Western Flower Thrip resistance: the story so far
By Joanna Wood
Western Flower Thrips (WFT), although non-native to the UK, have been a growing problem in many protected horticultural crops since the 1990’s. As growers are painfully aware this pest has developed resistance to many insecticides. WFT produces multi-generations each year which is why resistance builds so quickly. It has only become a problem pest for soft fruit in the last five years but is now particularly serious in everbearer strawberries. The pest is favoured by elevated temperatures that occur in polytunnel protected crops and recent mild winters have allowed the species to overwinter. When Tracer (spinosad) became available for use on protected strawberries in 2005 it was the only thing that worked and cleaned up the problem. However, the first case of WFT resistance to Tracer on strawberries was confirmed in 2007. This article will discuss the extent of the problem today and what growers should do to combat resistance.
Colin Cater from Landseer has been monitoring sites with Tracer resistant WFT and has come up with some surprising and valuable observations. Up until 2010 twelve sites have officially been found to have resistant WFT (resistance is confirmed when samples of WFT are tested in the lab with doses of up to 10x the normal rate of Tracer and are not killed!) Even at this stage Colin is at pains to say that the whole farm should not be written off. “What we find is that resistance can be confined to a set of tunnels and the rest of the farm is OK, as the pest tends to spread slowly.”
Where does WFT come from?
As WFT is not indigenous, where does it come from? This seems to be the crux of solving the problem, at least in the short term. Colin has noted that on sites where WFT has overwintered, the damage from WFT feeding on strawberry flowers and young developing fruitlets is gradual. However, on sites where there are new plants brought in there is dramatic, rapid and devastating damage. This strongly points to the problem of resistance being brought on to farms from outside sources.
Site history is key to understanding the issue. Firstly the locations of resistance are widespread in England (and probably Scotland) even though we suspect WFT cannot move far on its own. Secondly, Colin says that where the history of Tracer use can be traced: “we suspect field resistance or failure to control may occur after 10 to 12 applications. We believe this to be the pattern where the original population has been susceptible”. Of greater concern is the situation where imported WFT on plants may have already been exposed to Tracer as in this case the efficacy time-line will be even less. Unfortunately the more you spray the more you select out the Tracer resistant strains of WFT from the whole population.
A review of the dynamics of resistant WFT build up on farms delivers two main aspects: a slow build up since 2005 or a quick one from imported resistant populations. Some sites have shown a combination of both. There is some suggestion of a possible effect of local migration on resistance build up on three sites sited close together, but the common theme of the resistance sites studies is that the origin is from propagated plants. Spinosad has been used extensively for years by flower and ornamentals growers. To confirm a consistent theme, in one case where all the plants were from one propagation source, there had been a history of spinosad use;the plants were raised in glasshouses formerly in flower production.
Key aspects of best practice.
The importance of cultural management techniques to reduce WFT resistance is fundamental. Growers should note that WFT pupate and overwinter in the soil and growing media they need to adopt more robust strategies in following seasons. However, indications are that once a site is completely cleared, the risk to adjacent production sites is significantly reduced, on two sites which were devastated by WFT damage and subsequently cleared and taken out of production there were no infestation to tunnels immediately adjacent to those infected sites.
The best advice is to start clean. Ensure the original plant material is WFT free. Make sure you know the provenance of your plants. Minimise host plants for WFT by controlling weeds and mowing grass. Any discarded plant material, used straw or grow bags can harbour thrips and give them some protection over winter, where practical remove such material to increase winter kill by exposing sites. Introducing a more systemic rotation strategy should also be considered.
The latest advice stresses avoiding using Tracer where poor control has been noted in previous season everbearers. In fact don’t apply Tracer early season on sites where thrips are likely to have overwintered. Introduced predator species such as , Amblyseius cucumeris do offer good potential for biocontrol of WFT, ideally they need to be in place early enough to reduce emerging WFT numbers. If WFT populations keep increasing after predator options, growers should then consider applications of Tracer until later in the season.
Growers should consider carrying out a farm risk assessment of production areas to highlight likely trouble spots. The elements to consider are any history of WFT whatsoever and any risk from nearby production. If there is a history, then track back production in terms of the variety and the plant origin; whether there has been repeated use of the same site; the number of applications of Tracer; and the incidence of other pests which might necessitate use of an insecticide likely to disrupt predators. It is also important to see if a pattern of migration emerges. The best policy for new everbearer sites is to avoid proximity to known WFT hot spots and to avoid risky propagation material.
Looking to the future Colin Carter says, “the thing that worries me is that even if (and there is nothing on the horizon that I am aware of) a new effective insecticide becomes available, there is a great likelihood that resistance will occur again as there will be such a reliance on this one product.” The best hope for the future is various biocontrol methods now being comprehensively investigated in a HortLINK project started in 2009 with funding from the HDC.
Another interesting aspect of the biology of the pest is that resistance is recessive; it has been shown in Spain where Tracer has not been applied, the WFT population reverts to a sensitive strain, probably because the sensitive gene is dominant and is transferred to the progeny. Colin Carter is monitoring how long this process will take through testing a known resistant UK population.
Summary of key messages