Lethal time at different temperatures and date variety preference of the saw-toothed grain beetle in stored dates

doi:10.4236/as.2012.36095

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Vol.3, No.6, 789-794 (2012) Agricultural Sciences

http://dx.doi.org/10.4236/as.2012.36095

Lethal time at different temperatures and date variety

preference of the saw-toothed grain beetle in stored

dates

Mohammad Ali Al-Deeb

Biology Department, United Arab Emirates University, Al-Ain, UAE; [email protected]

Received 20 August 2012; revised 28 September 2012; accepted 3 October 2012

ABSTRACT

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

1. INTRODUCTION

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

[13].

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

M. A. Al-Deeb / Agricultural Sciences 3 (2012 ) 789-794

790

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. MATERIALS AND METHODS

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

55˚C.

2.3. Temperature Penetration Time in a Date

Mass

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

oven.

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).

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. RESULTS

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

Mass

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

freezer.

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-

M. A. Al-Deeb / Agricultural Sciences 3 (2012 ) 789-794

792

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. DISCUSSION

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

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-

namensis.

5. CONCLUSIONS

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

resistance.

REFERENCES

[1] Date Palm Culture in UAE (2009) Plant Tissue Culture

Laboratory. UAE University, Al Ain.

http://datepalm.uaeu.ac.ae/subpages/datepalmcultural.htm

[2] FAOSTAT (2012) FAO Statistics. Countries by commod-

ity. http://faostat.fao.org/site/339/default.aspx

[3] Calvin, D. (2001) Saw-toothed and Merchant Grain Bee-

tle. Cooperative Extension Service. Department of Ento-

mology, Pennsylvania State University, Altoona, SG-12.

[4] Al-Hafeedh, E.M. (2003) Date palm trees and dates in

Iraq and countries of the Gulf Cooperation Council.

Al-Yakoot Publication and Distribution, Amman.

[5] Najeeb, M.A. (2001) Pests and diseases of date palm.

Ministry of Agriculture and Water. Extension Directorate.

Kingdom of Saudi Arabia.

[6] Mason, L.J. (2003) Grain insect fact sheet. Cooperative

Extension Service. Department of Entomology, Purdue

University, West Lafayette.

[7] Kilpatrick, A.L., Zungoli, P.A. and Benson, E.P. (2004)

Saw-toothed grain beetle. Cooperative Extension Service.

Department of Entomology, Clemson University, Clem-

son.

[8] Mowery, S.V., Mullen, M.A., Campbell, J.F. and Broce,

A.B. (2002) Mechanisms underlying saw-toothed grain

beetle (Oryzaephilus surinamensis [L.]) (Coleoptera: Sil-

vanidae) infestation of consumer food packaging materi-

als. Journal of Economic Entomology, 95, 1333-1336.

doi:10.1603/0022-0493-95.6.1333

[9] Khamrunissa, B., Reddy, P.V., Leelaja, B.C., Rajashekar,

Y. and Rajendran, S. (2006) Studies on insect infestation

in chocolates. Journal of Stored Products Research, 43,

118-122. doi:10.1016/j.jspr.2006.02.001

[10] Throne, J.E. and Lord, J.C. (2004) Control of saw-

toothed grain beetles (Coleoptera: Silvanidae) in stored

oats by using an entomopathogenic fungus in conjunction

with seed resistance. Journal of Economic Entomology,

97, 1765-1771. doi:10.1603/0022-0493-97.5.1765

[11] Fang, L., Subramanyam, B. and Arthur, F.H. (2002) Ef-

fectiveness of spinosad on four classes of wheat against

five stored-product insects. Journal of Economic Ento-

mology, 95, 640-650. doi:10.1603/0022-0493-95.3.640

[12] Arthur, F.H. (2000) Immediate and delayed mortality of

Oryzaephilus surinamensis (L.) exposed on wheat treated

with diatomaceous earth: Effects of temperature, relative

humidity, and exposure interval. Journal of Stored Prod-

ucts Research, 37, 13-21.

[13] Subramanyam, B., Toews, M.D., Ileleji, K.E., Maier, D.

M., Thompson, G.D. and Pitts. T.J. (2006) Evaluation of

spinosad as a grain protectant on three Kansas farms.

Crop Protection, 26, 1021-1030.

doi:10.1016/j.cropro.2006.09.011

[14] Lelieveld, H. (2005) Improving Hygiene in the Food

Industry. GBR, Woodhead Publishing, Cambridge.

[15] Hilton, S.J. and Banks, H.J. (1997) Methyl bromide sorp-

tion and residues on sultanas and raisins. Journal of

Stored Products Research, 33, 231-249.

doi:10.1016/S0022-474X(97)00003-9

[16] Morrissey, W.A. (2004) Stratospheric ozone depletion

and regulation of methyl bromide. CRS Report for Con-

gress. RS20863.

http://www.ncseonline.org/nle/crsreports/04dec/RS20863.

pdf

[17] UNEP (2004) Montreal protocol talks to consider exemp-

tions from methyl bromide ban. Press Release, UNEP/211.

http://www.un.org/News/Press/docs/2004/unep211.doc.ht

M. A. Al-Deeb / Agricultural Sciences 3 (2012 ) 789-794

794

[18] Donahaye, E.J., Navarro, S. and Rindner, M. (1995) Low

temperature as an alternative to fumigation for disinfect-

ing dried fruit from three insect pests. Journal of Stored

Products Research, 31, 63-70.

doi:10.1016/0022-474X(94)00037-T

[19] Sheppard, K.O. (1984) Heat sterilization (superheating)

as a control for stored-grain pests in a food plant. In: F. J.

Baur (Ed.), Insect Management for Food Storage and

Processing. American Association of Cereal Chemists, St.

Paul, 193-200.

[20] Wright, E.J., Sinclair, E.A. and Annis, P.C. (2002) Labo-

ratory determination of the requirements for control of

Trogoderma variabile (Coleoptera: Dermestidae) by heat.

Journal of Stored Products Research, 38, 147-155.

doi:10.1016/S0022-474X(01)00011-X

[21] SAS (2001) SAS users guide: Statistics. 8th Edition, Sta-

tistical Analysis System Institute, Cary.

[22] Barreveld, W.H. (1993) Date palm products. FAO Agri-

cultural Services Bulletin No. 101.

http://www.fao.org/docrep/t0681e/t0681e00.htm#con

[23] Assiry, A.M. (2005) Penetration curves of solar heat into

date fruits as a mean to control insects. King Saud Uni-

versity.

http://faculty.ksu.edu.sa/Assiry/Documents/18_alhussein_

assiri.pdf

[24] CSIRO (2009) Heat treatment.

http://www.csiro.au/resources/graininheatdisinfestation.ht