Vol.3, No.6, 789-794 (2012) Agricultural Sciences
Lethal time at different temperatures and date variety
preference of the saw-toothed grain beetle in stored
Mohammad Ali Al-Deeb
Biology Department, United Arab Emirates University, Al-Ain, UAE; m_aldeeb@uaeu.ac.ae
Received 20 August 2012; revised 28 September 2012; accepted 3 October 2012
The saw-toothed grain beetle, Oryzaephilus su-
rinamensis (L.) (Coleoptera: Silvanidae), is the
most important insect pest attacking stored
dates in the United Arab Emirates. We sought to
determine the time required to cause 100%
mortality (lethal time) in O. surinamensis adults
incubated at different temperatures, to measure
temperature penetration time inside a date mass,
and to study date variety preference of O. suri-
namensis adults. To do this, adults of O. suri-
namensis were separately incubated at 22˚C,
50˚C, and 55˚C, for 5, 10, 20, 30, 60, 90, and 120
min, per temperature treatment; and we used
feeding choice tests on four date varieties (Khe-
las, Fardh, Lulu, and BuMaán). Results showed
that the lethal times of O. surinamensis adults,
incubated at 22˚C, 50˚C, and 55˚C, without
dates, were 5, 10, and 20 min, respectively; while
they were several times higher (30, 90, and 120
min, respectively) in the presence of dates. Re-
sults also showed that bigger date masses re-
quired either more heating or more freezing time
to reach lethal temperatures. We found Khelas
to be the least preferred date variety. Future
studies should evaluate if heat and chilling in-
jury affect postharvest date quality.
Keywords: Oryzaephilus surinamens is; Freezing;
Mortality; UAE
The date palm tree (Pho enix da ctylifera L.) is the most
important fruit tree in the United Arab Emirates (UAE).
Annual date production in the UAE had a 30-fold in-
crease from <8000 metric tons (MT) in 1971 to >240,000
MT in 1995 [1]. Additionally, the Food and Agriculture
Organization of the United Nations estimated date pro-
duction at 775,000 MT in 2010 [2]. Dates are attacked by
several insects, both in the field and during storage, but
mainly by the saw-toothed grain beetle, Oryzaephilus
surinamensis (L.) (Coleoptera: Silvanidae). This beetle is
widely distributed and commonly found in stored grain
[3]. In addition to the UAE, this species is also recorded
as one of the most important insect pests of stored palm
dates in Saudi Arabia, Egypt, Sudan, Jordan, Tunisia,
and Algeria [4,5]. Larvae can develop in flour, cereal
products, and many dried fruits, including dates. Adults
are 2.5 mm long and their bodies are very flat and
well-adapted for crawling into cracks and crevices [6].
The female adult lays its eggs either individually or in
small batches in and around a food supply. Development
from egg to adult takes approximately 2 months and
adult females typically live 6 - 10 months but can live up
to 3 years if conditions are ideal [7]. This insect can
penetrate soft packaging: in an experiment simulating
packaging flaws, females laid more eggs either inside or
near holes in plastic packaging film [8]. O. surinamensis
can also survive on chocolate and therefore, insect-proof
packaging and storage under hygienic conditions are
extremely important for avoiding infestation and cus-
tomer complaints [9].
Several control options against O. surinamensis exist,
including the use of the entomopathogenic fungus,
Beauveria bassiana [10]; reduced-risk, natural pesticides,
such as Spinosad [11]; and diatomaceous earth [12].
Spinosad is effective against O. surinamensis and other
insects that are common in stored grain for at least 6
months when applied as a single dose at 1 mg a.i./kg
Fumigation is the most effective method against in-
sects of stored products [14]. The fumigant methyl bro-
mide is widely used in many countries to control O. su-
rinamensis and other insect pests of stored products. In a
study, accumulation of bromide residues on sultanas and
raisins was directly related to both the number of fumi-
gations and the concentration of applied methyl bromide
[15]. Although an effective fumigant against many insect
Copyright © 2012 SciRes. OPEN ACCESS
M. A. Al-Deeb / Agricultural Sciences 3 (2012 ) 789-794
pests, methyl bromide is a gas that can deplete ozone
layer as it releases bromine approximately 40 times
faster than chlorine [16]. The Montreal Protocol banned
methyl bromide in 2005, and developing countries have
recommendations to gradually reduce its use towards
complete elimination by 2015 [17].
To avoid the use of chemicals and their potential side
effects, low and high temperatures can be used to kill
insects in stored products. One study found low tem-
perature treatment to be an effective alternative to fumi-
gation [18]. Heat sterilization (i.e., superheating) can also
be used as a control option for pests of stored grains
[19,20]. We, therefore, sought to determine more specif-
ics on how low and high temperature treatments affect O.
surinamensis adults that infest dates. Our main objec-
tives were: to determine the time required to cause 100%
mortality (=lethal time) in O. surinamensis adults incu-
bated at 22˚C, 50˚C, and 55˚C; to measure temperature
penetration time inside a date mass; and to study date
variety preference of O. surinamensis adults.
2.1. Insects
Adults of O. surinamensis were collected from in-
fested dates. Insects were mass-reared in the laboratory
on pesticide-free dates placed inside 2000-ml Sunpet®
jars (Sun Packing Systems FZC, UAE) made of polyeth-
ylene terephthalate (PET). Jars were incubated at room
temperature (25˚C ± 2˚C) in the entomology laboratory
of the Biology Department of UAE University.
2.2. Lethal Time at Different Temperatures
Adults of O. surinamensis were incubated at –22˚C for
5, 10, 20, 30, 60, 90, and 120 min. We selected this tem-
perature because it is the common temperature used in
many commercial freezers of homes and date-storing
facilities. The experiment included two treatments (in-
cubation with dates and incubation without dates, both at
–22˚C) and a control (incubation at room temperature, at
25˚C ± 2˚C). We incubated insects with dates similar to
their natural infestation, in which the insects are either
between or inside dates. Incubation of insects without
dates was representative of natural insect presence in the
cracks and the crevices of either a store or its machinery.
Each exposure time was tested in a separate experiment.
We initially separated unsexed adults of O. surinamensis
into plastic ziplock bags (10 cm × 15 cm), 20 insects per
bag. Each treatment had four replicates, amounting to
four ziplock bags and a total of 80 insects. We placed
200-g samples of dates inside each treatment bag. We
recorded the total number of dead insects after incuba-
tion, and used it to calculate percent mortality. Every
experiment was repeated 4 times. The above methodol-
ogy was also used to measure lethal time at 50˚C and
2.3. Temperature Penetration Time in a Date
Time (min) required for temperature penetration,
through a known depth inside a date mass, was measured
with a digital EasyLog EL-USB-5 thermometer (Lascar
Electronics, UK). Dates were placed inside 1500-ml
clear Sunpet® jars made of PET. The digital thermome-
ter was placed into the center of the date mass, inside the
jar, at equal distances from the outside environment
(Figure 1). The temperature was measured at two depths
(3.7 and 5.5 cm) inside the date mass while it was in a
22˚C freezer. The goal was to lower the date mass tem-
perature, at these two depths, from room temperature
(22˚C) to a low target temperature (17˚C). To measure
the effect of ventilation on temperature penetration time,
the same jar was replaced with cheesecloth #10. Similar
to above, the digital thermometer was placed inside a
date mass that was wrapped in cheesecloth, at equal dis-
tances (3.7 cm) from the outside environment. Our ex-
periment on heat penetration time was limited to one
preliminary experiment in which the temperature was
raised from 32˚C to 53˚C, at 3.7 cm deep, inside a date
mass that was placed inside a clear PET jar and a 55˚C
2.4. Date Variety Preference
A feeding choice test was conducted in the laboratory
to determine variety preference of adult O. surinamensis
for four date varieties that are commonly grown and con-
sumed in the UAE: Khelas, Fardh, BuMaán, and Lulu.
Figure 1. A clear polyethylene tere-
phthalate (PET) jar containing a digital
thermometer, placed equidistant from
all sides, to measure temperature inside
a date mass (3.7 cm deep).
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M. A. Al-Deeb / Agricultural Sciences 3 (2012 ) 789-794 791
They were selected for their high market value and pref-
erence by customers, who especially like the Khelas va-
riety. The feeding choice arena consisted of a square
plastic container (27 × 27 × 9.5 cm), covered by a lid that
was fitted tightly to prevent insect escape. We wrapped
100-g samples of dates, from each of the four varieties,
within single layers of cheesecloth. Wrapped dates were
placed at one of the four corners of the container. To
provide equal selection opportunity for the four varieties,
they were placed at equal distances from the insect re-
lease point. This was achieved by marking the two di-
agonals of the square container on its bottom, using a
black marker and a ruler, so that the line intercept was
the central release point. Two hundred unsexed adults of
O. surinamensis, initially placed inside a 15-ml blue
plastic cap tube (Sterilin, UK), were released into the
arena by inverting the tube at the center of the container.
The lid was attached and the arena was placed inside a
dark room that was maintained at 25˚C ± 2˚C. We used a
complete randomized design with date variety as the
fixed effect.
2.5. Statistical Analysis
We used arithmetic means to compare the lethal times
at 22˚C, 50˚C, and 55˚C. For the feeding choice ex-
periment, we tested for differences in O. surinamensis
adult numbers with an analysis of variance (ANOVA)
[21], with date variety as the classification variable.
ANOVA was performed with a SAS PROC GLM and
Fisher least significant difference, with significance set
at P < 0.05.
3.1. Lethal Time at Different Temperatures
The lethal time of O. surinamensis adults, incubated at
22˚C, was 20 min without dates but 120 min with dates
(Figure 2). The lethal time of O. surinamensis adults at
50˚C was 90 min and 10 min, with and without dates,
respectively; and, at 55˚C, 30 min and 5 min, with and
without dates, respectively (Figure 2).
3.2. Temperature Penetration Time in a Date
An average of 713 min was needed to decrease the
temperature from 22˚C to 17˚C, 3.7 cm inside a date
mass that was placed inside a clear plastic jar and a
22˚C freezer. Under the same temperatures, this time
increased to an average of 978 min when inside a bigger
date mass, 5.5 cm deep (Figure 3). The use of cheese-
cloth instead of the clear PET jar, however, decreased the
time to an average of 343 min, under the same tempera-
tures (Figure 4). Finally, an average of 350 min was
Figure 2. The effect of incubating Oryza-
ephilus surinamensis adults, either with or
without dates, on the time (min) required
to cause 100% mortality at 22˚C, 50˚C,
and 55˚C.
Figure 3. Effect of date mass depth on the
time (min) required to decrease tempera-
ture from 22˚C to 17˚C, as measured in-
side a date mass placed into a 22˚C
Figure 4. Effect of container ventilation
(clear polyethylene terephthalate [PET] jar
vs. cheesecloth) on the time (min) required
to decrease temperature from 22˚C to
17˚C, as measured inside a date mass
placed inside a 22˚C freezer.
needed to raise the temperature from 32˚C to 53˚C, 3.7
cm inside a date mass that was placed inside a clear PET
jar and a 55˚C oven.
3.3. Date Variety Preference
We found significant differences among the number of
O. surinamensis adults that infested the four date varie-
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M. A. Al-Deeb / Agricultural Sciences 3 (2012 ) 789-794
ties (F = 5.30; df = 3; P = 0.0265; Figure 5). Khelas was
the least infested variety (19.1 ± 1.8), followed by Bu-
Maán (23.5 ± 2.8), Lulu (27.9 ± 2.1), and Fardh (29.6 ±
1.1), which had the highest number on insects. Khelas
had significantly less insects, as compared to Lulu and
Fardh (t = 3.05063, df = 3, P = 0.0158; and t =
3.62405, df = 3, P = 0.0067, respectively), but was not
significantly different from BuMaán (t = 1.53678, df =
3, P = 0.1629). Fardh, Lulu, and BuMaán were not sig-
nifycantly different from each other.
4.1. Lethal Times at Different Temperatures
Extreme temperatures may become rapid, non-che-
mical alternatives to fumigation and chemical controls of
insects in stored products. Our study showed that O. su-
rinamensis adults can be killed by exposing them to
22˚C for 20 min in the absence of dates, and for a
longer time (120 min) in the presence of dates. Similarly,
lethal time increased from 10 to 90 min at 50˚C, and
from 5 to 30 min at 55˚C, in the absence and the pres-
ence of dates, respectively.
A longer lethal time when incubating insects with
dates most likely occurs because dates provide protection
to insects from rapidly changing temperatures inside
freezers and ovens. Some insects may seek shelter in the
small spaces between dates and others may hide either
under the fruit cap (perianth) or inside the dates. Fur-
thermore, since date temperature changes more gradually
than ambient surroundings, hiding insects have more
time to live. This latter explanation applies more to in-
sects that hide inside uncapped dates than those hiding
between dates. At 50˚C and 55˚C, heat transfer occurs
both by convection from air to dates and by conduction
through contact between outside warmer dates and
nearby dates that are less warm. Furthermore, the pres-
ence of a pit (seed) inside the date provides some level of
thermal insulation, which can delay both heat transfer,
Figure 5. Effect of date variety on the
number (mean ± standard error) of Oryza-
ephilus surinamensis adults attracted to
dates during a feeding choice test.
between dates, and mortality for hiding insects inside the
date. This possible delay of heat transfer occurs because
the pit has a wood-like structure due to its cellulose,
hemicellulose, and lignin components [22].
According to our results, approximately 12 hrs are
needed to decrease the temperature of dates from 22˚C to
17˚C, 3.7 cm inside a date mass placed inside a clear
PET jar and a 22˚C freezer. The freezing time was ap-
proximately 16 hrs when using a bigger date mass (5.5
cm deep), resulting in a 33.3% time increase.
The most important step to ensure successful cold
treatment is to keep both the packaging small enough and
freezer time long enough to ensure that a product com-
pletely freezes. Our study also shows that freezing time
can be shortened by placing dates inside ventilated con-
tainers (e.g., cheesecloth). By providing ventilation,
freezing time drops from an average of 713 to 343 min.
This 50% reduction in treatment time could considerably
decrease treatment cost. In fact, more time and cost re-
ductions could be achieved if dates were also placed
loosely in single layers (one date deep) on screen trays
during cold treatment so that cold air effectively chills
every date from all sides to reach the target lethal tem-
perature in a short time.
Our results show that the heating of dates to kill in-
sects also require treatment for several hours. Date qual-
ity in terms of either moisture loss or color change, is
less compromised with freezing. In fact, freezing is a
common food storage method that also helps retain food
vitamin content, color, flavor, and texture. One of our
preliminary experiments, however, showed that approxi-
mately 6 hrs were needed to raise the temperature from
32˚C to 53˚C, 3.7 cm deep inside a date mass that was
placed inside a clear PET jar and a 55˚C oven. Such a
long heating time (with even longer times for larger date
masses) may affect date quality, especially by darkening
the color. One study [23] found slight date color changes
and water losses, after using solar heat to control insects
on stored dates in Saudi Arabia. Most published research
on heat treated insect control methods are done on bulk
grains, where extended heating time does not affect grain
quality. In contrast, dates are mostly eaten fresh, without
processing; therefore, temperatures that successfully dis-
infest grain may negatively lower stored date quality.
Additional studies on the impact of heat treatments on
date fruit quality are needed to help identify the degree
of damage in the different date varieties. Nevertheless,
heat could be effectively used to disinfest machinery
plants, materials, and structures. Since O. surinamensis
adults die within a few minutes of either cold or heat
treatments (according to our results), heating and freez-
ing are non-chemical approaches that could be easily
used in date processing factories as alternatives to fumi-
gation. Flour mills and food processing plants in the
Copyright © 2012 SciRes. OPEN ACCESS
M. A. Al-Deeb / Agricultural Sciences 3 (2012 ) 789-794 793
United States and Canada already routinely incorporate
heat disinfestation into their pest control strategies by
using temperatures of 55˚C - 60˚C for 24 hr [24].
4.2. Date Variety Preference
Khelas was significantly less infested than other date
varieties, indicating less preference by O. surinamensis.
It was followed by BuMaán, Lulu, and Fardh, which
were not significantly different from each other. These
results show that Khelas is not highly preferred when
other varieties, such as Fardh, Lulu, and BuMaán, are
available. One possible explanation is that Khelas fruits
are very soft and become sticky during storage from their
production of a thick sugar solution. O. surinamensis
individuals cannot move freely within the date mass of
these sticky dates, can get stuck on the dates and date
container walls, and may eventually die. Other varieties,
such as Fardh, are neither soft nor become sticky during
long storage, possibly explaining preference by O. suri-
Our results showed that the lethal times of O. surina-
mensis adults incubated at 22˚C, 50˚C, and 55˚C, were
but a few minutes (5, 10, and 20 min, respectively) with-
out dates, yet several times higher (30, 90, and 120 min,
respectively) with dates. The results also showed that
bigger date masses took longer time to reach lethal tem-
peratures. To save time, energy, money, and postharvest
date quality, we recommend dates be frozen unpackaged,
in single layers, one date deep, and on screen trays so
that air can reach every date to chill it to its targeted tem-
perature. Consequently, this process will efficiently kill
O. surinamensis adults, and do so more safely than other
methods. Future studies, however, should evaluate if heat
and/or chilling injury from certain exposure times re-
duces date quality. In the choice test, Khelas was the
least preferred date variety as compared to Fardh, Lulu,
and BuMaán, yet insects still infested all of them. Future
studies should include more varieties and investigate
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