Cooking oils and “Triplex” in the control of Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) in farm-stored maize

The uses of some cooking oils viz., noug oil, soybean oil, sunflower oil, corn oil and olive oil, and the reputedly non-toxic botanical “Triplex”, were evaluated against maize weevil, Sitophilus zeamais, in stored maize grain under local Ethiopian storage conditions in 2003. For comparison, an untreated sample and the synthetic insecticides, malathion 5% dust and Actellic 2% dust, at the recommended rates of 0.05% (w/w) were included. All the cooking oils tested had a significant toxic effect on the weevils in stored grains. The oil treatments showed significantly higher mortality of adult weevils at each sampling period compared with the untreated grains. The cooking oil treatments also significantly reduced weight loss and grain damage as compared with the untreated control. The Triplex treatment was of comparable effectiveness to the synthetic insecticides, causing high mortality of adult weevils, low-percentage grain damage and low-percentage weight losses. These promising oils and Triplex could be recommended for use as part of an integrated pest management program in stored maize, particularly at low levels of pest infestation and, in the case of cooking oils, for grain intended for home consumption.     

Bioactivities of cymol and essential oils of Cupressus sempervirens and Eucalyptus saligna against Sitophilus zeamais Motschulsky and Tribolium confusum du Val

The essential oils extracted from Eucalyptus saligna and Cupressus sempervirens leaves were analysed by GC-MS and evaluated along with cymol, one of their main constituents for their repellent and toxic effects on Sitophilus zeamais and Tribolium confusum. Contact toxicity assayed by impregnation on filter paper discs or coating onto maize grains showed that these chemicals caused significant mortality of the test insects. Eucalyptus oil was more toxic than Cupressus oil to both insect species (LD₅₀=0.36 μl/cm² for S. zeamais and 0.48 μl/cm² for T. confusum) on filter paper discs, and was more toxic to S. zeamais on maize (LD₅₀=38.05 μl/40 g grain). Both oils considerably reduced the F₁ progeny production and grain weight loss. Moreover, both crude oil extracts produced a stronger repellent activity against the test insects than did cymol. These results suggest that the essential oils from E. saligna and C. sempervirens may be used in grain storage against insect pests.     

Efficacy of castor bean oil (Ricinus communis L.) against maize weevils (Sitophilus zeamais Mots.) in northwestern Ethiopia

The maize weevil, Sitophilus zeamais Mots. (Coleoptera: Curculionidae), undermines food security. The biocidal activity of castor bean oil (Ricinus communis L.) against S. zeamais, was studied at various doses at Dembecha, northwestern Ethiopia in 2013/14 (November–April). In the castor bean oil efficacy test, weevil mortality steadily increased with castor bean oil dose. According to the results of the ANOVA, number of dead weevils significantly varied between castor bean oil doses 1 h after treatment (F_{10, 21} = 117.6, p < 0.0001). Just 53% of the weevils were killed in one hour by applying 2 ml of the oil while doses higher than 2 ml killed greater than 85% of the weevils. Using Probit analysis, the LD₅₀ of using castor bean oil against maize weevils was calculated to be 2.04 ml. Therefore, 2 ml of castor bean oil was found sufficient to destroy 50% of the weevils. Higher doses of castor bean oil significantly reduced maize seed germination.     

Degradation of azadirachtin A on treated maize and cowpea and the persistence of Azadirachta indica seed oil on Callosobruchus maculatus and Sitophilus zeamais

Azadirachta indica seed oil has long been used in many parts of the world for the control of various insect pests. The quantification of its known insecticidal compound Azadirachtin A on treated commodities remains a challenge. The degradation of Azadirachtin A in treated cowpea and maize was determined with HPLC-MS as well as the toxicity of A. indica seed oil on their respective major pests is storage between 0 and 180 days. Azadirachtin A degraded slowly on treated maize from 1.31 mg/kg (0-day) to 0.38 mg/kg (180-day) while on cowpea it degraded from 1.14 mg/kg (0-day) to 0.43 mg/kg (180-day). A. indica oil caused a significant day-dependent mortality of adults Callosobruchus maculatus and Sitophilus zeamais and its effectiveness decreased with time. The tested oil was more persistent for inhibiting progeny production than on adult mortality. Further studies are needed to evaluate the quality of treated grains at different storage times.     

Bioefficacy of essential and vegetable oils of Zanthoxylum xanthoxyloides seeds against Acanthoscelides obtectus (Say) (Coleoptera: Bruchidae)

Experiments were conducted in the laboratory to evaluate the bioefficacy of essential and vegetable oils of Zanthoxylum xanthoxyloides (Rutaceae) against Acanthoscelides obtectus (Coleoptera: Bruchidae). The chemical composition of the essential oil and the fatty acid composition of the vegetable oil extracted from the seeds of Z. xanthoxyloides were determined. The insecticidal activities of these oils and the associated aromatized clay powder were evaluated against A. obtectus. Both oils were strongly repellent (100% repellency at 0.501 μl/cm2 essential oil and 3.144 μl/cm2 vegetable oil) and highly toxic (LC?? = 0.118 μl/cm2 for essential oil) to this beetle after contact on filter paper. The vapors of the essential oil were highly toxic to adult insects (LC?? = 0.044 μl/cm3), and the aromatized powder made from clay and essential oil was more toxic (LD?? = 0.137 μl/g) than the essential oil alone (LD?? = 0.193 μl/g) after 2 days of exposure on a common bean. Both oils greatly reduced the F? insect production and bean weight loss and did not adversely affect the bean seed viability. In general, the results obtained indicate that these plant oils can be used for control of A. obtectus in stored beans.     

Comparison of three different wood ashes and diatomaceous earth in controlling the maize weevil under laboratory conditions

We studied the insecticidal effects of ashes from three different tree species (common beech, Norway spruce, and black locust) on the mortality of Sitophilus zeamais adults under laboratory conditions. A diatomaceous earth formulation, SilicoSec^® was used as a positive control. We studied the contact effects of ashes on adults and the insecticidal effect of ash mixed with wheat grain. The experiment was carried out at three different temperatures (15, 20 and 25 °C) and at two different relative humidity [r.h.] values (55% and 75%). The study on contact effects (immediate mortality and delayed mortality) was carried out in Petri dishes, where we applied ash in two concentrations, 10 and 20 g/m². After 7 d there was more than 97% mortality for the beetles in all three wood ashes at 25 °C and 55% R.h., while after 14 d wood ash of Norway spruce showed the highest effectiveness (87%) at 20 °C and 55% R.h. 96% mortality was recorded at 20 °C and 55%, when SilicoSec^® was applied. The experiment with the mixture of ashes and grains was performed by mixing 2.5 or 5 w% of ash with wheat. In both approaches, we established that lower R.h. in combination with higher temperatures led to higher adult mortality rates. Regardless of the approach, the preparation concentrations did not influence adult mortality. The most efficient ash was that of Norway spruce, yet we attribute its effectiveness not only to its highest content of SiO₂ (11.68%) but also to the other ingredients in ash, which can enhance its hygroscopic properties. Our research demonstrated that wood ash can efficiently protect stored crops from maize weevils. However, before introducing wood ash into the systems of protecting stored wheat grains against harmful insects, the influence of concentrations of ashes and their hygroscopic properties should be studied.     

Chemical composition,insecticidal activity and persistence of three Asteraceae essential oils and their nanoemulsions against Callosobruchus maculatus (F.)

Essential oils from three local plants Ageratum conyzoides, Achillea fragrantissima and Tagetes minuta were obtained by hydrodistillation and analyzed using gas chromatography (GC) and GC/mass spectrometry (MS). Nanoemulsions from the obtained oils were prepared using the High Pressure Homogenization (HPH) technique. The plant oils showed considerable ovicidal, adulticidal and residual activities against the cowpea beetle, Callosobruchus maculatus, where A. conyzoides oil was the most toxic. Following a 24-h fumigation and a 48-h post exposure period, eggs were less susceptible to the oils than adults, where LC_50's ranged from 71.6 to 161.9 μl/l air and 19.2–77.8 μl/l air against eggs and adults, respectively. When mortality was counted 96 h after treatment, susceptibility of all stages was increased. When prepared as nanoemulsions and tested as fumigants, toxicity of oils was increased dramatically, where LC₅₀ values 96 h after treatment ranged from 16.1 to 40.5 μl/l air and 4.5–24.3 μl/l air against eggs and adults, respectively. In a contact toxicity bioassay, adults were susceptible to the oil-treated filter papers, where LC_50's 96 h after treatment ranged between (37.1–110.8 μl/cm²). When mixed with kaolin powder and tested as contact adulticides, activity of oils increased compared with their application alone. The plant oils showed a weak to moderate residual adulticidal activity, where A. conyzoides oil was the most effective. The study shows the potential of new ways of using the tested plant oils as natural grain protectants after the required toxicological assessments.     

Periodic disturbance time interval for suppression of the maize weevils,Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) in stored maize (Zea mays L.)

Sitophilus zeamais Motschulsky threaten maize (Zea mays L.) storage in the tropical regions of low-income countries where maize is a staple. Disturbance of maize during storage can be an alternative to chemical treatment in suppressing populations of S. zeamais. Disturbance has been successful against insect pests of stored products, i.e. bean weevils, Acanthoscelides obtectus (say) (Coleoptera: Chysomelidae: Bruchinae) and lesser grain borers Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae). Prior research has not established a standard disturbance interval. This study tested various disturbance intervals to see determine suppression of S. zeamais populations in stored maize with four treatments: stationary jars (control) and jars disturbed at intervals of 8, 12 and 24 h. Jars contained 1 kg of 16 ± 0.7% wet basis moisture maize and 234 ± 51 adult S. zeamais stored at 27 °C, 70% r. h. and 12 h light and dark phases. Periodically, electric motors rotated jars through about 1.25 revolutions in 3 s. Three jars were selected randomly from each treatment every 40 d up to 160 d for analysis. Reduction in S. zeamais populations was 75%, 95% and 94% for 8, 12 and 24 h disturbance intervals, respectively, compared to the stationary jars after 160 d. The average quality of maize in all disturbed jars was better than in stationary jars, respectively, for moisture content (13 ± 1.1% vs. 21 ± 1.0%), bulk density (553.5 ± 1.1 kg/m³ vs. 231.7 ± 1.2 kg/m³), broken corn and foreign material (9 ± 1.2% vs. 68 ± 3.9%), insect damage (70 ± 4.5% vs. 100 ± 0.0%), and mold damage (0.4 ± 0.2% vs. 27 ± 17.2%). Disturbances of 12h and 24 h (once per day) proved the best intervals in suppressing S. zeamais populations and maintaining the quality of maize, much longer than previously reported. Of these two intervals, the less frequent 24 h disturbance interval may be an effective non-chemical approach to control S. zeamais in maize stored by smallholder farmers, requiring less effort.     

The first pest of stored rice in East Asia? - A maize/rice weevil impression from the Tian Lu Shan site in China

In 2018, an impression of a maize weevil, Sitophilus zeamais MOTSCHULSKY. or a rice weevil, S. oryzae L., was discovered on a potsherd from the Tian Luo Shan site, Zhejiang Province, China. This sample is the earliest archaeological record of this insect in China and possibly the earliest pest to harm rice grains in East Asia. To investigate the history of the evolution and propagation of rice pest insects in prehistoric East Asia, this paper evaluated the impression of the insect pest on a stored product. However, it was revealed that the harmed object might not be limited to rice because of the insect's bigger body size. In the previous report written in Japanese on the discovery of the weevil impression, it was identified as S. zeamais/oryzae without showing concrete length data of archaeological and modern weevils or detail key points for identification. And the report mainly focused on the history of origin and propagation of product food pest in the world. Therefore, this paper discusses diagnostic key points for identification as maize/rice weevil, with providing these detail data again.     

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.     

Maize storage insects (Sitophilus zeamais and Prostephanus truncatus) prefer to feed on smaller maize grains and grains with color,especially green

Maize is the major food staple in Eastern and Southern Africa, and since production is seasonal, on-farm storage is practiced by farmers. Because stored maize is susceptible to pests, improved storage methods are being developed which need to be evaluated for their effectiveness in reducing losses. The standard count and weigh method (CWM) assumes storage insects attack grains of different sizes in equal measure, although this has not been tested. This study therefore analyzed storage insects’ feeding preferences for maize grains of different sizes. Maize grain was sieved and split into four size categories, colored with food dyes, remixed, and distributed into 48 jars, each with about 300 g of grain, artificially infested with maize weevils (Sitophilus zeamais) or larger grain borers (Prostephanus truncatus), 40 insects per jar. After two months, the grains were separated into their size categories (visually, by color), and grain damage and weight loss was estimated with the CWM. Results show that storage insects have a preference for feeding on smaller, rather than larger grains; and on colored rather than uncolored grains. Small-sized and colored grains were more likely to be perforated by insects and suffered more weight loss. This preference, observed for both species, is more pronounced in S. zeamais, who also distinguish between different colors, and prefer green. The size effect is, however, comparatively small and the average weight loss calculated by a size-adjusted formula is not significantly different from that calculated by the standard CWM. However, the resulting differences in weight losses between grains of different sizes are small. Weight loss calculated by weighted CWM, while in principle more accurate, does not in practice lead to significantly different estimates. Therefore, the results do not warrant a change in the conventional CWM for weight loss in stored grain.     

Grain-protectant compounds from Duguetia lanceolata (Annonaceae) derivatives: Bioassay-guided searching and toxicity against the maize weevil

In order to obtain bioactive allelochemicals from Neotropical Annonaceae, bioguided chromatographic fractionations with derivatives prepared from Duguetia lanceolata St.-Hil. leaves were performed using the maize weevil, Sitophilus zeamais Mots. (Coleoptera: Curculionidae), as bioindicator, which is a primary insect pest of stored cereals in tropical conditions. For this end, four phases (hexane, dichloromethane, ethyl acetate and hydroalcohol) were obtained firstly by liquid-liquid partition and tested through a residual contact bioassay (at 1500 mg L⁻¹) in order to verify their bioactivity on adults of the maize weevil. Only the hexanic fraction caused significant mortality (74%) after the tenth day of insect exposure on treated samples of grains corn. Thus, thisfraction was separated into seven new subfractions by means of vacuum column chromatography and solvents of increasing polarities. Subfraction 2 promoted 90% of mortality whereas subfraction 6 promoted 30% of mortality of maize weevils. The aromatic compound 2,4,5-trimethoxystyrene and a mixture of steroids [campesterol (8.44%) + stigmasterol (12.37%) + sitosterol (79.19%)] were isolated from subfraction 2 and promoted promising grain protective activity, in a manner comparable to a diatomaceous-earth based formulation (our positive control). Furthermore, the results reported in the present study demonstrate the potential application of derivatives of D. lanceolata leaves in integrated pest management (IPM) of stored grains, mainly in the framework of maize weevil management.     

Desiccation resistance and water balance in populations of the maize weevil Sitophilus zeamais

Water balance is a challenge for insects due to their small size and body with high surface area-to-volume ratios, and it is particularly important for insects that inhabit dry environments, such as stored grains, and warm climates. This subject has been curiously neglected in stored product insects, including the maize weevil Sitophilus zeamais Motschulsky, were broad population differences have been reported with management consequences. Here, we assessed variation in desiccation resistance and water balance among three strains of the maize weevil exhibiting differences in body mass. The survival of male and female weevils from three strains was therefore assessed at relative humidities ranging from 0 to 100%, in which their water content and water loss were also assessed. The effect of insect aggregation (1–25 insects per group) on water balance was also assessed, as was the water gain by the drinking of free water and drinking preference when exposed to insecticide-contaminated water. The median survival time (LT₅₀) of individual insects increased with relative humidity and varied significantly among the weevil strains. The larger strain exhibited 60–80% higher LT₅₀ at 50% relative humidity (r.h.) and above, with little difference between strains at 0% r.h. Heavier weevils exhibited higher water content and lower overall rates of water loss; these qualities made heavier weevils more resistant to dehydration. Larger groups of weevils facilitate water retention in individual insects. Weevils actively drink water to achieve favorable water balance when dehydrated, which is a major source of water intake. Under dehydration, weevils in general were not able to discriminate between insecticide-contaminated and uncontaminated water. However, females of the insecticide-susceptible strain avoided contaminated water.     

大孔树脂吸附分离藿香蓟花蓝色素的研究

采用大孔树脂对藿香蓟花蓝色素的提取液进行了吸附和洗脱试验,并确定藿香蓟花蓝色素的分离最佳工艺条件.结果表明,D301树脂对藿香蓟花蓝色素的静态吸附率达到90 mg/g,吸附蓝色素的平衡时间较短且重复使用20次后吸附性能依然稳定;上柱优化条件为流速为3 mL/min,质量浓度为0.2 mg/mL,温度为20℃;柱层析饱和吸附后用1 mol/L盐酸洗脱,洗脱率达到96.08％.     

Chemical bases for maize grain resistance to infestation and damage by the maize weevil,Sitophilus zeamais Motschulsky

It was suggested that the factors that confer resistance on a maize variety against Sitophilus zeamais Motschulsky are chemically based. To test this hypothesis, resistance indices of the weevil were assessed using a new scale developed by Nwosu et al. (2015a) after performing detailed chemical analysis of the whole-maize grain using twenty elite maize varieties under ambient temperature and relative humidity of 30.7 °C and 74.5% respectively. Increases in maize varietal crude fibre, phenolic acid and trypsin inhibitor increased significantly mortality of S. zeamais adults and reduced significantly survival rate of S. zeamais adults, percent of grain damage, percent of weight loss, weight of grain flour and oviposition by the adult females. The level of cystein proteinase inhibitor (a natural protectant of plants against insect attack) in the grains was not on its own responsible for increased mortality, reduced grain damage and reduced oviposition. The study identified crude fibre, phenolic acid and trypsin inhibitor of whole-maize grain as the bases for resistance. Their significant increase in grains resulted in low infestation. Whereas, increased protein, zinc, magnesium, calcium, sodium, phosphorus, manganese, iron, cobalt and starch rendered the elite maize grains susceptible to S. zeamais attack. Therefore, significant incorporation of the bases for resistance and significant reduction of the bases for susceptibility in maize grains during breeding programmes is advocated; provided this does not adversely affect palatability and nutritional needs of man. Eighty-five percent of the elite maize varieties were at least resistant to S. zeamais infestation and damage. The resistant varieties, particularly 2000SYNEE-WSTR and TZBRELD3C5 maize with high resistance should be promoted for growing. Antibiosis, antixenosis and preference were the mechanisms of maize grain resistance to S. zeamais attack.     

Efficacy of combining Niger seed oil with malathion 5% dust formulation on maize against the maize weevil, Sitophilus zeamais (Coleoptera: Curculionidae)

The combined effects of Niger seed oil and malathion, 5% dust, against the maize weevil, Sitophilus zeamais, were evaluated to determine the minimum effective rate(s) of the combinations that can provide adequate protection to maize seed against attack by weevils. Niger seed oil at the rates of 0%, 10%, 20%, 30%, 40%, 50% and 100% of the recommended application rate, 5 ml kg⁻¹, was combined with malathion at the respective rates of 100%, 50%, 40%, 30%, 20%, 10% and 0% of the recommended application rate, 0.5 g kg⁻¹. All combinations provided complete protection to maize seed from the maize weevil up to 90 days after infestation. To determine the residual effects of the treatments, weevils were reintroduced to the grain that had been treated 90 days previously. In addition to 100% malathion, 10% Niger seed oil + 50% malathion, and 20% Niger seed oil + 40% malathion, were fully effective in controlling S. zeamais for a further 156 days after this re-infestation. Therefore, these combinations could be considered as a potential component in an effort to establish integrated management of the maize weevil. Residual performance of both oil and malathion against the weevils was primarily affected by the dose of malathion, with higher doses of malathion providing greater protection for a longer period. Niger seed oil treatment lowered the level of seed germination at the application rate of 5 ml per kg of maize.     

An investigation of the extent of infestation of stored maize by insect pests in Rungwe District, Tanzania

Two hundred and sixty randomly selected subsistence farm households in Katumba ward, Rungwe District, Tanzania, were interviewed regarding methods that they use to prevent or control insect pests in stored maize. Maize samples were randomly collected from 130 out of 260 farm households and studied for infestation by insect pests, using incubation and inspection methods. About 93 % of the maize samples were found to be infested, either by weevils only (specifically Sitophilus zeamais and S. oryzae), by moths only (specifically Sitotroga cerealella), or by both weevils and moths. Both indigenous and improved varieties of maize were infested by the insect pests. The insect population density ranged from 0 to 210 per 120 maize kernels (or 51 g of maize), with an average of 80 insects per 120 maize kernels and a standard deviation of 51.44, amounting to 1,555 insects per kg of maize. Methods that farm households used to control the infestation of stored maize by insect pests were found to be inadequate. It was concluded that, with such high levels of infestation by insect pests, vulnerability of the subsistence farm households to food insecurity was inevitable. It was recommended that the government of Tanzania should make efforts to improve the resistance of maize grown in Rungwe district to Sitophilus and Sitotroga cerealella species, and that the affected farm households be encouraged to embrace new technologies, including genetic improvement of the resistance of maize to pests.     

Insecticidal and repellency activity of essential oil of Eucalyptus sp. against Sitophilus zeamais Motschulsky (Coleoptera, Curculionidae)

BACKGROUND: The objective of this work was to evaluate the insecticidal and repellent effect of essential oils of Eucalyptus dunnii, E. saligna, E. benthamii, E. globulus and E. viminalis in the control of Sitophilus zeamais in maize grains. RESULTS: The essential oils chemical composition showed that the E. benthamii presents the cis‐ocimene (56.88%) as major compound. Results permitted us to verify that the evaluated species presented different insecticidal action under Sitophilus zeamais. About 100% of mortality was achieved with doses of 65, 100 and 400 µL for E. dunnii, E. saligna and E. benthamii, respectively. After regression analysis the LD₅₀ values were calculated and E. dunnii was shown to be the most efficient (25.03 µL), followed by E. saligna (37.93 µL) and E. benthamii (121.09 µL). Using the previously calculated LD₅₀ values, the repellency activity was calculated, allowing us to conclude that all species presented significant values in terms of this important parameter. CONCLUSION: The essential oil of Eucalyptus sp. presented insecticidal and repellency against Sitophilus zeamais Motschulsky (Coleoptera, Curculionidae). Although from an economic point of view synthetic chemicals are still more frequently used as repellents, natural products (essential oils) have the potential to provide efficient and safer repellents for humans and the environment. Copyright © 2010 Society of Chemical Industry     

Allyl isothiocyanate actions on populations of Sitophilus zeamais resistant to phosphine: Toxicity,emergence inhibition and repellency

The risks associated with the use of synthetic insecticides have caused increased interest in the research of essential oils and their main constituents for use in the pest management of stored products. Allyl isothiocyanate (AITC) is the main component of mustard essential oil and has been reported as a potential replacement pesticide for conventional insecticides that control stored product insect pests. Here, we assessed the toxicity (including emergence inhibition) and repellent actions of AITC on Brazilian populations of the maize weevil Sitophilus zeamais (Coleoptera: Curculionidae) resistant to conventional insecticides (e.g., phosphine). We also evaluated physiological (e.g., respiration) and behavioral (e.g., walking and flight) traits of AITC-exposed insects. The AITC showed consistent insecticidal activity against the populations resistant to phosphine and other synthetic insecticides, with LC₅₀ values ranging from 1.5 to 2.9 μL L⁻¹. Significant inhibition of the offspring emergence was achieved after the exposure of parental adults to sublethal levels (i.e., LC₁ and LC₅) of AITC. Reductions in respiration rates were also registered in all the populations sublethally exposed to AITC. In all five populations, a high number of insects avoided AITC-treated (1.5 μL L⁻¹) grain masses, and although individuals of a phosphine-susceptible (i.e., Abre Campo) population increased walking and reduced flight activities, individuals of another phosphine-susceptible (i.e., Tunápolis) population exhibited higher flight activity under AITC exposure. Thus, our findings suggest that AITC is a potential tool that may be integrated into the control strategies of maize weevils where resistance to phosphine and other conventional insecticides is a problem.     

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.