A number of thrips species may become pests within greenhouses (Parrella, 1999); however, the main species of thrips which infest greenhouse crops are onion thrips (Thrips tabaci) and western flower thrips (F. occidentalis), both species are common worldwide and are a pest of a number of different crops. Thrips are small, elongated, dark-coloured insects which can move rapidly to avoid detection and may be concealed within buds and leaf axils. Identification of thrips is often by the damage this pest creates, which is a characteristic silvering of the leaf surface due to removal of the epidermal cell contents. Western flower thrips are most prevalent in the upper part of the plant, feeding on developing tissue such as growing points, flowers and fruitlets. Apart from causing damage and deformation (Fig. 1 ) to developing plant parts, thrips also transmit viruses such as the tomato spotted wilt virus which can be a significant disease in hydroponic tomato and capsicum crops. Population growth of western flower thrips is optimized at 25°C where the life cycle can be completed within 12–14 days. Thrips are spread via winged adults which can enter via vents, entrance ways and infested plant material such as seedlings and transplants. Blue sticky traps can be used to monitor and detect flying thrips adults to allow for the application of early control options.

Fig. 10.5. Deformation of cucumber fruit caused by thrips.

Thrips are another insect pest which has become increasingly difficult to control due to widespread genetic resistance to a range of chemical pesticide options. Prevention of infestations is vital for control of this pest and includes the use of fine mesh ‘thrips screens’ over vents and use of double-door entrances (Fig. 2 ). Control of thrips with insecticide sprays is difficult as complete coverage of spray material is required and thrips inhabit areas where spray penetration is not always achieved. Thrips develop resistance to chemical insecticides relatively quickly and most control compounds are limited to two to four applications per season. Different chemical classes of insecticides should be rotated to assist with the prevention of genetic spray resistance and combined with exclusion and other control methods. There are currently a number of predators available for biological control of thrips, these include predatory mites (Amblyseius cucumeris) which attack immature thrips and the soil-dwelling mite (Hypoaspis miles) (Parrella and Lewis, 2017). Minute pirate bugs (Orius spp.) may also be used for the control of adult and immature thrips.

Fig. 2 . Thrips screening over greenhouse vents.

Source: A Practical Guide Hydroponics and Protected Cultivation by Lynette Morgan

Image Source: https://www.planetnatural.com/pest-problem-solver/houseplant-pests/thrips-control/

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