Photo 2. Spotting on poinsettia leavers caused by western flower thrips, Frankliniella occidentalis.
Photo 3. Mottling on tomato fruits caused by Tomato spotted wilt virus transmitted by the western flower thrips, Frankliniella occidentalis..
Western flower thrips
Worldwide. Asia, North, South and Central America, the Caribbean, Europe, Oceania. It is recorded from Australia and New Zealand, but not from any Pacific island country.
Wide; it has been recorded on more than 250 plants in 65 families, although it is not sure if it breeds on all these or just feeds on them. Some examples are: soft fruit (plums, peaches, strawberries, grapes); flowers (Gladiolus, Impatiens, Gerbera, Chrysanthemum, poinsettia); vegetables (cucumber, tomato, capsicum, cabbages, beans), both in the field and in greenhouses. Many species of wild flowers are hosts.
Damage is caused by thrips in two ways. Direct damage results from feeding. The adults and nymphs have modified mandibles that puncture the cells of flowers and leaves to release their contents which they then suck up. Foliage becomes silvery, leaves and flowers become flecked, spotted and deformed (Photos 1&2), buds fail to open, scarring occurs on fruits of capsicum, cucumber and beans, and undersides of leaves show small black specks of faecal material.
Indirect damage is caused by infection of crops by viruses. Thrips spread Tomato spotted wilt virus (TSWV) (Photo 3). Symptoms of the virus vary with host, plant age, and temperature. The virus causes significant damage to vegetables in the Solanaceous family, such as tomatoes (Photo 3), potatoes and capsicum, but also lettuce. Many weeds, too, are hosts.
The eggs are kidney-shaped and laid in the flowers or leaves. The nymphs are pale yellow, thin and wingless, up to 1 mm long (Photo 4). They begin feeding immediately after hatching. There are two nymph stages. Towards the end of the second, the nymphs move down the plant to pupate in soil or in plant litter. There are two pupal stages and during this time the thrips are inactive and do not feed. Adults emerge a few days later; they are thin, ranging in colour from yellow through to light brown, 1.5-2 mm long, with two feathery wings (Photo 5). The life cycle varies from nine to more than 40 days in Australia, depending on temperature (Diagram).
All stages shelter in crevices or between touching leaves or flower parts. The length of the life cycle and life expectancy of the adults depends on temperature and also on the quality of the food. At 30°C the life cycle is about 12 days, while at 20°C it is about 20 days. The females live up to 90 days, whereas males live for about half that time.
Short distance spread is by flight; thrips are weak fliers, but often assisted by wind. Long distance spread is with infested plants associated with the horticultural trade or contaminated equipment.
Direct damage results in lost yield and/or inferior prices, as damage is unsightly - common in roses, strawberries, beans, capsicum and cucumbers.
Indirect damage by thrips as a vector of TSWV is common in lettuce, capsicum and tomato. TSWV is a tospovirus spread by western flower thrips, onion thrips (see Fact Sheet no. 117) and melon thrips (see Fact Sheet no. 106); however, the western flower thrips is the more important vector. TSWV has a very wide host range, and the only thrips that transmits the virus in a persistent way. This means that once the thrips picks up the virus through feeding, it retains it for life. Interestingly, the nymphs have to pick up the virus for the adults to be able to transmit it. Thrips need only a few minutes of feeding to transmit the virus.
Some countries have produced figures for the estimated costs of western flower thrips and TSWV. In the Netherlands, for instance, the annual cost of western flower thrips was put at US$30, with another US$19 million for TSWV. Furthermore, the estimated crop damage by all tospoviruses transmitted by western flower thrips exceeds US$1 billion per year.
Look for discoloured, deformed new growth and buds - when thrips feed on developing tissues, the cells are unable to expand and mature leaves and petals become distorted. Look for scaring on fruit, particularly noticeable on capsicum, and look for leaves which have a silvery appearance. Shake or tap flowers and shoots over white paper. Use yellow or blue sticky traps. Blue traps have the advantage that they are not as attractive to non-thrips species.
Natural enemies include Orius, Geocoris and Nabis species and also the larvae of lacewings, but all these are general predators. Also, predatory mites (Transieus and Amblyseius species) and predatory thrips (Haplothrips) are common, but do not adequately control thrips populations, except under greenhouse conditions, where they are used as part of IPM programs.
Western flower thrips is more difficult to control than other thrips species because it develops rapid resistance to pesticides. Cultural control options aim to prevent infection and minimise spread.
There are resistant varieties of cucumber and tomato to TSWV.
If thrips cause physical damage to the crop then insecticide sprays may be needed. However, there are problems using pesticides to control thrips. First, the insects are hidden within flowers and the leaves of shoots; secondly, the eggs are inserted into the leaves making it difficult for sprays to reach them; and thirdly, thrips rapidly become resistant to insecticides, so much so that there are large differences in the susceptibility of thrips populations to commonly used products.
Use insecticides as follows, but note that frequent use of broard spectrum synthetic insecticides may also lead to development of insecticide resistance in thrips populations:
AUTHOR Grahame Jackson
Photo 1 T Smith, University of Massachusetts.Bugwood.org. Photo 2 L Pundt, University of Connecticut. Photo 3 William M Brown Jt., Bugwood.org. Photo 4 Whitney Cranshaw, Colorado State University, Bugwood.org. Photo 5 Jack T Reed, Mississippi State University. Bugwood.org. Diagram Life cycle western flower thrips. University of Massachusetts.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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