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.     

Influence of rearing medium on size and weight of adults of four Sitophilus populations and on weight loss of host kernels (Coleoptera: Curculionidae)

Four populations of Sitophilus (S. zeamais Motsch. from Mexico, S. zeamais from Arkansas, U.S.A., S. oryzae (L.) and S. granarius (L.) were studied. Hard red winter wheat (“Cloud” variety), brown commercial sorghum, and yellow and white “dent” maize (mixed varieties) were used to determine the effect of parent-and progeny-rearing media on the adult elytron length in each population. Kernel weight loss caused by individuals of each population and weevil weights were determined.Elytron lengths of S. oryzae progeny were significantly (P < 0.05) but only slightly longer, when parents were reared in wheat rather than sorghum or maize; otherwise parent-rearing medium had no effect. Progeny-rearing medium, not parent size, was most responsible for differences in elytron lengths. S. granarius progeny from wheat were significantly larger than those from maize or sorghum; progeny from all other populations were significantly larger from maize than from wheat or sorghum.S. oryzae caused less weight loss in kernels of wheat and sorghum, and were significantly lighter (P < 0.05) than other insects. The Arkansas S. zeamais was significantly heavier than Mexican S. zeamais in wheat, otherwise their weights were similar. Weight losses (actual and percentages) of kernels of all grains were similar for Arkansas S. zeamais and Mexican S. zeamais. S. granarius were heavier and produced greater weight loss in wheat kernels than other populations did.     

Efficacy of layer grain treatment with thiamethoxam against adults of Rhyzopertha dominica (F.) and Sitophilus oryzae (L.)

The insecticidal effect of the neonicotinoid insecticide thiamethoxam, applied in layers of wheat, was studied in laboratory experiments, for the control of the lesser grain borer, Rhyzopertha dominica (F.) and the rice weevil Sitophilus oryzae (L.). In general, the increase of the size of the treated layer of wheat increased parental morality and decreased progeny production for both species. However, progeny production could not be totally avoided, even when the entire grain quantity was treated. Parental morality was lower for R. dominica than for S. oryzae, but the reverse was observed for progeny production for all treatments. Insect placement drastically affected thiamethoxam efficacy, since morality was generally higher when insects were forced to move downwards from the upper layer, as compared with insects that had been placed in the bottom part of the wheat column. Our study shows that thiamethoxam can reduce insect populations in partially treated wheat, at concentrations that are comparable with other, currently registered grain protectants.     

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

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

Evaluation of potential use of nootkatone against maize weevil (Sitophilus zeamais Motschulsky) and rice weevil [S. oryzae (L.)](Coleoptera: Curculionidae)

Nootkatone is a natural sesquiterpene ketone that shows insecticidal activity against insects and ticks. Its contact toxicity and repellency against two major stored-product insect pests, maize weevil (Sitophilus zeamais Motschulsky) and rice weevil [Sitophilus oryzae (L.)], were investigated in the current study. Contact toxicity was evaluated using a no-choice test with treated filter paper, while repellency was evaluated using a choice test with treated corn (for maize weevils) or wheat (for rice weevils). Nootkatone showed low contact toxicity (ranging from 0 to 51%) against the two weevil species at the tested concentrations (ranging from 11.58 μg/cm² to 1158.08 μg/cm²) on filter papers. In choice tests, corn treated with 0.10% or higher and wheat treated with 0.5% nootkatone or higher had significantly fewer maize or rice weevils compared with the solvent only treated control, indicating a repellent effect. The repellency percentage ranged between 46.3 and 93.1% against maize weevils and 39.2-67.2% for rice weevils.     

Resistance of maize to the maize weevil: III. Grain weight loss assessment and implications for breeding

The maize weevil (Sitophilus zeamais) is a significant pest of stored maize grain in tropical and subtropical regions worldwide. As a further step towards finding maize genotypes with acceptable levels of resistance to the maize weevil, 53 experimental maize hybrids that were generated in a North Carolina design II mating scheme, were evaluated for grain weight loss due to maize weevil damage under ambient temperature and humidity (ATH) conditions, in the on-station stock room at Harare, Zimbabwe. The study indicated that genotypic variation for grain weevil resistance was large (19%–57%) after five months of infestation by the maize weevils. A few F₂ populations with potential for use as sources of breeding new varieties for maize weevil resistance were identified. They displayed consistency of high performance (better than the standard control varieties) and showed a lower rate of grain weight loss due to the maize weevils over the five months in the stock room. The male and female GCA and SCA effects were highly significant (P ≤ 0.01) for maize weevil resistance, especially at two months after infestation, indicating that resistance was controlled by genes with both additive and non-additive effects, respectively. Further, the distribution of grain weight loss data for the hybrids was continuous and almost normal at the four intervals of data collection, clearly supporting that resistance was partial and possibly conditioned by minor genes with cumulative effects. Therefore, resistance could probably be improved by selection among the promising F₂ populations identified in this study. Although resistance was partial, a plot of grain weight loss of hybrids from the different categories demonstrated the advantage to farmers for growing a resistant variety (low percentage loss over time) compared to the susceptible ones that incurred significant grain weight losses. However, grain weight loss data were not significantly correlated with yield (r = 0.14; P > 0.05), suggesting that the traits are not mutually exclusive. Therefore, breeding for maize weevil resistance in these populations would not necessarily compromise grain yield. Our results demonstrate that there is potential in developing maize varieties with acceptable levels of maize weevil resistance through recurrent selection procedures which exploit both GCA and SCA effects with sustainable impact on food security and the environment.     

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.     

Comparison of seven methods for stored cereal losses to insects for their application in rural conditions

There are several methods for assessing harvest losses during storage with varying degrees of accuracy, reliability and practical application, however, field conditions may limit their use among subsistence farmers and traders in developing nations. We compared seven standard grain loss methods during natural and controlled infestation by Sitophilus oryzae L. in wheat and barley to select the best procedures for use in farm storage. Methods tested were: 1) visual inspection of infested lots, 2) uncorrected weight loss, 3) modified standard volume/weight ratio, 4) grain count and weight, 5) percentage of damaged grains converted to weight loss, 6) one thousand grain mass, and 7) one thousand grain mass including dust. Previously disinfested cereal lots (wheat, barley, 500 g fresh w) were exposed to 20 newly emerged adult S. oryzae for 90 d at 28 °C and 70% r.h. Naturally- infested wheat lots were also monitored under the same conditions. Before and after this period, fresh and dry weight, number, moisture content of uninfested and damaged grains, weight of 1 L of seeds and dust produced by insect activity, fresh and dry weight of 1000 kernels, and number of adult weevils was determined and applied to appropriate equations of methods (1), (2), (3), (4), (5), (6) and (7). Grain mass loss estimations varied by wide margins (9.3 ± 1.3% to 25.8 ± 2.3% in barley, 2.2 ± 0.7% to 12.5 ± 2.5% in wheat) depending on method employed, suggesting the need for careful selection of the most appropriate procedure under field conditions and farmer/trader interests. While accurate procedures such as method 7 provided dependable results, methods 3 and 5 appear most practical for wheat and barley.     

Relationship between seed physical traits and maize weevil (Sitophilus zeamais) damage parameters in selected Quality Protein Maize (QPM) varieties

The vulnerability of Quality Protein Maize (QPM) varieties to Sitophilus zeamais Motschulsky attack causes substantial postharvest loss to farmers. A study was conducted in the Institute of Agricultural Research and Training, Moor Plantation, Ibadan, Nigeria, to evaluate the response of the newly released QPM and High Protein Maize (HPM) varieties of the Institute to Sitophilus zeamais infestation. Five varieties comprising two QPM (ART/98/SW6-OB and ILE 1-OB); one HPM (ART/98/SW1) and two conventional maize varieties (KU1414SR/SR and TZPB) were used for the experiment. Two hundred seeds of each variety were artificially infested with eggs of Sitophilus zeamais and arranged in a completely randomized design with three replicates under two storage conditions (dark room and opened well-ventilated room) for three months. Data were collected on seed traits and weevil damage parameters. Results showed that there were significant differences among the varieties and between the storage conditions for most of the traits. Both QPM varieties significantly harbored Sitophilus zeamais as the number of emerged adult insects at 3 weeks after infestation (WAI), damage rating, total number of insects at termination and grain weight loss were highest in both, though higher in ILE 1-OB. The conventional inbred line (KU1414SR/SR) experienced less damage. Grain weight loss was positively correlated with insect damage rating, while seed coat thickness was negatively correlated with grain weight loss and other damage parameters. The released QPM varieties of the Institute are highly susceptible to Sitophilius zeamais infestation and therefore require genetic improvement. Seed coat thickness and damage ratings are important traits to be considered in improvement programmes for resistance to Sitophilus zeamais in QPM varieties.     

Efficacy of two Nigeria-derived diatomaceous earths against Sitophilus oryzae (Coleoptera: Curculionidae) and Rhyzopertha dominica (Coleoptera: Bostrichidae) on wheat

We investigated the effectiveness of two raw diatomaceous earths (DEs) obtained from Bularafa and Abakire districts in Yobe State, Nigeria, against the rice weevil, Sitophilus oryzae (L.) and the lesser grain borer, Rhyzopertha dominica (F.). Insecto, a commercialized DE, was also tested as standard check. Adults of the two species were exposed on wheat admixed with the respective DEs at a dose rate of 1000 ppm, at 30 °C and 55% RH. Relative to R. dominica, Insecto, Bularafa and Abakire DEs induced 14-d corrected adult mortalities of 73.6, 61.2 and 40.4%, respectively. Corrected 14-day adult mortalities against S. oryzae were 100, 100 and 81%, respectively. Mortality increased with increasing exposure duration. Bularafa and Insecto DEs were more effective than Abakire DE both in terms of adult insect mortality and F1 progeny suppression in the two species investigated. Both species avoided contact with DE-treated wheat. Bularafa and Insecto DEs contain higher levels of silica (81.0% and 87.0%, respectively) than Abakire DE (60.2%), and also have smaller particle sizes than Abakire DE. These differences in silica level and particle size probably explain why Bularafa and Insecto DEs are more effective against S. oryzae and R. dominica than Abakire DE. The decreasing order of efficacy of the three DEs against the two pest species is Insecto > Bularafa > Abakire. Bularafa DE has potential for the management of insect pests of stored grain in Nigeria.     

Development of Sitophilus zeamais Motschulsky,in maize kernels and pellets made from maize kernel fractions

As a means of studying certain aspects of resistance of maize varieties to Sitophilus zeamais, various types of pellets were prepared using ground maize and compared with whole, pericarpless, and germless kernels. Numbers of weevil progeny (from six females and three males, and usually a 7-day oviposition period) were greatest in pellets made of flour moistened with water containing agar. More progeny was produced in pellets than in whole kernels. Damaged kernels (hot-water treated, pericarpless, germless) were more susceptible than undamaged kernels and fewer weevil progeny were produced in pellets made of germless kernels than from pellets of whole kernels. Additions of small amounts of extra germ tissue to whole maize flour enhanced the susceptibility of pellets to the weevil, but adding larger amounts of germ of pericarp reduced progeny number, weight and developmental speed.     

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.     

Effects of flameless catalytic infrared radiation on Sitophilus oryzae (L.) life stages

A laboratory benchtop flameless catalytic infrared emitter was evaluated against all life stages of the rice weevil, Sitophilus oryzae (L.), an insect species associated with stored wheat. The emitted infrared radiation was in the 3-7 μm range. A non-contact infrared thermometer measured grain temperatures continuously during exposures of infested wheat. Insect mortality was a function of the final grain temperature attained. In general, higher grain temperatures were attained when using 113.5 versus 227.0 g of wheat, and at 8.0 cm from the emitter versus 12.7 cm, and during a 60 s exposure versus a 45 s exposure. Complete mortality of all life stages of S. oryzae was achieved at 8.0 cm from the emitter using 113.5 g of wheat, with a 60 s exposure; the mean grain temperatures attained ranged from 108.4 to 111.8 °C. The log odds ratio tests showed that eggs (0 days old) were the least susceptible stage to infrared radiation, followed by adults within kernels (28 days old), pupae (24 days old), young larvae (7 days old), larvae that were 14-21 days old, and adults (42 days old). These data using small amounts of grain indicate infrared radiation from the flameless catalytic emitter to be a viable option for disinfesting wheat containing various life stages of S. oryzae.     

Insecticidal effect of spinetoram against six major stored grain insect species

Spinetoram is a novel insecticide that belongs to the spinosyn class of insecticidal chemicals. The efficacy of spinetoram against numerous insect pest species in a variety of field crops has been well demonstrated. However, there are no data available for the effectiveness of spinetoram against stored grain insects. In the present study, we evaluated spinetoram as a grain protectant, against six stored-product Coleoptera. The species tested were: the rice weevil, Sitophilus oryzae, the lesser grain borer, Rhyzopertha dominica, the larger grain borer, Prostephanus truncatus, the confused flour beetle, Tribolium confusum, the granary weevil, Sitophilus granarius and the sawtoothed grain beetle, Oryzaephilus surinamensis. All species were tested at the adult stage, on wheat (or maize in the case of P. truncatus) treated to achieve spinetoram concentrations of 0.01, 0.1, 0.5, 1, 2, 5 and 10 ppm. Mortality was recorded after 1, 2, 7, 14 and 21 d of exposure, and 65 d later the wheat and maize were examined for offspring emergence. Among the species examined, P. truncatus and R. dominica were by far the most susceptible, given that mortality was close to 100% after 7 d on wheat treated or maize with 0.1 ppm of spinetoram. At this concentration, progeny production of P. truncatus and R. dominica was negligible. On the other hand, T. confusum was the least susceptible; mortality reached 95% only at 10 ppm, and only after 14 d of exposure. Similarly, O. surinamensis was of limited susceptibility to spinetoram; mortality reached 95% only after 14 d of exposure on wheat treated with 5 ppm. Nevertheless, offspring emergence of these species was extremely low. For S. granarius and S. oryzae, complete (100%) mortality was recorded after 14 d of exposure, at 0.5 and 1 ppm, respectively. At these concentrations or higher, progeny production was notably reduced. The results of the present study demonstrate that spinetoram is effective as a grain protectant, but its efficacy varies according to the target species, concentration and exposure interval.     

Responses of phosphine susceptible and resistant strains of five stored-product insect species to chlorine dioxide

Adults of phosphine susceptible laboratory strains and phosphine resistant field strains of five stored-product insect species were exposed in vials with 0 or 10 g of wheat for different time periods to 0.54 g/m³ (200 ppm) of chlorine dioxide gas. After exposure, adult mortality was determined 5 d later at 28 °C and 65% r.h. The 5-d mortality was 100% in laboratory and field strains of the red flour beetle, Tribolium castaneum (Herbst); sawtoothed grain beetle, Oryzaephilus surinamensis (L.); lesser grain borer, Rhyzopertha dominica (F.); maize weevil, Sitophilus zeamais Motschulsky; and rice weevil, Sitophilus oryzae (L.) that were exposed in vials with 10 g of wheat to chlorine dioxide for 26, 16, 24–34, 18–24, and 15–18 h, respectively. Corresponding exposure durations for these species and strains in vials without wheat were 15, 3, 18–20, 7–15, and 5–7 h, respectively. Dosages of chlorine dioxide producing 99% mortality (LD₉₉) of T. castaneum, O. surinamensis, R. dominica, S. zeamais, and S. oryzae strains in vials with wheat ranged from 14.79−22.57, 8.20–8.41, 15.79–21.60, 10.66–14.53, and 7.67–12.20 g-h/m³, respectively. In vials without wheat, corresponding LD₉₉ values for T. castaneum, R. dominica, and S. zeamais strains were 6.51–8.66, 11.46–23.17, and 5.79–10.26 g-h/m³, respectively. LD₉₉ values for O. surinamensis and S. oryzae could not be computed, because of 100% mortality after a 3–5 h exposure to chlorine dioxide. No adult progeny production of T. castaneum and O. surinamensis was observed after 8 weeks in control and chlorine dioxide-exposed samples. Adult progeny production of Sitophilus spp. was found only in the control samples. The dosage for 99% adult progeny reduction relative to control for R. dominica strains ranged from 10.07 to 18.11 g-h/m³. Chlorine dioxide gas is effective in killing phosphine susceptible and resistant strains of five stored-product insect species and suppressing adult progeny production of three out of the five species.     

Detection of External and Internal Insect Infestation in Wheat by Near-Infrared Reflectance Spectroscopy

Near-infrared spectroscopy (NIR) in the reflectance mode was investigated for the rapid, automatic and non-destructive detection of insect stored-grain pests external or internal to wheat kernels. Convincing calibration performance was obtained for external adult Oryzaephilus surinamensis (L) (saw-toothed grain beetle) in unmilled samples including varieties Beaver (soft wheat) and Mercia (hard wheat) at two moisture contents. With this large substrate variability, the method could differentiate between uninfested samples and samples with approximately 270 insects kg⁻¹ or more. Milling made no improvement. Large spectral differences were observed between uninfested kernels and kernels infested internally with Sitophilus granarius (L) (grain weevil) larvae or pupae, arising from both a changed chemical composition and physical structure. Single uninfested and infested kernels were discriminated by their second derivative (d²) spectra. For both external and internal infestation there was substantial evidence that insect protein and/or chitin and moisture were being detected. NIR should be useful as a rapid method of detection.     

Effects of zeolite formulations on the mortality and progeny production of Sitophilus oryzae and Oryzaephilus surinamensis at different temperature and relative humidity levels

The insecticidal potential of two natural zeolite formulations, Zeocem and Mycostop, was tested against the rice weevil Sitophilus oryzae (L.) and the sawtoothed grain beetle Oryzaephilus surinamensis (L.) on wheat, at the dose rates of 0.25, 0.50, 0.75, and 1 g/kg. The bioassays were carried out under laboratory conditions at three temperature levels (20, 25, and 30^οC) and two relative humidity (r.h.) levels (55, 75%). Mortality was recorded after 7, 14, and 21 d of exposure and progeny production was recorded 65 d later. Complete (100%) mortality was achieved only for O. surinamensis adults with Zeocem for most temperature and r.h. combinations tested. Progeny production for this species was extremely low. In contrast, mortality for S. oryzae was much lower and did not exceed 87% after 21 days at any of the combinations tested, while progeny production was high. For O. surinamensis exposed to Mycostop, mortality after 21 days did not exceed 97% for all temperature and r.h combinations, while overall progeny production was low. Moreover, S. oryzae mortality was generally low and did not exceed 56%, while progeny production for this species was high for most temperature and r.h. combinations. Despite the fact that some general trends were evident, the effect of temperarature and r.h. in zeolite efficacy was often found inconclusive. The results of the present study illustrated the insecticidal potential of zeolites for the contol of certain stored product insect species, which requires additional investigation.     

Grain loss caused by Tribolium castaneum, Sitophilus oryzae and Acanthoscelides obtectus in stored durum wheat and beans treated with Beauveria bassiana

The effect of the fungal entomopathogen Beauveria bassiana (Balsamo) Vuillemin (Hyphomycete) on the losses caused to durum wheat and beans by storage insects was investigated. Grains were infested with Tribolium castaneum (Herbst), Sitophilus oryzae (L.) and Acanthoscelides obtectus (Say). Beauveria bassiana was produced in inoculated autoclaved rice. The spore formulation (ground rice and B. bassiana) was applied to grain (wheat or bean) and shaken to evenly cover the grain. Adults of T. castaneum or S. oryzae were added to wheat and adults of A. obtectus to bean. Five replicates were set up for each treatment and controls (milled rice but no conidia). The insecticidal effect of B. bassiana was tested by measuring the fresh weight and weight loss of grains after four months of storage. Wheat grains infested with S. oryzae without the conidia was significantly more damaged by weevils than grains treated with B. bassiana. The mean fresh weight of grains with the conidia was significantly greater (18.4%) than the corresponding mean without the fungus when S. oryzae were present. Percentage weight loss decreased by 81.5% and was significantly smaller than the loss from the untreated grain. Significant differences were not found in the fresh weight of seed infested with T. castaneum or A. obtectus in treated or untreated grain nor in the percentage weight loss of grains infested with these insects, with and without B. bassiana.     

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 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.