The warm months of the year starting this September are reckoning. Rose growers across the country are already arming themselves to combat the almost imminent upsurge of thrips in their crops. This is because in spite of great advances in insectpest control strategies in horticultural systems, thrips still remains to be one of the most challenging pest to control across the world. In Kenya, Rose growers have witnessed increasing difficulty in controlling the most common and problematic thrips species in Roses, the Western flower thrips (Frankliniellaoccidentalis). Historically, thrips management in crop production systems relied heavily on the use of chemical pesticides (insecticides) that are usually foliar applied. No grower wants to wake up one morning only to find their most valuable varieties have succumbed to a thrips attack at the peak season. Yet this is a reality most growers have to contend with now. In high-value horticultural and ornamental crops, the perceived risks of insect damage have often encouraged growers to make intensive insecticide applications. This of course has come at a steep price, as widespread resistance of thrips to most classes of insecticides including but not limited to, Carbamates, Organophosphates and Pyrethroids used for its management, has been reported.
This high propensity of thrips in developing insecticide resistance has been attributed to its biological attributes. Indeed, thrips have proven notoriously difficult to control owing to their small sizes, high rates of reproduction and cryptic (hiding) habits. A female thrips can produce up to 250 eggs during its life cycle. This quickly leads to great numbers of thrips infesting individual plants. An infestation of thrips may begin through insects being introduced into the greenhouse along with planting material and later in the season, adult thrips may fly from outside a process that is commonly referred as thrips influx. Thrips influx is very common in farms located in areas where crops such as Wheat which are good alternative hosts to thrips are grown. Growers also have to be aware of resident thrips which are basically thrips populations which thrive in the greenhouse in spite of regular chemical sprays. The fact is that some stages of the thrips are not susceptible to insecticide application and as a result continues to multiply and increase. These include the eggs which are laid in the leaf tissue and pupal stages which shelter under soil and debris. The spread of thrips can be both active (flight) and/or a passive process (floating on air currents).
An Integrated Management Approach:
Most Sustainable Tool in Thrips Management
As highlighted earlier, insecticide resistance is just one the issues growers have to contend with in control of thrips. Increasingly, growers are facing more stringent market demands with mounting pressure to reduce the number of molecules and observe maximum residue levels (MRLs) on their produce. This turn of events has of course cast growers in an unenviable situation. The reality is that no single control measure can effectively manage thrips population and this calls for an emphasis on an integrated management approach. In order to counter insecticide resistance in the western flower thrips, several insecticide resistance management (IRM) programs have been developed around the world. The core objective of IRM programs is to maximize the life-span of efficacy for insecticides. Research has shown that successful IRM programs rely on non-insecticidal tactics, such as biological and cultural controls and host plant resistance to reduce population pressures. Also of importance is the rotation among insecticides of different mode of action classes, resistance monitoring, sampling to determine the need for insecticide applications and education to assure proper implementation. Growers need to adopt a proactive rather than a reactive approach in implementation of IRM programs in order to maximize their chances of success. The focus should be on more judicious insecticide use, and this can be accomplished with the development of well-founded economic thresholds.
The Role of Bio-control Agents
One of the main challenges in the use of insecticides as highlighted earlier is the fact that resident thrips population are always present as some stages of the thrips are not susceptible to insecticide application, specifically eggs and pupae. In addition, the risk of influx thrips from outside can catch a grower off-guard leading to surprise losses. To get rid of thrips eggs it’s advisable to use cultural methods such as knocking down damaged & open flowers. Koppert has also recently introduced ENTONEM based on the juvenile stage of the beneficial nematode Steinernema feltiae. ENTONEM tackles the pupal stages of thrips which shelter under soil and debris. It is important to ensure high moisture for a number of days to enhance efficacy of the product. The soil dwelling predatory mite MACRO-MITE (Macrochelesrobustulus) can also be applied on growing media to target thrips pupae. Besides, it also feed on eggs, larvae and pupae of sciarid flies (Sciaridae), preferring smaller larvae.
Predatory mites remain one of the most promising biological control agents for the control of thrips in roses. The generalist predator Amblyseiusswirskii such as SWIRSKI-MITE from Koppert predates, reproduces and develops on first larval instar of thrips. Additionally, it also predates on eggs and initial larvae of both greenhouse whitefly and tobacco whitefly; and is also known to feed on the younger phases of spider mites. Amblyseiuscucumerisis sometimes used to control thrips particularly in vegetable crops that produce a lot of pollen as an alternative food source; otherwise its not advisable for use in ornamental crops such as Roses. Amblyseiusswirskiiis a better option as it develops much more quickly than A.cucumeris if the prey is available. In the near future, Koppert Kenya will introduce LIMONICA (Amblydromaluslimonicus) an even more superior predator than Amblyseiusswirskii which predates on both first and second larval instar stages of thrips.
Role of Mechanical Methods
Although predatory mites will only target the larval stages of thrips, it is important to note that both adult and larval stages contribute to plant damage. A holistic IPM strategy for thrips management should combine predatory mites and mass trapping of the adult thrips with blue sticky traps which have an adhesive on both sides to traps insects that land on it. The traps are not only a key preventive strategy as they trap any adults before they breed, but they also take care of influx thrips before it is too late. Western flower thrips in particular has good sensitivity to UV, green-yellow and blue colour while Onion Thrips (Thrips tabaci) is attracted to both blue and yellow colour. However, it is important to note that the wavelength of colour that a trap emits is key to its efficiency to attract thrips, so the trap must be of high quality. In greenhouses roses, mass trapping of thrips has been achieved by setting up blue sticky traps such as HORIVER-TR from Koppert at a rate of at least 1 card per 10m2 (1000 cards per Hectare). The cards are suspended 30 cm or less above the crop and are distributed evenly throughout the greenhouse. The use of Kairomones (thrips attractant – both males & females) in combination with sticky cards, lures thrips out of their hiding places (e.g. tightly closed plant parts like floral buds and unopened flowers) toward the sticky traps. Growers who combine sticky cards with Kairomones such as LUREM-TR from Koppert have reported that the sticky cards captured 43% more thrips compared to those without.
Growers who have successfully adopted mass trapping of thrips with sticky traps have reported that thrips control has become much easier. They have been able to reduce their thrips sprays by up-to 50%, even during the warm season when thrips pressure is high. The combination of the beneficial nematode Steinernema feltiae and soil dwelling predatory mite MACROMITE (Macrochelesrobustulus)to tackle the pupal stages of thrips, predatory mite such as Amblyseiusswirskii to deal with the larvae stages of thrips and blue sticky cards to trap the adult population drastically reduce the need for frequent insecticide sprays. Most important, growers can expect reduced flower stem losses due to thrips damage and enjoy better returns and increased market access. By Purity Kabuba& Patrick Koome