Feeding of the cabbage stem flea beetle Psylliodes chrysocephala on high and low glucosinolate cultivars of oilseed rape


Barlet, E.; Mithen, R.; Clark, S.J.

Entomologia Experimentalis et Applicata 80(1): 87-89

1996


Entomologia
Experimentalis
et
Applicata
80:
87-89,
1996.
87
©1996
Kluwer
Academic
Publishers.
Printed
in
Belgium.
Feeding
of
the
cabbage
stem
flea
beetle
Psylliodes
chrysocephala
on
high
and
low
glucosinolate
cultivars
of
oilseed
rape
E.
Bartlet',
R.
Mithen
2
&
S.
J.
Clark
3
'Entomology
&
Nematology
Department
&
3
Statistics
Department,
IACR
Rothamsted,
Harpenden,
Herts
ALS
2JQ,
UK
2
Brassica
and
Oilseeds
Research
Department,
John
Innes
Centre,
Colney,
Norwich,
NR4
7UH,
UK
Accepted:
November
7,
1995
Key
words:
secondary
plant
metabolites,
isothiocyanates,
variability,
plant
genetics,
Chrysomelidae
Introduction
The
cabbage
stem
flea
beetle,Psylliodes
chrysocepha-
la
L.,
is
an
important
pest
of
winter
oilseed
rape
in
Europe
(Bromand,
1990).
In
laboratory
tests,
it
fed
only
on
plants
containing
glucosinolates,
secondary
plant
compounds
ubiquitous
in
the
Brassicaceae
(B
art-
let
&
Williams,
1991).
Glucosinolates
stimulated
the
feeding
of
the
cabbage
stem
flea
beetle
when
added
to
agar,
and
the
amount
of
feeding
that
occurred
increased
with
increasing
glucosinolate
concentration
(Bartlet
et
al.,
1994).
Thus,
higher
levels
of
feeding
by
the
cabbage
stem
flea
beetle
might
be
expected
on
plant
tissue
with
high
levels
of
glucosinolate.
Glucosinolate
levels
in
the
cotyledons
of
oilseed
rape
reflect
those
in
the
seed
(Milford
et
al.,
1989).
In
field
trials
Williams
(1989)
found
no
consistent
difference
in
cabbage
stem
flea
beetle
damage
on
the
cotyledons
of
single
low
(high
seed
glucosinolate)
and
double
low
(low
seed
glucosinolate)
commercial
oilseed
rape
cultivars.
This
may
have
been
because the
difference
in
glucosinolate
concentration
between
single
and
double
low
cvs
was
insufficient
to
cause
appreciable
differences
in
feeding.
When
Giamoustaris
&
Mithen
(1995)
assessed
levels
of
cabbage
stem
flea
beetle
damage
on
field
plots
of
28
lines
of
rape,
they
found
that
flea
beetle
damage
to
the
leaves
was
positively
correlated
with
total
leaf
glucosinolate
concentration
(r
2
=0.39,
0.50
in
2
years'
field
trials),
which
varied
by
up
to
nine-fold
across
the
different
lines.
However,
the
lines
that
they
test-
ed
included
breeder's
lines
of
rape,
which
differed
in
the
concentration
and
profile
(proportions
of
differ-
ent
glucosinolate
types)
of
aliphatic
glucosinolates
in
the
leaves,
and
there
was
evidence
that
glucosinolate
type
also
affected
the
amount
of
flea
beetle
damage.
In
particular,
lines
with
a
high
proportion
of
3-butenyl
glucosinolate
appeared
to
suffer
increased
flea
beetle
attack.
These
laboratory
experiments
assessed
the
feed-
ing
of
the
cabbage
stem
flea
beetle
on
seedlings
of
commercial
and
breeder's
lines
of
oilseed
rape
in
rela-
tion
to
their
glucosinolate
concentration,
testing
only
rape
lines
with
the
conventional
oilseed
rape
profile
of
aliphatic
glucosinolates.
Materials
and
methods
Post-diapause
cabbage
stem
flea
beetles
were
col-
lected
from
crops
of
winter
oilseed
rape
between
September
and
February,
kept
in
plastic
boxes
(210
x
100
x
80
mm)
at
10
°C,
L12:D12,
and
fed
on
rape
leaves
(cv.
Ariana).
To
reduce
variability,
only
male
beetles
were
tested.
They
were
starved
for
24
h
before
the
start
of
the
experiment.
Eight
oilseed
rape
lines
were
tested.
These
were:
two
single
low
cvs
(Willi
and
Bienvenu),
three
double
low
cvs
(Ariana,
Falcon
and
Apache),
one
breeding
line
with
high
levels
of
aliphatic
glucosinolates
(412/3)
and
two
(NKA
and
NKB)
with
low
levels
of
aliphat-
ic
glucosinolates
(Giamoustaris
&
Mithen,
1995).
Seeds
were
sown
in
seed
trays
(215
x
360
x
55
mm)
and
transplanted
to
pots
(90
mm
diameter)
when
they
reached
the
cotyledon
stage
(growth
stage
1.0,
Sylvester-Bradley,
1985).
Eight
seedlings,
one
of
each
cultivar,
were
transplanted,
in
random
order,
to
form
a
ring,
within
each
of
18
replicate
pots.
Ten
beetles
were
introduced
to
each
pot of
rape
seedlings,
which
was
88
Table
I.
Feeding
of
cabbage
stem
flea
beetles
on
seedlings
of
rape
lines
with
varying
levels
of
aliphatic
glucosinolates
(n
=
18)
Line
Glucosinolate
level
(pmol
I
dry
weight)
Mean
feeding
login
(no.
holes
per
Aliphatic
Indole
Total
seedling
+
1)
NCS*
non-NCS
s.e.
=
0.093
4123
0.61
6.52
4.71
11.92
0.66
Willi
0.49
9.38
11.95
21.99
0.68
Bienvenu
0.40
1.75
6.15
8.38
0.64
Ariana
0.51
1.56 9.97
12.16
0.44
Falcon
0.24
0.35
5.42
6.08
0.77
Apache
0.27
0.22
4.03
4.58
0.71
NKA
0.22
0.11
4.95
5.38
0.57
NKB
0.22
0.12
6.14
6.55
0.61
*
Glucosinolates
releasing
isothiocyanates
on
hydrolysis
by
myrosinase.
then
encased
by
a
glass
cylinder
(100
mm
diameter,
180
mm
high),
covered
at
the
top
with
nylon
mesh.
The
beetles
were
allowed
to
feed
for
48
h,
after
which
the
feeding
holes
on
each
seedling
were
counted.
On
the
same
day
as
the
experiment
was
started,
a
sample
of
seedlings
of
each
cv
(approx.
10
g
fresh
weight)
was
harvested
from
each
seed
tray,
and
immediately
frozen,
by
placing
on
solid
carbon
dioxide
in
an
insulated
box.
The
samples
were
then
lyophilised
and
ground
to
a
powder.
Their
glucosinolate
content
was
analysed
as
in
Magrath
et
al.
(1993).
The
amount
of
feeding
(number
of
feeding
holes
per
seedling)
that
occurred
on
different
oilseed
rape
lines
was
analysed
using
analysis
of
variance,
and
cultivar
means
were
compared
using
an
F-test.
The
raw
data
were
transformed
(logio(n
+
1
))
before
analy-
sis,
to
standardise
the
variance.
The
transformed
data
were
also
regressed
on
each
of
aliphatic,
indole
and
total
glucosinolate
concentration
separately,
using
lin-
ear
regression,
and
t-tests
were
used
to
test
whether
the
slope
differed
significantly
from
zero.
Analyses
were
done
using
the
statistical
package
Genstat
5
(Genstat
5
Committee,
1993).
Results
Aliphatic
glucosinolate
concentration
varied
ca.
30-
fold
across
the
different
rape
lines
(Table
1),
but
the
total
glucosinolate
concentration
of
the
rape
cvs
var-
ied
only
ca.
five-fold.
It
is
perhaps
surprising
that
the
glucosinolate
content
of
Bienvenu,
a
single
low
cv,
is
lower
than
that
of
Ariana,
a
double
low
cv,
but
seed
glucosinolate
levels
can
vary
greatly
between
different
batches
of
the
same
seed
(Glen
et
al.,
1990).
Feeding
by
the
cabbage
stem
flea
beetle
differed
significant-
ly
amongst
the
rape
lines
(F7,136
=
2.33;
P
=
0.028).
Untransformed
means
ranged
from
2.3
(Ariana)
to
5.7
(Falcon)
holes/plant,
but
were
not
linearly
related
to
the
levels
of
aliphatic
(slope
=
-0.0033,
t
1
42
=
0.46),
indole
(slope
=
-0.0133,
t142
=
-1.45)
or
total
glucosi-
nolates
(slope
=
-0.0019,
t142
=
-0.41).
Plots
of
the
data
did
not
suggest
that
any
higher
order
or
non-linear
relationships
existed.
Discussion
No
relationship
was
found
between
glucosinolate
con-
tent
and
feeding
by
the
cabbage
stem
flea
beetle.
Bart-
let
et
al.
(1994)
found
that
a
5000-fold
increase
in
the
glucosinolate
concentration
of
agar
led
to
only
a
seven-fold
increase
in
feeding
by
this
beetle.
Thus,
the
difference
in
total
glucosinolate
concentration
found
between
the
rape
lines
used
in
these
experiments
(ca.
five-fold)
may
not
have
been
enough
to
cause
demonstrable
effects
on
feeding.
This
does
not
explain
why
a
positive
correlation
was
found
in
the
field,
where
variation
in
glucosinolate
concentration
was
still
relatively
low
(Giamoustaris
&
Mithen,
1995).
However,
the
amount
of
damage
that
occurs
in
the
field
is
affected
not
only
by
involatile
feeding
stimulants
such
as
glucosinolates
(which
will
affect
the
amount
that
each
individual
beetle
feeds),
but
also
by
volatile
attractants
(which
will
affect
the
num-
ber
of
insects
present
in
each
plot).
It
is
known
that
the
89
cabbage
stem
flea
beetle
is
attracted
by
isothiocyanates,
hydrolysis
products
of
glucosinolates
that
are
released
by
oilseed
rape
(Bartlet
et
al.,
1992).
The
correlation
between
glucosinolate
content
and
flea
beetle
damage
in
the
field
could
therefore
be
partly
due
to
increased
flea
beetle
attraction
towards
plants
giving
off
higher
levels
of
isothiocyanates.
Attraction
by
isothiocyanates
is
probably
of
less
importance
in
laboratory
bioassays,
when
the
insects
are
placed
in
the
proximity
of
the
rape
plants
at
the
start
of
the
experiment.
Furthermore,
only
certain
glucosinolate
types
(for
example,
3-butenyl
glucosinolate)
metabolise
to
release
isothiocyanates.
The
concentration
of
isothiocyanate-releasing
glucosi-
nolates
varied
little
amongst
the
lines
tested
in
the
cur-
rent
experiment,
whereas
those
previously
tested
in
the
field
exhibited
a
twenty-fold
variation
in
concen-
tration
of
these
glucosinolates.
Further
experiments
to
test
cabbage
stem
flea
beetle
feeding
on
rape
lines
with
similar
total
glucosinolate
levels
but
different
aliphatic
glucosinolate
profiles
are
planned.
The
glucosinolate
levels
that
were
measured
are
those
the
beetles
would
have
encountered
at
the
start
of
the
experiment,
and
these
could
have
altered
during
the
experimental
period.
The
glucosinolate
composi-
tion
of
rape
changes
with
both
plant
growth
stage
(Mil-
ford
et
al.,
1989),
and
with
cabbage
stem
flea
beetle
damage
(Koritsas
et
al.,
1991).
In
future
experiments,
it
would
thus
be
desirable
to
measure
the
glucosinolate
content
of
plants
at
both
the
beginning
and
the
end
of
the
experimental
process.
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E.
&
I.
H.
Williams,
1991.
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flea
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