mm long and dark in colour, with dark brown wings and a pattern of grey, light chestnut and black patches.
Verticillium lecanii is a mould which occurs frequently and often attacks arthropods, among other things. It is used in association with the predatory mites.
Sucking moths which attack thrips larvae are also mentioned in the literature (Hypoaspis aculeifer).
There is interest among researchers in a number of moulds that damage insects, such as Metarhizium spp., Paecilomyces fumosoreus and Beauveria bassiana. It is the fruiting bodies which achieve the infection: they allow the mycelium of the fungus to regenerate, and this mycelium is able by the use of various enzymes or by physical pressure to pass through the insects’ cuticle. Inside, the fungus proliferates and infects the body of the insect, which dies 2 to 14 days after contact with the spores. All appearances are, following the first results, that Beauveria bassiana is highly pathogenic for many glasshouse pests such as thrips, mites, whitefly, caterpillars and spiders.
Thrips are not easy to deal with during the flowering season because they hide in the flowers and flower buds.
The eggs, laid in the parenchyma cells of young leaves, petals and stalks, are sheltered from insecticides.
The pupae, camouflaged in dark places such as in the ground or the underneath of pots, are also hard to get at with plant hygiene treatments.
What makes chemical measures comparatively hard is the inaccessibility of the thrips. Moreover, the Californian thrips arrived in Europe with a resistance to many pesticides already acquired. The different families of chemical must be used in rotation or resistances will appear.
The constant development of the regulations and homologations of phytosanitary treatment products, and the differences in regulations according to each country make it impossible for us to include updated information on homologations. Each producer will have to contact his local plant protection bureau to obtain the latest updates concerning the regulations and use of phytosanitary products. We strongly advise testing beforehand on a plant sample in order to measure the chemical’s activity (establishing the dose) and any effect on the plant (plant poisoning).
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