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Integrated Protection of Stored Products
IOBC Bulletin/wprs Vol. 27 (9) 2004
pp. 93-102
93
Estimation of population density and spatial pattern of stored paddy
rice insect species using un-baited traps
Maria Otília Carvalho1*, António Ferreira Barbosa1, Pedro Marques2, Blaine Timlick3,
Cornel Adler4 and António Mexia1
1CEFA/IICT, Lisbon; Portugal
2APARROZ, Alcácer do Sal; Portugal
3 Canadian Grain Commission, Winnipeg; Canada 4 BBA, Berlin; Germany
*Corresponding author e-mail: motiliac@netcabo.pt
Abstract: Studies were conducted in five paddy rice stores and one silo used for rice storage to
determine the insect species associated with stored rice, their abundance and spatial pattern from
November 2002 to April 2003. In each store and silo, un-baited Pitfall and Storgard WB Probe II traps
were placed. Additionally, during a period of six weeks, un-baited Pitfall traps were placed in the silo,
near each Storgard WB Probe II position to compare quantitative trap catches. In addition, Storgard
Dome traps were used, with attractants (food and pheromones) to monitor the occurrence of pest
insects on the floor below the aerated silos for grain drying. The traps were observed weekly and the
insects counted and identified. The Storgard WB Probe II traps captured significantly more insects and
more species than did Pitfall traps. Ptinus raptor (Sturm), Cryptophagus saginatus (Sturm), Coni-
nomus constricutus (Westwood), Gnathocerus cornutus (F.) and Sitophilus zeamais (Motschulshy)
were the most abundant species from about 21 insect species identified. In all five-paddy rice stores,
Iwao’s regression analysis pattern suggests a definite tendency of aggregation for the insect species
discovered. In addition, insect surveys were conducted in a rice mill. In the stores of the processors,
un-baited Storgard WB Probe II and Pitfall traps were placed in the paddy, brown and white rice
(bagged storage). To survey insect species in the factory, Stogard Dome traps were placed on the
floor, between machinery, and Stogard Thinline traps, on the walls. The most abundant insect species
recorded were Sitophilus zeamais and Cryptolestes ferrugineus (Stevens) and the greatest abundance
was discovered in the brown rice.
Key-words: paddy rice, trap, stored product insect, density, spatial pattern, Iwao’s regression
Introduction
Portugal consumes the largest quantity of white rice in Europe, and consequently a large
number of farmers and industries are associated with rice production, transportation, and
processing. In Portugal, rice is a seasonal crop. Planting of rice takes place in April and
harvest is near the end of August. Rice is stored as paddy on farm or in co-operatives in
horizontal warehouses or vertical silos, until the end of winter.
Like all stored grains after harvest, rice is permanently at risk of insect infestation.
Because of this, those involved in product storage and processing often use direct application
of chemicals to the rice for insect control. Due to consumer concerns over the presence of
pesticide residues in rice, there is an urgent need to develop integrated pest management
strategies to reduce these residual treatments. An important element in developing an
integrated pest management strategy to preserve quality of rice, is sampling. By utilizing
appropriate sampling procedures, it is possible to detect insect species infesting the
commodity, to determine the nature of their populations, such as density and dispersion. This
will ultimately help in maximizing the effectiveness of insect control actions. It is also
possible to estimate a threshold or a tolerance level for initiating control measures and
94
consequently this will help in designing cost-effective sampling plans for estimating insect
density and categorizing the infestation level.
The aim of this work was to detect insect species from paddy rice stored in bulk on farm
warehouses, in silos and in a rice mill. The paddy and brown rice were stored in bulk and the
white rice was stored in polyethylene bags. The study included detection of the density, the
spatial pattern of the most abundant insect species collected in farm storage and a comparison
of the total trap catches from two types of traps without lures.
Materials and methods
Collecting sampling data
Farm storage
To detect and study the relative density and spatial pattern of insects associated with paddy
rice; trials were conducted in six storages containing paddy rice, situated in Alcácer do Sal,
Portugal. Five of these storages were horizontal and the sixth was a silo. Sampling lasted from
7 November 2003 to 11 April 2003 (Table 1).
Rice mill
During the course of the year producers, deliver paddy rice to the rice mill situated in
Santiago do Cacém, Portugal. Sampling at this facility, was carried out in the horizontal
storage and in the mill from mid April to 18 July 2003 (Table 2).
Sampling procedure
Farm storage
The number and type of traps used in each place are presented in Table 1. For the paddy rice in
bulk, Pitfall and Probe traps (Storgard WB Probe II traps) were used without lures (Table 1). In
the silo trial, nine Pitfall traps were placed within approximately 30 cm of the Probe traps to
compare trap catch efficacy during nine weeks, from 2 January to 14 March, 2003 (Table 1).
Also at the silo structure site, Storgard Dome traps containing standard attractant oil
without pheromone were placed on the floor below the aerated silos for grain drying. These
dryers were located adjacent to the main paddy rice silos. Pheromone lures for Tribolium
castaneum (Herbst) and T. confusum du Val was used in these traps only during the first four
weeks of trials. The traps were inspected weekly and insects were counted and identified.
Rice mill
Bulk storage at the rice mill was sampled using Pitfall and Probe traps without lures. Store 1
contained paddy and brown rice in bulk, Store 2 finished product in polyethylene bags of a
capacity of 1.1 tonne : 143 bags of white rice and 90 bags of by-product. Inside the mill, 5
Stogard Dome traps were placed on each of the 3 floors. The traps were placed on the floor
among machinery components. Besides the standard attractant oil, pheromone dispensers with
new formulation lures to attract Lasioderma serricorne (F.), Tribolium castaneum, T.
confusum, and Trogoderma species were used and replaced every four weeks. Stogard
Thinline traps with new lures for Lasioderma serricorne, Plodia interpunctella (Hb.) and
Trogoderma species, were also placed on the factory walls. The traps were inspected weekly
from May to July. During each inspection, the insects were counted and identified.
Environmental conditions
For determining environmental conditions in the farm stores, a thermo hygrograph was placed
in the silo structure and in the rice mill. Temperature probes were used in the paddy rice
stored in the farm storages.
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Table 1. Farm storage data: locations sampled, quantity of paddy rice in each storage type and
number of traps used, and the trial period.
Type of trap
Local
Total of
paddy rice
(tonnes) Pitfall Storgard WB
Probe II
Storgard
Dome
Trial period
Warehouse Store 1 510 10 – – 7 Nov. 03 – 24 Jan. 03
Warehouse Store 2 590 10 – – 19 Dec. 02 – 9 Jan. 03
Warehouse Store 3 300 10 3 – 7 Nov. 02 – 7 Mar 03
Warehouse Store 4 240 10 – – 7 Nov. 02 – 5 Dec. 03
Warehouse Store 5 2017 – 6 – 21 Jan. 03 – 7 Mar. 03
Silo 300 9 9 – 7 Nov. 02 – 28 Mar. 03 Silos
structure Floor* – – 12 7 Nov. 02 – 11 Apr. 03
* Dome traps used on the floor, outside the silos and dryers but inside the building.
Table 2. Rice mill data: locations sampled, quantity of rice in each storage type and number of
traps used, and the trial period.
Type of trap
Local
Total o
f
rice
(tonnes) Pitfall Storgard
WB Probe II
Storgard
Dome
Storgard
Thinline
Trial period
paddy rice 946 8 8 – – 15 Apr. – 18 Jul.03 Warehouse
Store 1 brown rice 2000 9 7 – – 15 Apr. – 18 Jul.03
Factory - – – 15 10 24 May – 18 Jul. 03
Warehouse
Store 2 white rice 257 8 4 – – 24 May – 6 Jun.03
Statistical Analysis
The mean number of adult insects caught each week, was determined and used to calculate
the mean crowding ( *
x) according the equation created by Lloyd (1967) and expressed as:
1)( _
2
_
*
x
s
xx ,
where, *
x = mean crowding, _
x= mean insects (
_
x>0); s2= variance of the mean. The mean
and variance necessary to calculate mean crowding can be based on different sample sizes and
on several sets of samples obtained from different locations and at various sampling times
(Subramanyam and Hagstrum, 1996). The mean crowding is used to describe the mean
number of other individuals per individual in an average sample unit and these indices
expressed the level of "crowding" in a given unit of habitat. The patchiness linear regression,
examines the relationship between the Lloyd's mean crowding ( *
x) and the mean insects ( _
x):
*
x = +
_
x. The y-intercept, , has been interpreted as the average number of other
individuals living in the same sample unit or quadrat per individual as is termed as the index
of basic contagion. The measure of clump size is given as ( +1). The slope , also called the
density-contagiousness coefficient, is used as a measure of the dispersion of the basic unit and
indicates the spatial pattern of the clump (Iwao, 1968; Southwood, 1978; Davis, 1994).
96
Table 3. Total of adults and the percentage of the total, of each insect species associated to stored product caught in the traps in the stores, silo
and on the floor of the silos structure (farmers) and in the store and the rice mill.
Farmer’s storage Rice mill
Paddy rice Warehouse Store Bags Factory
Warehouse
Stores Silo Floor*Paddy rice Brown rice White rice Floor
Total % total Total %total Total %total Total %total Total %total Total %total Total %total
Coleoptera
Anthicidae
Anthicus floralis L. 1 2.6E-03
Bostrychidae
Ryzopertha dominica (F.) 2 2.7E-3
Carabidae
Harpalus rufipes (Degeer) 1 2.6E-3
Cryptophagus
Cryptophagus saginatus Sturm 119 0.31 24 0.02 4 0.01
Curculionidae
Sitophilus spp 8 0.02 48 0.05 356 0.73 281 0.67 79 0.11 61 0.49
Laemophloeidae
Cryptolestes sp. 1 2.6E-3 4 4.1E-3 1 2.0E-3
Cryptolestes ferrugineus Steph. 132 0.31 639 0.87 + 44 0.35
Lathridiidae
Coninomus constrictus Gyllenhal 150 0.39 96 0.10 1 2.0E-3
Coninomus nodifer (Westwood)
Coninomus bifasciatus (Reitter) 1 2.6E-3
Mycetophagidae
Litargus balteatus J. Leclerc 3 0.01
Typheae stercorea (L.) 7 0.02 1 0.01
Nitidulidae
Carpophilus dimediatus (F.) 0 0 52 0.11
Ptinidae
Ptinus raptor Sturm 77 0.20 647 0.67 6 0.01
Silvanidae
Ashaverus advena (Waltl) 3 0.01 2
96
97
Farmer’s storage Rice mill
Paddy rice Warehouse Store Bags Factory
Warehouse
Stores Silo Floor*Paddy rice Brown rice White rice Floor
Total % total Total %total Total %total Total %total Total %total Total %total Total %total
Monotoma sp. 1 2.6E-3
Oryzaephilus surinamensis (L.) 11 0.03 7 0.01 2 4.1E-3 3 4.1E-3
Staphilinidae 5 0.01 1 0.01
Tenebrionidae
Gnathocerus cornutus F. 2 0.01 124 0.13 24 0.05 1 1.4E-3
Tribolim castaneum (Herbst) 6 0.01 1 2.4E-3 4 0.01 16 0.13
Tribolim cofusum du Val 1 0.01
Hemiptera
Anthocoridae
Lyctocoris campestris F. 1 2.6E-3
Hymenoptera
Pteromalidae
Anisopteromalus calandrae
(Howard) 6 0.01 3 4.1E-3
Dinarmus basalis Ashm. 2 0.01
Lariophagus distinguendus (Förster) 1 2.6E-3
Chalcydidae
Cerocephala sp. 1 0.01
Lepidoptera
Gelechiidae
Sitotroga cerealella (Olivier) 16 0.02 37 0.08
Pyralidae
Plodia interpunctella (Hb.) 4 4.1E-3
Psocoptera + + + + + +
Total 389 1.00 972 1.00 489 1.00 420 1.00 736 1.00 - - 125 1.00
* Dome traps used on the floor, outside the silos and dryers but inside the building.
97
98
Results
Environmental conditions
Farm storage
The mean temperature of paddy rice in the silo was 21.4 2.0ºC. In the silo structure, the
mean temperature was 14.6 0.4 ºC and the relative humidity was 72.5 1.6%. The
temperature ranged from 10.5ºC during mid January to 17ºC in the first week of
December. The relative humidity varied from 57% in the first week of April to 80.5%,
during the first half of December.
Rice mill
During June and July 2003 the mean temperature in the factory was 25.6 0.4ºC and
ranged from 24.5ºC to 26.5ºC. The relative humidity was 60.5 4.1% and varied from
46% to 70%.
Insect detection
The total number of insects and the percentage of the total of each insect species caught
in the traps in stores, silo and on the floor of the farm silos structure as well as those
insects caught in the stores and the rice mill are presented in Table 3.
Twenty-eight insect species of insects were identified, 21 in the farm storage and
nine in the rice mill. Species of Psocoptera were not identified among these species. The
percentages shown in the following sections do not result the 100% of the population,
there was a residual population that consisted of various storage insects of about 14
species of low percentage each.
In the farm storage the three main species were:
Stores – Coninomus constrictus (39%), Cryptophagus saginatus (31%) followed
by Ptinus raptor (20%).
Silo:
opaddy rice – P. raptor (67%), Gnathocerus cornutus (20%) and C. constrictus
(10%).
ofloor – Sitophilus spp. (73%), which 92% was S. zeamais and 8% S. oryzae,
followed by Carpophilus dimidiatus (10%).
In the rice mill the main species were:
Stores:
opaddy rice - Sitophilus spp., (67%), of which 63% of the total was S. zeamais
and 27% S. oryzae, and Cryptolestes ferrugineus (31%)
obrown rice – C. ferrugineus (87%), Sitophilus spp. (11%), which 67% was S.
zeamais and 23% S. oryzae, and Psocoptera
owhite rice (big bags) – Psocoptera
Factory:
ofloor – Sitophilus spp. (49%), which 61% was S. zeamais and 39% S. oryzae, C.
ferrugineus (35%) and Tribolium castaneum (13%)
owalls – the Stogard Thinline caught no insects.
Psocoptera were constantly present at low populations and were associated with
paddy rice and white rice. High population levels of Psocoptera were also present in
brown rice. Additionally, some parasitoids of Sitophilus larvae and other coleopterans
were caught during trials, primarily Anisopteromalus calandra (Howard).
99
Comparing trap catches of Pitfall and Probe traps
The results of trap catches from Pitfall and Probe traps are presented in Table 4. In the
Probe-traps 345 adults of 10 different insect species were caught while during the same
period, the Pitfall traps caught 16 adults of five insect species.
Table 4. Insect species associated to stored products caught by nine Pitfall and nine
Probe traps at the surface of a paddy rice silo from 2 January to 14 March
2003.
Type of trap
Species Pitfall Probe
Cryptolestes sp. 0 1
Sitophilus zeamais 4 15
Sitophilus oryzae 1 2
Coninomus nodifer 0 1
Coninomus constrictus 1 67
Ptinus raptor 9 228
Oryzaephilus surinamensis 0 3
Ashaverus advena 1 1
Gnathocerus cornutus 0 24
Sitotroga cerealella 0 3
Psocoptera + +
Total 16 345
Relative density and spatial pattern of insect species
associated with paddy rice in the farmer’s storage
Two population proprieties, namely density and dispersion, were examined for the most
abundant insect species present in the farm storage. These species were Coninomus
constrictus, Cryptophagus saginatus, Gnathocerus cornutus, Ptinus raptor and
Sitophilus spp..
Density
The relative density of the five insect species caught in silos and stores are presented in
Fig. 1. Ptinus raptor was the most abundant species collected in the silo and it was
detected also in the flat stores and on the floor of silo structure. It was caught until the
end of March.
The relative density of Sitophilus spp., especially S. zeamais, was particularly high
in the product remains on the floor under the silo type dryer. It was significantly lower
in the product but it was also detected in the traps placed in the paddy rice stored in
stores and silo.
Gnathocerus cornutus was caught until the end of February, mainly in silo and its
population was very low in stores and on the floor of silos structure.
The mycetophagous species, C. constrictus and C. saginatus were also caught in
the traps placed in the paddy rice, in stores and silo. C. constrictus was caught during all
trials while C. saginatus was caught mainly in the stores and until the end of February.
100
Coninomus constrictus
0.00
0.50
1.00
1.50
2.00
2.50
Mean trap catches
Stores Silo floor
Cryptophagus saginatus
0.00
0.50
1.00
1.50
2.00
Mean trap catches
Gnathocerus cornutus
0.00
1.00
2.00
3.00
4.00
Mean trap catches
Ptinus raptor
0.00
5.00
10.00
15.00
Mean trap catches
Sitophilus spp.
0.00
1.00
2.00
3.00
4.00
5.00
6.00
14-11-02
21-11-02
28-11-02
5-12-02
12-12-02
19-12-02
26-12-02
2-1-03
9-1-03
16-1-03
23-1-03
30-1-03
6-2-03
13-2-03
20-2-03
27-2-03
6-3-03
13-3-03
20-3-03
27-3-03
3-4-03
10-4-03
mean trap catches
Fig. 1. Relative density of Coninomus constrictus, Cryptophagus saginatus, Gnatho-
cerus cornutus, Ptinus raptor and Sitophilus spp, caught by traps placed in the
stores, silos and in the floor of silos structure.
Spacial pattern
The Iwao’s patchiness regression estimates for adults of the five insect species are
shown in Table 5.
Except for the _
*xx pairs of adults of C. constrictus, all pairs of the other four
insect species followed well the regression.
The slope- classified as aggregated, the spatial pattern of habitat utilization by the
adults of the five species: very strong tendency of aggregation for C. saginatus,
101
Sitophilus spp. and C. constrictus; weak but with a definitive tendency of aggregation
for adults of P. raptor and G. cornutus.
The y-intercept negative, obtained for adults of C. saginatus, Sitophilus and C.
constrictus, can likely be interpreted as the effects of intraspecific competition among
adults in the same colony. For C. constrictus, the results demonstrated a weak
probability (p = 0.13) for the presence for one adult no influence the presence of another
where the clump size ( +1) is a single individual. For G. cornutus, this probability is
higher (p= 0.51) and the results show a very weak tendency for the presence of a
colony rather than a single adult. For P. raptor, the values of ( +1 = 1+0.78 0.31)
suggesting that more than one individual exist together in the same quadrat.
Table 5. Iwao’s patchiness regression for adults of five insect species associated with
paddy rice.
Insect species naSE SEt /SE SEt /)1( r2
Cryptophagus saginatus 21 -1.53 0.54 -2.84 (P=0.01) 10.37 1.12 8.36 (P=0) 0.81
Sitophilus spp. 45 -0.76 0.27 -2.86 (P=0.01) 3.45 0.19 13.07(P=0) 0.89
Coninomus constrictus 33 -1.32 0.86 -1.55 (P=0.13) 5.97 1.01 4.90 (P=0) 0.53
Ptinus raptor 38 0.78 0.31 2.56 (P=0.01) 1.21 0.08 2.53 (P=0) 0.86
Gnathocerus cornutus 25 0.11 0.16 0.66 (P=0.51) 1.25 0.17 1.50 (P=0) 0.71
a number of _
*xx pairs used in the regression
Discussion
The most abundant insect species reported in farm storage (C. saginatus and C.
constrictus) tend to be associated with moulds and accumulations of residues (P. raptor)
and are usually an indication of deteriorating grain due to poor storage conditions, or
lack of an effective management program. However, the main insect species discovered
in the rice mill are important rice pests (Sitophilus spp. and C. ferugineus).S. zeamais
and S. oryzae were the two species identified, although S. zeamais was the dominant
species in all records from the farm storage to the rice mill.
Results tend to indicate that paddy rice stored on the farms surveyed increases in
moisture because of increasing humidity over the storage life. This increase in paddy
rice moisture favours mould growth and therefore the presence of the mycetophagous
species of insects (Adler, 1998). This implies the risk of mycotoxin production if critical
moisture contents are exceeded and it would thus be recommended to avoid such
conditions by drying procedures.
When conditions tend to be cooler, trap captures may not truly reflect the actual
populations as inactivity of the pest species and/or migration may have affected
different species to a different extent. Possibly examining grain samples from various
positions of the grain stores may give a more accurate picture in future surveys.
The number of insect species associated with the processing facility compared to
the trap captures at farmers storage decreases significantly, while the presence of
primary insect pests such as Sitophilus spp, in paddy rice, and C. ferrugineus, in brown
rice stand out dramatically. The presence of Psocoptera, especially in the rice mill in
brown rice was quite consistent.
102
The most abundant insect species present in the farmers’ storage, were recorded
mainly in the vertical silo, as G. cornutus and P. raptor; in stores and silo, C. saginatus
and C. constricutus; while Sitophilus spp. were discovered mainly in the residues of
product under dryers. In addition, the adults of these insect species were distributed in
an aggregate pattern. In the colonies, between individuals, C. saginatus, Sitophilus spp.
and C. constrictus may show some repulsion interaction and competition; the presence
of an adult of G. cornutus does not influence the presence of other and the clump size is
a single individual and for adults of P. raptor, more than an individual tend to live
together in the same quadrat.
The Probe trap was significantly the most efficient trap when compared with the
Pitfall trap, regarding the number of adults and the insect species recorded in each type.
The efficacy of Probe traps have also been recorded from several authors (Lippert &
Hagstrum, 1987; Fargo et al., 1994; Hagstrum et al, 1998; Hagstrum, 2000).
Acknowledgements
The authors express their gratitude to Dr. Jordi Riudavets for his contribution in this
study. We also thank APARROZ and paddy rice farmers association for allowing the
conductance of the trials in their stores and silos structures and especially the manager
and friend João Reis Mendes. We thank SEAR, for allowing the trials in the rice mill,
especially Dr. Romano Mancini and Altino Teixeira; and Trécé (Salinas, EUA),
specially Bill Lingren and Selina AA-Stewart for their supply of the Storgard Dome and
Thinline traps.
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