Genetic diversity and its geographic structure in Sitophilus oryzae (Coleoptera; Curculionidae) across India – implications for managing phosphine resistance

The rice weevil, Sitophilus oryzae, is a serious global pest of stored grains. Fumigation with phosphine gas is the primary control method for S. oryzae, but the indiscriminate and prolonged use of phosphine gas has led to the development of heritable resistance. Developing and implementing an effective phosphine resistance management strategy for S. oryzae relies on an understanding of its genetic diversity and any structuring of that diversity geographically. We therefore sequenced the mitochondrial cytochrome c oxidase subunit I gene from 143 S. oryzae specimens collected from 37 locations across India, and from that assessed the genetic diversity of the species and its phylogeographic structuring. In addition, we compared the genetic diversity in Indian S. oryzae populations (the hypothesised origin of this beetle) to global populations. Genetic diversity was low in Indian S. oryzae, with only eight haplotypes (including two very common haplotypes) identified. The low level of mitochondrial diversity observed in this species appears typical of stored product pests, perhaps suggesting that low mitochondrial diversity is associated with repeated phosphine fumigations, which may eliminate low frequency haplotypes. The genetic diversity of S. oryzae in India is, however, higher than in many other countries, though comparable levels were identified in China. There was no evidence of population genetic structure across India, with most haplotypes found in three of the broad biogeographic regions. This lack of phylogeographic structuring indicates significant gene-flow across India, most likely through the incidental anthropogenic transport of this relatively poor (or reluctant) flyer. The major practical implication is that phosphine resistance management for S. oryzae needs to be dealt with country wide, as populations are not isolated.     

Microsatellite markers for Rhyzopertha dominica (Coleoptera: Bostrichidae)

Rhyzopertha dominica (F.) is a destructive insect pest of stored products. Due to this pest's broad host range, short life cycle, and high reproductive capacity, it has rapidly spread throughout the world. Understanding the population structuring of this important pest is vital for developing and implementing an effective integrated pest management strategy. In this study, we isolated 19 novel polymorphic microsatellite loci from an enriched genomic library based on a biotin/streptavidin capture protocol. The genetic parameters were estimated based on 80 individual R. dominica from two natural populations. The numbers of alleles ranged from 3 to 10, and sixteen loci exhibited polymorphic information contents (PICs) greater than 0.5. The observed (H_O) and expected (H_E) heterozygosities ranged from 0.033 to 0.900 and 0.310 to 0.882, respectively. Six locus/population combinations significantly deviated from Hardy–Weinberg equilibrium (HWE). These microsatellite markers will provide a valuable resource for studying the population genetic structure of this species.     

Significant population genetic structuring in Rhyzopertha dominica across Turkey: Biogeographic and practical implications

Rhyzopertha dominica is a key pest of stored grain. Understanding the movement of this beetle on broad geographic scales is crucial, particularly when developing strategies to prevent the spread of phosphine resistance. We assessed population genetic structuring in this pest across Turkey, using a combination of mitochondrial (cytochrome oxidase I) and microsatellite markers. In addition, we screened samples for Wolbachia, as this endosymbiont has previously been suggested to be associated with low mitochondrial genetic diversity in this beetle. Mitochondrial genetic diversity was low, with only six haplotypes identified. The genetic diversity was, however, substantially higher than that previously found in Australia or India, suggesting that R. dominica may have originated in the Middle East. Wolbachia were detected only at a single site, indicating they are not impacting the mitochondrial genetic diversity of R. dominica across Turkey. Microsatellite markers indicated there is significant geographic genetic structuring across Turkey, even among sites less than 100 km apart, suggesting there is little movement of beetles across regions within the country. This contrasts with the significantly higher levels of gene-flow found in Australia and the United States. We suggest that the limited movement of beetles across Turkey may be due to a combination of the historically localised agricultural practices (which limits anthropogenic movement among regions), and the mountainous landscape (which limits active flight among regions). Our results demonstrate that the movement of stored product pests may differ significantly across studies conducted in different countries. As a consequence, phosphine resistance management strategies must incorporate region specific information on the extent of beetle movement.     

Anisole is an environmentally friendly fumigant for postharvest pest control

Plant essential oils have demonstrated insecticidal effects but not many are as effective as currently used fumigants. We evaluated anisole, a volatile compound extracted from aniseed, for control of four insect pests including adult rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae), granary weevil, Sitophilus granarius (L.) (Coleoptera: Curculionidae), and confused flour beetle, Tribolium confusum Jacquelin du Val (Coleoptera: Tenebrionidae), and a fresh produce product pest, western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), in the laboratory tests. Anisole fumigations were effective against all four pests. At a dose of 100 μL/L, anisole fumigation completely controlled S. granarius and S. oryzae in 24 and 16 h, respectively. At a dose of 150 μL/L, complete control of the two weevils was achieved in 16 and 8 h, respectively. T. confusum was completely controlled in 16 and 8 h with anisole fumigations at 400 μL/L and 600 μL/L respectively. F. occidentalis was 100% controlled in 8 and 6 h treatments with anisole at doses of 150 μL/L and 300 μL/L, respectively. The median lethal times (LT₅₀) of anisole fumigation at 50 μL/L were 0.6 and 0.5 h for S. granarius and S. oryzae, respectively. LT₅₀ for T. confusum at 600 μL/L was 2.9 h. In a 16 h confirmatory test in a mini silo, anisole fumigation achieved 100% control of rice weevil at different depth of stored corn. This study demonstrated that anisole fumigation was effective against insects. Therefore, anisole fumigation has potential as an environmentally friendly, alternative fumigant for postharvest pest control.     

Biology,morphology and molecular characterization of Sitophilus oryzae and S. zeamais (Coleoptera: Curculionidae)

The present study focuses on the variations in biology, morphology and mitochondrial cytochrome oxidase I (mtCOI) sequences of the two Indian populations of the rice weevil Sitophilus oryzae (L.) and S. zeamais (Motschulsky) (Coleoptera: Curculionidae), which occur on cereals as storage grain pests. These two species are the important pest of major importance and morphologically are very similar to each other. The biological study reveals that the developmental period, longevity, mating and premating period are longer in S. zeamais compared to S. oryzae. The developmental periods of eggs, first to fourth instar larvae, pupae and adults were 6.9, 5.8, 7.0, 8.4, 7.5, 12.5 and 3.5 days, respectively with total life cycle duration of 51.6 days in S. zeamais; and it was 5.5, 5.0, 5.7, 6.5, 7.0, 7.4 and 2.4 days, respectively with total life cycle duration of 39.1 days in S. oryzae. Morphometrics of the life stages revealed significant differences in the dimensions of length and width except in case of eggs and fourth instar larvae. Since, morphological identification were quite confusing and difficult, some characters such as dentations on fore tibia, gular suture on rostrum, apex of 8th tergite and variation in the spermatheca, are brought out as additional diagnostic characters. This study evaluated the utility of mtCOI sequences for quick and accurate species diagnosis of S. oryzae and S. zeamais independent of life stages and sex. The phylogenetic analyses revealed that among the four species of the genus Sitophilus, S. oryzae and S. zeamais cluster together (1.0 PP and 84% ML) and are closer to S. linearis (Hbst.) than S. granarius (L.).     

Stored grain pest prevalence and insecticide resistance in Egyptian populations of the red flour beetle Tribolium castaneum (Herbst) and the rice weevil Sitophilus oryzae (L.)

As a consequence of the widespread use of insecticides against insect pests in grain warehouses and in the food industry, insecticide resistance has greatly increased among these species all over the world. Nonetheless, insecticide resistance in stored grain insects in Egypt was little studied. Therefore, our study aimed to forecast infestation risks and determine the insecticide resistance level in Egyptian populations of the main insect pests of stored grains. To achieve this goal, we surveyed the populations of stored-product insects in wheat in two different sites in the Alexandria governorate (Egypt) between June and August 2017. Moreover, insecticide resistance levels in Egyptian populations of Tribolium castaneum (Herbst) and Sitophilus oryzae (L.) collected from wheat storage facilities (granaries and flour mills) in Alexandria governorate were also determined. Three contact insecticides, namely malathion, pirimiphos-methyl and cypermethrin, were tested. Tarsal contact bioassays on filter paper were carried out on adults following FAO methods and concentration-mortality lines were estimated to determine the resistance ratios for each insecticide and population. The data from the monitoring of insects revealed that T. castaneum and S. oryzae were the most common species infesting wheat in both Alexandria granaries and mills. Populations of S. oryzae were generally more resistant to malathion than the populations of T. castaneum. However, T. castaneum populations were more resistant to pirimiphos-methyl than the S. oryzae populations. Both populations of T. castaneum and S. oryzae were susceptible to cypermethrin. The detected levels of insecticide resistance indicate that this phenomenon takes place and should be a concern in Egyptian populations of T. castaneum and S. oryzae requiring monitoring and design of resistance management practices.     

Efficacy of diflubenzuron plus methoprene against Sitophilus oryzae and Rhyzopertha dominica in stored sorghum

The efficacy of diflubenzuron (1 mg kg⁻¹)+methoprene (1 mg kg⁻¹) against Sitophilus oryzae (L.) and Rhyzopertha dominica (F.) in sorghum was evaluated in a silo-scale trial in southeast Queensland, Australia. Sorghum is normally protected from a wide range of insects by mixtures of grain protectants. The chitin synthesis inhibitor diflubenzuron was evaluated as a potential new protectant for S. oryzae in combination with the juvenile hormone analogue methoprene, which is already registered for control of R. dominica. Sorghum (ca 200 t) was treated after harvest in 2000 and assessed for treatment efficacy and residue decline during 6.5 months storage. The reproductive capacity of S. oryzae and R. dominica was greatly reduced in bioassays of treated sorghum throughout the trial, and efficacy remained relatively stable during the trial. An initial exposure of S. oryzae adults to treated sorghum for 2 weeks reduced F₁ progeny production of all strains by 80.8-98.8%, but a second exposure of 4 weeks reduced F₁ progeny production by 98.5-100%. In addition, the reproductive capacity of any S. oryzae progeny produced was greatly reduced. Exposure of R. dominica adults to treated sorghum for 2 weeks reduced F₁ progeny production of all strains by 99.6-100%, including a methoprene-resistant strain. The results indicate that S. oryzae or R. dominica adults invading sorghum treated with diflubenzuron (1 mg kg⁻¹)+methoprene (1 mg kg⁻¹) would be incapable of producing sustainable populations.     

Evaluation of diatomaceous earth formulations enhanced with natural products against stored product insects

The insecticidal effect of prepared insecticide formulations labelled as Natural P, Inert Natural P and Py EC on Sitophilus oryzae, Rhyzopertha dominica and Tribolium castaneum have been evaluated on wheat grains. Formulation Natural P contains diatomaceous earth (DE), amorphous silica gel (3%), pyrethrin, flax oil, lavandin essential oil (EO) and un-activated yeast. Formulation Inert Natural P contains DE, amorphous silica gel, lavandin EO and food grade bait whereas formulation Py EC contains pyrethrin, piperonyl butoxide (PBO), flax oil, polysorbate, methyl oleate and amorphous silica gel (5%). Celatom® MN-51 (diatomaceous earth – DE) was used as a standard insecticide. Inert Natural P and Natural P were applied as dust at four different doses and Py EC was applied as emulsions on grain by spraying. All three formulations showed higher insecticidal efficacy and higher progeny inhibition on all three tested insect species and had lower impact on wheat bulk density reduction compared with Celatom® MN-51. The LD₅₀ and LD₉₀ values of Inert Natural P were 48.7 and 163.7 ppm respectively for S. oryzae, 15.2 and 178.0 ppm for R. dominica and 115.2 and 171.3 ppm for T. castaneum. The LD₅₀ and LD₉₀ values of Natural P were 83.6 and 97.9 ppm respectively for S. oryzae, 19.5 and 97.9 ppm for R. dominica and 75.4 and 105.6 ppm for T. castaneum. Applied Py EC at concentration of 2.0 ppm a.i. pyrethrin exhibited 100% mortality after 2 d of S. oryzae and T. castaneum and after 6 d of R. dominica. In addition, all three formulations caused significant reduction of progeny (F1) population compared to control, providing promising approach of integrated pest management strategy.     

Effects of spinosad on the heat tolerance and cold tolerance of Sitophilus oryzae L. (Coleoptera: Curculionidae) and Rhyzopertha dominica F. (Coleoptera: Bostrichidae)

Sitophilus oryzae and Rhyzopertha dominica are serious insect pests of stored products in Sri Lanka. Currently pirimiphos methyl and phosphine fumigation are used as control measures but grain handlers seek for alternatives. Exposure to high or low temperature is popular in stored-product insect pest management but is expensive. Spinosad is effective against certain stored-product insects but has not yet been tested for its synergy with heat or cold. This experiment was conducted to evaluate the effect of spinosad on the heat tolerance and cold tolerance of S. oryzae and R. dominica adults. The experiment was a two-factor factorial, complete randomized design with four replicates. The spinosad concentration and exposure period were changed. Adults of S. oryzae and R. dominica were first exposed to a series of spinosad concentrations. Later they were held at higher (40 °C) or lower (6–11 °C) temperatures than room temperature for different durations.Pre exposure of S. oryzae adults to Spinosad at 18 ppm or above synergized the adult mortality at high or low temperature showing a dose response. Pre-exposure of R. dominica adults to spinosad concentrations 12.5 ppm or higher synergized the mortality at high temperature whereas the spinosad synergized the mortality of R. dominica at low temperature when exposed to 6.25 ppm or higher concentrations; the effects followed a dose response. This study shows that heat and cold tolerance of S. oryzae and R. dominica adults are reduced by pre-exposure to spinosad. Therefore, spinosad is a potential grain protectant at high or low temperatures against these two insect species.     

Breed assignment test in four Italian beef cattle breeds

The assessment of a method able to assign individuals to the breed of origin is needed to certify origin and quality of livestock products. A set of 21 microsatellites was tested for breed identification in four native Italian beef breeds: Chianina, Marchigiana, Romagnola, and Piemontese. Two statistical approaches, based on maximum likelihood and on a Bayesian method, were evaluated. Different marker sets, chosen in order of the highest gene diversity and F_ST estimates were also tested. The Bayesian method performed better, achieving a correct assignment rate of about 90% even with six microsatellites. The marker sets with the highest gene diversity were shown to perform best. Considering a threshold probability of 90%, only 52.5% of the genotypes were correctly allocated. Such results are mainly due to the low genetic differentiation estimates among breeds (F_ST = 0.049). These findings suggest that markers with high gene diversity and the presence of private alleles should be investigated and the Bayesian method used.     

Insecticidal efficacy of diatomaceous earth against Sitophilus oryzae (L.) (Coleoptera: Curculionidae) and Tribolium confusum du Val (Coleoptera: Tenebrionidae) on stored wheat: influence of dose rate, temperature and exposure interval

Laboratory experiments were conducted in order to assess the insecticidal effect of a diatomaceous earth formulation (Silicosec^®, Biofa GmbH, Germany) against Sitophilus oryzae and Tribolium confusum on stored wheat. Adults of the two species were exposed on wheat treated with diatomaceous earth at four dose rates: 0.25, 0.5, 1 and 1.5 g/kg of wheat, respectively. For each dose rate, the treated wheat was placed at 22°C, 25°C, 27°C, 30°C and 32°C. Dead adults were counted after 24 and 48 h, 7 and 14 d of exposure. After the 14-d interval, the live adults were removed and placed for 7 d in untreated wheat (in the case of S. oryzae) or untreated flour (in the case of T. confusum), and the production of F₁ was recorded. For both species, dose rate, temperature and exposure interval significantly affected mortality (P<0.001). Mortality was higher at longer exposure intervals. The efficacy of SilicoSec against S. oryzae increased with temperature, but for T. confusum mortality was lower at 32°C, compared to 30°C, for 24 and 48 h exposure intervals. Tribolium confusum proved less susceptible to SilicoSec than S. oryzae. In general, the rates of 1 and 1.5 g/kg of wheat provided a satisfactory level of protection against the two species examined. For S. oryzae, F₁ emerged only at 22°C, in wheat treated with 0.25 or 0.5 g/kg. However, for T. confusum, F₁ were recorded at 22°C for 0.5 g/kg and at 22°C, 25°C, 27°C and 30°C for 0.25 g/kg.     

Identification of genomic region associated with rice weevil resistance in sorghum [Sorghum bicolor (L.) Moench]

The rice weevil Sitophilus oryzae (L.) is an important pest of stored sorghum (Sorghum bicolor L. Moench). Quantitative trait loci (QTL) associated with rice weevil resistance was analyzed using a F₁₀ recombinant inbred line (RIL) population derived from the cross 654 (susceptible) × LTR108 (resistant). Grain weight losses (GWL), flour production (FP), and percentage kernels damaged (PKD) were evaluated across two trails. A total of 21 QTLs for GWL, FP, and PKD were identified. The phenotypic variation explained by individual QTL varied from 4.14% to 15.51%. Both parents contributed resistance alleles. Several main effect QTLs affecting GWL, FP, and PKD were mapped to the same regions on chromosome 2. The identification of genomic regions associated with rice weevil resistance will be useful to understand the resistance mechanism and improve rice weevil resistance of sorghum by marker-aided approaches.     

The use of modified atmospheres to control Sitophilus zeamais and Sitophilus oryzae on stored rice in Portugal

This study reports the efficacy of using CO₂ against Sitophilus zeamais and Sitophilus oryzae as an alternative treatment to fumigation for rice stored in a rice mill in Portugal. The trials were conducted in a silo containing 40 tonnes of polished rice and in four hermetic big bags of 1 tonne capacity; two with paddy and two with polished rice. The composition of the atmosphere ranged from 90 to 95% CO₂ and 0.7–2.1% O₂. Three trials were carried out at different temperatures and treatment times; stored rice in the silo at 29.6 ± 0.1 °C for 26 days (first trial), at 34.1 ± 0.2 °C for 10 days (second trial), and in big bags at 22 °C for 26 days (third trial).To evaluate the efficacy of each treatment, metal cages with 16 g of infested rice where placed at bottom, middle, top and surface of the polished rice in the silo. Four replications of each type of infested rice containing one-week-old S. zeamais adults, or eggs of S. zeamais or S. oryzae, were incubated in the laboratory, at the same temperature as in the silo, to serve as a control.In all modified atmosphere treatments adults of S. zeamais, and eggs of both S. oryzae and S. zeamais, showed mortality close to 100% and no F1 emergence was recorded in any treatment sample. This was the first time that a Portuguese rice mill used modified atmospheres.     

Effects of habitat fragmentation on genetic diversity and population differentiation of Liposcelis bostrychophila badonnel (Psocoptera: Liposcelididae) as revealed by ISSR markers

Liposcelis bostrychophila is a globally important stored product pest that is adapted to many habitats and is difficult to control due to insecticide resistance. We evaluated effects of habitat fragmentation on the genetic diversity and differentiation of L. bostrychophila populations. These genetic data were collected in Wuhu city, China using inter-simple sequence repeat (ISSR). A total of 108 discernible DNA fragments were detected, 107 were polymorphic and the percentage of polymorphic bands (PPB) was 99.1%. Shannon’s information index (I) and Nei’s gene diversity (He) from four populations were ordered (high to low) as herbstore > flourmill > bedroom > library. Population differentiation (Gst = 0.41) was average in these populations. Analysis of molecular variation (AMOVA) showed that variation existed mainly within populations (68.1%). At the same time, gene flow (Nm = 0.72) was interpreted to be moderate. Cluster analysis showed a small degree of genetic distance among different isolates and permitted grouping the isolates of L. bostrychophila into three distinct clusters. The study demonstrated clear genetic effects of habitat fragmentation on L. bostrychophila populations. Genetic diversity decreased and genetic differentiation increased between remnant populations when habitats were fragmented. Gene flow is not impeded by habitat fragmentation due to the impact of human activities. These findings are of great use for the prevention and control of this pest.     

Effect of paddy variety,milling status and aeration on the progeny emergence of Sitophilus oryzae L. (Coleoptera: Curculionidae)

In Sri Lanka, insects cause enormous losses in stored paddy; the rice weevil Sitophilus oryzae is a devastating insect. The magnitude of losses in stored paddy caused by insects varies with characteristics of paddy but has not yet been fully investigated. Due to the increased concerns on the use of synthetic chemicals, safer alternative pest management strategies for stored paddy are needed. Objectives of this study were to determine the effect of paddy variety, milling status and nature of aeration on the progeny emergence of S. oryzae. The experimental design was a three-factor factorial, completely randomized design (CRD).Freshly emerged S. oryzae adults were introduced to un-milled paddy or milled/polished rice belonging to traditional and improved varieties. The samples were maintained either open or airtight, and the progeny adults emerged was determined at monthly intervals. Progeny emergence was lower in the traditional varieties, un-milled paddy and under air-tight condition compared to improved varieties, milled/polished rice and aerated samples, respectively. Overall, the improved variety Bg 300 exhibited the maximum resistance to infestation by S. oryzae. Furthermore, white-colour and long-grain varieties produced lower progeny of S. oryzae than red-colour and short-grain varieties, respectively. It is concluded that the maximum protection in paddy/rice from insect infestation during storage can be achieved by using traditional varieties, stored as un-milled paddy (without dehusked) under airtight condition.     

Rolliniastatin-1, a bis-tetrahydrofuran acetogenin: The major compound of Annona mucosa Jacq. (Annonaceae) has potent grain-protective properties

The widespread use of synthetic insecticides results in insecticide-resistant populations of maize weevil, Sitophilus zeamais Motsch. (Coleoptera: Curculionidae), a primary pest species of stored corn worldwide. Thus, new active ingredients with different modes of action are needed for integrated pest management (IPM) of stored grains. Thus, toxicological bioassays (using S. zeamais as bioindicator) associated to chromatographic techniques were performed to isolate insecticidal compounds from the grain-protective ethanolic extract of Annona mucosa Jacq. (Annonaceae) seeds. The acetogenin bis-tetrahydrofuran rolliniastatin-1 was identified as major bioactive constituent from A. mucosa seeds based on bioassay-guided fractionation, along with rolliniastatin-2, deethylrollinastatin-1, jimenezin and fractions containing triglycerides. This compound produced 51.1% of mortality of adult weevils when applied at 57.66 mg kg⁻¹, drastically reducing F₁ progeny and their damage to corn grains. Although the acute toxicity level was lower than that with the formulation based on diatomaceous earth at a concentration 17.3 times higher (Insecto®^, at recommended rate) used as a positive control, rolliniastatin-1 protected grain statistically similar to that of the positive control. Furthermore, bioassays indicate that compounds of different chemical natures have a synergistic effect on the overall biological activity of seed derivatives of A. mucosa. Efficacy and technical viability of the process to obtain rolliniastatin-1 from the seeds of A. mucosa should allow the production of a botanical insecticide to control populations of S. zeamais at corn warehouses.     

Chemical,physical, and sensory changes in rice subjected to UV-C radiation and its acceptability to rice weevil Sitophilus oryzae (L.) (Coleoptera: Curculionidae) and humans

The objective of this study was to evaluate the effect of ultraviolet C (UV–C) radiation treatment of rice on both the development of S. oryzae and changes in the quality of rice Oryza sativa (L.) (Poales: Poaceae) grains. Husked rice grains were treated with UV-C, and subsequently placed alternately with the control treatment (without UV-C) in an arena. Subsequently, 500 adult S. oryzae were placed in the center of the arena, with free access to both samples for 30 days. On the 30th day of the experiment, a greater number of adult S. oryzae were identified as consuming grains subject to the control treatment; these same S. oryzae also showed greater emergence on the 40th, 50th, and 60th day. In the group treated with UV-C, reduced acidity, protein solubility, K₂₃₂, K₂₇₀, luminosity, and damage were observed; however, increases in gravimetric and volumetric yields were also observed. The sensorial analysis of the cooked grains showed that grains treated with UV-C had less acceptance and purchase intention by human panelists, mainly due to the odor attribute. Further studies are still needed to identify metabolites formed in rice subjected to UV-C; however, it is noteworthy that some S. oryzae are able to recognize irradiated rice as distinct from control.     

The toxicity effects of atmospheres with high content of carbon dioxide with addition of sulphur dioxide on two stored-product pest species: Sitophilus oryzae and Tribolium confusum

Modified atmospheres (MAs) are safe and environmentally friendly alternatives to control pests in stored products. However, to accommodate the requirements of the food industry, the control of insects by a MA can be a too lengthy process. This paper describes the potential of sulphur dioxide as an additive to reduce the long lethal exposure of modified atmospheres (MA) enriched with carbon dioxide for major stored product pests. Specifically, we evaluated whether the addition of SO₂ (0–30,000 ppm) to a high-CO₂ content of 70%–95% MA could enhance its insecticidal effect for the control of Sitophilus oryzae and Tribolium confusum. The addition of 15,000 ppm and 30,000 ppm of SO₂ to 95% CO₂ enhanced control up to 100% in comparison to CO₂ alone for S. oryzae and T. confusum adults when treated for one day in all of the substrates tested. However, the effectiveness of adding SO₂ at reduced contents was lower and depended on the substrate and pest species considered. The addition of SO₂ also increased the mortality of all of the developmental stages of S. oryzae. The increase in mortality with the addition of SO₂ was also observed when included with 70% CO₂. Therefore, the addition of SO₂ can be considered a feasible means of shortening the length of treatment necessary to achieve the control of these two pests using a high-CO₂ MA.     

Field validation of phosphine efficacy on the first recorded resistant strains of Sitophilus granarius and Tribolium castaneum from the Czech Republic

This study first estimated the current state of phosphine sensitivity (using a knock-down/KT₁₀₀/Degesch kit) in Sitophilus granarius (23 strains) and Tribolium castaneum (8 strains) in Czech Republic grain stores. The resistance of S. granarius (21.7% resistant strains; coefficient of resistance KT₁₀₀ ranged from 0.5 to 2.3 among strains) was substantially lower and less frequent than that of T. castaneum (87.5% resistant strains; coefficient of resistance KT₁₀₀ ranged from 0.9 to 52.5 among strains). The phosphine efficacy of the laboratory and field (i.e., resistant) pest strains was validated during commercial fumigation when suboptimal tarpaulin sealing resulted in low-concentration phosphine exposure (Ct products ranged from 5.9 to 7.4 g*hr/m³). Although even low-dose fumigation led to 100% adult mortality of both laboratory and field strains of S. granarius and laboratory strains of T. castaneum, the mortality of the field strain of T. castaneum ranged from 47% to 95%. Larval emergence from the fumigated commodity samples with pest eggs was zero or near zero for laboratory strains, while 1.3–6.0 (S. granarius) and 63.7–80.00 (T. castaneum) field-strain larvae emerged per sample (100 g). This study shows that although a high proportion of the tested pest populations were still sensitive, several T. castaneum populations showed an elevated level of resistance that may decrease field fumigation efficacy, especially under suboptimal phosphine dosage conditions.     

Insecticidal and residual effect of three pyrethroids against Sitophilus oryzae (L.) (Coleoptera: Curculionidae) on stored wheat

Laboratory tests were conducted to assess the use of the pyrethroids, deltamethrin, beta-cyfluthrin and alpha-cypermethrin at the rates of 0.125 and 0.25 ppm, as grain protectants in stored wheat against the rice weevil, Sitophilus oryzae (L.). For this purpose, clean untreated wheat was sprayed with these insecticides and stored for 6 months. During this period, four bioassays were carried out, in order to evaluate the residual efficacy of each pyrethroid. In each bioassay, treated wheat was infested with S. oryzae adults, and dead insects were counted after 1, 2 and 7 d of exposure. In addition, S. oryzae progeny production was estimated on each bioassay, until the production of the F₄ generation. The results indicated that deltamethrin and beta-cyfluthrin, both at 0.25 ppm, were significantly more efficient than the other treatments. Efficacy was notably higher after 7 d of exposure, than after 1 and 2 d. At the same exposure level, 1, 2 and 3.5 months after treatment, at 0.25 ppm, mortality for deltamethrin was approximately 89%, 92% and 86%, respectively, while the corresponding percentages for beta-cyfluthrin were 97%, 83% and 62%, and for alpha-cypermethrin 50%, 49% and 33%. However, at the rate of 0.125 ppm, mortality was ?58% in all cases. In all treatments appearance of F₁-F₄ generations was observed, with the exception of both deltamethrin and beta-cyfluthrin at 0.25 ppm, where only a few F₁ adults were noted, without the production of subsequent generations.