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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a hole into the kernel, deposits the egg, and then secretes a mucilaginous plug to enclose the egg as the ovipositor is withdrawn.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 References Anonymous. 2008. Granary weevil Sitophilus granarius (L.) Canadian Grain Commission. http://www.grainscanada.gc.ca/storage-entrepose/pip-irp/gw-cg-eng.htm Anderson, D.M. 1987. Larval beetles (Coleoptera). In: Insect and Mite Pests in Food: An illustrated key. (Ed. J.R. Gorham). USDA Handbook No. 655, United States Department of Agriculture: Washington DC, USA. Vol 1, pp 95-114. http://www.afpmb.org/pubs/tims/tg27/docs/Insect%20and%20Mite%20Pests%20in%20Food%20G orham.pdf 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: 4145. http://www.bashanfoundation.org/dilantha/dilanfingerprinting.pdf Bousquet, Y. 1990. Beetles associated with stored products in Canada: An identification guide. Agriculture and Agri-Food Canada. http://home.cc.umanitoba.ca/~fieldspg/fields/beetles.pdf Brader, B., Lee, R.C., Plarre, R., Burkholder, W. et al. 2002. A comparison of screening methods for insect contamination in wheat. 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Purdue University, Department of Entomology. http://extension.entm.purdue.edu/publications/E-237.pdf Neethirajan, S., Karunakaran, C., Jayas, D.S., White, N.D.G. 2007. Detection techniques for storedproduct insects in grain. Food Control, 18:157162. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T6S-4HH81D92&_user=139754&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000011578&_versio n=1&_urlVersion=0&_userid=139754&md5=7c69d2a0a876fc1ad83c2dc0b6308cb5 Niewiada, A., Nawrot, J. et al. 2005. Some factors affecting egg-laying of the granary weevil (Sitophilus granarius L.) Journal of Stored Products Research, 41(5): 544-555. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T8Y-4FRJKVK1&_user=139754&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000011578&_versio n=1&_urlVersion=0&_userid=139754&md5=8fde099348b9bdd77683f031093ae9de Obrepalska-Steplowska, A., Nowaczyk, K., Holysz, M., Gawlak, M., Nawrot, J. 2008. 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