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SID 5

Research Project Final Report
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SID 5 (2/05)
Project identification
1.
Defra Project code
2.
Project title
PS2710
Evaluating the response of UK strains of Thrips tabaci to
deltamethrin
3.
Contractor
organisation(s)
Rothamsted Research
Harpenden
Herts.
AL5 2JQ
54. Total Defra project costs
5. Project:
Page 1 of 12
£
33,888
start date ................
01 April 2006
end date .................
31 December 2006
6. It is Defra’s intention to publish this form.
Please confirm your agreement to do so. ................................................................................... YES
NO
(a) When preparing SID 5s contractors should bear in mind that Defra intends that they be made public. They
should be written in a clear and concise manner and represent a full account of the research project
which someone not closely associated with the project can follow.
Defra recognises that in a small minority of cases there may be information, such as intellectual property
or commercially confidential data, used in or generated by the research project, which should not be
disclosed. In these cases, such information should be detailed in a separate annex (not to be published)
so that the SID 5 can be placed in the public domain. Where it is impossible to complete the Final Report
without including references to any sensitive or confidential data, the information should be included and
section (b) completed. NB: only in exceptional circumstances will Defra expect contractors to give a "No"
answer.
In all cases, reasons for withholding information must be fully in line with exemptions under the
Environmental Information Regulations or the Freedom of Information Act 2000.
(b) If you have answered NO, please explain why the Final report should not be released into public domain
Executive Summary
7.
The executive summary must not exceed 2 sides in total of A4 and should be understandable to the
intelligent non-scientist. It should cover the main objectives, methods and findings of the research, together
with any other significant events and options for new work.
1) Onion thrips, Thrips tabaci, is a prevalent pest of many cultivated crops throughout Europe
and various other parts of the world. It infests glasshouse and field vegetables such as leeks and
salad onions, and primarily causes direct feeding damage that disfigures the leaves. Pesticides are
the main option for control but this approach is challenging due to problems with delivery and
contacting thrips. There is also the risk of insecticide resistance developing, illustrated by
widespread loss of control with organophosphates since the early 1990s.
2) There are anecdotal reports that poor efficacy of pyrethroids against UK T. tabaci may, in part,
be a consequence of resistance. Resistance to this class of chemicals has been confirmed in other
parts of the world, eg. to deltamethrin in New Zealand, to lambda-cyhalothrin in the USA and to
cypermethrin in Nicaragua. However, in general there is much less known about this
phenomenon in T. tabaci than in the closely-related species Western flower thrips, Frankliniella
occidentalis.
3) The purpose of the work was to provide information on the response of contemporary UK
strains of T. tabaci to deltamethrin to assess the level and prevalence of resistance, which could
then be used as a basis for informing the approval of new compounds and the development of
management strategies utilising all the chemical classes available. The work ran for 9 months,
providing time to complete three main objectives: (i) validate and refine the bioassay method; (ii)
obtain and quantify the responses of UK samples to a range of concentrations of deltamethrin;
and (iii) analyse data and prepare a final report.
4) Procedures for culturing, handling and testing samples of T. tabaci followed those developed
in New Zealand by Dr Nicolas Martin and colleagues. This allowed the response of UK samples
to deltamethrin to be compared directly with data for New Zealand ones. This approach is
particularly valuable when there is no proven baseline susceptible strain against which data from
field samples can be compared.
5) Live samples of T. tabaci were collected from leeks or salad onions at seven sites in England
from May to August 2006 (Figure 1). They were transported to Rothamsted accompanied by
records of host plant, place and date of origin and any insecticide treatment history.
SID 5 (2/05)
Page 2 of 12
6) Samples were reared in glass jars containing tubular segments of organically-grown leeks at
25oC under a 16/8 light/dark photoperiod. New plant material was added every 2-3 days.
Generation time (adult to adult) was approximately three weeks.
7) Bioassays used discs cut from the white part of organically-grown leeks. Each disc was dipped
into a solution of deltamethrin and allowed to dry in a fume hood. Adult thrips were placed in 50
mm diameter plastic Petri dishes containing one leek disc. Petri-dishes were stored at 25oC under
a 16/8 h light/dark photoperiod and responses were assessed after 24 h.
8) Initial bioassays used two samples that had not previously been exposed to insecticides. These
showed comparable dose-response relationships to ones confirmed as resistant in New Zealand.
Consequently, all 10 UK samples were exposed to a wide range of deltamethrin concentrations
rather than just three diagnostic concentrations (as originally proposed). Data were subjected to
probit analysis for statistical comparisons of concentration-response relationships.
9) All UK samples showed levels of resistance to deltamethrin comparable or only slightly lower
than the maximum level reported from New Zealand. Unlike in the New Zealand study, no
deltamethrin-susceptible samples were found. The resistance mechanism(s) are unknown at
present but are likely to extend to other synthetic pyrethroids approved for thrips control. As a
result, we conclude that these compounds are unlikely to give satisfactory control of this pest in
this country.
10) Results were disseminated through contact with Defra-PSD and IRAG-UK and an article in
December’s issue of Herb News. A summary of the results was distributed by email to advisors and
leek and salad onion growers (in November 2006). Also, an ‘established problem for resistance to
pyrethroids in Thrips tabaci’ has been added to the IRAG Pest Resistance Matrix (collated by Bill
Parker at ADAS).
The work complements an ongoing Defra-funded project at HRI-Warwick on the phenology,
dynamics and management of T. tabaci (HH3116 TFV).
Project Report to Defra
8.
As a guide this report should be no longer than 20 sides of A4. This report is to provide Defra with
details of the outputs of the research project for internal purposes; to meet the terms of the contract; and
to allow Defra to publish details of the outputs to meet Environmental Information Regulation or
Freedom of Information obligations. This short report to Defra does not preclude contractors from also
seeking to publish a full, formal scientific report/paper in an appropriate scientific or other
journal/publication. Indeed, Defra actively encourages such publications as part of the contract terms.
The report to Defra should include:
 the scientific objectives as set out in the contract;
 the extent to which the objectives set out in the contract have been met;
 details of methods used and the results obtained, including statistical analysis (if appropriate);
 a discussion of the results and their reliability;
 the main implications of the findings;
 possible future work; and
 any action resulting from the research (e.g. IP, Knowledge Transfer).
SID 5 (2/05)
Page 3 of 12
1. Background
Onion thrips, Thrips tabaci, is a prevalent, polyphagous pest of many cultivated crops throughout
Europe and various other parts of the world. It infests glasshouse and field vegetables such as
leeks and salad onions, and primarily causes direct feeding damage that disfigures the leaves.
Pesticides are the main option for control but this approach is challenging due to delivery/contact
problems. There is also the risk of resistance developing, illustrated by widespread loss of control
with organophosphates since the early 1990s. Furthermore, there are anecdotal reports that poor
efficacy of pyrethroids against UK T. tabaci may, in part, be a consequence of the evolution of
resistance. Resistance has been confirmed to deltamethrin in New Zealand (Martin et al., 2003),
to lambda-cyhalothrin in the USA (Shelton et al., 2003) and to cypermethrin in Nicaragua (Rueda
& Shelton, 2003) However, in general there is much less known about this phenomenon in T.
tabaci than in the closely-related and highly polyphagous species Western flower thrips,
Frankliniella occidentalis.
2. Scientific objectives
The purpose of the work was to provide information on the response of contemporary UK strains
of T. tabaci to deltamethrin as a basis for informing the approval of new compounds and the
development of management strategies utilising all the chemical classes available. The work
complemented an ongoing Defra-funded project at HRI-Warwick on the phenology, dynamics
and management of T. tabaci (HH3116 TFV). The work ran for 9 months, providing time to
refine the bioassay method and quantify the responses of field-collected strains to diagnostic
doses of deltamethrin. The project had three major objectives, all of which were either
successfully met or exceeded.
Objective 1: Validate bioassay and establish diagnostic concentrations
Initially, an untreated strain of T. tabaci currently being maintained in a glasshouse at HRIWarwick will be established in culture at Rothamsted and used to validate the bioassay procedure
published by Martin et al. (2003). It is intended that all rearing and testing will be done on
organically grown leeks (Allium porrum) purchased from a local supermarket. This will preclude
any requirement to culture plants at Rothamsted. Insects will be reared on tubular sections of leek
in thrips-proof containers, and maintained at 25°C in a 16h light: 8 h dark photoperiod regime.
For bioassays, leaf discs cut from leek plants will be dipped into aqueous solutions of
deltamethrin and placed into Petri-dishes. Ten adult thrips will be placed in each dish and
mortality recorded after 24 h at 25°C. Each concentration will be replicated at least three times.
Initial bioassays will establish the concentration-response relationship for the HRI strain, and
based on probit analysis three diagnostic concentrations of deltamethrin (equating approximately
with the LC50, LC90 and 5 x LC90 of the lab strain) will be identified for monitoring of field
samples.
Amendment: instead of using diagnostic concentrations, a full range of insecticide
concentrations were applied to each strain. The resulting larger data sets allowed the calculation
of probit lines and LC50 values. This altered approach was necessary because a deltamethrinsusceptible strain was not found and therefore an initial calculation of diagnostic concentrations
to be applied to subsequent UK samples could not be made.
Objective 2: Collection and testing of field strains
Between June and October 2006, at least 10-12 samples will be obtained from commerciallygrown crops of leeks (and possibly salad onions) identified in consultation with HRI and HDC,
transferred to Rothamsted, and reared for one generation to provide uniformly-aged adults for
SID 5 (2/05)
Page 4 of 12
bioassays. Laboratory rearing will not only overcome the likely substantial differences in age of
field-caught insects, but will also preclude the possibility of insects having been exposed to
insecticides or other crop protection agents prior to testing. At least three batches of ten insects
will be used for each concentration (plus untreated controls) for each sample. Bioassays showing
greater than 20% mortality will be repeated until control mortality is within acceptable limits.
Amendment: some strains were reared for more than one generation in order to obtain sufficient
numbers for bioassays.
Objective 3: Analysis and presentation of data
Probit- or arcsin-transformed mortality data will be subjected to analyses of variance to quantify
the extent of heterogeneity in response between samples and to compare these data to ones for the
HRI ‘standard’. Since our bioassay method is designed to reproduce accurately that adopted by
Martin et al. (2003), data published by the latter for a reference susceptible strain and ones
exhibiting up to 500-fold resistance to deltamethrin will enable an estimation of the extent of
resistance in UK samples. Results will be presented to Defra in a final report to be submitted in
December 2006.
Amendment: an analysis of variance was not used. Instead, full probit lines were fitted for each
sample tested (see amendment described in Objective 1).
3. Materials and methods
The procedures for culturing, handling and testing strains of T. tabaci followed those developed
in New Zealand by Martin et al. (2003) assisted by a productive dialogue with Dr Nicolas Martin
on technical aspects of this project. A big advantage of adopting the same experimental
techniques is that bioassay data for deltamethrin in the UK can be compared directly with
equivalent data for susceptible and resistant strains from New Zealand. This is particularly
valuable when there is no proven baseline susceptible UK strain readily available, against which
data from field strains can be compared.
Collection of Thrips tabaci samples
Ten live T. tabaci strains were collected as samples (Table 1) taken from leeks or salad onions at
a range of field sites in England from May to August 2006 (Figure 1). For each sample, plants
were taken from scattered positions throughout the collection site. They were then immediately
transported to Rothamsted Research by car (in sealed large bags), first class post or next day
courier (in sealed robust boxes). Each sample was accompanied by a record of host plant, place
and date of origin and any insecticide treatment history.
Rearing of Thrips tabaci
Rearing methods were based on those described in Martin et al. (2003). Thrips strains and
bioassays used untreated leek discs (Allium porrum) transferred using tweezers. Thrips were
moved using a slightly wetted fine paint brush.
T. tabaci adults and nymphs from each sample were transferred to 500 ml wide-mouthed glass
jars containing tubular segments of leek leaves positioned on a few layers of paper tissue. Each
jar was then covered with two paper tissues (Kimwipes: Lite 200, Kimberley Clark) held in place
by a metal screw lid. Each lid contained a circular hole (47 mm in diameter) to allow air flow.
Jars were stored at 25oC under a 16/8 h light/dark photoperiod. New segments of leek were added
every 2-3 days and rotting pieces removed. Generation time (adult to adult) took approximately
three weeks.
SID 5 (2/05)
Page 5 of 12
Thrips tabaci bioassays
The initial objective was to establish in culture at Rothamsted a strain (Warwks1) of T. tabaci
being maintained in a glasshouse at HRI-Warwick. This was to be used to validate the bioassay
procedure published by Martin et al. (2003). However, this strain proved to take a long time to
bulk-up, so bioassays commenced on other strains (Notts1(2) and Yorks1(10)) that had also not
received treatment with insecticides. These proved to show similar concentration-response
relationships to the New Zealand resistant strain (Figure 2). As a result, it was decided that all ten
UK strains would be exposed to a full range of deltamethrin concentrations to allow complete
probit analysis instead of the initial proposal of using just three diagnostic concentrations of
deltamethrin (90 thrips in total) on all but the first, untreated, strain tested.
All rearing and testing was done using organically-grown leeks (Allium porrum) purchased from
local supermarkets. To gain sufficient numbers for bioassay, most strains were reared for one to
four generations. At the appropriate period after setting up a new generation, rearing jars were
inspected each morning for newly-emerged female adults. If more than fifty were present they
were collected using a battery-powered hand-held insect pooter, cooled for half an hour in a
fridge and then transferred (by gently tapping the inverted pooter) into a Petri dish, positioned on
an ice block to reduce insect activity, prior to bioassay.
Bioassays were based on those described in Martin et al. (2003) using young, adult female thrips
as larvae vary in size and quality. Discs, 25 mm in diameter, were cut from the white part of
untreated leeks. Each disc was dipped for 10 s in one of a range of dilutions of formulated
deltamethrin (Decis, Bayer CropScience: 25 g/l EC) and allowed to dry for approximately 20
minutes in a fume hood. Dilutions were done using 0.05% Agral. Controls were dipped in 0.05%
Agral alone. For each replicate, approximately 10 adults were transferred from their holding Petri
dish, to a 50 mm diameter plastic bioassay Petri dish (Vulcan Plastics) containing one leek disc.
The dishes were then stored at 25oC under a 16/8 h light/dark photoperiod and the number of live
and dead individuals in each recorded after 24 h. Thrips were considered dead if they failed to
move when they were gently touched with the tip of a paint brush. LC50 values were calculated
by probit analysis using the POLO program (LeOra Software, Menlo Park, California). Results
presented are based on bioassays testing between 108 and 376 thrips per strain (Table 2).
4. Results
The responses to deltamethrin of the ten UK T. tabaci strains tested are shown in Table 2. All
showed similar responses, with seven strains showing resistance levels statistically the same as
that published for the highly-resistant reference strain from New Zealand (Martin et al. (2003).
Two strains showed slightly lower, but still substantial, resistance to deltamethrin. As a result,
none of the UK samples were susceptible to this compound, ie. none were close in response to the
susceptible reference strain from New Zealand. It would therefore appear that resistance to
deltamethrin is widespread in Thrips tabaci in England.
5. Discussion and implications
The leek disc-dip bioassay used in this project simulates field spraying where insects are exposed
to residues on the host plants. The presence of the discs encourages thrips to feed and contact the
insecticide. Eight of the UK T. tabaci strains (collected from Lincolnshire, Yorkshire,
Warwickshire, Norfolk and Herefordshire) showed statistically similar LC50 values to that
obtained by Martin et al. (2003) for a highly deltamethrin-resistant New Zealand strain. The
remaining two UK strains (collected from Yorkshire and Norfolk) showed lower LC50s.
However, these were still far greater than for a deltamethrin-susceptible New Zealand strain also
tested by Martin et al. (2003).
SID 5 (2/05)
Page 6 of 12
In England the average field rate for deltamethrin used for T. tabaci control is 300 mls of Decis in
60 litres of water per hectare (D Norman, pers comm.). This dose is equivalent to 12.5 mg ai/l
(12.5 ppm). Martin et al. (2003) used a slightly higher dose, 20 mg ai/l deltamethrin, as a
diagnostic for resistance because it consistently killed 90% of susceptible thrips. This diagnostic
was included in the current study which applied a wide range of concentrations up to 5000 mg
ai/l. The LC50s for all the UK samples tested ranged from ~350 to over 1500 mg ai/l. Samples
showing these levels of resistance should easily survive a field application of deltamethrin.
Therefore, it would appear that high resistance to deltamethrin is widespread, at least in England,
in agreement with ongoing reports that synthetic pyrethroids are no longer giving control of onion
thrips. The two strains from Lincolnshire showed the highest LC50 values. However, there was no
obvious overall association between variation in LC50 and collection region. Furthermore, the
level of resistance to deltamethrin shown by the three strains that had been collected from
insecticide-untreated crops were not significantly different to the strain (Lincs1) showing the
highest LC50 value.
It is likely, but not yet proven, that resistance to deltamethrin in T. tabaci extends to all
pyrethroids. However, nothing is yet known about the mechanism/s conferring resistance.
Furthermore, the implications on resistance to insecticides that are not pyrethroids,
particularly OPs, neonicotinoids and spinosad, will depend on the mechanism. If it is based on
alteration to the target site (a sodium channel protein), it is not likely to confer cross-resistance to
other classes of insecticides. However, if the mechanism is based on metabolic changes it could
confer resistance to OPs, but it is less likely to confer resistance to neonicotinoids or spinosad
although this can only be ruled out by bioassays on live insects. Martin et al. (2003) suggest that
resistance to the OPs, diazinon and dichlorvos, in New Zealand onion thrips appears to have
different characteristics to resistance to deltamethrin and therefore may be being conferred by
different mechanisms. However, some OPs, e.g. chlorpyrifos and methamidaphos, have given
good control of onion thrips in that country.
6. Opportunities for future work
Future work should assess whether resistance to deltamethrin confers cross-resistance to other
compounds, such as neonicotinoids and spinosad, which are also used for onion thrips control,
and more generally should assess any variation in response to alternative insecticides.
Martin et al. (2003) discuss indications, from laboratory and field observations, that resistance to
deltamethrin declines fairly rapidly when thrips are not exposed to synthetic pyrethroid
insecticides. It would therefore be interesting to see if similar declines occur for resistance in UK
onion thrips when the use of these compounds is reduced either in the laboratory or in the field
(due to changes in control strategies stemming from our findings). However, the consistency of
responses across strains observed in this study suggest that resistance is relatively stable.
7. Actions
Onion and leek growers now have hard evidence and therefore the motivation to modify and
implement a pyrethroid resistance management strategy for onion thrips. However, this is a
polyphagous pest and all relevant groups of growers need to be involved with its implementation
if it is to be successful in reducing pyrethroid resistance in UK onion thrips.
After consultation with David Richardson at Defra-PSD, it is likely that there will be a
recommendation that SOLAs using deltamethrin should have the following placed on the product
labels:
"All samples of onion thrips (collected from leeks and salad onions) recently tested have shown
high resistance to deltamethrin and this is likely to extend to other synthetic pyrethroids approved
SID 5 (2/05)
Page 7 of 12
against this pest. As a result, it is highly unlikely that these compounds will give any control of
onion thrips and should therefore not be used."
8. Publications and other outputs
Article in Herb News (December 2006): Thrips on chives and other herbs.
Pest Intelligence Notes on Vegetable Pests (written for DEFRA by Mike Lole at ADAS: due to
be published in Spring 2007).
A summary of the results showing resistance to pyrethroids was distributed by email to advisors and
leek and salad onion growers in November 2006.
An entry stating an established problem for resistance to pyrethroids in Thrips tabaci has been added
to the IRAG Pest Resistance Matrix (collated by Bill Parker at ADAS).
SID 5 (2/05)
Page 8 of 12
9. Tables
Table 1. Origins and insecticide treatment histories of UK Thrips tabaci and standard New
Zealand strains.
_____________________________________________________________________________________________
Samples
Origin
Crop
Treatment1
Date
_____________________________________________________________________________________________
NZ-S2
Wellington and Pukekohe
Onions
Sept’99/Feb’2000
Untreated
NZ-R3
Canterbury
Onions
Feb’2000
Unknown
_____________________________________________________________________________________________
Warwks1
Notts1
Here1
Lincs1
Nor1
Nor2
Nor3
Lincs3
Yorks1
Yorks2
Wellesbourne, Warws
Leeks
30/5/06
Untreated
Edwinstowe, Notts
Leeks
23/7/06
Untreated
Evesham, Heref/Worcs
Onions
24/7/06
Pyrethroids, Dimethoate, Spinosad
Grantham, Lincs
Onions
14/8/06
Pyrethroids, Dimethoate
Southery, Norfolk
Leeks
16/8/06
Pyrethroids, Dimethoate
Southery, Norfolk
Leeks
16/8/06
Pyrethroids, Dimethoate
Southery, Norfolk
Leeks
16/8/06
Pyrethroids, Dimethoate
Holbeach, Lincs
Onions
16/8/06
Pyrethroids
Selby, N. Yorks
Leeks
28/8/06
Untreated
Selby, N Yorks
Leeks
28/8/06
Pyrethroids
_____________________________________________________________________________________________
1
Recent treatment history
New Zealand deltamethrin-susceptible strain (from Martin et al. 2003)
3
New Zealand deltamethrin-resistant strain (from Martin et al. 2003)
2
Table 2. Response of New Zealand and UK Thrips tabaci strains (arranged by LC50) to
deltamethrin in leaf-dip bioassays. Sample numbers are shown in parenthesis.
______________________________________________________________
Sample
N1
LC502
95% CI3
Slope
______________________________________________________________
NZ-S4
Unknown
2.82
2.38-3.26a
1.3
NZ-R5
Unknown
1596
945-2445d
1.8
____________________________________________________
Yorks1
Nor1
Yorks2
Warwks1
Nor2
Nor3
Notts1
Here1
Lincs3
Lincs1
190
165
202
171
376
189
251
249
344
108
352
646
747
796
845
857
894
962
1170
1583
229-621b
538-795bc
480-1048bcd
530-1162bcd
551-1389bcd
419-1312bcd
505-1220bcd
635-1229cd
549-1603bcd
949-2574bcd
2.8
4.2
2.7
2.6
1.5
2.9
3.1
3.1
3.0
3.1
______________________________________________________________
1
Total number of thrips tested in each sample
Concentation (mg active ingredient per litre) resulting in 50% dead
3
Confidence limits at 95%; values followed by the same letter do not differ significantly
4
New Zealand deltamethrin-susceptible strain (based on raw data from Nicolas Martin)
5
New Zealand deltamethrin-resistant strain (based on raw data from Nicolas Martin)
2
SID 5 (2/05)
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10. Figures
Figure 1. Map showing origins and order of collection of UK Thrips tabaci samples.
Untreated
Treated
9 10
2
4
8
5
1
3
SID 5 (2/05)
Page 10 of 12
6
7
Figure 2. Probit line responses to deltamethrin of New Zealand standards and UK Thrips tabaci
strains.
8.0
99
90
6.0
70
50
5.0
30
4.0
10
3.0
Mortality (percentage)
Mortality (probits)
7.0
1
2.0
0.1
1
10
100
1000
10000
Log concentration (ppm)
New Zealand standards
English strains
References to published material
9.
This section should be used to record links (hypertext links where possible) or references to other
published material generated by, or relating to this project.
SID 5 (2/05)
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NA Martin, PJ Workman & RC Butler (2003) Insecticide resistance in onion thrips (Thrips
tabaci) (Thysanoptera: Thripidae). New Zealand Journal of Crop and Horticultural Science
31, 99-106.
A Rueda & AM Shelton (2003) Development of a bioassay system for monitoring susceptibility
in Thrips tabaci. Pest Management Science 59, 5536-558.
AM Shelton, BA Nault, J Plate & J-Z Zhao (2003) Regional and temporal variation in
susceptibility to lambda-cyhalothrin in onion thrips, Thrips tabaci (Thysanoptera:
Thripidae), in onion fields in New York. Journal of Economic Entomology 96, 1843-1848.
SID 5 (2/05)
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