Resistance in UK populations of Tuta absoluta
The importance of varying the chemistry used for treatments against any pest species must not be underestimated because continuous selection pressure from one chemical group can lead to resistance. Tuta absoluta seems to have an extraordinary ability to develop systems that break down certain types of chemicals.
The three insecticides used in the original UK IPM programme (i.e. spinosad, chlorantraniliprole and indoxacarb) were from different Insecticide Resistance Action Committee (IRAC) Mode of Action Classification Groups and, together with the biological control agent, should have formed a robust resistance management strategy.
In February 2015, a British tomato grower reported concern over poor results with spinosad against Tuta absoluta on his nursery. There soon followed similar reports from three other British tomato growers in other parts of the country. At about the same time, a Scandinavian grower reported poor results with spinosad in a population recently inherited from a Spanish supplier.
One British grower had also experienced a treatment failure with indoxacarb in a Tuta absoluta population believed to have arrived on produce imported from Italy.
Resistance to spinosad
There can be many reasons for poor results from an insecticide application other than resistance to that product.
After investigating other possible explanations, the troublesome populations were tested in laboratory bioassays at Rothamsted Research and this was followed by more in-depth molecular studies at Exeter University. The results confirmed resistance to spinosad in three Tuta absoluta populations at levels that would be expected to compromise the efficacy of the insecticide when applied at the field rate. A further population had lower levels of resistance, while some exhibited full susceptibility. Although resistance declined in the Scandinavian population in the absence of further spinosad selection pressure, this did not seem to be the case with the UK populations.
The team at Exeter have now identified the molecular basis of the spinosad resistance in the UK Tuta absoluta populations.
Even where UK growers still obtain acceptable results from spinosad, it is recommended that they reduce the risk of selecting resistant individuals by limiting its use to one application per crop.
Tolerance to chlorantraniliprole
Two Tuta absoluta populations have also shown tolerance to chlorantraniliprole. The levels of tolerance were relatively modest in the field populations but further selection of one of these strains in the laboratory rapidly resulted in potent resistance to this compound.
The team at Exeter identified a mutation responsible for the resistance, which has also been reported in Tuta absoluta populations from Greece and Italy.
These findings suggest that UK growers should restrict their use of chlorantraniliprole, alternating with insecticides from other Mode of Action Classification Groups, to prolong its useful life.
Enhancing the efficacy of indoxacarb
With the help of Dupont UK, TGA trials have shown that the efficacy of indoxacarb (as Steward®) can be greatly enhanced by incorporating the adjuvant Codacide into the spray. Codacide is believed to aid the penetration of the active ingredient into the plant tissue where it has a much greater impact on the mining caterpillars.
What is parthenogenesis?
Parthenogenesis is a form of reproduction in which an egg can develop into an embryo without being fertilised by a sperm.
A peer-reviewed study from the University of Liege (Belgium) in 2012 indicated that some female Tuta absoluta exhibited parthenogenesis.
It was feared that the use of the Isonet T mating disruption system could select for such females – just as the use of certain insecticides can select for resistance to that particular chemistry. If so, this would clearly compromise the efficacy of the mating disruption technique.
Can the mating disruption technique select for parthenogenesis?
The team at Exeter University investigated the reproductive capacity of virgin females of Tuta absoluta collected from a UK tomato crop before and after the use of Isonet T. They demonstrated that this UK population could reproduce parthenogenetically and observed a small but significant increase in the rate of parthenogenesis associated with the use of Isonet T. Other changes were recorded in the population, with a significant reduction in the number of eggs produced, a delay in the onset of egg laying and increased adult longevity post-Isonet T.
They concluded that the low rate of parthenogenetic reproduction was unlikely to result in loss of efficacy of the mating disruption technique. However, the observed changes in longevity and egg laying could influence the IPM programme.
What should growers do about parthenogenesis?
Most UK growers continue to obtain very good results with the mating disruption technique. However, at least one grower has reported reduced efficacy.
It is important that the industry remains aware of the possibility of selecting for parthenogenesis and that experimental work continues, with regular monitoring of UK populations all-year-round to detect any further changes.
The content on this page was authored for AHDB by Dr Rob Jacobson (Rob Jacobson Consultancy Ltd).
AHDB is grateful to all the TGA members who contributed to the trials that developed the current IPM strategies. Particular thanks to Peter Bell, Richard Bezemer, Colin Bridges, Roly Holt, Paul Howlett, Brian Moralee and Phil Morley, as well as our overseas colleagues at Horticilha, Portugal.
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