DNA identification of two laboratory colonies of the weevils, Sitophilus oryzae (L.) and S. zeamais Motschulsky (Coleoptera: Curculionidae) in Taiwan

DNA amplification fingerprinting (DAF) and nuclear ribosomal DNA (nrDNA) amplification were used to discriminate between two laboratory colonies of two closely related species of weevils: the rice weevil, Sitophilus oryzae (L.), and the maize weevil, S. zeamais Motschulsky. For DAF, three sets of primers (aldolase, prolactin receptor, and interleukin-1β) were used for identification by polymerase chain reaction (PCR) and agarose gel electrophoresis, and the highly similar patterns of the resultant amplicons reconfirmed that the two weevils are closely related. The fragments of nrDNA amplification showed that for S. oryzae and S. zeamais, the homologies of the nucleotide sequences of the internal transcribed spacer regions, ITS-1 and ITS-2, were unusually high, at 96% and 97%, respectively. Based on the ITS-1 and ITS-2 sequences, two species-specific primer sets were designed: with the primer set ITS3/So, the predicted 450 bp DNA fragment was yielded with S. oryzae genomic DNA after PCR amplification (n=10), but no PCR product was obtained with S. zeamais (n=10); with the primer set ITS3/Sz, the same 10 S. zeamais specimens yielded a 550 bp DNA fragment, but S. oryzae yielded no amplicons. In view of the difficulty of distinguishing between these two closely related species, the specificity and availability of these two primer sets might prove to be a useful tool for distinguishing between them. However, the nrDNA sequences of the ITS-1 and ITS-2 regions of geographically isolated populations of both weevils still need to be elucidated, and the applicability of this technique to different geographical populations will need to be confirmed by further study.     

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.     

Altered susceptibility of granary weevil Sitophilus granarius (L.) (Coleoptera: Curculionidae) populations to insecticides after selection with pirimiphos-methyl and deltamethrin

Toxicity of the contact insecticides dichlorvos, malathion, chlorpyrifos-methyl, pirimiphos-methyl, deltamethrin and cypermethrin to granary weevil Sitophilus granarius (L.) adults from five populations previously selected with deltamethrin and pirimiphos-methyl, and two populations that had no contact with pesticides over an interval of 12 generations, was investigated in the laboratory by application to filter paper. The populations originated from storage facilities situated in different parts of the former Yugoslavia.A population originating from Apatin and one from Belgrade Port were selected three times each at the LD₅₀ level with deltamethrin and pirimiphos-methyl, respectively. In separate experiments, weevils from Belgrade Port were selected once at the pirimiphos-methyl LD₇₀ level and Bijeljina and Kikinda populations both once at the deltamethrin LD₇₀ level. All selection doses were at a level obtained from determining weevil mortality after 24 h of exposure to treated filter paper.Compared with the toxicity to laboratory weevils, the weevils from Apatin after the third selection were found to be 32.1 and 51.9 times less susceptible to deltamethrin at the LD₅₀ and LD₉₅ levels, respectively, while those from Belgrade Port were 2.7 and 3.2 times less susceptible to pirimiphos-methyl. Selection of Belgrade Port weevils at the pirimiphos-methyl LD₇₀ did not significantly affect their susceptibility to that insecticide but it caused a significant decrease in deltamethrin toxicity, which is indicative of a cross-resistance between the two compounds. Selection of Bijeljina weevils caused an increased resistance to deltamethrin, while selection of Kikinda weevils had little effect on their susceptibility to dichlorvos, but it caused a significant decrease in malathion and cypermethrin toxicity, and resistance to deltamethrin markedly increased (RR=238.8 at LD₅₀ and 660.8 at LD₉₅ levels, compared with laboratory weevils). Chlorpyrifos-methyl was the most toxic insecticide to all populations, while cypermethrin was the least toxic compound.     

Efficacy of Silicosec, filter cake and wood ash against the maize weevil, Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) on three maize genotypes

The effectiveness of the diatomaceous earth Silicosec, a mineral industrial filter cake and domestic wood ash, applied at three different rates for the control of the maize weevil, Sitophilus zeamais, on three maize genotypes was determined. Treatment with Silicosec was the most effective followed by filter cake and wood ash. The treatments reduced progeny emergence, percentage grain damage and grain weight losses, but did not affect percentage seed germination. Grain treated with wood ash at all rates resulted in a relatively low mortality 3 days after infestation as compared to other treatments. However, all treatments caused high mortality (97-100%) after 15 days of exposure. Therefore, Silicosec, filter cake and wood ash can be considered as potential components of an integrated pest management strategy against the maize weevil.     

Susceptibility to contact insecticides of granary weevil Sitophilus granarius (L.) (Coleoptera: Curculionidae) originating from different locations in the former Yugoslavia

The insecticides dichlorvos, malathion, chlorpyrifos-methyl, pirimiphos-methyl, deltamethrin and cypermethrin were applied to filter paper to assess their toxicity to granary weevil Sitophilus granarius adults. Based on discriminating doses obtained from tests on a susceptible laboratory population, the susceptibility of 12 populations originating from different storage facilities (11 in Serbia and 1 in Bosnia-Herzegovina) was tested. The facilities included silos, floor stockpiles and attic stockpiles. Weevils originating from Apatin, Belgrade Port, Bijeljina and Kikinda were submitted to toxicity testing, and determination of ld-p lines, LD values and levels of susceptibility/resistance. Chlorpyrifos-methyl proved the most toxic and cypermethrin the least toxic insecticides against all populations. Dichlorvos, malathion and pirimiphos-methyl had the least toxic effect on weevils originating from Belgrade Port and Kikinda, While deltamethrin was most toxic to weevils from Belgrade Port, and least toxic to weevils from Kikinda. The resistance ratios (RR) for deltamethrin at the LD₅₀ and LD₉₅ levels 48 h after exposure to treated filter paper were 11.2 and 14.5 for Bijeljina weevils, and 20.9 and 25.5 for Kikinda weevils.     

Ovarian physiology and age-grading in the rice weevil, Sitophilus oryzae (Coleoptera: Curculionidae)

Physiological and morphological changes in the ovarian system in rice weevils, Sitophilus oryzae (L.) (Coleoptera: Curculionidae), reared on wheat seeds were studied as a function of adult age, mating status, and nutrient availability. X-ray analysis was used to determine time of adult eclosion and the duration of development of pre-emergent weevils within the seeds, a process that lasted almost 4 days at 25°C and 60% r.h. There was no follicular differentiation in pre-emergent weevils. Oocyte maturation began after adults emerged from the seeds and started to feed. There was a significant increase in mean germarium length and the size of proximal follicles within the first 5 days when newly emerged weevils were mated and fed ad lib. Maximum number of follicles and mature eggs per ovariole in mated females occurred at 5-30 days of age. The number of mature eggs decreased in 60-days-old weevils, at the same time that adult mortality increased. Development of the ovarian system was much slower in unmated females than in mated females. Although there was follicle development in unmated females, ovulation never occurred and no eggs were laid. Starvation of mated females resulted in a rapid reduction in numbers of follicles and mature eggs, probably as a result of oosorption. Females were categorized into two nulliparous and three parous stages according to ovarian development and the degree of accumulation of follicular relics. Parity was directly correlated with both weevil age and the number of progeny produced and was the physiological basis used to construct an age-grading model for this species. The method will be useful for determining the age structure and reproductive potential of rice weevil populations in the field.     

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.     

The impact of grain quantity on the biology of Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae): oviposition, distribution of eggs, adult emergence, body weight and sex ratio

Studies were conducted to observe the effect of grain quantity on the oviposition, distribution of eggs, adult emergence, adult body weight and sex ratio of Sitophilus zeamais, an important pest of maize. Three quantities of maize grains (200, 400 and 800 grains/glass jar of 1 l) were used under laboratory conditions (30°C and 70% r.h.). Twenty five male/female pairs of maize weevil were introduced into each glass jar for 12 d. The greatest and lowest number of eggs was laid on batches of 800 and 200 grain kernels, respectively. The total number of grains attacked followed a similar trend. The aggregation of eggs expressed as a ratio of the variance to mean increased as the amount of maize grains was increased. The aggregation parameter k ranged from 2.35 on 200 grains to 4.49 on 800 grains. On average, a significantly higher proportion of grains were infested (90%) when less grain was present. Emergence of adult weevils was, however, maximal (472 individuals/glass jar) when the grain quantity was highest. The mean weight of the emerged adults was not significantly influenced by grain quantity, but female weevils were heavier than males (mean weight 3.16 vs. 3.05 mg) irrespective of maize quantity and eggs laid. Sex ratio (males/100 females) of emerged adults did not differ among treatments. The importance of grain quantity, and of oviposition, in internally feeding granivores and the possibility that there is an adaptive reproductive strategy are discussed.     

Insecticidal and antifeedant effects of Junellia aspera (Verbenaceae) triterpenes and derivatives on Sitophilus oryzae (Coleoptera: Curculionidae)

Triterpenes from Junellia aspera (Gillies ex Hook) (Verbenaceae) and chemical derivatives were evaluated for their antifeedant and toxic effects against adults of Sitophilus oryzae (L.). Five triterpenoids were acutely toxic by ingestion. The compounds maslinic acid (II), daucosterol (III), and 3β-hydroxy-12α-bromine-(28→13)-oxide-oleanane (IX) showed the highest toxic effects, while oleanolic acid (I) and oleanonic acid (VII) showed less toxicity. Daucosterol also exhibited important feeding deterrence activity. No toxicity was observed when the assayed compounds were topically applied.     

Comparative study of field and laboratory evaluations of the ethnobotanical Cassia sophera L. (Leguminosae) for bioactivity against the storage pests Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) and Sitophilus oryzae (L.) (Coleoptera: Curculionidae)

The powdered leaves of Cassia sophera along with hot- and cold-water leaf extracts of this plant were tested in laboratory experiments in the UK and in field trials in Tamale, Northern Ghana, using traditional storage containers, to determine their inhibitory and toxic effects against Sitophilus oryzae and Callosobruchus maculatus infestation of stored rice and cowpea, respectively. Laboratory and field experiments with cowpea showed that the use of C. sophera hot-water extracts was more effective at reducing C. maculatus infestation and adult emergence on cowpea than the traditional leaf-powder application (1% and 5% w/w) or the use of a cold-water extract of C. sophera. Hot-water extracts of C. sophera might be a more effective technique of applying the plant material on to stored cowpea than using powdered C. sophera leaves, the currently used application by small-scale farmers. In contrast, experiments with S. oryzae on rice showed that C. sophera leaf powder (5% w/w) effectively reduced adult emergence in the laboratory, but this could not be confirmed under field conditions. The hot and dry climatic conditions in the field might impart a natural protection against rice infestation by S. oryzae, making the use of protectants and pesticides less necessary for farmers. This was supported by the negligible rice grain damage after 6 months of field storage and by the failure of the S. oryzae population to establish itself under field conditions. The implications of using botanicals in pest control are discussed.     

Fecundity and ability of the parasitoid Lariophagus distinguendus (Hymenoptera: Pteromalidae) to find larvae of the granary weevil Sitophilus granarius (Coleoptera: Curculionidae) in bulk grain

Fecundity and host finding behavior in the storage environment were examined for the parasitoid Lariophagus distinguendus (Förster), a potential agent for biological control of stored-product pest beetles. Larvae of the granary weevil, Sitophilus granarius (L.), in wheat were used as hosts. The fecundity varied considerably among strains of different geographical origin, from 1.3±2.4 to 92.5±23.6 offspring per female parasitoid. Host finding ability was examined in a small acrylic cylinder (about 42 kg wheat), a silo-bin (20.5 t wheat grain), and the flat storage grain bin in a commercial facility (3000 t rye). Experiments in the cylinder and the silo-bin revealed that the number of hosts found by the parasitoids decreases with increasing depth. Moreover, parasitoids were more likely to find hosts in the cylinder than under field storage conditions in a silo-bin. This indicates that the density of parasitoids per unit volume of the grain might be an important factor in determining how deep they penetrate a grain layer. In the silo-bin and the commercial grain store, parasitoids were able to find and parasitize hosts located up to 4 m vertically and horizontally from the release point. The significance of these results with respect to the suitability of L. distinguendus for biological control of the granary weevil is discussed.     

Screening of Chinese medicinal herbs for bioactivity against Sitophilus zeamais Motschulsky and Tribolium castaneum (Herbst)

Extracts of 40 species of Chinese medicinal herb from 32 different botanical families were screened for contact, fumigant and feeding-deterrent activities against the two stored-grain insects Sitophilus zeamais and Tribolium castaneum. Thirty Chinese medicinal herbs exhibited insecticidal or feeding-deterrent activities against the two species of insects. Extracts of Artemisia argyi, Dictamnus dasycarpus, Evodia rutaecarpa, Litsea cubeba, Narcissus tazetta var. chinensis, Polygonum aviculare, Rhododendron molle, Sophora flavescens, Stemona sessilifolia, Tripterygium wilfordii, and Torreya grandis were most active.     

Survival and reproduction of Tribolium castaneum (Herbst), Rhyzopertha dominica (F.) and Sitophilus oryzae (L.) following periods of starvation

This study determined the starvation tolerance of Tribolium castaneum (Herbst), Rhyzopertha dominica (F.) and Sitophilus oryzae (L.) in terms of both adult survival and reproduction, the impact of starvation on reproduction not having been studied before. Experiments were conducted at 30 °C and 55% or 70% r.h. using a laboratory strain and a field strain of each species. The number of progeny was a better indicator of the impact of starvation on a species than adult survival. Tribolium castaneum was the most tolerant species, requiring up to 35 d starvation before no progeny were produced. Rhyzopertha dominica and S. oryzae required up to 8 d starvation before no progeny were produced. The results suggest that hygiene will have a greater impact on populations of S. oryzae and R. dominica than T. castaneum.     

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.     

The effect of wheat grain composition, cuticular lipids and kernel surface microstructure on feeding, egg-laying, and the development of the granary weevil, Sitophilus granarius (L.)

Adult feeding intensity, oviposition, and larval development of Sitophilus granarius (Coleoptera: Curculionidae) were observed on grain from three Polish wheat varieties (Begra, Korweta, and LGR 896/64a) washed with petroleum ether to remove cuticular lipids. Extraction of lipids did not cause any statistically significant changes in the physicochemical, biochemical and technological (milling, rheological and baking) properties of the wheat grain studied. Wheat grain washed with petroleum ether did not show any visible changes in the surface and morphology of the outer layer of the wheat grain. However, differences were noted in the microstructure of the kernel surface. Grain with a thicker seed coat (LGR 896/64a) was infested at a lower rate than the other varieties. Of the 18 hydrocarbons extracted from the grain surface, three compounds - n-heptacosane (C₂₇), n-nonacosane (C₂₉) and n-hentriacontane (C₃₁) - were found in significant amounts. In general, beetles produced 64-95% less dust and laid 7-16% fewer eggs in kernels from which cuticular lipids had been removed. This implies that these compounds have a major role in food selection and the search for an oviposition site prior to grain infestation.     

Residual effects of deltamethrin and malathion on different populations of Sitophilus granarius (L.) on treated wheat grains

The impact of 2-, 7-, 14-, 30-, 90-, 150- and 720-day-old deposits of deltamethrin, applied with or without the synergist piperonyl butoxide (PBO), and of malathion, on adults of different populations of granary weevil Sitophilus granarius on wheat was investigated in the laboratory. The insecticides used were commercial formulations and their application rates were as recommended: deltamethrin (dustable powder) 0.5 mg a.i./kg, deltamethrin + PBO (1:10) (emulsifiable concentrate) 0.25 mg a.i./kg, and malathion (dustable powder) 10 mg a.i./kg. The weevil populations examined were: (a) a laboratory population, (b) field populations with different susceptibility to some insecticides as established previously, and (c) populations selected in the laboratory with deltamethrin or pirimiphos-methyl.The 2-day-old deposit of malathion caused complete mortality of all weevil populations after 7 and 14 days. The corresponding deposit of deltamethrin was 100% effective only against the laboratory weevils after 7-14 days, while deltamethrin at the lower level formulated with PBO caused about 90% mortality of laboratory weevils and much lower levels of kill among field and selected weevils.Deposits of deltamethrin and malathion up to 90 days old killed all field weevils after 14 days of exposure. The 150-day-old deposit of deltamethrin was also 100% effective against field weevils exposed for 14 days, while the effectiveness against selected populations was around 50%. Malathion deposits of the same age gave 40-50% mortality of field weevils, and 4-68% mortality of selected weevils. The 720-day-old deposits of malathion were ineffective against all weevil populations, while the mortality of laboratory weevils after 14 days contact with deltamethrin deposits of the same age was 76%, and that of field and selected weevils about 50%.     

Resistance of Tripsacorn to Sitophilus zeamais and Oryzaephilus surinamensis

One strategy that has been used to find germplasm for developing improved plant varieties is to test ancestral germplasm for the desired traits. Although the progenitors of commercial maize are not known, a hybrid (called Tripsacorn) developed from a perennial teosinte, Zea diploperennis, and eastern gamagrass, Tripsacum dactyloides, resembles the earliest known samples of primitive domesticated maize. We tested resistance of whole Tripsacorn to the primary storage pest (primary storage pests can infest intact kernels) the maize weevil, Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae), and resistance of ground Tripsacorn to the secondary storage pest (secondary pests usually cannot infest intact kernels) the sawtoothed grain beetle, Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae). Tripsacorn was immune to attack by S. zeamais. The weevils were unable to lay eggs in the Tripsacorn, and we hypothesized that the hardness of the fruitcase was responsible for lack of weevil oviposition. Oryzaephilus surinamensis were able to complete immature development on ground Tripsacorn, but duration of development was longer and weight of emerged adults was less than for beetles developing on wheat. Hardness of the fruitcase may have been a primitive mechanism of defense against insects and other pests, but probably would not be an acceptable trait in commercial varieties. It remains to be determined whether the possible antibiotic effect demonstrated in ground Tripsacorn would be a useful trait in commercial maize hybrids.     

Survival of Sitophilus oryzae (L.) on wheat treated with diatomaceous earth: impact of biological and environmental parameters on product efficacy

A series of experiments was conducted to evaluate the effects of temperature, relative humidity (r.h.), population density, concentration, exposure interval, and residual aging on susceptibility of Sitophilus oryzae (L.), the rice weevil, to diatomaceous earth (DE). In the first experiment, hard red winter wheat was treated with 300 ppm of the Protect-It™ formulation of DE, and 10, 20, or 30 1-2 week-old mixed-sex adult weevils were exposed on 35 g of wheat for 1 week at combinations of 22°C, 27°C, or 32°C; 40%, 57%, or 75% r.h. No weevils survived when exposed at 40% or 57% r.h., but at 75% r.h. survival was related to both population density and temperature. A higher percentage of adults survived when 30 were exposed compared to 10 and 20, and within each density, survival decreased with increasing temperature. No F₁s were produced at any r.h. on wheat held at 22°C. At 27°C and 32°C, the maximum number of F₁s was produced on wheat held at 75% r.h. In the second experiment, wheat was treated with 25%, 50%, 75%, or 100% of the label rate of 300 ppm, and 10 mixed-sex adult S. oryzae were exposed on 35 g of wheat for either 1, 2, or 3 weeks at 27°C, 57% and 75% r.h. Survival decreased with increasing exposure interval and concentration, but within exposure interval and concentration, survival was usually greater at 75% versus 57% r.h. In the final experiment, wheat was treated with 300 ppm, held at 22°C and 27°C, 57% r.h., and bioassayed at monthly intervals for 3 months by exposing 20 adult mixed-sex S. oryzae on 35 g of wheat for 1 or 2 weeks. At each month, survival of S. oryzae was greater when exposed at 22°C compared to 27°C and when exposed for 1 week compared with 2 weeks. Survival gradually increased with each monthly bioassay, except for those conducted at 3 months. Results of these studies show that S. oryzae is susceptible to DE, but survival of exposed insects will depend in part on the temperature and r.h. humidity (or grain moisture content) at which they are exposed. Survival is directly related to temperature, and as r.h. increases either higher concentrations or longer exposure intervals will be necessary to maintain a certain level of mortality. There may also be a loss of efficacy with residual aging.     

Toxicity of ethyl formate to adult Sitophilus oryzae (L.), Tribolium castaneum (herbst) and Rhyzopertha dominica (F.)

Fumigations were conducted using a continuous flow-through laboratory process to maintain constant concentrations of ethyl formate and low levels (<0.8%) of respiratory carbon dioxide. The procedure minimised the effects of sorption by exposing test insects without media and minimised the effect of carbon dioxide by use of continuous flow. The concentration×time (Ct) products of ethyl formate for adult Sitophilus oryzae, Tribolium castaneum and Rhyzopertha dominica at 25 °C and 70% relative humidity for the 6 h exposure were, respectively: (1) LD₅₀ 107.8, 108.8 and 72.8 mg h L⁻¹ and (2) LD_99.5 207.4, 167.1 and 122.2 mg h L⁻¹. Endpoint mortality was reached within 24 h of initial exposure.     

Influence of grain type on the susceptibility of different Sitophilus oryzae (L.) populations, obtained from different rearing media, to three diatomaceous earth formulations

Bioassays were carried out to assess whether the commodity, from which adults of the rice weevil Sitophilus oryzae emerged, influences the insecticidal efficacy of three diatomaceous earth (DE) formulations: Protect-It™, PyriSec^® and DEBBM. Protect-It™ is a DE formulation that contains 10% silica gel, while PyriSec^® and DEBBM are enhanced DEs that contain natural pyrethrum and the plant extract bitterbarkomycin, respectively. The S. oryzae populations tested were reared on wheat, barley or maize and the susceptibility of each to the DE formulations was assessed on all three commodities. The DE application doses were: 500 ppm for Protect-It™ and PyriSec^®; 150 and 75 ppm for DEBBM. Mortality of S. oryzae adults was counted 7 and 14 d after their exposure on the treated commodities. Bioassays were carried out at 25 °C and 55% r.h. Barley-reared S. oryzae were the most tolerant of all formulations and treated commodities, whereas maize-reared were the most susceptible ones. DE effectiveness was always lower in maize than in wheat or barley irrespective of the commodity from which the populations were obtained. Furthermore, Protect-It™ and PyriSec^® were more effective than DEBBM in wheat or barley, but not in maize.