Download Diagnostic Methods for Granary weevil Sitophilus granarius

Diagnostic Methods
for
Granary weevil
Sitophilus granarius
This Diagnostic Protocol can be constantly updated and is only correct at time of printing (Thursday
8th of June 2017 at 03:33:58 PM).
The website http://www.padil.gov.au/pbt should be consulted to ensure you have the most current
version before relying on the information contained.
Introduction
Granary weevil Sitophilus granarius (Linnaeus, 1758), along with the closely related rice weevil, is
among the most destructive of all stored grain insects. The larvae develop inside kernels of whole
grain in storage, thus making an infestation difficult to remove in the milling process. The granary
weevil is mainly a pest of stored grains, especially in bulk storages, grain mills and elevators, and
does not develop in loose flour or products unless it is compacted. It is more common in cooler areas
than rice weevil.
Apart from the indirect effects, arising from the production of heat by the insects, the major effect of
infestation by Sitophilus spp. is the damage to grain by feeding activities of the adults and the
development of immature stages within the grain. This not only reduces the grain quality but also
produces a considerable amount of grain dust mixed with frass (Longstaff 1981, Anon 2008).
People who are regularly exposed to weevil infested grain may develop allergic responses including
rhinitis, pruritis or marked asthma. This may be due to a weevil protein in mill dust that results in
sensitization in those exposed continuously (Lunn 1966).
Biology
Stages of Development
Description of life stages
Complete development time for the life cycle of S. granarius averaged 44 days (range 39-50 days)
at near optimum conditions of 27 ± 1oC, and 69 ± 3% RH. ( Kirkpatrick & Wilbur (1965).
Detail on lifecycle if relevant
Females lay between 50 and 400 eggs during their lifetime and usually deposit one egg in each grain
kernel. Egg, larval and pupal stages develop inside the grain, and are rarely seen outside. The larval
stages feed on the internal portions of whole grains, making early detection of infestations difficult
(Lyon, undated, Woodbury 2008).
The female drills a hole into the kernel, deposits the egg, and then secretes a mucilaginous plug to
enclose the egg as the ovipositor is withdrawn. The plug rapidly hardens, leaving a small raised area
above the seed surface, which provides the only external evidence that the kernel is infested. Eggs
may be laid anywhere in the kernel, but few are laid in the embryo. In wheat, most eggs are
deposited at the end farthest from the embryo. Sometimes, more than one egg may be laid in a single
grain but it is rare for more than one larva to develop to maturity, because of cannibalism (Longstaff
1981, Anon 2008, Woodbury 2008, Niewiada et al. 2005).
Not all excavated holes are used for oviposition; some are abandoned and others are expanded into
feeding holes (Campbell 2002).
Woodbury (2008) found that females are deterred from feeding or ovipositing into grain kernels that
already contain an egg plug but the mechanism is not known.
Like several other grain infesting insects, female S. granarius prefer to lay their eggs into clean
grain rather than on grain on which weevils have fed previously. They also lay their eggs away from
the embryo, thus preventing egg injury by other beetles (Niewiada et al. 2005).
In wheat of high moisture content (>13.5%) at 26-30oC, infestations can develop rapidly with eggs
hatching in 3 days, larvae maturing in 18 days, and pupae in 6 days (Lyon, undated). Infestation can
begin at temperatures as low as 15oC (Anon. 2008).
There are four larval instars all of which remain within the grain (Fig. 1). Immediately on hatching,
the first instar feeds by burrowing through the tissues of the grain. At the end of the fourth instar the
larva uses a mixture of frass and larval secretion to close off the end of the burrow, to form a pupal
cell. Under normal developmental conditions, weevil larvae allow their frass to accumulate around
them inside the grain in which they are feeding. However, if the carbon dioxide level exceeds 5%,
the fourth instar larva makes a small hole in the grain and ejects much of the frass. The larva then
assumes a prepupal form for a short period before transforming into the pupa (Longstaff 1981).
Mature 4th-instar larvae, prepupae, pupae, and newly emerged adults occupy most of the space
within a wheat grain, since there is not enough space or food for a second insect to develop
regardless of the number of eggs deposited (Kirkpatrick & Wilbur 1965).
The newly emerged adult remains inside the grain for several days for it's cuticle to harden and
mature. The time varies from 5 days at 25oC to 80 days at 12.5oC (Longstaff 1981). It then chews its
way out of the kernel, leaving a characteristically large and roughly oblong emergence hole with
ragged edges (Figs. 2, 3) (Bousquet 1990, Kirkpatrick & Wilbur 1965). After leaving the kernel,
females move to a surface above the food to release sex pheromone to attract males for mating
(Mason 2003).
Life cycle can be as short as about 5 weeks in summer, but may take up to 20 weeks in cooler
temperatures. Adults live 7 to 8 months, with one pair of weevils producing up to 6,000 offspring in
one year (Lyon, undated, Anon. 2008). Several studies have found that egg and pupal development
rates are not affected by relative humidity, whereas all four larval instars developed more rapidly as
relative humidity increased (Schwartz & Burkholder 1991). Granary weevils feign death by drawing
their legs close to the body, falling and remaining immobile when disturbed (Lyon, undated).
Dispersal
Adults are flightless, and the species is usually restricted to stored grain (Bousquet 1990) but they
may walk great distances inside warehouses and bulk grain storages.
Weevils from infested grain may become entrained in a product stream prior to packaging in
processing plants, and may be carried in or on packaging materials such as boxes and pallets.
Survival
S. granarius has been shown to occur and survive outside grain storage facilities in temperate
climates. Untreated grain residues outdoors may be readily colonised, and become potential sources
for re-infestation of grain in storage (Kucerová et al. 2005).
Figure 1. Larva and pupa of Sitophilus granarius. Image: Agriculture Western Australia
http://agspsrv34.agric.wa.gov.au/Ento/pestweb/images/granaryweevil2degesch.jpg
Host Range
Adults and larvae feed on a wide variety of grains, such as wheat, oats, rye, barley, oats, sunflower
seed, bird seed and corn (Figs. 3, 4). Sitophilus spp. have been reported to prefer large seeds for
oviposition; larger seeds are more likely to be parasitized or contain multiple eggs (Lyon, undated,
Campbell 2002).
Suitability of various grains for granary weevil development (in order from highest to lowest) is
wheat, rye, barley, corn, and oats. Development times are longer on corn, presumably because of its
hard, horny endosperm (Schwartz & Burkholder 1991).
Distribution
Widely distributed throughout Australia.
Transmission
Dispersal
Adults are flightless, and the species is usually restricted to stored grain (Bousquet 1990) but they
may walk great distances inside warehouses and bulk grain storages.
Weevils from infested grain may become entrained in a product stream prior to packaging in
processing plants, and may be carried in or on packaging materials such as boxes and pallets.
Establishment Potential
S. granarius is already established in Australia in cooler southern areas.
Detection
Detection Method
Balasubramanian, A., Jayas, D.S., Fernando, W.G.D., Li, G., White, N.D.G. 2007. Sensitivity
analysis of DNA fingerprinting technique for detecting insect fragments in wheat flour. Canadian
Biosystems Engineering, 49: 41-45.
http://www.bashanfoundation.org/dilantha/dilanfingerprinting.pdf
Brader, B., Lee, R.C., Plarre, R., Burkholder, W. et al. 2002. A comparison of screening methods
for insect contamination in wheat. Journal of Stored Products Research, 38: 7586. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T8Y-43YY60T9&_user=139754&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000011578&_versio
n=1&_urlVersion=0&_userid=139754&md5=6f080a6a1f8b79906d6c1e754bbe0ef2
Fornal, J., Jelinski, T., Sadowska, J. et al. 2007. Detection of granary weevil Sitophilus granarius
(L.) eggs and internal stages in wheat grain using soft X-ray and image analysis. Journal of Stored
Products Research, 43: 142-148.
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T8Y-4JT8DKV1&_user=139754&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000011578&_versio
n=1&_urlVersion=0&_userid=139754&md5=ac446f70c87fd81bc488ea597d6f8e77
Obrepalska-Steplowska, A., Nowaczyk, K., Holysz, M., Gawlak, M., Nawrot, J. 2008. Molecular
techniques for the detection of granary weevil (Sitophilus granarius L.) in wheat and flour. Food
Additives and Contaminants: Part A - Chemistry Analysis control Exposure & Risk Assessment,
25(10): 1179-1188.
Rajendran, S. 2005. Detection of insect infestation in stored foods. Advances in Food and Nutrition
Research, 49: 163-232.
Rotondo, G., Germinara, G.S., De Cristofaro, A. 2000. Immuno-osmophoretic technique for
detecting Sitophilus granarius (L.) infestations in wheat. Journal of Stored Products Research,
36(2): 153-160
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T8Y-3YJYDG56&_user=139754&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000011578&_versio
n=1&_urlVersion=0&_userid=139754&md5=e0c2970a4756f72b4445ef5f1608e71a
Schatzki, T.F., Wilson, E.K., Kitto, G.B., Behrens, P., Heller, I. 1993. Determination of hidden
Sitophilus granarius (Coleoptera: Curculionidae) in wheat by myosin ELISA. Journal of Economic
Entomology, 86(5): 1584-1589.
Symptom Description
S. granarius feed on the individual grains, leaving only the hulls, and a severe infestation can reduce
stored grain to a mass of hulls and flour-like, powdery frass. Infested grains exhibit holes where
adults have emerged (Figs. 3, 4). Grain which floats on the surface of water often indicates larval
damage inside the kernel.
A wheat grain normally contains only sufficient endosperm to support the development of a single
larva. A comparison of weight loss caused by adult feeding showed that Rhyzopertha dominica
adults are the most destructive, followed by S. granarius. If left undisturbed, S. granarius and R.
dominica adults can reduce wheat grains to a shell of bran. Production of frass is an important byproduct of insect activity during infestation, because frass pollutes the grain bulk making the grain
unsuitable for human and often livestock production. Frass produced by S. granarius consists
mainly of exuvia, faeces, and urinary products. (Campbell & Sinha 1976).
Symptom Images
Figure 2. Scanning electron microscope image of a Sitophilus granarius (x 50). Note the elongated
snout or rostrum, with the chewing mouthparts at the end. These weevils cannot fly. Artificially
coloured by computer. Image: The Natural History Museum,
London. http://piclib.nhm.ac.uk/piclib/www/image.php?search=weevil&getprev=59906
Figure 3. Adult Sitophilus granarius in wheat. Image: Clemson University - USDA Cooperative
Extension Slide
Series, Bugwood.org http://www.ipmimages.org/browse/detail.cfm?imgnum=1435092
Figure 4. Adult Sitophilus granarius infesting maize seed. Image: CSIRO Entomology, Australia.
http://www.scienceimage.csiro.au/index.cfm?event=site.image.detail&id=397
Sites of Infection/Infestation
The egg, larvae and pupae stages occur inside the grain, and are rarely seen outside of the grain,
making early detection of infestations difficult (Figs. 1, 5, 6). Feeding activity of larvae reduces the
quality of the grain by creating insect "flour" and frass as well as heating the grain (Longstaff 1981,
Lyon, undated, Woodbury 2008). Adults and larvae feed on a wide variety of grains, such as wheat,
oats, rye, barley, oats, sunflower seed, bird seed and corn (Lyon, undated).
Identification
Morphological Methods
Egg: opaque, shining white, ovoid to pear-shaped in form, widest below middle, bottom broadly
rounded, neck narrowing gradually toward top, which is somewhat flattened and bears a small
rounded protuberance that fits into a cap or plug that cements the egg in place. Length 0.68 to 0.80
mm, width about 0.33 mm (Cotton 1921).
Larvae: Larvae are legless, white to creamy white with a small, tan head (Lyon, undated). Mature
larva 2.5 to 2.75 mm in length; a pearly white, footless grub, fleshy and very thick-bodied, ventral
outline being approximately straight while dorsal outline is almost semicircular. Head and
appendages of head similar in every respect to those of Sitophilus oryzae. Thoracic segments
similar in external appearance to those of S. oryzae. The abdominal segments are similar in form to
those of S. oryzae with the following exceptions which afford the best characters for distinguishing
between larvae of these two species: First four abdominal segments divided above into three
distinct areas, praescutum, scutum, and scutellum, whereas in the larva of S. oryzae the first three
only of the abdominal segments are so divided. Middle lobe of the hypopleurum of the abdominal
segments of S. granarius is provided with a seta. This seta is lacking in larva of S. oryzae (Cotton
1921).
Figure 5. Sitophilus granarius larvae. Image: US Department of Agriculture, Agricultural
Research Services.
Pupae: Have snouts like adults (Lyon, undated). Uniformly white when first formed; length 3.75
to 4.25 mm, width 1.75 mm. Tips of elytra attaining fifth abdominal segment, inner wings
rudimentary and almost completely concealed by elytra. Tips of metathoracic tarsi extending
beyond tips of elytra. Head rounded, rostrum elongate. Head has two prominent spines toward
vertex, a group of two small spines and two spinules on each side above eyes, two pairs of small
spines near anterior margin and one on each side of front between eyes, three pairs of spines on
rostrum between frontal ones and base of antenna, a pair of small ones on rostrum midway between
base of antenna and tip of rostrum, a pair on sides of rostrum between latter pair and tip of rostrum,
and two pairs of minute spines on tip of rostrum.
Prothorax provided with one pair of antero-marginal setigerous tubercules, one pair of anterolateral, two pairs of medio-lateral, and four pairs of dorsal setigerous tubercules; also a pair of
minute medio-lateral ventral spines. Mesonotum and metanotum normally each provided with three
pairs of spines; one or more pairs often missing.
Abdomen with seven distinct dorsal tergites, the seventh being much larger than rest. Dorsal area
of each armed with a pair of large spines and a pair of smaller ones.
Lateral area of each tergite armed with a spine, at base of which is a small seta. Epipleural lobes
each obscurely armed with two minute setae. Ninth segment armed as usual with two prominent
pleural spines (Cotton 1921).
Figure 6. Sitophilus granarius pupa showing appendage development. Image: US Department of
Agriculture, Agricultural Research Services.
Adult: Adult S. granarius are larger than S. oryzae or S. zeamais (Mason 2003).
Adults are a shiny reddish-brown to blackish colour and lack distinguishing marks (Figs. 7, 8). Body
length 3- 5 mm, with a head that is prolonged into long, slender rostrum, which is about one quarter
the length of the body or two thirds as long as the thorax (Fig. 9). Rostrum slender, cylindrical,
finely and sparsely punctuate. Thorax sparsely punctuate. Elytra deeply striate, striae punctured at
bottom, not serrate; intervals smooth, alternately wider and more elevated, especially towards the
base; the sutural with a row of elongate punctures. Ventral surface coarsely and less densely
punctured than in S. oryzae.
S. granarius has only rudimentary wings, and is not capable of flight. The elytral intervals are at
least as wide as the striae, the strial punctures small and clearly separated, and the pronotal
punctures distinctly elongate (Fig. 14) (Bousquet 1990, Cotton 1921, Lyon, undated).
Males have the abdominal sterna V and VI distinctly deflected, and the rostrum usually wider and
less elongate. Females do not have the abdominal sterna V and VI deflected, and the rostrum is
usually narrower and more elongate (Bousquet 1990).
Figure 7. Adult Sitophilus granarius, showing uniform body colour. Image: US Department of
Agriculture, Agricultural Research Services.
Figure 8. Sitophilus granarius lateral view. Image: Museum Victoria, courtesy of
www.padil.gov.au
Figure 9. Sitophilus granarius head, side view. Image: Museum Victoria, courtesy of
www.padil.gov.au
Figure 10. Sitophilus granarius rostrum. Image: Museum Victoria, courtesy of www.padil.gov.au
Figure 11. Sitophilus granarius elytra. Image: Museum Victoria, courtesy of www.padil.gov.au
Figure 12.
Sitophilus granarius posterior. Image: Museum Victoria, courtesy of
www.padil.gov.au
To sex adult Sitophilus:
Females: Rostrum relatively long and narrow, punctures along rostrum in regular rows and not
touching each other.
Males: Rostrum relatively short and wide, punctures along rostrum large & irregular, not in rows,
and often touching each other (Walker 2008b, Halstead 1963).
Further distinguishing characters are provided by the basal groove and the median area on the 6th (4th
visible) abdominal sternite. The median area is limited by the basal groove and the apical furrow,
which is at the base of the fixed apical process. In the male, the basal groove is broader and the
median area narrower than in the female. On the midline the basal groove represents one quarter to
one third of the toal length of the male sternite but only one eighth to one fifth of that of the female
sternite.
When the abdomen is seen in profile, the basal groove is deeper and the median area much more
convex in the male than in the female. These sternal characteristics, which are easily observed at
magnifications of 20 - 50X, are reliable regardless of weevil size, can be used to supplement the
rostral differences (Lum & Baker 1975).
Potentially misidentified species
S. oryzae - body length 2.3 - 3.5 mm, pronotum with round punctures, elytra colour with four
distinct reddish-brown spots. Sitophilus granarius has a body length of 3 - 5 mm, pronotum with
oval punctures (Fig. 13), elytra colour uniform (Walker, 2008a).
Figure 13. Pronotum of Sitophilus oryzae, showing the rounded punctures. Image: Museum
Victoria, courtesy of www.padil.gov.au
S. granarius is distinct from S. oryzae in having the hind wings reduced, the elytral intervals at least
as wide as the striae, the strial punctures small and clearly separated, and the pronotal punctures
distinctly elongate (Fig. 14) (Bousquet 1990).
Figure 14. Pronotum of Sitophilus granarius showing the elongated punctures. Image: Museum
Victoria, courtesy of www.padil.gov.au
Biochemical Methods
Leelaja, B.C., Rajashekar, Y., Rajendran, S. 2007. Detection of eggs of stored-product insects in
flour with staining techniques. Journal of Stored Products Research, 43: 206-210.
Molecular Methods
Obrepalska-Steplowska, A., Nowaczyk, K., Holysz, M., Gawlak, M., Nawrot, J. 2008. Molecular
techniques for the detection of granary weevil (Sitophilus granarius L.) in wheat and flour. Food
Additives and Contaminants: Part A - Chemistry Analysis Control Exposure & Risk Assessment,
25(10): 1179-1188.
Further Information
Contacts
Mr Rob Emery, Agriculture Western Australia.
Email: [email protected]
Phone: 08 9368 3247
Acknowledgements
Protocol author: Julianne Farrell
Email: [email protected]
Dr Barry Wallbank reviewed the draft Protocol
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Additives and Contaminants: Part A - Chemistry Analysis Control Exposure & Risk Assessment,
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