4-Pyridyl carbonyl compounds as thrips lures: Effectiveness for western flower thrips in Y-tube bioassays

Article abstract of DOI:10.1021/jf800863t

In a search for chemical lures to manage the cosmopolitan crop pest western flower thrips (WFT), Frankliniella occidentalis, a Y-tube olfactometer was used to screen 20 compounds, including 18 4-pyridyl compounds. Comparison of Y-tube results for New Zealand flower thrips (NZFT), Thrips obscuratus, with field trapping data for ethyl nicotinate and ethyl isonicotinate, suggested that the minimum attractive dose (MAD) of an odor compound, where significantly (p < 0.05) more than 50% of thrips walked up the odor-laden arm, provided a measure for selecting compounds to evaluate for potential lure efficacy in the field. Eighteen synthetic 4-pyridyl compounds were tested on female WFT in a Y-tube olfactometer and four 4-pyridyl carbonyl compounds had MADs lower than the known WFT attractants p-anisaldehyde (MAD 10^-3 μL) and ethyl nicotinate (10^-2 μL): methyl isonicotinate (10^-6 μL), ethyl-2-chloropyridine-4-carboxylate (10^-6 μL), ethyl isonicotinate (10^-4 μL) and methyl 4-pyridyl ketone (10 ^-5 μL). The suitability of MAD for selecting compounds for further evaluation of practical lure efficacy is discussed. Comparisons of activities within homologous series of esters and ketones showed that attractant activity decreased with chain length. 4-Formyl pyridine was an attractant at a dose of 10^-5 μL, but was repellent at higher doses (10^-2- 10° μL).

Full text of DOI:10.1021/jf800863t

6554 J. Agric. Food Chem. 2008, 56, 6554–6561
4-Pyridyl Carbonyl Compounds as Thrips Lures:
Effectiveness for Western Flower Thrips in Y-Tube
Bioassays
,
†
‡
‡
MELANIE M. DAVIDSON,* NIGEL B. PERRY, LESLEY LARSEN,
‡
†
†
VANESSA C. GREEN, RUTH C. BUTLER, AND DAVID A. J. TEULON
New Zealand Institute for Crop and Food Research Limited, New Zealand
In a search for chemical lures to manage the cosmopolitan crop pest western flower thrips (WFT),
Frankliniella occidentalis, a Y-tube olfactometer was used to screen 20 compounds, including 18
4-pyridyl compounds. Comparison of Y-tube results for New Zealand flower thrips (NZFT), Thrips
obscuratus, with field trapping data for ethyl nicotinate and ethyl isonicotinate, suggested that the
minimum attractive dose (MAD) of an odor compound, where significantly (p < 0.05) more than 50%
of thrips walked up the odor-laden arm, provided a measure for selecting compounds to evaluate for
potential lure efficacy in the field. Eighteen synthetic 4-pyridyl compounds were tested on female
WFT in a Y-tube olfactometer and four 4-pyridyl carbonyl compounds had MADs lower than the known
-
3
-2
WFT attractants p-anisaldehyde (MAD 10 µL) and ethyl nicotinate (10 µL): methyl isonicotinate
-6
-6
-4
(
10 µL), ethyl-2-chloropyridine-4-carboxylate (10 µL), ethyl isonicotinate (10 µL) and methyl
-pyridyl ketone (10 µL). The suitability of MAD for selecting compounds for further evaluation of
-
5
4
practical lure efficacy is discussed. Comparisons of activities within homologous series of esters and
ketones showed that attractant activity decreased with chain length. 4-Formyl pyridine was an attractant
-
5
-2
at a dose of 10 µL, but was repellent at higher doses (10 -10° µL).
KEYWORDS: Western flower thrips; Frankliniella occidentalis; New Zealand flower thrips; Thrips
obscuratus; pyridine derivative; olfactometer; attractant; repellent
INTRODUCTION
be used in push-pull strategies, in which a repellent pushes
thrips off a crop and an attractant pulls thrips to a trap (16).
Western flower thrips (WFT), Frankliniella occidentalis
Pergande (Thysanoptera), are major cosmopolitan pests on a
variety of vegetable, fruit and ornamental crops (1). WFT are
difficult to manage because of their small size (<3 mm), cryptic
habits and resistance to a number of insecticides (2). As with
other thrips pest species, their response to color has been
exploited through the use of colored sticky traps for monitoring
populations (3, 4). Thrips have also been shown to respond to
olfactory cues, including alarm, sex, and aggregation phero-
mones (5–7) and plant-derived compounds (8–11). These
responses could be used in pest management applications.
Attractants could be used as lures with colored sticky traps to
measure a thrips population in a crop (improved monitoring)
We have discovered a new class of thrips lures, 4-pyridyl
carbonyl compounds, effective at increasing trap capture of
onion thrips (OT), Thrips tabaci Lindeman, and New Zealand
flower thrips (NZFT), Thrips obscuratus Crawford, in field trials
using water traps (17). The purpose of this study was to
investigate the responses of WFT toward this class of com-
pounds. In New Zealand, WFT are a key pest in greenhouses
but are not found in very high numbers outdoors, so outdoor
water trap bioassays were not appropriate. Screening compounds
in greenhouses is problematic because, with the restricted area
and reduced airflow, an active compound in one trap can
interfere with catches of neighboring traps. We therefore needed
a method to screen a large number of compounds that would
indicate which compounds could warrant further testing in the
greenhouse.
(12), to remove thrips from a crop (mass trapping) (13), to attract
thrips to an insecticide or predator/parasitoid (lure and kill), or
to bring thrips into contact with a pathogen/virus that infected
thrips then spread to others in a population [autodissemination
Y-tube olfactometers have been used to test the responses of
WFT to various compounds (11, 18). Y-tube testing has several
advantages: assays are not dependent on variable natural thrips
populations; dose-response measurements can be done; both
repellent and attractant activities can be investigated; and only
small amounts of compounds are needed (milligrams versus
grams for field trials using water traps). On the other hand,
(
14) or lure and infect (15)]. Attractants and repellents could
*
Author to whom correspondence should be addressed (telephone
+
64 3 325 9442; fax +64 3 325 2074; e-mail davidsonm@crop.cri.nz).
†
Private bag 4704, Christchurch, New Zealand.
‡
Department of Chemistry, University of Otago, P.O. Box 56,
Dunedin, New Zealand.
1
0.1021/jf800863t CCC: $40.75  2008 American Chemical Society
Published on Web 07/04/2008

Thrips Lures
J. Agric. Food Chem., Vol. 56, No. 15, 2008 6555
Figure 1. Compounds tested as thrips attractants in a Y-tube olfactometer [* see Koschier et al. (11)].
Y-tube experiments only measure a walking response whereas
thrips in the field or greenhouse are likely to be both flying and
walking, so it is not obvious that Y-tube results will predict
practical potencies. However, pheromones which attracted WFT
in Y-tube experiments (19) also increased catches on blue sticky
traps in greenhouses (6). Furthermore, both p-anisaldehyde and
ethyl nicotinate were significantly attractive to WFT in Y-tube
experiments at a range of doses (11) and both of these
compounds have given increased trap catches of WFT in
greenhouse and open field experiments (9, 10).
1-(4-Pyridyl)-1-butanol was prepared by a Grignard reaction.
Magnesium turnings (5.1 g, 3.2 eq.), a few crystals of I and a stirring
bar were placed in a 500 mL three-necked round-bottom flask. The
flask was fitted with a dropping funnel containing propyl bromide (17.8
2
mL, 3 eq.) in anhydrous Et
Et O (100 mL) and a condenser. The reaction was carried out under
. Once the Grignard reagent had been made, Et
O was added (∼100
mL) and the reaction was cooled to -5 °C. 4-Formylpyridine (6.24
mL, 65.4 mmol) in anhydrous Et O (30 mL) was added dropwise over
0 min at -5 °C. The mixture was stirred for 2 h, then poured onto
2
O, a dropping funnel containing anhydrous
2
N
2
2
2
4
crushed ice. Sulfuric acid (15% v/v) was added until the solution had
pH ∼1 and the mixture was stirred for 10 min. The solution was made
basic again with aqueous NaOH (20% w/w). The layers were separated
Here we report close similarity of Y-tube results between
laboratories, results on NZFT suggesting a Y-tube measure for
indicating which compounds may be worth testing in a
commercial greenhouse, and results for 18 4-pyridyl carbonyl
and related compounds showing both attractant and repellent
activities toward WFT in a Y-tube olfactometer.
and the aqueous layer was extracted with 10% MeOH/CHCl
00 mL). The combined organic extracts were dried (Na SO
3
(2 ×
3
2
4
) and
solvent removed in Vacuo to give the crude product as a brown oil
(
(
[
∼10 g). The oil was purified by silica gel column chromatography
gradient elution from CH Cl to EtOAc) to give 1-(4-pyridyl)-1-butanol
18085-89-7] as a pale yellow oil (5 g, 50% yield) with appropriate
2
2
1
MATERIALS AND METHODS
H NMR (300 MHz, CDCl ) signals: δ 0.94 (3H, t, J 7 Hz), 1.41 (2H,
3
m), 1.70 (2H, m), 4.70 (1H, dd, J 5, 7 Hz), 7.27 (2H, d, J 6 Hz), 8.54
Sources of Compounds. Eleven of the 20 compounds tested (Figure
) were from the commercial sources previously listed (17) and pyridine
1
(
2H, d, J 6 Hz) ppm; 96% pure by H NMR.
1
4
-methanol was from Merck, all with stated purities of 97-99%.
n-Propyl 4-pyridyl ketone was synthesized by oxidation of 1-(4-
1
n-Propyl isonicotinate (estimated by H NMR to be 98% pure),
isopropyl isonicotinate (98% pure), ethyl 2-chloropyridine-4-carboxylate
pyridyl)-1-butanol. Pyridinium chlorochromate (4.27 g, 19.84 mmol)
was suspended in dry CH Cl (50 mL) in a round-bottom flask under
. After stirring for 5 min, 1-(4-pyridyl)-1-butanol (2.0 g, 13.23 mmol)
in dry CH Cl (10 mL) was added. After 2 h anhydrous Et O (70 mL)
was added, the supernatant was decanted and the residual black gum
was washed with anhydrous Et
O (4 × 100 mL). The combined organic
fractions were concentrated in Vacuo, and the black tar-like substance
2
2
(
99% pure), and 4-(1,3-dioxolan-2-yl)pyridine (98% pure) were syn-
N
2
thesized as described elsewhere (17). iso-Pentenyl isonicotinate (3-
methyl-but-2-enyl isonicotinate [CAS Registry No. 638203-02-8], 96%
pure) was synthesized by a method similar to that for isopropyl
isonicotinate (17).
2
2
2
2

6556 J. Agric. Food Chem., Vol. 56, No. 15, 2008
Davidson et al.
was subjected to two stages of silica gel column chromatography (3:1
in CHCl
3
. 4-(1,3-Dioxolan-2-yl)pyridine, pyridine 4-methanol, and
EtOAc:hexanes, then EtOAc as eluants) to give n-propyl 4-pyridyl
4-cyanopyridine were solid at room temperature and were first dissolved
in CHCl (1:1).
ketone [1701-71-9] as a yellow-green oil (520 mg, 27% yield) with
3
1
appropriate H NMR (300 MHz, CDCl
3
) signals: δ 1.01 (3H, t, J 7
Bioassay. An individual female thrips of unknown age that had been
starved for 24 h was released into the base of the stem of the Y-tube
using a small aspirator, after disconnecting the silicone tubing con-
necting the base of the Y-tube to the pump. Most thrips walked up
into the stem of the Y-tube within a few seconds, at which time the
silicone tubing was reconnected to the base of the glass Y-tube. The
end position of a thrips was recorded when the thrips had reached
the far end of one arm (odor or clean-air) and only those thrips that
walked to the end of an arm within 3 min were recorded. Generally
more than 90% of thrips introduced into the Y-tube walked to the end
of one of the arms. After every five recorded thrips the Y-tube and
vials were rotated 180° to avoid position effects. After 25 recorded
thrips, the Y-tube and vials were thoroughly cleaned with acetone
Hz), 1.78 (2H, sextuplet, J 7 Hz), 2.95 (2H, t, J 7 Hz), 7.72 (2H, d, J
5
Hz), 8.81 (2H, d, J 5 Hz) ppm; 96% pure.
Di-isopropyl isonicotinamide was obtained during an unsuccessful
attempt to prepare ethyl 3-bromopyridine-4-carboxylate by the method
of Epsztajn et al. (20). n-Butyl lithium (8.27 mL, 1.6 M in hexanes,
1
3.23 mmol) was added to di-isopropylamine (13.36 mmol, 1.95 mL)
in dry THF (10 mL) at -78 °C and stirred for 0.5 h (to make lithium
di-isopropylamine, LDA). The mixture was warmed to room temper-
ature for 20 min, then cooled to -78 °C again. Ethyl isonicotinate
(
1.00 g, 6.62 mmol, 0.91 mL) was dissolved in dry THF (30 mL) and
cooled to -78 °C under Ar, and the freshly prepared LDA was added
dropwise. The mixture was stirred for 1 h, then allowed to warm to
room temperature for 0.5 h. The reaction mixture was cooled to -78
(
99.5%, Merck) and allowed to dry before repeating the experiment.
For each dose there were three replicates of 25 recorded thrips.
Statistical Analysis. Data were analyzed with a binomial generalized
linear model with a logit link (21), with the number to chose the odor-
laden arm as the positive response, out of the total number of thrips to
walk up either arm. Comparisons between different doses of a given
compound were made as contrasts within the analysis of deviance done
as part of the analysis (21). Tests for preference for a given dose over
the control were made with t tests of the parameters estimated on the
transformed (logit) scale. On the logit scale, 50% has a value of zero,
so if the parameter for a given dose is significantly greater than zero
on the logit scale, significantly more than 50% of the thrips walked up
the odor or clean-air arm. Confidence limits of 95% for the percentage
of thrips walking up the odor arm were calculated on the transformed
°
C again and 1,2-dibromotetrafluoroethane (6.62 mmol, 0.79 mL) in
THF (10 mL) was added slowly. The reaction was stirred at -78 °C
for 1 h, then stirred at room temperature overnight. Water (40 mL)
was added, the organic layer was separated, and the aqueous layer was
washed with CHCl
were dried (Na SO
3
(3 × 100 mL). The combined organic solutions
2
4
) and the solvent removed to give the crude
compound, which was subjected to silica gel column chromatography
8:2 benzene:acetone) to give di-isopropyl isonicotinamide (220 mg,
(
1
1
6% yield) [77924-05-1] with appropriate H NMR (300 MHz,
CDCl ) signals: δ 8.66 (2H, d, J 6 Hz), 7.19 (2H, d, J 6 Hz), 3.69 (1H,
3
bs), 3.55 (1H, bs), 1.54 (6H, bs) and 1.16 (6H, bs); 95% pure.
Insects. A laboratory colony of WFT was maintained on potted
flowering chrysanthemums, Dendranthema grandiflora (cv. Onyx time
yellow). The colony was established in 2001 from thrips collected from
commercial greenhouses (Auckland, New Zealand), with yearly
introductions of WFT from greenhouses (Canterbury and Auckland,
New Zealand). The plants used for the colony were held in three
temperature-controlled Perspex cages (5 plants per cage) with a
temperature maintained at 25 °C under a 16:8 h light:dark cycle. A
fresh uninfested plant was placed in each cage every 3 days and the
oldest plant was removed at the same time. Female Thrips obscuratus
were collected from flowering gorse (Ulex europaeus L.) or cabbage
trees (Cordyline australis Forst f.) near Lincoln, Canterbury the day
before the Y-tube experiments.
Y-Tube Olfactometer. The responses of thrips toward odor
compounds were evaluated in a glass Y-tube olfactometer following
the method described by Koschier et al. (11). Briefly, the Y-tube had
two branching arms at an angle of approximately 45° leading into a
single tube (stem), all 60 mm long with an internal diameter of 5 mm.
The Y-tube was held at an inclining position of 25°. Glass vials (4
mL) were attached to each arm via Wheaton Micro Kit adapters. Air
was drawn through activated charcoal before entering the vials and
the Y-tube apparatus, using a suction pump (AR Harris Co. Ltd., New
Zealand) producing an airflow of 5 cm/s through each arm and 10 cm/s
through the stem of the Y-tube. Air was drawn through the Y-tube for
(
(
logit) scale, and then back-transformed. A significance level of 5%
p < 0.05) was used in all analyses.
RESULTS AND DISCUSSION
Female WFT and NZFT were used, rather than both males
and females, because a lure for female thrips could reduce egg
laying and thus population growth (unmated females can lay
viable eggs) (1). We previously found a strong correlation
between the responses of male and female NZFT to the
compounds tested in water traps outdoors (17). Furthermore,
females are larger than males and thus easier to manipulate.
Y-Tube Interlaboratory Comparisons. p-Anisaldehyde and
ethyl nicotinate (structures in Figure 1) are the most studied
thrips attractants (17), so we tested these in the Y-tube
olfactometer as positive controls for comparisons with results
from other laboratories. The responses of female WFT to these
compounds reported by Koschier et al. (11) fell within the 95%
confidence limits calculated in the present study (Figure 2).
This shows that thrips behavior in Y-tube olfactometers can be
consistent between laboratories, and between thrips populations.
However, there were differences between the two studies. For
example, Koshier et al. found that 63% (p < 0.05) of female
3
0 min before introducing the first thrips. Connections between the
activated charcoal, vials, Wheaton adapters, Y-tube and suction pump
were with silicone tubing. The Y-tube and Wheaton adapters were held
within a box, the interior of which had been painted gray in order to
prevent external stimuli influencing thrips behavior, which was located
in a darkened room at 22 ( 2 °C. The Y-tube was lit from above
-
5
WFT walked up the odor-laden arm containing 10
µL
p-anisaldehyde, but in the present study, only 52% walked up
the odor-laden arm at this dose (p ) 0.36) (Figure 2). This
could be due to differences in the solvents used for the dilutions.
Koschier et al. used paraffin, which does not evaporate as
quickly as hexane or chloroform, the solvents used in the present
study.
We also tested isobornyl valerate as an aggregation phero-
mone for WFT in our Y-tube olfactometer, and found significant
attractant activity (results not shown) at the same levels as
reported by Hamilton and Kirk, despite differences in Y-tube
designs (22).
(
approximately 42 cm away from a light source) with a halogen light
providing 780 lx at the Y-tube level. The vial on the clean-air arm of
2
the Y-tube contained filter paper (1 cm Whatman no. 1) dosed with
1
0 ° µL of either hexane or chloroform (Pronalys AR), depending on
which solvent was used to dilute a compound. Preliminary assays had
shown that these solvents did not elicit a response from female WFT
in the Y-tube. The vial for the odor-laden arm contained filter paper (1
2
cm Whatman no. 1) dosed with 10 ° µL of a diluted or undiluted
volatile compound. Generally an undiluted sample of a compound was
tested first, then 10X or 100X dilutions until the percentage of thrips
walking up each arm was similar. Most compounds were diluted in
hexane to obtain different concentrations, but 4-formylpyridine, and
Interpreting Y-Tube Results for Evaluating Lure Efficacy.
The aim of this study was to use Y-tube screening results to
select compounds for further evaluation of their efficacy as thrips
4-(2-hydroxyethyl) pyridine were not soluble in hexane and were diluted

Thrips Lures
J. Agric. Food Chem., Vol. 56, No. 15, 2008 6557
Figure 3. Mean percentage (95% confidence limits) of New Zealand flower
thrips females to walk up the odor-laden arm containing regioisomeric
pyridyl esters in a Y-tube olfactometer.
Figure 2. Mean percentage (95% confidence limits) of western flower
thrips females to walk up an odor-laden arm containing known thrips
attractants in a Y-tube olfactometer. Open squares are values from
Koschier et al. (11).
We have shown highly significant differences in catches of
female NZFT in water traps baited with either ethyl nicotinate
(
(
average of 158 times more than control) or ethyl isonicotinate
average of 32 times more than control) (structures in Figure
lures in practical applications. The dose-response curves from
Y-tube experiments, e.g. for p-anisaldehyde and ethyl nicotinate
in Figure 2, offer several measures of potency: the range of
doses at which significantly (p < 0.05) more than 50% of the
thrips walked up the odor-laden arm; the minimum attractive
dose within this range; or the maximum proportion of thrips to
walk up the odor-laden arm. Furthermore, some compounds can
be attractants at lower doses, but repellents at higher doses (see
below).
1
) (17). When we tested these compounds in the Y-tube with
female NZFT we found that ethyl nicotinate had a thousand-
-6
fold lower MAD (10
µL) than the regioisomeric ethyl
isonicotinate (10- µL) (Figure 3), corresponding to the signifi-
cantly higher efficacy in the field bioassay. Therefore we discuss
the results of our screening compounds against WFT in the
Y-tube (below) primarily in terms of the MAD, as an indication
of lure efficacy.
3
4
-Pyridyl Esters as WFT Attractants. The three isomeric
The concentration of a compound in the atmosphere around
a lure will depend very strongly on air flow [see discussion
and references in Miller et al. (23)], but this concentration will
always decline with distance from the lure, by diffusion into a
greater volume of air. Therefore, a compound that is attractive
at a lower concentration (i.e., a lower dose in the Y-tube) may
be detected by thrips at a greater distance from the lure, so
potentially more thrips will be attracted. Based on this argument,
the minimum dose to result in significantly (p < 0.05) more
than 50% of thrips walking up the odor-laden arm in the Y-tube
esters ethyl picolinate, ethyl nicotinate and ethyl isonicotinate
(structures in Figure 1) were tested at a range of doses in the
Y-tube on WFT females (Figures 2 and 4). Ethyl isonicotinate
showed the lowest MAD (10- µL) so further work focused on
4-pyridyl derivatives. The reduced heterocyclic analogue of ethyl
nicotinate, ethyl isonipecotate (Figure 1), was significantly
attractive at 10 ° µL but not at 10 µL (Table 1, only tested
at these two doses) so no further work was done on this class
of compound.
4
-2
Five other 4-pyridyl esters were tested (Figure 4 and Table
-
6
(
minimum attractive dose, MAD) seems a logical choice for
1). The methyl homologue was more active (MAD 10 µL)
than ethyl isonicotinate, and the n-propyl homologue was less
active (MAD 10- µL). The branched chain isopropyl isoni-
indicating a compound’s potential lure efficacy if it remains
attractive at higher doses. However, the MAD, as with the
response of thrips toward all of the doses tested, is based on a
significance test and is thus necessarily affected by trial size,
i.e. number of thrips per replicate, number of replicates per dose
per compound. Therefore, the response of thrips toward doses
of a given compound, including the MAD, must be interpreted
cautiously between this and other studies.
3
-2
cotinate had a similar MAD (10 µL) to its straight chain
isomer, n-propyl isonicotinate (Figure 4). The higher molecular
weight iso-pentenyl isonicotinate was inactive at the highest
(10 ° µL) dose tested (Table 1). Thus MAD decreased with
decreasing molecular weight. One possible explanation for this
structure-activity relationship is the decrease of vapor pressure

6558 J. Agric. Food Chem., Vol. 56, No. 15, 2008
Davidson et al.
Figure 4. Mean percentage (95% confidence limits) of western flower thrips females to walk up an odor-laden arm containing 4-pyridyl esters in a Y-tube
olfactometer.
Table 1. Mean Percentage (95% Confidence Limits) of Female Western Flower Thrips To Walk up an Odor-Laden Arm of a Y-Tube Olfactometer for Pyridyl
Compounds Tested at One, Two, or Three Doses
dose (µL)
-
4
10-3
-3
10-2
10-1
-1
compound
5 × 10
5 × 10
5 × 10
10°
ethyl isonipecotate
49 (38, 60)
63 (51, 73)
49 (38, 60)
iso-pentenyl isonicotinate
di-isopropyl isonicotinamide
propyl 4-pyridyl ketone
a
a
81 (71, 89)
47 (36, 58)
55 (43, 65)
a
4
4
1
-cyanopyridine
39 (28, 50)
61 (50, 72)
-(1,3-dioxolan-2-yl)pyridine
-(4-pyridyl)-1-butanol
pyridine 4-methanol
4-(2-hydroxyethyl) pyridine
48 (37, 59)
63 (51, 73)
47 (36, 58)
52 (41, 63)
79 (68, 86)
a
b
48 (37, 59)
59 (47, 69)
a
b
Solid at room temperature, dissolved in chloroform (1:1). Not soluble in hexane, diluted in chloroform.
with increasing molecular weight (Figure 6), resulting in fewer
molecules of higher molecular weight compounds per unit
volume to affect the thrips’ behavior. Bengtsson et al. (24) and
Heath and Tumlinson (25) have discussed the importance of
taking vapor pressures into account for insect pheromones.
However, ethyl 2-chloropyridine carboxylate (MAD 10- µL)
was about a hundred times more active than its nonchlorinated
analogue ethyl isonicotinate (Figure 4), despite having a much
lower predicted vapor pressure (Figure 6). A GC-MS analysis
confirmed the 99% purity of the ethyl 2-chloropyridine car-
boxylate sample, with a 1% level of an isomer. The GC retention
time, which is related to vapor pressure (25), was much greater
than the retention time of ethyl isonicotinate, confirming the
relative vapor pressures.
6

Thrips Lures
J. Agric. Food Chem., Vol. 56, No. 15, 2008 6559
Figure 6. 4-Pyridyl carbonyl compounds’ effects on WFT females in the
Y-tube bioassay (log minimum attractive dose, MAD; p < 0.05) and vapor
pressures (log Torr at 25 °C, calculated by ACD software, in the Chemical
Abstracts Registry file). 4-Formylpyridine was repellent at higher doses.
it at higher doses. Koschier et al. found that another aromatic
aldehyde, salicaldehyde (Figure 1), was significantly repellent
to WFT at two doses in their Y-tube tests, but did not show
significant attractant activity at lower doses (11). The repellent
activity of compounds such as salicaldehyde and 4-formyl
pyridine could be used in push-pull strategies, with a repellent
to push thrips off a crop and an attractant to pull thrips to a
trap (16).
In the light of this finding of a dose-dependent bimodal
response for 4-formyl pyridine, the decrease in attractiveness
of p-anisaldehyde at the highest dose tested (10 ° µL) observed
by us (Figure 2) and by Koschier et al. (11) is interesting. In
our results (Figure 2) the decrease in attractiveness between
-1
1
0
and 10 ° µL is statistically significant, so p-anisaldehyde
may be repellent at higher doses. This result means that
compounds that we only tested at one or two doses (Table 1)
could show attractant or repellent activities at other doses (see
below).
Figure 5. Mean percentage (95% confidence limits) of western flower
thrips females to walk up an odor-laden arm containing 4-pyridyl ketones
or an aldehyde in a Y-tube olfactometer.
4
-Pyridyl Alcohols and Other Derivatives as WFT At-
tractants and Repellents. 4-(1,3-Dioxolan-2-yl)pyridine is an
acetal derivative of 4-formyl pyridine (Figure 1) that was
4
-Pyridyl Ketones and an Aldehyde as WFT Attractants
-1
-3
and Repellents. Ethyl 4-pyridyl ketone, which we found to be
attractive to NZFT and onion thrips in field trapping experiments
significantly attractive to WFT at 5 × 10 and 5 × 10 µL
doses (Table 1). 4-Cyanopyridine is an analogue of 4-formyl
pyridine (Figure 1) that was significantly repellent to WFT at
(
17), is an analogue of methyl isonicotinate (Figure 1). We
-1
tested methyl, ethyl and n-propyl 4-pyridyl ketones against WFT
the single 5 × 10 µL dose tested (Table 1). This compound
was not tested further because it is classified as a harmful
chemical, which would limit practical applications (the other
4-pyridyl compounds in this study are generally classified as
irritants, rather than harmful, in Material Safety Data Sheets).
Another alcohol, 4-(2-hydroxyethyl) pyridine (Figure 1) at-
tracted WFT at a 10 ° µL dose, but not at two lower doses
(Table 1). This compound is a 4-pyridyl analogue of 2-phe-
nylethanol, a common component of floral scents that has been
shown to attract NZFT and onion thrips (17).
in the Y-tube (Table 1 and Figure 5). The MAD decreased
-
5
-2
along the homologous series: methyl 10 µL; ethyl 10 µL;
and propyl was not active at the doses tested (Table 1). As
with the isonicotinate esters (above), this decline in activity
could be due to decreasing vapor pressure (Figure 6). Ethyl
4
-pyridyl ketone has a very similar molecular size and shape
to methyl isonicotinate, but is about ten thousand times less
active (Figure 6).
4
-Formyl pyridine is both a homologue of the 4-pyridyl
ketones, with hydrogen instead of the alkyl groups, and an
The unplanned synthesis of N,N-di-isopropyl isonicotinamide
(Figure 1) led to the discovery of thrips attractant activity for
another type of 4-pyridyl carbonyl compound, the isonicotina-
mides. N,N-Di-isopropyl isonicotinamide was significantly at-
analogue of benzaldehyde (Figure 1). 4-Formyl pyridine was
-5
significantly attractive at a dose of 10 µL, but was significantly
repellent at higher concentrations: only 15% of thrips chose the
-
2
-1
odor arm of the Y-tube at a 10 µL dose (Figure 5). Terry et
al. (26) found that ꢀ-myrcene, a compound found in cones of
the cycad, Macrozamia lucida, attracted its obligate pollinator,
Cycadothrips chadwicki, at low doses in a Y-tube, but repelled
tractive to WFT at the one 5 × 10 µL dose tested (Table 1),
which is surprising from a compound with two isopropyl
substituents and a low predicted vapor pressure (<0.001 Torr
at 25 °C, calculated by ACD software, in the Chemical Abstracts

6560 J. Agric. Food Chem., Vol. 56, No. 15, 2008
Davidson et al.
Registry file). N,N-Diethyl isonicotinamide (not yet tested) is a
-pyridyl analogue of the most well-known insect repellent, N,N-
diethyl m-toluamide (DEET), active against many insect species
including mosquitoes (27).
(5) de Kogel, W. J.; van Deventer, P. Intraspecific attraction in
the western flower thrips Frankliniella occidentalis; indications
for a male sex pheromone. Entomol. Exp. Appl. 2003, 107, 87–
4
8
9.
(
(
6) Hamilton, J. G. C.; Hall, D. R.; Kirk, W. D. J. Identification of a
male-produced aggregation pheromone in the western flower thrips
Frankliniella occidentalis. J. Chem. Ecol. 2005, 31, 1369–1379.
7) Teerling, C. R.; Pierce, J. R.; Borden, J. H.; Gillespie, D. R.
Identification and bioactivity of alarm pheromone in the western
flower thrips Frankliniella occidentalis. J. Chem. Ecol. 1993, 19,
Selected 4-Pyridyl Carbonyl Compounds as Efficacious
WFT Lures. From the Y-tube results discussed above, we
selected four 4-pyridyl carbonyl compounds for testing in a
commercial greenhouse as potential WFT lures. These were
methyl and ethyl isonicotinate, methyl-4-pyridyl ketone, and
ethyl-2-chloropyridine-4-carboxylate, which had the lowest
MADs and no significant repellent activity. Yellow sticky traps
treated with methyl or ethyl isonicotinate did trap significantly
more female WFT (up to 14 times) than untreated traps (28).
Traps treated with methyl-4-pyridyl ketone trapped more female
WFT (up to 4.5 times) than control traps, but traps treated with
ethyl-2-chloropyridine-4-carboxylate did not increase the capture
of female WFT (28).
6
81–697.
(
(
8) Kirk, W. D. J. Effect of some floral scents on host finding by
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3.
9) Teulon, D. A. J.; Penman, D. R.; Ramakers, P. M. J. Volatile
chemicals for thrips (Thysanoptera: Thripidae) host finding and
application for thrips pest management. J. Econ. Entomol. 1993,
86 (5), 1405–1415.
The result for ethyl-2-chloropyridine-4-carboxylate was
surprising, since it was a potent attractant for WFT in Y-tube
experiments (Figure 4). The predicted vapor pressure for
ethyl-2-chloropyridine-4-carboxylate is much lower than for
the other selected compounds (Figure 6) so it could be that
the odor plume emanating from this compound was small
and/or short-lived in the greenhouse. Also, in the Y-tube there
is a continuous airflow carrying molecules that do vaporize
directly to the thrips. Therefore, we propose future selection
criteria for thrips attractants for field trials are a low MAD
in Y-tube experiments combined with moderate vapor
pressure.
The WFT results given above, taken with the results for
OT and NZFT in our field trial study (9), show that 4-pyridyl
carbonyl compounds are potent attractants for several thrips
species. We now have an example of a 4-pyridyl amide as
an attractant, in addition to the ester and ketone derivatives,
and the aldehyde (4-formyl pyridine) was found to be an
attractant at a low dose. In our field trial paper we pointed
out that thrips may never have encountered such 4-pyridyl
carbonyl compounds in nature, since they have not been
reported from thrips, and only rarely recorded at low levels
in plants (references in (9)). We propose that the synthetic
(10) Teulon, D. A. J.; Hollister, B.; Butler, R. C.; Cameron, E. A.
Colour and odour responses of flying western flower thrips: wind
tunnel and greenhouse experiments. Entomol. Exp. Appl. 1999,
93, 9–19.
(11) Koschier, E. H.; de Kogel, W. J.; Visser, J. H. Assessing the
attractiveness of volatile plant compounds to western flower thrips
Frankliniella occidentalis. J. Chem. Ecol. 2000, 26, 2643–2655.
(
12) G o´ mez, M.; Garcia, F.; GreatRex, R.; Lorca, M.; Serna, A.
Preliminary field trials with the synthetic sexual aggregation
pheromone of Frankliniella occidentalis on protected pepper and
tomato crops in south-east Spain. IOBC/WPRS Bull. 2006, 29,
1
53–158.
(
13) Kawai, A.; Kitamura, C.; Studies on population ecology of Thrips
palmi Karny, X. V. Evaluation of effectiveness of control methods
using a simulation model. Appl. Entomol. Zool. 1987, 22, 292–
3
02.
(14) Ignoffo, C. M. Strategies to increase the use of entomopathogens.
J. InVert. Pathol. 1978, 31, 1–3.
(
15) Smits, P. H.; Van Deventer, P.; De Kogel, W. J. Western flower
thrips: reactions to odours and colours. Proc. Exp. Appl. Entomol.
N.E.V. 2000, 11, 175–180.
(16) van Tol, R. W. H. M.; James, D. E.; de Kogel, W. J.; Teulon,
D. A. J. Plant odours with potential for a push-pull strategy to
control the onion thrips Thrips tabaci. Entomol. Exp. Appl. 2007,
122, 69–76.
4
-pyridyl carbonyl compounds bind to olfactory receptors
for natural allelochemicals, because of their structural
similarity to benzene carbonyl derivatives commonly found
in flower scents (see ref 9).
(17) Teulon, D. A. J.; Davidson, M. M.; Hedderley, D. I.; James, D. E.;
Fletcher, C. D.; Larsen, L.; Green, V. C.; Perry, N. B. 4-Pyridyl
carbonyl and related compounds as thrips lures: effectiveness for
onion thrips and New Zealand flower thrips in field experiments.
J. Agric. Food Chem. 2007, 55, 6198–6205.
ACKNOWLEDGMENT
(
18) de Kogel, W. J.; Koschier, E. H.; Visser, J. H. Y-tube olfactometer
to determine the attractiveness of plant volatiles to western flower
thrips. Proc. Exp. Appl. Entomol., N.E.V. Amsterdam 1999, 10,
131–135.
We thank Willem Jan de Kogel of Plant Research Interna-
tional, The Netherlands, for teaching M.M.D. the Y-tube
methodology, leAnne Glennie for assisting with syntheses, Rob
McGregor for constructing the Y-tubes, and Catherine Sansom
for GC-MS.
(19) Kirk, W. D. J.; Hamilton, J. G. C. Evidence for a male-produced
sex pheromone in the western flower thrips Frankliniella occi-
dentalis. J. Chem. Ecol. 2004, 30, 167–174.
(
20) Epsztajn, J.; Plotka, M. W.; Grabowska, A. Application of
organolithium compounds in organic synthesis. Part 19. Synthetic
strategies based on aromatic metallation. A concise regiospecific
synthesis of 3-halogenated picolinic and isonicotinic acids. Synth.
Commun. 1997, 27, 1075–1086.
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