Photo 1. Silvery-white feeding patches on the leaves of shallot from infestations of the onion thrips, Thrips tabaci.
Photo 3. Colours of onion thrips, Thrips tabaci, vary from yellow to brown, through green; compare Photo 2.
Worldwide. Asia, Africa, North, South and Central America, the Caribbean, Europe, Oceania. It is recorded from American Samoa, Australia, Federates States of Micronesia, Fiji, New Caledonia, New Zealand, Northern Mariana Islands, Palau, Papua New Guinea, Solomon Islands, Tonga, and Vanuatu.
Onions are a preferred host, but infestations also occur on onion relatives, beans, brassicas, carrot, cotton, cucurbits, legumes, papaya, pineapple, potato, tobacco, tomato and many ornamentals. At least 25 plant families are infested.
Thrips mouthparts are beak-shaped; the left mandible is much larger than the right, and forms a narrow stylet used to pierce the cell wall of plant tissues. They suck out the contents of cells, leaving air-filled spaces beneath the surface, giving the plants a silvery (and in some case a silvery-bronze) look in sunlight (e.g., on capsicum, cucumber and leek). It is a common symptom on leeks, onion and shallots (Photo 1). The feeding produces distorted leaves (e.g., bronze blisters on cabbage), discolouration (e.g., capsicum flowers), withering from water loss (e.g., onion) and death (especially seedlings of onion and cabbages).
Thrips are parthenogenetic, that means females do not need males in order to produce fertile eggs. Eggs are white, kidney-shaped, about 0.25 mm long and 0.12 mm wide. Males occur, but they are rare. The eggs are placed singly in slits in leaves and flowers. They hatch in 2-4 days, and the larvae - they look like small adults without wings - are yellow, and group together in leaf axils.
The larvae moult once, and then when fully formed drop to the soil and develop into pupae. The adults emerge a few days later; they are yellow and brown with red eyes (Photos 2-4), and about 1.5 mm long. The entire life cycle, from egg to adult is about 11 days at 30°C.
Thrips are not strong fliers, but they move short distances (a few metres) from dying weeds to crop plants, and over long distances (tens or hundreds of kilometres) when large numbers are caught in warm updrafts. Spread also occurs with the international trade of pot plants and cut flowers.
The adults and young (larvae) do the damage. Reduction of yields is common and death of entire crops can occur. In the USA, 50 adult thrips per onion results in a 50 per cent loss of yield.
Tiny black faecal specks occur on leaves and flower parts and make plants unattractive to consumers, and this adds to the economic loss to growers.
Indirect damage occurs as the thrips spread Tomato spotted wilt virus, a serious disease of many vegetables, especially tomato, capsicum and lettuce (but not onions). Weeds serve as an important reservoir for the virus. Symptoms are purple-brown blotches on stems, ring patterns and blotches with haloes on leaves, and discoloured flowers. Young stages of the thrips acquire the virus within 15 minutes of feeding on infected plants and, a few days later, when adult, they transmit the virus, and continue doing so for the remainder of their lives.
Look for thrips on the leaves or in the buds and flowers. This is usually done by shaking, tapping or washing the plants.
Many natural enemies of Thrips tabaci have been reported, but few, if any, are specific to the pest. Predators include bugs, beetles, lacewings, flies, mites and predatory thrips, and there are wasps and fungal parasites. Some predators consume large number of thrips each day, but many also feed on other insects, and give them higher priority. IPM programs for greenhouses use yellow sticky cards for scouting, sanitation, commercial preparations of the fungus Verticilium lecanii, predatory mites, lacewings, and strategic use of insecticides.
White onion varieties are more tolerant of thrips than red. There are varieties of tomato and capsicum that have resistance to Tomato spotted wilt virus.
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
Information from CABI (2014) Thrips tobaci (onion thrips) Crop Protection Compendium (http://www.cabi.org/cpc/). Photos 1&2 Rehan Silva University of Queensland.
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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