Identification
Most adult thrips are about 1 – 2 mm long, slender insects that possess piercing-sucking mouthparts. Barely visible to the naked eye, they have long fringes on the margins of both pairs of their long, narrow wings. Their juveniles (called nymphs) are slender and elongate and lack wings. Most thrips range in colour from translucent white or yellowish to dark brown or black. Out of the more than 6,000 species of thrips, the western flower thrips (WFT) tops the ‘least wanted’ list among flower growers worldwide as the most damaging and hardest to control.
Biology
Knowledge of thrips biology and damage are important in understanding the challenges associated with developing a sound management programme. A thrips life cycle consists of an egg stage, two nymphal stages, two pupal stages, and an adult. The life cycle generally takes two to three weeks to complete. This, however, is temperature dependent with the optimum range between 26oC and 29oC during which the life cycle may be completed in 7 to 13 days. Adult females can live for up to 45 days laying 150-300 eggs.
Thrips management is extremely difficult due to several biological characteristics. Thrips eggs are inserted into leaf or petal tissue and are thus protected from insecticides. The eggs hatch into nymphs which usually remain protected in flower buds or foliage terminals. They pass through two nymphal stages, both of which feed in these protected areas. Toward the end of the second nymphal stage, the insects stop feeding and move down into the soil or leaf litter to pupate. The thrips pass through two ‘pupal’ stages (prepupal and pupal), during which no feeding and little movement occurs. While in these pupal stages in the soil, they are protected from insecticides directed at the crop
Adults emerge from the soil and feed in protected areas of the plant such as flowers and terminals. Adults fly readily and can be carried on wind currents or on clothing to greenhouses near infested fields. They can fly from sprayed to unsprayed areas or move in or out of a greenhouse through doors or vents.
Damage
Thrips nymphal and adult stages feed by piercing plant cells with their mouthparts and sucking out the cellular contents. The damage to plant cells can result in deformation of flowers, leaves, and shoots. There is often silvery streaking and flecking on expanded leaves. Thrips often deposit tiny greenish-black faecal specks on leaves when they feed. Crops with high levels of Nitrogen are attacked more due to abundance of amino acids and proteins; female’s productivity is also enhanced. Besides direct feeding damage, the western flower thrips has the ability to cause indirect damage by transmitting the tospoviruses: impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TMSV) to a wide variety of greenhouse plants. Both direct and indirect damage may result in huge economic losses to greenhouse producers.
A part from the cryptic (hidden) habitats (unopened terminal and flower buds) highlighted above, other reasons that make thrips difficult to manage in greenhouses include their broad host range, high reproductive capacity, rapid life cycle (egg to adult), small size (2mm long), and resistance to insecticides. As such, the only way to effectively deal with thrips in greenhouse production systems is by taking a holistic (integrated) approach by implementing a variety of strategies.
Start clean
Making sure the young plants are clean is an important part of keeping a crop thrips-free. Transplants should be inspected for thrips before being placed in a greenhouse. Granted, this may be too labour intensive and time consuming especially when large quantities of plant material are involved but sticky card traps placed among the new plants for a day or two will quickly indicate the presence of thrips.
Exclusion
Staying clean would involve excluding thrips from the greenhouse as well as eliminating sources of thrips infestation. Where feasible, covering the openings to greenhouses is a very effective delivering a reduction of up to 70% in pest problems. Screening to exclude thrips must be very fine. Such screening, known as microscreen, has a maximum hole size of 192μm (0.037mm2). This, however significantly reduces air flow when placed over vents, and growers must modify ventilation systems to compensate for this.
Weeds and plant debris
Weed control is essential for a successful thrips control program. Certain weeds, particularly those in the Composite and Solanaceae families, and those with yellow flowers, attract thrips adults serving as refuge both for thrips and tospoviruses. As such weeds must be removed from both inside and around the greenhouse. It is also essential to immediately remove plant debris and bloomers from the greenhouse or placed into containers with tight-sealing lids, and not in the open as is common practice in most farms. Thrips adults will abandon desiccating plant material for the main crop.
Mass trapping
Mass trapping through the use of yellow or blue sticky traps placed just above the crop canopy is another method that has been shown to be effective in reducing population of flying insects (pests and beneficial). Research has shown that blue sticky traps catch more thrips than yellow ones, and in the same vein, stop the wearing of blue or yellow clothing in the greenhouse!
A recent development in the area of mass trapping is the use of sex aggregation pheromone lures that increase the number of thrips adults captured on sticky traps by attracting thrips out of their hiding with the added advantage of increasing their exposure to insecticide spray applications.
Other cultural and/or physical management strategies that may be implemented include overhead irrigation/showering/misting, which has shown to decrease thrips populations by creating an unfavourable environment for thrips; placing a barrier (e.g. polythene) underneath plants on beds thus preventing thrips from entering the soil to pupate; and use of trap or lure crops, which are plants that are more preferred by thrips thereby attracting thrips away from the main crop. Such plants may then be spayed with an insecticide, removed from the greenhouse, or inoculated with biocontrol agents. Marigold (Tagetes spp) has been shown to be very attractive to thrips.
Monitoring
Monitoring is important for early detection of a thrips infestation; determining the numbers of thrips present in the greenhouse; detecting seasonal trends in thrips population over the year; and in assessing the effectiveness of management strategies implemented. Yellow or blue sticky cards are the easiest way to monitor for thrips. Gently tapping the terminal buds and flowers onto a white piece of paper and using a 10x magnifying lens to examine the insects that fall out is an easy way to check for thrips. Be sure to record population levels.
Biological control
Biological control of thrips relies on the use of natural enemies including predatory mites and pirate bugs, entomopathogenic (or insect-killing) fungi, and entomopathogenic nematodes.
Predatory mites
The predatory phytoseiid mites, Amblyseius cucumeris, Iphiseius degenerans, and A. swirskii, are well suited for immature thrips control on greenhouse crops. Like thrips, they prefer small niches where contact between predator and prey is maximized. These predators feed on pollen when thrips populations are low and must be introduced before a thrips population has built up to damaging levels. The mites establish themselves on leaves, usually on the undersides, and are most effective in attacking 1st instar thrips nymph. They use their mouthparts to pierce the thrips and suck out the cellular fluids.
The predatory mite Amblyseius cucumeris (AMBLYTECH C®) regulates thrips populations by feeding on the 1st and 2nd instar nymphs. It should be applied on tender shoots and applied frequently based on pest pressure.
Another predatory mite, Hypoaspsis miles resides either in the soil or growing medium feeding on the pupal stage and should be applied on moist soil/medium.
The predaceous mites will establish themselves on a crop, mate and reproduce in the greenhouse. These mites are susceptible to many insecticide sprays. Contact your supplier for specific information on compatible pesticides and favourable environmental conditions.
Pathogens
Several pathogens have been investigated for control of thrips. The entomopathogenic fungus Beauveria bassiana (BEAUVITECH®) has been shown to be very effective in managing thrips populations in cutflowers where relative humidity is high. The fungus is capable of infecting both adult and juvenile thrips. Frequent usage will ensure the fungus is present on most crop foliage affecting juvenile and adult thrips. After application allow 24 hours before spraying a fungicide. The product is compatible with most insecticides.
Entomopathogenic nematode Steinernema feltiae (NEMATECH S®)is effective in infecting the soil-dwelling prepupal and pupal stages. The nematode also kills sciarid flies larvae in the soil.
The key to implementing a successful biocontrol programme is to release biocontrol agents early enough in the cropping cycle. It is important to note that natural enemies will not regulate an already established or existing high thrips population because it takes time from release before natural enemies will lower thrips populations below damaging levels. Moreover, natural enemies will not eradicate (neither will insecticides) thrips.
Chemical control.
Chemical insecticides play an integral role in thrips management programs, more so because the tolerance of thrips damage on greenhouse-grown ornamental crops is very low. Insecticides with contact or translaminar activity are generally used to control thrips, because systemic insecticides typically do not move into flower parts (petals and sepals) where thrips normally feed.
High volume sprays are typically required to reach thrips that are located in hidden areas of plants such as flower buds. Most available insecticides have no activity on eggs and pupae. Repeat frequent applications are therefore warranted to kill the life stages that were not affected by previous application. However, frequent applications may lead to the development of insecticide resistance, another elephant in the room of thrips management, and possible plant injury (phytotoxicity).
In conclusion…
Thrips has been, and still is, a difficult insect pest to control or regulate in greenhouse production systems leading many to believe that we have reached an impasse regarding its management. Dealing with thrips, therefore, requires a holistic approach integrating the methods detailed above. Key in this approach is knowledge about the biology of the pest, and indeed all other relevant pests, the crop (s), the cropping systems, pest management options etc. Dudutech’s range of sustainable pest management solutions are backed up by training courses covering these very topics. Knowledge, we believe is the best weapon against pests!
John Ogechah is the Training Manager at Dudutech
Anthony Rono is a Senior Technical Liaison Officer at Dudutech.