A method of assessing rice yield losses caused by

ENTOMOPHAGA, I6
(2), r971, r85-I9I
A METHOD OF ASSESSING RICE YIELD LOSSES
CAUSED BY THE STEM BORERS RUPELA ALB/NELLA
AND DIATRAEA SACCHARALIS IN SURINAM
AND THE ASPECT OF ECONOMIC THRESHOLDS (*)
by
J.B. M. VAN DINTHER
Laboratory of Entomology, Agricultural University,
\Vageningen, Netherlands.
By comparing the mean panicle weight (al per uninfested stem to the mean
panicle weight (b) per infested stem, yield loss can be calculated from the equation
loss = (a-h) n.p. where n represents the number of panicles per sq.m. or ha and
p the percentage of infestation.
Losses caused by Rupela turn out to be small or moderate. Larval development
takes place within one internode and the nodal septum is not destroyed. Chemical
control measures are not considered to be economically feasible.
Damage inOicted by Diatraea may be considerable.
Injury is not limited
to one internode and several nodes are often pierced. Since the more serious
infestations by this borer mainly seem to occur randomly in the form of local
erruptions, forecasting the outbreak of this insect in a rice region probably does
not give a sufficient clue for a pin-pointed chemical control. A preventive control
over the ,,·hole area is too expensive. Thus in Surinam insecticides are not used
against stem borers.
Separated sowing and harvesting periods with the shortest
duration possible, and stubble burning and clean cultivation of fallow fields have
proved valuable in restricting borer infestation. A further depressive action can
be produced by the side-effect of insecticides commonly applied against non-borer
pests such as waterweevils, leafminers, leaf-eating caterpillars, <lelphacids and
seed bugs.
Rupela albinella (CR.), a white satin-like moth of the family
Schoenobiidae and Diatraea saccharalis (F.), a brownish straw coloured
moth of the family Crambidae, are the two stem borers of rice in Suri­
nam. As rice is the most important crop in this country, much attention
has been given to insects which attack this plant and in particular
the stem borers. Measurement of the damage caused by these borers
has been carried out to determine the extent of their deleterious effect.
Data on yield losses have been obtained from a research in the
8 000 hectares " vVageningen " rice area in Western Surinam where
(1) Presented at the symposium
24th sept. 1970.
OILB
on borers of graminaceous plants. Paris,
186
J.
B.
M.
VAN DINTHER
two crops are grown annually. About 4 000 ha is under cultivation
from April to September, and approximately 6 000 ha from October
to March.
Damage assessment
During the last week preceding harvest 200 plants, corresponding
to 1 000-1 500 tillers, were collected per field (12 ha). They were
randomly sampled by walking diagonally across the field.
In the laboratory stems were cut open lengthwise in order to
determine whether they contained borers. Panicles were cut off
and classified in five groups, according to what the stems contained:
1) Rupela larvae of the first three instars, 2) older developmental
stages of Rupela, 3) Diatraea larvae of the first three instars, 4) older
developmental stages of Diatraea, 5) no borers.
TABLE 1
Weights in grammes of dried, ripe panicles (moisture content 16 %)
of infested and non-infested stems, collected from 25 fields.
Panicle weight of stems infested by
Field
< Rupela
l
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
T
2.1
2.6
2.4
2.1
2.4
1.8
3.1
2.8
2.5
2.4
2.7
2.5
2.7
2.5
2.2
2.7
2.7
2.7
2.5±0.07
> Rupela
2.2
2.0
2.5
2.7
2.0
2.2
1.6
1.7
1.8
2.1
2.3
1.6
2.6
2.3
2.2
2.1
2.5
2.4
2.2
2.4
2.0
1.7
2.4
2.2
2.6
2.2±0.06
< Diatraea
2.2
2.0
2.0
2.4
2.0
2.5
2.3
2.2±0.08
Panicle weight
> Diatraea
of non-infested
stems
1.7
2.0
2.2
2.3
1.9
1.9
1.4
1.5
1.8
1.9
2.1
1.3
2.1
1.4
1.7
1.8
2.0
2.0
1.7
2.0
1.7
1.4
1.8
1.9
2.2
l.8±0.05
2.5
2.2
2.7
2.8
2.5
2.4
2.4
2.5
2.4
2.4
2.6
1.8
3.1
3.0
2.8
2.7
3.1
2.9
2.9
2.7
2.3
2.5
3.0
2.7
2.7
2.6±0.06
Each weight listed already represents a mean of at least 50 panicles. <
of first three instars,
>
=
=
borer in later stage of development.
larvae
METHOD OF ASSESSING RICE YIELD LOSSES
187
Panicles collected from stems that showed a Rupela or Diatraea
moth exit hole belonged to group 2 and group 4, respectively. Panicles
were also weighed after drying at 500C for about 14 hr. Their moisture
content then amounted to 16 %Table 1 shows the data obtained from 25 fields sown with the
variety Dima. From these results it can be seen that in tillers infested
by Rupela larvae of the fourth instar or older the gross mean panicle
weight is 0.4 g less than the mean panicle weight of non-infested tillers;
for Diatraea this difference is 0.8 g.
When plants are attacked in their tillering stage by D. saccharalis,
the growing points of the young stems are often destroyed, thus causing
the symptoms of" dead heart" and " discoloured leaf sheath". These
plants may subsequently give rise to new tillers. However, as a rule
this compensation effect is of little value since, under the extensive
rice cultivation practised, harvesting starts at the optimal overall
ripening condition of the fields. Newly formed tillers often have
insufficient growing time to produce mature panicles. In a number
of cases panicles are not formed at all. Infestation during and after
the heading stage frequently results in bad setting of the grain; empty
panicles can also be found. Since the failure of panicle formation
was not taken into account in sampling the mean weight loss caused
by Diatraea was fixed at 1.0 g per panicle to provide a margin of
error.
Panicles were then analysed and special attention was also paid
to grain quality. It was found that stem infestation by Rupela and/or
Diatraea reduced the number of grains per panicle and increased the
percentage of "wind rice, "i.e.· spikelets that fail to form grain or have
aborted or badly dwarfed kernels (Table 2). Significant differences
in grain quality were not found (Table 3).
Further panicle density in the field was ascertained by counting
the number of panicles per sq.m. Sampling spots were chosen at
random. �aximum and minimum values obtained from 100 spots were
375 and HO, respectively; the mean value was 170.
By comparing the mean panicle weight (a) per uninfested stem
to the mean panicle weight (b) per infested stem, the yield loss can be
calculated from the equation Loss
(a-b) n.p, where n represents
the number of panicles per sq.m. or ha, and p the percentage of infes­
tation. The expected yield in absence of borers equals a.n.
Table 4 shows the relation between the percentage of infestation
and crop loss. It appears that losses due to Rupela activities are small
to moderate in contrast to losses that can be caused by Diatraea. In
the Wageningen scheme and also in rice growing regions elsewhere
in Surinam D. saccharalis occurs generally in low to medium numbers
compared to the abundant Rupela moths. In 1958 the ratio 1 : 22 was
found for the Wageningen area.
=
a
106.4±3.0
88.5
I24.I
I20.9
114.5
96.7
109.2
103.4
92.0
115.I
96.7
120.5
116.0
95.2
I05.9
97.4
N
2.44±0.02
2.53
2.43
2.45
2.50
2.34
2.47
2.37
2.48
2.43
2.47
2.56
2.50
2.35
2.37
2.42
0.11±0.02
2.33±0.01
2.41
2.35
2.34
2.41
2.22
2.35
2.25
2.39
2.31
2.34
2.39
2.30
2.29
2.31
2.36
N
2.6I±0.0l
2.67
2.60
2.63
2.66
2.56
2.62
2.53
2.65
2.59
2.63
2.66
2.59
2.53
2.55
2.62
N
0.00±0.02
2.6I±O.Ol
2.67
2.6I
2.64
2.67
2.56
2.62
2.52
2.65
2.57
2.63
2.65
2.58
2.54
2.55
2.62
I
c
I
b
(excl. " wind rice "),
Weight of 100 grains
in grammes
" ),
in grammes
(incl. " wind rice
Weight of 100 grains
d
N
7.4±0.4
6.5
7.8
8.4
7.2
II.I
6.8
8.I
7.6
7.3
6.7
5.0
4.1
7.4
8.7
9.0
4.9±0.5
12.3±0.3
ll.7
I2.3
13.3
I2.0
I6.I
I2.I
I2.6
Il.3
11.4
12.8
ll.6
13.1
11.6
11.4
11.7
I
% " wind rice "
e
of
IO x IOO grains; column
e
gives the weight of " wind rice " of the analysed panicles.
N
c
and d
0.03±0.00
(l.03
0.04
0.03
0.06
0.03
0.04
0.03
0.03
0.03
0.03
0.02
0.03
0.04
0.04
o.oa
0.03±0.00
0.06±0.00
0.05
0.06
0.06
0.06
0.07
0.06
0.06
0.05
0.05
0.05
0.05
0.06
0.05
0.05
0.05
I
rice ", in grammes
Weight of" wind
Each of the values listed in the first four columns represents a mean, viz. in column a of 100 panicles, in columns, b,
14.8±3.6
91.6±2.0
·r
v
83.0
98. a
100.9
94.5
78.8
89.3
94.7
86.5
106.0
81.2
91.5
98.1
85.8
98.4
87.6
I
Grains per panicle
1
2
3
4
5
6
7
8
9
IO
11
I2
13
I4
15
Field
Analysis of ripe panicles (moisture content 16 %) of infested (I) and non-infested (N) stems.
TABLE 2
::i;
t:<:I
;:a
�
tj
....
z
<1
�
z
�
to
;-<
i-'
00
00
METHOD OF ASSESSING RICE YIELD LOSSES
189
TABLE 3
Kernel analysis of ripe panicles of infested (I) and non-infested (N) stems.
% green
% crack
Field
% chalky
I
N
I
N
I
N
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1.9
2.9
7.4
2.4
1.5
2.5
4.2
5.0
4.1
5.0
5.7
1.5
5.1
4.6
4.0
2.5
2.4
6.8
2.7
1.8
2.9
3.0
4.4
5.0
5.9
6.2
1.4
4.6
5.8
4.5
o.o
0.3
0.2
1.0
1.4
0.6
0.3
0.6
1.4
0.6
0.9
0.7
0.5
0.2
0.0
0.1
0.4
0.2
1.2
1.1
0.8
0.3
0.5
0.8
0.3
0.9
0.7
0.2
0.1
o.o
2.7
3.8
3.3
1.5
6.7
3.0
9.2
2.9
3.5
3.2
4.6
6.4
1.6
3.1
2.5
2.6
3.8
4.3
1.2
6.4
2.9
8.3
2.5
2.8
2.7
4.7
6.0
1.2
3.0
1.9
T
3.9±0.4
4.0±0.4
0.6±0.1
0.5±0.1
3.9±0.5
3.6±0.5
Each of the percentages listed already represents a mean of 10 x 100 kernels.
TABLE 4
Relation between percentage of infestation and crop loss at a panicle
density of 170 per sq.m.
Crop loss in kg/ha
% infestation
Rupela
a-b
5
10
20
40
80
a-b
=
=
0.4 g
34
68
136
272
544
Diatraea
a-b
=
1.0 g
85
170
340
680
1360
difference between mean panicle weight of non-infested and infested stems
(moisture content 16 %).
Discussion
As a striking outcome from the damage assessment work it was
found that injury caused by Rupela is mild. Larval development
takes place within one internode and the nodal septum is not destroyed.
In most cases an infested tiller contains one larva only.
190
J.
B. M. VAN DINTHER
Since the economic level of damage is low relative to the numbers
of Rupela larvae found, chemical control measures against this borer
are not considered to be economically feasible. In this respect the
effect of daily rain showers, which are quite normal during much of
the long rainy season (April-June) and to a lesser degree during the
other rainy season (December-January), must also be taken into account
since rainfall seriously limits the residual action of insecticides.
Injury by D. saccharalis differs from that caused by Rnpela
because larval development is not restricted to one internode. Stem
tissues are badly damaged and several nodes are often pierced. Plants
may lodge as stems become weakened. In some fields production
can be reduced by more than 50 %.
The flight periods of Rupela in the Wageningen area have been
studied by means of light traps (YAN DINTHER, 1962), but nothing
is known with regards to D. saccharalis. This stem borer is only very
slightly attracted to common electric light.
The use of sex traps, e.g. traps of the type applied by PEREZ
& LONG (1964), may elucidate the flight activity of Diatraea. However,
the fact that this moth mainly occurs haphazardly as local erruptions
rather than seriously infesting a whole area will give problems in its
control. Thus forecasting the outbreak of this insect in a rice region
probably does not give a sufficient clue for a pin-pointed application
of chemical control measures. As an alternative, a preventive control
over the entire area would be too expensive since sufficient results
can only be expected after three sprayings. In small scale trials,
WouTERS (1965) obtained fair control results from 3 to 4 sprayings
with endrin.
The possibility of treating only that area which is sown during
the last 2-3 weeks of the sowing period, needs more attention. It is
well known that borer infestation in late fields is often higher than
in the early sown fields.
In Wageningen separated sowing and harvesting periods, with
as short a duration as possible, in addition to stubble burning and
clean cultivation of fallow fields, are practiced. Since the reclamation
of the area these measures, gradually carried out more carefully,
have proved valuable in limiting borer infestation. Diapausing Rnpela
larvae remaining in the stubble are also destroyed.
A further depressive action can be ascribed to the side-effect
of a more intensified application of insecticides against non-borer
pests such as water weevils (Helodytes sp.), leafminers (Hydrellia sp.),
leaf eating caterpillars (Spodoptera frugiperda SMITH & ABB., Mocis
latipes GUEN.), delphacids (Sogatodes orizicola Mum) and seed bugs
(Oebalus poecilus DALL.).
The mean percentages of total infestation by Rupela and Dia­
traea dropped accordingly by degrees from 31 % in 1956 to 8 % in
METHOD OF ASSESSI�G RICE YIELD LOSSES
191
1961 for the crop of the April-September season, and from 40 % to
15 % for the crop of the October-March period. These were calculated
by sampling 100 tillers per field of 12 ha shortly before harvesting.
More recently, attention has become focussed on the possibility
of reducing losses due to borers by using their natural enemies. Since
D. saccharalis can be mass reared on an artificial diet ( VAN DINTHER
& GoossENs, 1970), the role which parasites of this borer play can be
studied under experimental conditions.
RESUME
Methode de determination des pertes de recolte du riz
causees par les foreurs des tiges
Rupela albinella et Diatraea saccharalis en Surinam
et
seuils
economiques.
En comparant le poids moyen d'un epi (a) des tiges non-attaquees avec le
poids moyen d'un epi (b) des tiges attaquees la perte de recolte se laisse calculer
selon l'equation Perte
au metre Carre
=
(a-b) n.p., dans laquelle n represente le nombre d'epis
a l'hectare et p le pourcentage d'attaque.
Les pertes causees par Rupela se trouvent etre moderees (Tableau 4), le deve­
OU
loppement larvaire ayant lieu dans un seul entre-nceud. Une lutte chimique est
consideree comme non profitable d'autant que les precipitations limitent l'action
residuaire des insecticides.
Le degat provoque par Diatraea ne se circonscrit pas a un entre-nceud. Une
chenille parcourt deux entre-nceuds ou plus et la perte est souvent considerable.
Les infestations graves semblant intervenir au hasard sous forme de pullulations
locales un pronostic d'apparition de ce foreur dans une vaste region ne donnera
pas un indice propre pour une lutte chimique qui veut se limiter aux rizieres serieu­
sement menacees. Une lutte preventive pour toute la region n'est pas acceptable
economiquement. Par suite, des insecticides ne sont pas appl iques contre les
foreurs du riz en Surinam.
Des semailles et recoltes separees et d'une duree la plus courte possible, la destruc­
tion des chaumes par le feu et le nettoyage complet des champs en friche ont
prouve etre de valeur pour restreindre les infestations dans la region de ·wage­
ningen, Surinam de l'ouest, ou le riz est cultive deux fois par an. En outre une
action limitative
parait due a l'effet des insecticides generalement appliques
contre les autres insectes nuisibles du riz comme les curculionides attaquant les
racines et les feuilles, les chenilles defoliatrices, les punaises et les delphacides.
REFERENCES
DrnTHER, J. B. lVI. VAN. - 1962.
albinella (Cn.) in Wageningen,
geschool Gent, 27, 829-836.
Flight periods of the white rice borer Rupela
Surinam (South America). - .Med. Landbouwho­
D1:-<THER, J.B. M. VAN & GoossENS ,P.A. -1970.
Rearing ofDiatraea saccharalis
on diets in Surinam.
Ent. exp et appl., 13, 320-326.
:
PEREZ, R. & LoNG, YV. H.
196{.. Sex attractant and mating behaviour in
sugarcane borer.
J. econ. Entomol., 57, 688-690.
-
-
(F.)
the
A. - 1965. The control of pests and diseases at the Wageningen
rice project in Surinam. Congress Agr. Res. in the Guianas. - Agr. exp. Sta.
vVouTERs, L. J.
Paramaribo, Bull., 82, 381.