The dynamics of Thrips tabaci(Thysanoptera, Thripidae) populations on UK Allium

crops

Joshua Burnstone

Project Background• I have just completed the second year

of a CASE studentship sponsored by the BBSRC and Speciality Produce Ltd a major UK producer of salad onions

• I work at Warwick HRI Wellesbourne which is a department of the University of Warwick

The Problem• Thrips tabaci is a polyphagous pest of

the full range of Allium crops grown in the UK

• Around 11,000 ha of onions are planted in the UK each year - 20% of which are Green or ‘Salad’ onions

• In addition around 2000 ha of Leeks are planted each year

The Problem• In 1995 around 54% of all UK Allium

crops treated with insecticides were treated for thrips

• By 2003 83% of all Allium crops treated with insecticide or nematicide were being treated specifically for Thrips tabaci

The Problem• Despite extensive pesticide use,

growers are reporting very poor levels of control

• Lambda-cyhalothrin and Deltamethrinare becoming unusable due to resistance

• Other pesticides are proving ineffective for control as well

The Problem – key points• Thrips tabaci represents a major problem to

UK salad onion growers• Currently available chemical treatments are

ineffective– Resistance– Shape and complexity of Allium plant forms

protects cryptic thrips• Improvements to control strategies will

require a more complete understanding of Thrips tabaci: Its biology and behaviour

A Thrips tabaci population through the year• Spring

– Arrival in the crop– Early population development

• Early Summer– Population growth– Intra-plant distribution and targeting the pest

• Late Summer and Autumn– Flight behaviour and late summer dispersal events– The autumn population surge

• Winter– Overwintering strategies– Alternative hosts

Spring

• Initial infestation of crops occurs in spring via aerial invasion

• Thrips tabaci first caught on sticky traps between mid-April and early June

• Sticky trap captures always occur before thrips first detected in crops

• Overwintered crops may suffer particularly badly with a steep initial increase in population size if adult thrips have overwintered on them

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3-Apr 23-Apr 13-May 2-Jun 22-Jun 12-Jul 1-Aug 21-Aug 10-Sep 30-Sep

Mea

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2004 2005 2006

The numbers of adult thrips captured on sticky traps in commercial crops in Cambridgeshire in 2004-6 (averaged over sites)

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Early Summer• Populations of Thrips tabaci increase slowly

in early summer• Thrips numbers are very variable on

individual plants• Many growers believe this is the key period

for pesticide use• The thrips are cryptic and timing of control

applications must be well planned– An experiment was designed to attempt to improve

our understanding of when they are most vulnerable

Percentage of adult Thrips tabaci population occupying the different sections of salad onion plants through the day

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Time of day: 1 being dawn and 5 being dusk

Apical half of leavesBasal half of leavesStem

Early Summer• Thrips tabaci exhibits a diurnal

periodicity in its intra-plant distribution• Cryptic thrips should be more

vulnerable to control at midday• Growers have noted a decrease in

thrips problems following periods of intense rainfall and irrigation– An experiment was designed to investigate

these possibilities

Adult Thrips tabaci on salad onion crops at Warwick HRI between 10/7/06 and 27/8/06

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Larval Thrips tabaci on salad onion crops at Warwick HRI between 10/7/06 and 27/8/06

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Late Summer and Autumn• Thrips tabaci flight activity increases

dramatically in July and August• Thrips numbers on plants remain steady

during this period• What are the key factors influencing this

distinctive behavioural event?

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Late Summer and Autumn• Following the extensive flight activity in July

and August few further flights seen• Large dispersal event is followed by an

energetic period of population expansion• Populations peak in September and October• This sudden expansion is brief; larvae will be

almost completely absent from the population by early November

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25-Apr 9-May 23-May 6-Jun 20-Jun 4-Jul 18-Jul 1-Aug 15-Aug 29-Aug 12-Sep

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Thrips on plants Thrips on traps

Thrips tabaci caught on sticky traps at one site in Cambridgeshire in 2005 andthe number of Thrips tabaci found per Leek plant at the same location

Winter• Thrips tabaci overwinter as adults in the UK• Leek is a preferred overwintering host• Thrips colonise the innermost leaves of the

plant and remain low at the point of leaf branching

• Thrips huddle together in large groups and can survive extreme conditions including bring frozen

• A key alternative host is Winter Wheat

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27-Jun-05

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AdultsLarvae

Mean numbers of Thrips tabaci on a single plot of Leek plants at Warwick HRI between June 2005 and June 2006

Future Work• Continue to monitor field populations • Lab investigations into:

– Development– Flight– Distribution– Activity and behaviour

Thanks for listening!