Sitophilus oryzae (lesser grain weevil)
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- Habitat List
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Growth Stages
- List of Symptoms/Signs
- Biology and Ecology
- Natural enemies
- Notes on Natural Enemies
- Detection and Inspection
- Similarities to Other Species/Conditions
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Sitophilus oryzae (Linnaeus)
Preferred Common Name
- lesser grain weevil
Other Scientific Names
- Calandra bituberculatus (F.)
- Calandra frugilegus (De Geer)
- Calandra funebris (Rey)
- Calandra granarius (Stroem)
- Calandra oryxae var. minor
- Calandra oryzae Linnaeus
- Calandra sasakii Tak.
- Calendra oryzae Linnaeus
- Curculio oryza Linnaeus
- Curculio oryzae Linnaeus
- Diocalandra oryzae Linnaeus
- Sitophilus oryzae var. minor
- Sitophilus sasakii (Takahashi)
International Common Names
- English: rice weevil
- Spanish: gorgojo común del arroz; gorgojo del arroz
- French: charançon du riz
- Portuguese: gorgulho do arroz; gorgulho do milho
Local Common Names
- Denmark: risbille
- Germany: Käfer, Reis-; Rüssler, Reis-
- Iran: susske bereng
- Israel: chidkonit haorez
- Italy: calandra del riso
- Japan: ko-kokuzo
- Netherlands: rijstklander
- Norway: rissnutebille
- Turkey: pirinc biti
- CALAOR (Sitophilus oryzae)
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Metazoa
- Phylum: Arthropoda
- Subphylum: Uniramia
- Class: Insecta
- Order: Coleoptera
- Family: Dryophthoridae
- Genus: Sitophilus
- Species: Sitophilus oryzae
Notes on Taxonomy and NomenclatureTop of page
The taxonomy of the Sitophilus group has been confused until recently, so that the value of much of the earlier literature on these insects has been reduced, because of the difficulty of knowing the species to which it refers.
First described by Linnaeus in 1798 as Curculio oryza, the first named species of the group was later revised by De Clairville and Scheltenburg in 1798 as Calandra oryzae, which uses the commonest generic synonym for Sitophilus. Many workers subsequently recognized that two distinct forms of the species existed, which were described as the 'large' and 'small' forms. In 1855, Motschulsky recognized the large form as a distinct species, which he named Sitophilus zeamais. Unfortunately, few workers recognized this revision and the name Calandra oryzae continued to be applied to all insects in this complex. Takahashi in 1928 and 1931 complicated matters by raising the small form to specific status as Calandra sasakii. This confused situation continued until 1959, when Floyd and Newsom (1959) revised the complex; this was followed by a further revision by Kuschel (1961). In these revisions it was shown that Linnaeus originally described the smaller species and that Motschulsky's description of the larger species was valid. Both species were therefore placed in the genus Sitophilus with the specific names proposed by Linnaeus and Motschulsky.
Unfortunately, the size difference between S. oryzae and S. zeamais is not consistent, so it is not possible to be sure that references to the large and small forms of Calandra oryzae refer to S. zeamais and S. oryzae, respectively. Therefore the only true and unconfused synonym of S. oryzae is Calandra sasakii; in pre-1960s literature, C. oryzae 'small' and 'large' forms could refer to either S. zeamais or S. oryzae, and it is also possible that some references to 'S. oryzae' in the 1960s and early 1970s literature actually relate to S. zeamais misidentified by use of old keys. The genus Sitophilus and its species may be identified using the keys of Gorham (1987) or Haines (1991).
DescriptionTop of page
Eggs, Larvae and Pupae
These developmental stages are all found within tunnels and chambers bored in the grain and are thus not normally seen. The eggs are shiny, white, opaque and ovoid to pear-shaped. The larva is white, stout and legless. The pupa is also white but has legs, wings, and the snout of the fully-grown weevil.
Usually red-brown, dull with coarse microsculpture. Scutellum usually with lateral elevations closer together than their length and evidently more than half as long as scutellum.
Males with median lobe of aedeagus evenly convex dorsally in cross section.
Females with lateral lobes of internal, Y-shaped sclerite broader and rounded apically, more narrowly separated.
S. oryzae and S. zeamais are almost indistinguishable from each other externally; identification is by exmaination of the genitalia. Both have the characteristic rostrum and elbowed antennae of the family Curculionidae. The antennae have eight segments and are often carried in an extended position when the insect is walking. Both species usually have four pale reddish-brown or orange-brown oval markings on the elytra, but these are often indistinct. (See also S. zeamais.)
Both species can be separated from S. granarius by the presence of wings beneath the eltyra (absent in S. granarius) and by having circular, rather than oval, punctures on the prothorax.
DistributionTop of page
S. zeamais and S. oryzae are found in all warm and tropical parts of the world, but S. oryzae may also be found in temperate climates. The earlier confusion over the identity of S. zeamais and S. oryzae, and the fact that most of the major basic studies were made before the confusion was resolved, means we cannot be sure to which of the species many of the observations refer. For a distribution map, see S. zeamais data sheet.
The detailed map plotted on the basis of actual country records gives a falsely restricted distribution. These pests are carried all over the world in grain shipments and can establish themselves wherever there is food and where grain moisture and temperature are favourable. In various locations, one species may be more common than the other. A global survey of resistance to pesticides (Champ and Dyte, 1976) contains detailed location lists for both species.
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 27 Feb 2020
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Cameroon||Present||Ngamo et al. (2007)|
|Central African Republic||Present||CABI Data Mining (2001)|
|Chad||Present||Trematerra et al. (2003)|
|Congo, Republic of the||Present||CABI (Undated)||Original citation: Munyuli bin Mushambanyi T (2003)|
|Côte d'Ivoire||Present||CABI Data Mining (2001)|
|Egypt||Present||CABI Data Mining (2001)|
|Ethiopia||Present||CABI Data Mining (2001)|
|Kenya||Present||Likhayo and Hodges (2000)|
|Mali||Present||CABI Data Mining (2001)|
|Morocco||Present||Benhalima et al. (2004)|
|Nigeria||Present||CABI Data Mining (2001)|
|Senegal||Present||CABI Data Mining (2001)|
|Somalia||Present||CABI Data Mining (2001)|
|South Africa||Present||CABI Data Mining (2001)|
|Sudan||Present||CABI Data Mining (2001)|
|Tanzania||Present||CABI Data Mining (2001)|
|Bangladesh||Present||CABI Data Mining (2001)|
|Bhutan||Present||CABI Data Mining (2001)|
|China||Present||CABI Data Mining (2001)|
|-Anhui||Present||Wu and Yan (2018)|
|-Fujian||Present||Wu and Yan (2018)|
|-Guangxi||Present||Wu and Yan (2018)|
|-Henan||Present||Wu and Yan (2018)|
|-Jiangsu||Present||Wu and Yan (2018)|
|-Jiangxi||Present||Wu and Yan (2018)|
|-Shandong||Present||Wu and Yan (2018)|
|-Sichuan||Present||Wu and Yan (2018)|
|-Yunnan||Present||Wu and Yan (2018)|
|India||Present||Kumar et al. (2005); Rajendran et al. (2001); Kumawat (2007)|
|-Andhra Pradesh||Present||Visalakshi Mahanthi et al. (2005)|
|-Arunachal Pradesh||Present||Pathak and Jha (2003)|
|-Assam||Present||Pathak and Jha (2003)|
|-Bihar||Present||CABI Data Mining (2001)|
|-Delhi||Present||Saravanan and Gujar (2006)|
|-Karnataka||Present||CABI Data Mining (2001)|
|-Manipur||Present||CABI Data Mining (2001)|
|-Meghalaya||Present||Pathak and Jha (2003)|
|-Mizoram||Present||Pathak and Jha (2003)|
|-Nagaland||Present||Pathak and Jha (2003)|
|-Odisha||Present||CABI Data Mining (2001)|
|-Punjab||Present||Rajendran et al. (2001)|
|-Sikkim||Present||Pathak and Jha (2003)|
|-Tamil Nadu||Present||Kumar et al. (2005)|
|-Tripura||Present||Pathak and Jha (2003)|
|-West Bengal||Present||CABI Data Mining (2001)|
|Indonesia||Present||CABI (Undated a)||Present based on regional distribution.|
|-Java||Present||CABI Data Mining (2001)|
|Iran||Present||Forghani and Marouf (2015); CABI Data Mining (2001)|
|Iraq||Present||CABI Data Mining (2001)|
|Israel||Present||CABI Data Mining (2001)|
|Japan||Present||CABI Data Mining (2001)|
|Malaysia||Present||CABI Data Mining (2001)|
|Nepal||Present||CABI Data Mining (2001)|
|Pakistan||Present||CABI Data Mining (2001)|
|South Korea||Present||CABI Data Mining (2001)|
|Sri Lanka||Present||CABI Data Mining (2001)|
|Taiwan||Present||CABI Data Mining (2001)|
|Austria||Present||CABI Data Mining (2001)|
|Belarus||Present||Ovcharenko et al. (2004)|
|Belgium||Present||CABI Data Mining (2001)|
|Czechia||Present||Stejskal et al. (2004)|
|Germany||Present||CABI Data Mining (2001)|
|Greece||Present||CABI Data Mining (2001)|
|Italy||Present||CABI Data Mining (2001)|
|Poland||Present||CABI Data Mining (2001)|
|Russia||Present||Ovcharenko et al. (2004)|
|Spain||Present||Riudavets et al. (2002)|
|Switzerland||Present||CABI Data Mining (2001)|
|Ukraine||Present||Mishchenko et al. (2000)|
|United Kingdom||Present||CABI Data Mining (2001)|
|Canada||Present||Karunakaran et al. (2003)|
|Cuba||Present||CABI Data Mining (2001)|
|Dominica||Present||Tilley et al. (2007)|
|United States||Present||CABI (Undated a)||Present based on regional distribution.|
|-California||Present||CABI Data Mining (2001)|
|-Florida||Present||CABI Data Mining (2001)|
|-Georgia||Present||CABI Data Mining (2001)|
|-Hawaii||Present||CABI Data Mining (2001)|
|-Indiana||Present||CABI Data Mining (2001)|
|-Kansas||Present||CABI Data Mining (2001)|
|-Michigan||Present||CABI Data Mining (2001)|
|-Minnesota||Present||CABI Data Mining (2001)|
|-Oklahoma||Present||CABI Data Mining (2001)|
|-South Carolina||Present||CABI Data Mining (2001)|
|Australia||Present||CABI (Undated a)||Present based on regional distribution.|
|-New South Wales||Present||CABI Data Mining (2001)|
|-Queensland||Present||CABI Data Mining (2001)|
|-Victoria||Present||CABI Data Mining (2001)|
|New Zealand||Present||CABI Data Mining (2001)|
|Argentina||Present||CABI Data Mining (2001)|
|Bolivia||Present||CABI Data Mining (2001)|
|Brazil||Present||CABI Data Mining (2001)|
|Peru||Present||CABI Data Mining (2001)|
Habitat ListTop of page
Hosts/Species AffectedTop of page
Both S. oryzae and S. zeamais are able to develop on a wide range of cereals and also on processed cereal products such as pasta. However, food preferences of the two species are variable; it is clear that S. zeamais is predominantly found associated with maize grain, whereas S. oryzae is associated with wheat.
In the case of rice, detailed surveys in Indonesia have shown that S. zeamais is dominant on milled rice, whereas S. oryzae is more common on paddy (rough rice). Laboratory studies have shown that this is a result of their different rates of increase on these two forms of rice (Hussain et al., 1985). It is not yet known whether these relationships with the form of rice hold true throughout the tropics, but imports of milled rice into the UK from many countries are much more frequently infested by S. zeamais than by S. oryzae.
Both species are able to breed on dried cassava and have been reported as frequent pests of this commodity. A few strains of S. oryzae have been found which can develop on grain legumes (Coombs et al., 1977): peas, lentils and green or black gram are the pulses most often attacked by these strains.
Although S. oryzae is primarily a pest of stored products, it can also attack cereal plants in the field.
Host Plants and Other Plants AffectedTop of page
|Cicer arietinum (chickpea)||Fabaceae||Other|
|Hordeum vulgare (barley)||Poaceae||Other|
|Lens culinaris subsp. culinaris (lentil)||Fabaceae||Other|
|Manihot esculenta (cassava)||Euphorbiaceae||Main|
|Oryza sativa (rice)||Poaceae||Main|
|Pennisetum glaucum (pearl millet)||Poaceae||Other|
|Pisum sativum (pea)||Fabaceae||Other|
|Secale cereale (rye)||Poaceae||Other|
|Sorghum bicolor (sorghum)||Poaceae||Main|
|stored products (dried stored products)||Main|
|Triticum aestivum (wheat)||Poaceae||Main|
|Triticum spelta (spelt)||Poaceae||Main|
|Vicia faba (faba bean)||Fabaceae||Other|
|Vigna angularis (adzuki bean)||Fabaceae||Other|
|Vigna radiata (mung bean)||Fabaceae||Other|
|Vigna unguiculata (cowpea)||Fabaceae||Other|
|Zea mays (maize)||Poaceae||Main|
Growth StagesTop of page Post-harvest
SymptomsTop of page The eggs, larvae and pupae are not normally seen because they develop inside intact grains. The larvae chew large, irregular holes in the germ and endosperm of the kernel. Adult emergence holes (about 1.5 mm diameter) with irregular edges are apparent some weeks after the initial attack. Two rice weevils may develop at the same time on two sides of a single kernel. Adults can be found wandering over the surface of grain. In a heavy infestation, the only part of a grain that remains is the shell of the kernel perforated by adult feeding and emergence holes.
List of Symptoms/SignsTop of page
|Seeds / internal feeding|
Biology and EcologyTop of page
The earlier confusion over the identity of S. zeamais and S. oryzae, and the fact that most of the major basic studies were made before the confusion was resolved, means we cannot be sure to which of the species many of the observations refer. The development of the two species is clearly very similar, but there are probably a number of differences in the effects of environmental factors. Thus, the information given below may be taken as generally applicable to both species, but it should be remembered that there may be specific differences in details.
The biology of S. zeamais and S. oryzae has been reviewed in detail by Longstaff (1981). The adults are long-lived (several months to one year). Eggs are laid throughout most of the adult life, although 50% may be laid in the first 4-5 weeks; each female may lay up to 150 eggs. The eggs are laid individually in small cavities chewed into cereal grains by the female; each cavity is sealed, thus protecting the egg, by a waxy secretion (usually referred to as an 'egg-plug') produced by the female. The incubation period of the egg is about 6 days at 25°C (Howe, 1952). Eggs are laid at temperatures between 15 and 35°C (with an optimum around 25°C) and at grain moisture contents over 10%; however, rates of oviposition are very low below 20°C or above 32°C, and below about 12% moisture content (Birch, 1944).
Upon hatching, the larva begins to feed inside the grain, excavating a tunnel as it develops. There are four larval instars: in English wheat at 25°C and 70% RH, pupation occurs after about 25 days, although development periods are extremely protracted at low temperatures (e.g. 98 days at 18°C and 70% RH). Pupation takes place within the grain; the newly developed adult chews its way out, leaving a large, characteristic emergence hole. Total development periods range from about 35 days under optimal conditions to over 110 days in unfavourable conditions (Birch, 1944; Howe, 1952). The actual length of the life cycle also depends upon the type and quality of grain being infested: for example, in different varieties of maize, mean development periods of S. zeamais at 27°C and 70% RH have been shown to vary from 31 to 37 days.
Bhuiyah et al. (1990) determined the egg, larval and pupal periods for S. oryzae on maize in the laboratory as 5-6 days, 16-20 days and 8-9 days, respectively, at 23-35°C and 79-87% RH. The longevity of adult males and females was 114-115 days and 119-120 days.
Trematerra et al. (1996) developed a method for analysis and comparison of the development rate of S. oryzae on different cereals (Triticum aestivum, T. dicoccum, T. durum, T. monococcum and T. spelta). Yoon et al. (1997) constructed a matrix model of S. oryzae populations based on degree-days. The behavioural activity of S. oryzae towards intact and damaged kernels of Triticum aestivum, T. durum, T. dicoccum, T. monococcum and T. spelta was investigated by Trematerra et al. (1999).
Although both species are capable of flight, S. zeamais has a greater ability and tendency to fly (Giles, 1969). Where grain is stored on small farms, S. zeamais is thus more likely than S. oryzae to fly to the ripening crop in the field and establish an infestation in the grain before harvest.
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
|Bacillus thuringiensis thuringiensis||Pathogen|
Notes on Natural EnemiesTop of page
Both S. zeamais and S. oryzae are commonly parasitized by pteromalids (and occasionally other Hymenoptera). Common pteromalid parasites found in the Tropics include Anisopteromalus calandrae, Lariophagus distinguendus and Choetospila elegans [Theocolax elegans].
ImpactTop of page
S. oryzae is universally regarded as one of the most destructive primary pests of stored cereals such as barley, maize, rice and wheat. It does not often breed in non-cereal foods, although it does attack split peas and pasta. It can attack cereal plants in the fields. Voracious feeding on whole grains by this insect results in weight loss, fungal growth, quality loss through an increase in free fatty acids and it can even completely destroy stored grain in all types of storage. Invasion by this primary pest may cause grain heating and may facilitate the establishment of fungal colonies, secondary insect pests, and mite pests.
S. oryzae and S. zeamais are very important pests of cereals. In maize or sorghum, attack may start in the mature crop when the moisture content of the grain has fallen to 18-20%. Subsequent infestations in storage result from the transfer of infested grain into stores or from the pest flying into storage facilities, probably attracted by the odour of the stored grain.
In stored maize, heavy infestations of these pests may cause weight losses of up to 30-40%, although losses are commonly 4-5%.
Generally, both adults and larvae feed on whole cereal grains, including wheat, rice, barley, maize, groundnuts, cassava, beans, millet, and sorghum; but the females can lay eggs and develop on solid products made of cereals, such as pasta. S. oryzae can infest maturing grain, especially maize in the field, in the southern USA and in other warm and tropical regions.
Boles and Pomeranz (1978) reported a wide variation in numbers of progeny observed among barley samples from different locations as well as among samples of individual varieties. Blue aleurone-layered barleys produced slightly fewer progeny of S. oryzae than the white aleurone-layered barleys. Singh et al. (1991) reported varietal susceptibility of barley grains to S. oryzae.
In Egypt, weight losses attributable to S. oryzae and S. granarius in grain stored, under natural conditions, at 25°C and 70% RH, were about 79-81% in barley (Koura and El-Halfawy, 1972).
According to Kamel and Zewar (1973), an increase of 1% in mean infestation resulted in a decrease of 0.35% in the weight of maize kernels and a 0.41% weight decrease in millet kernels. Losses in stored grains of five high-yielding hybrid varieties of maize were determined in Uttar Pradesh, India, by Karan Singh et al. (1974). Significant differences in weight loss were found between the varieties and varied from 1.3% to 4.5%. Rodriguez (1976) reported damage to stored maize in the Mexican state of Yucatan. Weight loss during storage (4-5 months) reached 30%, and most damage was caused by S. oryzae.
When 170 1-day-old adults of S. oryzae were released into 2 kg sacks of maize, 88.51% infestation and 38.12% weight loss of grains were recorded by Bhuiyah et al. (1990) after 6 months. Loss in weight and viability in five maize hybrids revealed a significant difference due to hybrids, pests and their interactions (Kurdikeri et al., 1993). Percentage seed damage and loss in weight increased with the increase in storage period in all the maize hybrids, while the viability of seeds decreased (Kurdikeri et al., 1994).
Khare et al. (1974) reported that over an 18 month period, the loss in protein content in damaged grain varied from 8.76 to 50.85 mg/g. Matioli (1981) reported that weight loss, frass volume and percentage of damaged grains varied according to the variety of maize. Pericarp hardness was associated with resistance at low rates of infestation, but at higher infestation rates, carbohydrate content seemed to be more important. According to the variety concerned, the endosperm, pericarp and embryo were the preferred food (De and Prakash, 1989).
According to Rubbi and Begum (1986), in Bangladesh the population of Sitotroga cerealella was highest, followed by S. oryzae and then Rhyzopertha dominica, and the percentage loss in weight of the rice followed the same order.
Sittisuang and Imura (1987) reported that brown rice lost 19% of initial kernel weight over 14 weeks of infestation with S. oryzae. In India, stored rice (unhusked) samples, drawn from six districts of Himachal Pradesh, were infested with S. oryzae (69%); the average weight loss in storage was 2.11%, and ranged from 1.09% to 3.10% (Thakur and Sharma, 1996). The effect of feeding by larvae and adults of S. oryzae on the weight of rice and wheat grain was determined in laboratory tests. The maximum weight loss caused to single kernels by individual larvae was 57% for rice and 19% for wheat (Banerjee and Nazimuddin, 1985).
In Egypt, weight losses attributable to S. oryzae and S. granarius in grain stored, under natural conditions, at 25°C and 70% RH, ranged from 56-74% in rice (Koura and El-Halfawy, 1972).
In Maharashtra, India, some hybrids and varieties of sorghum were less susceptible to attack by S. oryzae than local varieties (Borikar and Tayde, 1979). Torres et al. (1996) conducted laboratory observations with 29 sorghum varieties to study resistance to S. oryzae. The relative resistance of 36 improved and local sorghum varieties were also assessed in Nigeria (at Samaru) (Bamaiyi et al., 1998).
In Georgia, USA, 21 varieties of sorghum were assessed for losses during storage for up to 9 weeks at 30°C and 72% RH. The loss in grain sample weight from damage by S. oryzae varied from 4 to 52% (McMillian et al., 1981).
Weight loss in sorghum grains in large and small grains was 0.32 and 0.41%, respectively (Shazali, 1987). Threshed grain was more susceptible than unthreshed grain; more progeny were produced on threshed than on unthreshed sorghum (Wongo and Pedersen, 1990).
On wheat after 65 days, the average proportion of damaged grains ranged from 0.05% to 42%; the loss in weight ranged from 0.43% to 21.02% (Singh and Mathew, 1973).
A survey in Uttar Pradesh, India, showed that the weight loss after storage for 6 months varied from 0.06 to 9.7%, and the loss in viability from 7.0% to 22.0%. In 12 districts of the Indian Punjab, after about 8-10 months storage on the farm, the weight loss was about 2.5% (Bhardwaj et al., 1977).
In the UK, wheat grain infested with S. oryzae and stored at 27°C and 70% RH for 14 weeks revealed losses of up to 38%, and a loss of 18% of the original protein fraction (Francis and Adams, 1980).
Levchenko and Imshenetskii (1984) reported that of 10 wheat varieties studied, there were significant differences in the damage done by S. oryzae and S. granarius.
In the tropics, cereal grain insects including S. oryzae cause an estimated overall yield loss of up to 30%, especially under inadequate storage methods (Singh and Benazet, 1974). In Ethiopia, the major species of stored product pests include S. oryzae, S. granarius and S. zeamais on whole grains. A weight loss of 4.2% of wheat stored in traditional stores was attributed to Sitophilus spp. (Hulluka, 1991).
In Egypt, weight losses attributable to S. oryzae and S. granarius in grain stored under natural conditions, at 25°C and 70% RH, ranged from 36-40% in wheat (Koura and El-Halfawy, 1972).
Other Crop Losses
Bandyopadhyay and Ghosh (1999) investigated the loss of stored rice and wheat under different climatic conditions in West Bengal, India. Grain damage was found throughout the year in all the localities, and ranged from 3 to 15.5% at Purulia, 4.7 to 23.4% at Kalyani and 4.4 to 20.4% at Cooch Behar on rice, and 4.3 to 21.8%, 6.4 to 25.5% and 4.5 to 22.2% on wheat, respectively.
In Egypt, on local maize, millet and wheat varieties in storage, the relationship between total infestation and external infestation, weight loss, the number of insect fragments in milled grain and the moisture content of the kernels were investigated by Omar and Kamel (1980-1981). Weight loss was negligible unless infestation exceeded 5%, but otherwise the correlation between weight loss and total infestation was linear, with variations for different grain types and insect species. The correlation between insect fragments and total infestation was positive up to a certain infestation level (varying with grain type and insect species) but became negative when infestation exceeded that level. The moisture content of infested grains was positively correlated with the level of infestation in all cases.
Detection and InspectionTop of page
Flight traps will collect S. zeamais, but seldom S. oryzae (which rarely flies). In milled rice stores, bag traps baited with brown rice have captured both species (Hodges et al., 1986). Disturbance of the grain causes adult Sitophilus spp. to migrate upwards and become visible on the surface.
A male aggregation pheromone attracts both sexes in S. oryzae (Phillips and Burkholder, 1981). Trematerra and Girgenti (1989) investigated the influence of pheromone and food attractants on trapping S. oryzae. Levinson et al. (1990) confirmed the activity of 4S,5R sitophinone and 2S,3R-sitophilate for S. oryzae, S. zeamais and S. granarius using electro-antennogram-recordings. The effect of insect age on the response of Sitophilus species to 4S,5R-sitophilure and food volatiles was reported by Wakefield (1998). For further information, see Plarre (1998).
A WB Probe II Trap was used to monitor insect activity in grains in the laboratory at 24±1°C and 70±5% RH (Trematerra, 1998). In all cereals examined, the traps trapped more S. oryzae than Tribolium castaneum and Oryzaephilus surinamensis. The effectiveness of a 'stored grain insect trap' was tested on heavily infested paddy and wheat grains in India (Rajkumar and Anitha, 1998). The results showed that all larvae and adults of S. oryzae were collected. The collection of larval forms with this trap is reported for the first time.
Similarities to Other Species/ConditionsTop of page
S. oryzae and S. zeamais can be separated from S. granarius by the presence of wings beneath the elytra (absent in S. granarius) and by having circular, rather than oval, punctures on the prothorax.
Some molecular and morphological markers for the diagnosis of S. oryzae and S. zeamais are reported in Hidayat et al. (1994).
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
Cultural Control and Sanitary Methods
Good store hygiene plays an important role in limiting infestation by S. oryzae and S. zeamais. The removal of infested residues from last season's harvest is essential.
Grain may be protected by the admixture of insecticide. Sitophilus spp. have a low susceptibility to synthetic pyrethroids but are readily killed by organophosphorous compounds such as fenitrothion and pirimiphos-methyl. Grain stocks may be fumigated with phosphine to eliminate existing infestation, but these treatments provide no protection against re-infestation. Sitophilus spp., particularly in the pupal stage, have a lower natural susceptibility to the fumigant phosphine and to carbon dioxide used in controlled atmosphere storage than do other species tested and thus inadequate treatments are particularly likely to result in some survival.
In laboratory tests to determine the toxicity of deltamethrin, fluvalinate, chlorpyrifos-methyl, etrimfos and malathion against Sitophilus zeamais and S. oryzae, etrimfos was found to be the most toxic insecticide to S. oryzae (Srinivasacharayulu and Yadav, 1997).
A mixture of fenitrothion, esfenvalerate and piperonyl butoxide was found to be effective against S. oryzae in stored rice until 180 days after treatment (Pinto et al., 1997).
The effects of phosphine on the pupae of S. oryzae, S. zeamais and S. granarius were studied at 15°C. No significant differences were found in pupal mortality between phosphine and the mixture with carbon dioxide (Goto et al., 1996). Mixtures of phosphine plus carbon dioxide reduced levels of resistance to phosphine in populations of S. oryzae and Rhyzopertha dominica (Athie et al., 1998).
Carbonyl sulfide (as gas), carbon disulfide (as liquid) and ethyl formate (aqueous solution) were tested as fumigants in silos of wheat in Australia. Control of S. oryzae, Tribolium castaneum and Rhyzopertha dominica was 99-100% (Desmarchelier et al., 1998).
Raised levels of carbon dioxide are known to be toxic to many insect species, but S. oryzae has previously been shown to be one of the more tolerant species to this treatment. Annis and Morton (1997) reported acute mortality for all life stages of S. oryzae exposed to 15-100% carbon dioxide at 25°C and 60% RH.
The rates of carbon dioxide production and oxygen consumption by adult S.oryzae on wheat indicated that caution was needed when interpreting fumigant dosage/response data obtained in sealed systems where carbon dioxide concentrations exceed about 1% and changes in respiratory physiology start to occur (Damcevski et al., 1998).
The effectiveness of controlled atmosphere was verified using generators of inert gases, such as carbonic anhydride and nitrogen, for the disinfestation of wheat stored in vertical silos and horizontal stores (Contessi, 1999). No pest survived at environmental temperature 27°C and temperature of the cereal mass approx. 24°C, but Sitophilus survived when the treatment was less than 12 days. It is suggested that this technique could be used as an alternative to fumigation with toxic gases.
Experiments were conducted using 36 different diatomaceous earths or formulations collected from the USA, Mexico, Canada, Australia, Japan, China and Macedonia. The results indicated that the efficacy of diatomaceous earth against insects depended on different properties of the diatom particles (Korunic, 1997). The source of diatomaceous earth, insect species, grain moisture content, temperature, method of application and duration of exposure all factors influenced the mortality of stored-product insects. For S. oryzae some diatomaceous earths had increased efficacy at lower temperatures and others had decreased efficacy at lower temperatures (Fields and Korunic, 2000).
The effect of low temperatures on S. oryzae and S. zeamais was investigated by Nakakita et al. (1997). Both hatching and metamorphosis of each species were inhibited at 10°C. Population increase of S. oryzae was completely suppressed at 15°C, while a small number of F1 beetles of S. zeamais emerged.
When the pupae of S. oryzae, Corcyra cephalonica and Sitotroga cerealella were exposed to temperatures of 35-45°C for 24-72 h, S. oryzae was the most vulnerable species. A very high incidence of sterility was induced in the adults emerged from pupal exposures at 40°C (Sharma et al., 1997).
Beckett et al. (1998) used conductive heating to quickly obtain and maintain moderate temperatures in grain while minimizing grain moisture loss (Beckett et al., 1998).
S. oryzae was more susceptible to gamma radiation than S. granarius. Doses of Ú1.0 kGy resulted in 100% mortality within 3-6 days for S. granarius, and within 4 days for S. oryzae (Ignatowicz, 1997). A sterilizing dose of gamma radiation from Cobalt-60 was determined for adults of S. oryzae, S. zeamais, S. granarius on rice, maize and wheat grains as 70, 60 and 80 Gy, respectively (Franco et al., 1997).
Radio frequency and microwave dielectric properties of stored-grain insects were investigated and their implications for potential insect control are reported by Nelson et al. (1997, 1998).
Laboratory studies were conducted on different sorghum varieties (Leuschner and Manthe, 1996) to study the relationship between resistance to S. oryzae and grain nutrient content (Torres et al., 1996). The relative resistances of 36 improved and local sorghum varieties were assessed in Nigeria (Bamaiyi et al., 1998). Eight land races of sorghum collected in Ethiopia showed significant variation by genotype in soluble phenolic content suggesting that the soluble phenolic content could be used as an indicator of resistance (Ramputh et al., 1999).
Chunni and Singh (1996) evaluated 64 wheat varieties for resistance to S. oryzae. Singh et al. (1998) screened 15 varieties of maize and Thakur (1999) and Thakur and Sharma (1996) screened 20 rice varieties.
Biological control has not been practised against these species. There may be some potential for the development of pest management strategies that favour the action of natural parasites.
A number of plant extracts have been tested for activity against S. oryzae including Ocimum basilicum, Capsicum frutescens, Piper guineense, Tetrapleura tetraptera and Eichhornia crassipes (Gakuru and Foua-Bi, 1996); Dicoma sessiliflora and Neorautanenia mitis (Chimbe and Galley, 1996); Ricinus communis (Mahgoub and Ahmed, 1996); Labrador tea (Ignatowicz and Wesolowska, 1996); Melilotus officinalis and M. albus (Ignatowicz, 1997); Withania somnifera (El-Lakwah et al., 1997); Gardenia fosbergii (Kestenholz and Stevenson, 1998); many Asteraceae (Ignatowicz, 1998); Thujopsis dolabrata var. hondai (Ahn et al., 1998); Eucalyptus tereticornis (Khan and Shahjahan, 1998); Allium sp. (Trematerra and Lanzotti, 1999); Decalepis hamiltonii (George et al., 1999); Chenopodium multifidum, Flaveria bidentis, Aristolochia argentina and Tagetes erecta (Broussalis et al., 1999).
Mohapatra et al. (1996) reported that alcohol extracts of Azadirachta indica were superior to aqueous extracts providing 100% protection to rice grains for 6 weeks at a concentration of 1%. The repellent effect of a neem formulation extracted from seeds was evaluated by Suss et al. (1997). Imti and Zudir (1997) reported the efficacy of neem leaf and kernel powders. Sharma (1999) suggested that neem products can be mixed with stored maize to protect the grains up to 9 months from the attack of the major pests. The average mortality of S. oryzae adults treated with Neemazal-W was high and reached 100% at all tested concentrations 14 days post-treatment (El-Lakwah and El-Kashlan, 1999).
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