Efficacy of filter cake and Triplex powders from Ethiopia applied to wheat against Sitophilus zeamais and Sitophilus oryzae

The efficacy of filter cake and Triplex powders applied to wheat was evaluated in the laboratory against the maize weevil, Sitophilus zeamais Motschulsky and rice weevil, Sitophilus oryzae (Linnaeus)—two most common insect pests associated with stored grain in Ethiopia. Efficacy of these powders was determined by exposing 20 adults of each species to 100 g of wheat treated with 0, 100, 500, 700 and 1000 mg/kg of filter cake and Triplex. Adult mortality was determined 7 and 14 d after exposure. In addition, adult progeny production, percentage of insect damaged kernels, and percentage of grain weight loss at each species-powder-concentration-time combinations were determined after 42 d. The 7 and 14 d mortality was 100% for adults of both species exposed to 1000 mg/kg of filter cake; only the 14 d mortality of Sitophilus species was 100% for adults exposed to 700 mg/kg. Mortality of S. oryzae adults was 100% when exposed for 14 d to1000 mg/kg of Triplex. Mortality of S. zeamais never reached 100% in any Triplex treatments. Adult progeny production of S. zeamais was completely suppressed at filter cake concentrations of 700 and 1000 mg/kg, whereas 1000 mg/kg was necessary for complete suppression of S. oryzae adult progeny production. Complete suppression of adult progeny production was not observed in any Triplex treatments. Complete reduction in percentage of insect damaged kernels and percentage of grain weight loss were obtained when S. zeamais and S. oryzae adults were exposed to 1000 mg/kg of filter cake; similar reductions with S. zeamais occurred only at 1000 mg/kg of Triplex. In the case of S. oryzae, complete reduction of insect damaged kernels and grain weight loss were not achieved at any concentration of Triplex. These powders can be used as alternatives to chemical insecticides for management of Sitophilus species.     

Tolerance of Sitophilus zeamais (Coleoptera: Curculionidae) to heated controlled atmosphere treatments

Combination heat and controlled atmosphere (CA) postharvest phytosanitary treatments are environmentally friendly alternatives to chemical fumigants. A controlled atmosphere/heating block system (CA-HBS) was used to rapidly assess tolerances of adult maize weevil, Sitophilus zeamais, both under regular air (RA) and CA (1% O₂ and 15% CO₂) conditions. In the RA treatment, thermal death kinetics for S. zeamais adults were determined at temperatures between 46 °C and 52 °C at a heating rate of 5 °C/min. The results showed that thermal death curves of S. zeamais adults followed a 0th-order kinetic reaction model. The required holding times for achieving 100% mortality were 165, 40, 14, and 4 min at 46, 48, 50 and 52 °C, respectively. The activation energy for killing S. zeamais adults was 526.7 kJ/mol. The effects of CA at various temperature-time combinations and heating rates on insect mortality were evaluated. The mortality of S. zeamais adults was higher under CA treatment than in the heat treatment alone. The slowest heating rate (0.1 °C/min) achieved the highest insect mortality in CA treatments but lowest mortality in RA treatments. The information obtained from the CA-HBS can be used to develop combination heat and CA treatments against S. zeamais.     

Paddy rice stored under hermetic conditions: The effect of relative humidity,temperature and storage time in suppressing Sitophilus zeamais and impact on rice quality

The aim of this study was to analyse the effect of relative humidity in suppressing Sitophilus zeamais, in paddy rice stored under hermetic conditions, during four and seven months, at different average temperatures, as well as the impact on rice quality.Hermetic bags, GrainPro^® SuperGrainbag^® Farm™, were used to store two rice varieties under three different relative humidities: 67%, 75% and 85% RH, and average temperatures of 14 °C, 17 °C and 24 °C, both monitored by Hobo^® Data loggers, with the probe placed inside the bags. CheckpointII Portable O₂ and CO₂ Gas Analyzer was used to assess gas contents on the top and bottom of each bag. At the end of the trials, paddy samples were collected to estimate water activity (a_w). The rheology behaviour of rice pastes prepared with race flour obtained from the different treatments was also evaluated, using a controlled stress rheometer.The results showed that the response of the stored-product insects changes with environmental conditions, O₂ and CO₂ contents. Other parameters were considered; a_w increased with relative humidity and temperature, but decreased with storage time. The relative humidity played an important role, together with the increase of temperature, in suppressing insect populations. A modified atmosphere was naturally produced inside the hermetic bag, under 85% RH, with low O₂ and high CO₂ contents, at different average temperatures, 14 °C and 17 °C. These results demonstrated that S. zeamais can survive, but has no progeny. Under the same conditions, but at the higher average temperature of 24 °C, S. zeamais attained 100% mortality before producing progeny.The increase on respiration rate, registered by CO₂ increase and O₂ decrease, for higher RH values, reduced the viscoelastic functions and changed the starch gelatinization point of Indica and Japonica rice.The results obtained showed that storing paddy hermetically, at low relative humidity, did not change atmospheric content and maintained the viscoelastic functions of the rice pastes.     

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.     

Influence of medium and physical disturbances during rearing on development and numbers of Sitophilus progeny

Four Sitophilus populations (Sitophilus zeamais Motschulsky from Mexico, S. zeamais from Arkansas, U.S.A., Sitophilus oryzae L., and Sitophilus granarius L.) were studied. Hard red winter wheat, sorghum, and maize were used to determine effects of rearing media on progeny production of insects from the four populations.Mechanically disturbing infested samples to remove newly-emerged adult weevils every 48 hr reduced numbers of progeny surviving to the adult stage, especially for Arkansas S. zeamais and S. granarius reared in any of the three grains.With one exception, reproduction for all populations in all three grains was highest when parents were reared in wheat; other differences are discussed. Also, progeny developmental periods for all four populations were shortest when parents were reared in wheat. Progeny of all four populations had the longest developmental periods when reared in maize regardless of the medium in which their parents were reared.In 250 g samples of wheat, and 125 g samples of sorghum, 50 parent S. oryzae produced statistically as many progeny as in 500 g and 625 g samples of wheat and 250, 500, and 625 g samples of sorghum, respectively. In maize, 625 g samples produced significantly more progeny than 500 g samples.Sex ratios of progeny from each grain were 1:1, except that females of S. oryzae reared in maize outnumbered males.     

Effect of CO2 modified atmosphere packaging on aflatoxin production in maize infested with Sitophilus zeamais

The weevil Sitophilus zeamais (Motschulsky), the maize weevil, is a pest of stored maize that can cause feeding damage and lead to the proliferation of toxigenic fungi. The application of modified atmospheres with a high concentration of CO₂ is an alternative method for the control of S. zeamais and the inhibition of fungal growth. The objectives of the study were to determine the effect of S. zeamais infestation, grain damage and grain moisture content on aflatoxin production by Aspergillus flavus on maize, and the impact of high CO₂ modified atmosphere packaging on pest infestation and aflatoxin production. Mycotoxin production was only recorded when maize was infested with S. zeamais and had A. flavus inoculum. However, production of mycotoxins was not recorded when the maize was mechanically damaged and stored at 18% moisture content, indicating that the biological activity of the insect was determinant in the production of mycotoxins. The high CO₂ modified atmosphere packaging tested (90% CO₂, 5% O₂ and 5% N₂) prevented mycotoxin production.     

Susceptibility of selected maize seed genotypes to Sitophilus zeamais (Coleoptera: Curculionidae)

Studies were carried out under laboratory conditions of 25 ± 2 °C and 70 ± 5% RH in an incubator to determine the relative susceptibility of eighteen maize genotypes to attack by the maize weevil, Sitophilus zeamais Motschulsky. The experiment was laid out in a Completely Randomized Design (CRD) with three replications. Adult mortality, number of F₁ progeny, weevil development time, susceptibility index, percentage seed damage, percentage weight loss, and weight of powder produced were determined after storage period of three months. The susceptibility index was determined using Dobie’s formula and the genotypes were classified into different susceptibility groups. The genotypes exhibited varying degrees of susceptibility to S. zeamais attack. Only Aseda was regarded as resistant and TZE-Y POP STR as moderately resistant to S. zeamais. Kpari-Faako, Tintim, WACCI-M-1215, WACCI-M-1594 and Wang-Dataa were regarded as moderately susceptible to S. zeamais. However, Abontem, Bihilifa, Ewul-Boyu, Sanzal-Sima, TZE-I 17, WACCI-M-1205, WACCI-M-1508 and WACCI-M-1510 were regarded as susceptible genotypes. Furthermore, Aburohemaa, Obaatanpa and Omankwa were regarded as highly susceptible to S. zeamais. Number of F₁ progeny, seed damage, seed weight loss, weight of dust produced and seed moisture content had positive relationship with varietal susceptibility while adult weevil mortality, median development period and seed germination after infestation had an inverse relationship with susceptibility in maize. The use of insect resistant genotypes would offer a sustainable way of minimizing postharvest losses of seeds in storage especially for smallholder farmers who keep harvested grains for future use as food and seed.     

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.).     

Efficacy of atmospheric and pressurized carbon dioxide or air against Sitophilus zeamais Motchulsky (Coleoptera: Curculionidae) and Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) in milled rice

Carbon dioxide gas was evaluated in the laboratory for control of Sitophilus zeamais Motchulsky and Tribolium castaneum (Herbst) in milled rice using a specially designed pressure chamber. Tests were conducted at atmospheric pressure with 60, 80 and 100% carbon dioxide and with 100% carbon dioxide pressurized to 4, 6 and 8 bars. LT₉₉ were ≤79, ≤78, and ≤148 h for all life stages of S. zeamais exposed to 60, 80 and 100% carbon dioxide, whereas, they were ≤59, ≤51 and ≤66 h for T. castaneum. Pure carbon dioxide controlled the adult stage, but there tended to be no significant differences of carbon dioxide concentrations on mortality of immature stages. When pure carbon dioxide was pressurized, mortalities of the two insect species were increased significantly. LT₉₉ values decreased with levels of applied pressure: ≤29, ≤9.0 and ≤4.8 h for S. zeamais and ≤15, ≤5.8 and ≤2.3 h for T. castaneum at pressures of 4, 6 and 8 bars, respectively. Sitophilus zeamais was more tolerant than T. castaneum to carbon dioxide gas. The adult was the most susceptible stage, however, immature stages responded differently to carbon dioxide gas applied at varying concentrations and pressure levels. Adult mortalities were lower when air was used in the pressure chamber instead of carbon dioxide. Pressurized carbon dioxide at relatively low pressure (4–8 bars) was found effective in controlling all live stages of S. zeamais and T. castaneum in milled rice with shorter exposure times than at atmospheric pressure using an inexpensive set of equipment as compared to high pressure carbon dioxide fumigation (20–30 bars).     

Pasta preference and ability to penetrate through packaging of Sitophilus zeamais Motschulsky (Coleoptera: Dryophthoridae)

Results of bioassays of the maize weevil, Sitophilus zeamais, on different types of special commercial pasta are reported. The attraction to eight types of pasta was compared: barley and buckwheat, dietetic, egg pasta, gluten-free, spelt and lentils, tricolour pasta (a mixture of semolina, tomato, and spinach), vitamin enriched, and whole wheat semolina. The results obtained demonstrated that adult S. zeamais revealed significant preferences for whole wheat semolina and tricolour pasta, followed by vitamin enriched pasta, egg pasta, spelt and lentils pasta, gluten-free pasta, barley and buckwheat pasta, and dietetic pasta. The ability of S. zeamais to penetrate packages of commercial pasta was also determined using a folded carton box or a plastic pillow pouch with or without gussets. Sixteen types of commercial pasta packages were compared. S. zeamais was found inside packages of barley and buckwheat pasta, durum wheat pasta, egg pasta, five cereals pasta, kamut pasta, rice pasta, spelt pasta, spelt and lentil pasta, tricolour pasta, vitamin enriched pasta, and whole wheat pasta. The adults entered into carton boxes and infested the pasta through openings not well sealed by glue. The large or enlarged air vent micro-holes present on polypropylene pasta packages facilitated the entry of adults.     

The effectiveness of low temperature (5 °C) on Sitophilus oryzae (L.), Sitophilus zeamais (Motch.) and Sitophilus granarius (L.) in wheat grain: The impact of pre-acclimation

Laboratory trials were conducted in wheat grain to assess the effectiveness of low temperature (5 °C), applied at intervals of 5, 7, 9, 11, 20, 25, 30 and 35 days (d), as a control measure for stored product insect pests of the genus Sitophilus, and compared the results with the impacts of 7 d pre-acclimation at 15 °C and non-acclimation. Mortality was determined after 1, 2 and 7 d of recovery in the laboratory at 25 ± 1 °C and 60 ± 5% r. h., while progeny were assessed 7 weeks later. Acclimation had the strongest impact on S. oryzae after 5 d exposure to 5 °C and 7 d recovery, while the same effect on S. zeamais was shown after 20 and 25 d of exposure and 1 d recovery, and the same impact on S. granarius occurred after 30 and 20 d of exposure and 1 and 7 d recovery, respectively. Low temperature was found to be the most effective against S. oryzae, and the least effective against S. granarius. Mortality was high (≥96%) in both S. oryzae test groups after 11 d of exposure and 7 d recovery, S. zeamais mortality was high (>97%) after 35 and 30 d of exposure and 2 and 7 d of recovery, respectively, and S. granarius mortality was ≤87% in all test variants. Total reduction (100%) of non-acclimated S. oryzae, S. zeamais and S. granarius progeny required 8, 25 and 30 d of exposure to 5 °C, respectively, while acclimated weevils required 11, 30 and 30 d, respectively.     

A study on the feasibility of quantifying the population density of stored product insects in air-tight grain storage using CO2 concentration measurements

This study was aimed at estimating the number of insects per 1 kg of paddy (IPK) in an air-tight mock-up silo by CO₂ concentration monitoring. The first experiment was to determine the respiration rates of adult Sitophilus zeamais, Rhyzopertha dominica, and Tribolium castaneum. CO₂ concentrations were recorded from groups of 50, 100 and 200 insects with and without 125 g of brown rice. The respiration rate was calculated from the slope of the CO₂ concentration curve. A sample size of at least 100 insects was recommended. In 100-insect group, with the presence of food the respiration rates of S. zeamais, R. dominica, and T. castaneum were 9.57–14.13, 1.96–3.93 and 4.59–11.76 μl_CO2/insect−h, respectively. In the second experiment, S. zeamais populations at actual IPK = 2, 0.5, 0.25, 0.125 and 0.0625 in a 1.618 m³ silo filled with paddy were quantitatively determined. Similarly, R. dominica and T. castaneum populations at actual IPK = 0.5, 0.25, 0.125 and 0.0625 in 0.064 m³ silos were estimated in the third experiment. In each trial, small containers filled with a known number of adult insects along with brown rice were buried in the silo. Additionally, a control silo containing only disinfested paddy was set up in parallel. The insect population density was calculated from the difference in the slopes of the CO₂ curves between the infested and control silos divided by the respiration rate of one insect. On average, for each species and each infestation level the estimated population density was not greater than twice of the actual ones. Although several assumptions (e.g., silos being completely sealed, only one species and one life stage of infesting insects) had to be made, monitoring CO₂ concentrations could potentially be an effective tool for determining insect population density during grain storage.     

Efficacy of phosphine and insect penetration ability in ZeroFly® bags

The deltamethrin incorporated woven polypropylene ZeroFly® storage bag is a promising novel technology for grain storage. However, if grain stored in ZeroFly bags gets infested and has to be fumigated using phosphine (PH₃), data on the effectiveness of such treatments are needed. Additionally, obtaining field data on ability of stored-product insect pests to breach ZeroFly bags would facilitate insect management. Therefore, efficacy of PH₃ in immature and adult Sitophilus zeamais (Motschulsky), Prostephanus truncatus (Horn), Rhyzopertha dominica (F.) and Tribolium castaneum (Herbst) in experimental cages in maize stored in 100-kg polypropylene (PP), jute and ZeroFly bags was investigated. Post-fumigation mortality of adults was recorded after 7 d, and after 7 wk for immatures. The ability of either S. zeamais or P. truncatus to penetrate fabric of PP, jute and ZeroFly bags was assessed. Phosphine efficacy was good in all the three types of bags and resulted in complete mortality of adults and immatures of the four species tested. Sitophilus zeamais and P. truncatus were more successful in penetrating the PP bag fabric and on average made 84 and 780 holes per bag over a 4 mo-period, respectively; this was followed by jute with 37 and 614 holes. The ZeroFly bag was harder to breach and ≤3 holes per bag were made for both species. This study shows that PH₃ is highly efficacious in insects that infest maize stored in ZeroFly bags, and that these bags are not easily penetrated by stored product insect pests. Hitherto, ZeroFly bags are a good technology for storing grain that is not infested, and fumigation using PH₃ can be effectively conducted if infestation occurs. Therefore, ZeroFly bags can be incorporated in integrated stored product insect management (IPM) programs for bagged grains.     

Industrial-scale radio frequency treatments to control Sitophilus oryzae in rough,brown, and milled rice

Many previously studies have confirmed that radio frequency (RF) treatments have the potential to be developed as non-chemical alternative disinfestation methods. However, most of them are conducted in laboratory scale RF systems, and it is necessary to scale up the treatment protocol for industry scale applications. A pilot-scale, 27.12 MHz, 6 kW RF system was used to simulate the continuous industrial processing and finally estimate the heating efficiency and throughput for controlling adult rice weevils, Sitophilus oryzae (L.), in milled, brown, and rough rice. An electrode gap (11.5 cm) was chosen based on the appropriate heating rate (6–8 °C/min). RF heating uniformity in the three types of rice samples was improved by adding 50 °C forced hot air surface heating, sample movements on the conveyor belt at a speed of 12.5 m/h, and holding in 50 °C hot air for 6 min. The final industrial RF treatment achieved a complete mortality of adult S. oryzae and provided acceptable quality attributes in moisture content, water activity, color, protein, free fatty acid, and ash. The average heating efficiency and throughput of the RF treatments were 77.7, 76.3, and 74.3%, and 268.8, 247.3, and 224.8 kg/h for milled, brown, and rough rice, respectively. The industrial scale-up studies provide alternative physical methods for disinfesting milled, brown, and rough rice to replace chemical fumigation.     

New insecticide delivery method for the control of Sitophilus zeamais in stored maize

Maize kernel deterioration caused by the action of insects has led to an urgent need to develop new control methods against the maize weevil, Sitophilus zeamais, one of the major pests found in silo bags during the storage of maize. Here, we evaluated the insecticidal efficiency of plasticized and unplasticized cotton matrices (deliveries), loaded with R-(+)-pulegone, (−)-carvone, 2-decanone and trans-2-hexenol against S. zeamais. R-(+)- pulegone was the only compound that produced weevil mortality. Plasticized delivery loaded with R-(+)-pulegone achieved a 90% mortality on the 12th day, with mortality values recorded of above 96% over the course of 30 days. R-(+)-pulegone from plasticized delivery was released more slowly compared to unplasticized delivery. Moreover, delivery loaded with R-(+)-pulegone did not show phytotoxicity in maize kernels. Hence, due to its effectiveness against the weevil and the lack of phytotoxic activity against maize kernels, plasticized delivery loaded with R-(+)-pulegone represents a promising material for S. zeamais control. However, large-scale studies are needed in order to evaluate its potential use in grain storage systems.     

Progress towards a lure to attract three stored product weevils, Sitophilus zeamais Motschulsky, S. oryzae (L.) and S. granarius (L.) (Coleoptera: Curculionidae)

The behavioural responses of Sitophilus granarius, S. oryzae and S. zeamais to synthetic 4S,5R-sitophinone alone and in combination with volatiles from kibbled carob have been investigated with a view to the development of a single lure to attract all three species. Sitophilus zeamais and S. oryzae were shown to respond to 4S,5R-sitophinone at amounts as low as 0.025 ng. Sitophilus granarius gave a significant response to 50 ng 4S,5R-sitophinone. Volatiles from kibbled carob were also shown to attract all three species. This is the first time that attraction of S. zeamais and S. oryzae to carob volatiles has been demonstrated. Fresh lures containing 4S,5R-sitophinone and carob volatiles attracted significantly more insects in pitfall bioassays for all three species than when either component was used alone. However, after 6 weeks a significant increase in response compared to the control was found only for S. oryzae and S. zeamais. The effect of insect age on response to the pheromone and carob volatiles was also studied for all three species. The response to carob volatiles decreased with increasing insect age for all three species. A significant response to 4S,5R-sitophinone was found only for 1-2-day-old adults of S. oryzae and S. zeamais but for all ages tested of S. granarius. The effect of the combination of pheromone and carob volatiles also decreased with increasing insect age.     

Effectiveness of spinosad and spinetoram against three Sitophilus species: Influence of wheat endosperm vitreousness

The influence of wheat endosperm vitreousness on the effectiveness of spinosad and spinetoram against lab populations of Sitophilus granarius, Sitophilus oryzae (from Serbia) and Sitophilus zemais (from Serbia and Slovenia) were evaluated in a laboratory conditions. Characteristics of two test wheat varieties were determined, i.e. a variety with high (HVWG) and another with low (LVWG) endosperm vitreousness. Spinosad and spinetoram were applied at the doses of 0.5, 1 and 2 mg/kg (ppm) to both wheat varieties. Mortality was assessed after 7, 14 and 21 days, while impact on progeny production/reduction PR (%) was assessed after 8 weeks. All doses of spinosad applied to the HVWG variety and 1–2 mg applied to the LVWG variety achieved high mortality (93–100%) of S. granarius after 14 days, while high mortality of the other test species (97–100%) was achieved by 1–2 mg doses on the HVWG and 2 mg on LVWG varieties. After 14 days, all doses of spinetoram achieved high mortality (96–100%) of S. granarius on both wheat varieties, while high mortality of S. oryzae (97–100%) and both populations of S. zeamais (93–100%) was achieved using 1–2 mg doses on the HVWG and 2 mg dose on the LVWG variety. The highest S. granarius PR (>90%) was found in both wheat varieties treated with 2 mg/kg of spinosad or spinetoram, while the highest PR of S. zeamais was observed in HVWG treated with 2 mg/kg of spinosad and 1–2 mg/kg of spinetoram, and in LVWG treated with 2 mg/kg of spinetoram. The greatest PR (91–94%) of S. oryzae in both wheat varieties was observed only in grain treated with 2 mg/kg spinetoram. In general, endosperm vitreousness was found to influence the effectiveness of spinosyns to all Sitophilus species, especially their doses not causing high mortality.     

Electrophoretic analysis of amylase isozymes in geographical strains of Sitophilus oryzae (L.), S. zeamais Motsch., and S. granarius (L.) (Coleoptera: Curculionidae)

Electrophoretic analysis of adult intestinal tracts indicated that two strains of Sitophilus oryzae (L.) (Japan and Tanzania-90) lacked the major α-amylase isozyme present in 12 other geographical strains. Amylase zymograms of adults of 17 strains of Sitophilus zeamais Motschulsky were not only different from those of S. oryzae but showed significant between-strain variation. Strains of S. zeamais could be arranged into approximately four groups based on isoamylase patterns. There was no apparent correlation between geographical origin or culture diet and isoamylase pattern within these groups. One strain of Sitophilus granarius (L.) (3SG) had an additional minor isozyme not present in five other strains of this species. Although the biological or functional significance of the wide strain to strain variation in isoamylase patterns of S. zeamais has yet to be determined, the qualitative and quantitative differences in amylases between these weevil species indicate these enzymes may be useful chemotaxonomic markers. In addition to differences in amylase composition, there was significant strain variation in weights of wheat-fed weevils from each species.     

Impact of moisture content and maize weevils on maize quality during hermetic and non-hermetic storage

The objective of this study was to determine the impact of moisture content and Sitophilus zeamais Motschulsky on maize quality during hermetic and non-hermetic storage conditions. Commercial Channel 211-97 hybrid maize kernels were conditioned to 14, 16, 18, and 20% moisture content (wet basis), and then three replications of 300 g of maize grain were stored in glass jars or triple Ziploc^® slider 66-μm(2.6-mil) polyethylene bags at four conditions: hermetic with weevils, hermetic no-weevils, non-hermetic with weevils, non-hermetic no-weevils. All jars and bags were stored in an environmental chamber at 27 °C and 70% relative humidity for either 30 or 60 d. At the end of each storage period, jars and bags were assessed for visual mold growth, mycotoxin levels, gas concentrations, pH level, the numbers of live and dead S. zeamais, and maize moisture content. The maize stored in non-hermetic conditions with weevils at 18 and 20% exhibited high levels of mold growth and aflatoxin contamination (>150 ppb). Conversely, very little mold growth was observed in maize stored in hermetic, and no aflatoxins were detected in any moisture level. CO₂ increased and O₂ gradually decreased as storage time increased for maize stored in hermetic conditions (with or without weevils) in all moisture level. No significant difference in pH was observed in any storage conditions (P < 0.05). Total mortality (100%) of S. zeamais was observed in all hermetically stored samples at the end of 60 days storage. Moisture content for hermetically stored maize was relatively constant. A positive correlation between moisture content and storage time was observed for maize stored in non-hermetic with weevils (r = 0.96, P < 0.05). The results indicate that moisture content and the number of S. zeamais weevils plays a significant role in maize storage, both under hermetic and non-hermetic conditions.