Synergistic effects of insect-resistant maize and Teretrius nigrescens on the reduction of grain losses caused by Prostephanus truncatus (Horn.)

Prostephanus truncatus is a pest that causes serious losses in stored maize (Zea mays L.) especially in developing countries. This research was conducted to investigate the use of post-harvest insect resistance maize in combination with biological control of P. truncatus by the predator Teretrius nigrescens to reduce maize storage losses. We studied the population dynamics of P. truncatus with and without a predator in combination with susceptible maize and resistant maize to insects under laboratory conditions. This study confirms that P84c3 is a resistant variety against P. truncatus. Maize resistant kernels had a reduction of 30% losses in comparison with susceptible kernels. Significant and favorable interactions were observed between P84c3 maize and presence of T. nigrescens. A dramatic reduction of 80% in progeny number, 81% grain weight loss, and 75% frass production caused by P. truncatus was observed when the predator was used in combination with P84c3. Resistant maize reduced the prey development time and consequently the insect density allowing the predator to control more effectively the population. Prey/predator proportion on resistant maize was significantly higher in comparison with susceptible kernels; thus, giving a more effective pest population control by the predator. These results demonstrated that the combination of post-harvest insect resistance maize with the predator T. nigrescens reduces grain maize losses by P. truncatus.     

Influence of commodity on the population growth of the larger grain borer,Prostephanus truncatus (Horn) (Coleoptera: Bostrychidae)

In this study, we compared different amylaceous commodities for their suitability for population growth of the larger grain borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrychidae), which is an important pest of stored maize in the tropics. In this context, we conducted three different series of tests. In the first test, we compared whole grains: whole barley, peeled barley, maize, whole oats, peeled oats, peeled rice, rough rice, rye, triticale, and wheat. Only maize was found suitable for P. trunctatus to reproduce and significantly increase its population, but there were certain commodities where surviving adults were found. In the second series of tests, we evaluated the percentage of cracked maize kernels on P. truncatus population growth. In this test, we found that the numbers of alive P. truncatus adults in the vials that contained 100% cracked kernels was higher than these with lower percentages of cracked kernels (5–50%) or no cracked kernels (0%). Finally, in the third series of tests, we compared eleven different types of amylaceous commodities with maize. These commodities were: whole maize (whole kernels), whole oat flakes, maize flour, whole barley flour, pasta, white soft wheat flour, whole soft wheat flour, white hard wheat flour, whole hard wheat flour, whole rye flour, and semolina. The results indicated that only whole maize and maize flour were suitable for P. truncatus population growth. Nevertheless, in some of the other commodities tested, there was a low number of adults and immatures that were able to survive 60 days after the incorporation of the parental adults. The results of the present study indicate that from the commodity range tested here, only maize was suitable for P. truncatus population growth. Nevertheless, the marginal numbers of surviving individuals in the non-maize commodities and the role of these substrates as potential “non-preferred food vehicles” that may contribute to further spread of this species should be tested in more detail.     

The biology,ecology and management of the larger grain borer,Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae)

The invasive larger grain borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae), originating from Meso-America, is a devastating stored product pest of maize (Zea mays L.) and cassava (Manihot esculenta). This species can destroy up to 40% of stored maize within four months. Despite four decades of research, P. truncatus has spread throughout Africa, while decimating maize and cassava production. Given the continuing problems with P. truncatus, the likelihood for its continued range expansion under climate change, and its status as a species of concern in many countries, the objective of this review was to provide an updated and comprehensive freely accessible record of the distribution and spread, biology, development, and ecology, host preferences, chemical ecology, detection, and monitoring, and management of P. truncatus. We conducted a search of the literature from 1911 to 2021 using Google Scholar and Web of Science to find all papers related to key search terms. We found that P. truncatus has been recorded in 36 countries across the globe, including 21 now in Africa. A recent predictive model found that the insect has been limited to tropical and subtropical regions but could likely spread to temperate regions as temperatures rise with climate change. Conspecifics respond to their two component, male-produced aggregation pheromone early after eclosion, but quickly switch to other cues as older adults. At close-range, P. truncatus may use food cues, but host volatiles are not involved in long-range host finding of commodities. Research on managing P. truncatus has mostly focused on chemical control to the detriment of other tactics, with the most promising tactic likely to be the different hermetic storage technologies. Many outstanding areas of basic behavior and ecology remain to be assessed for P. truncatus. We highlight specific areas that should be prioritized for further work in order to better manage and reduce the impact of this invasive insect pest.     

Effect of wood ash and conidia of Beauveria bassiana (Balsamo) Vuillemin on mortality of Prostephanus truncatus (Horn)

A series of experiments was carried out to investigate the effect of treating maize with wood ash and entomopathogenic fungus on the mortality of the larger grain borer Prostephanus truncatus. Jars of maize were treated with ash and/or conidia of Beauveria bassiana at doses of 0.0005, 0.005, 0.05 and 0.2 g conidia/g ash/100 g maize. Adult P. truncatus were placed on the maize and numbers of live and dead insects were assessed weekly.Ninety-five per cent mortality of P. truncatus was observed after 1 week and 100% mortality after 2 weeks in maize treated with the highest dose of conidia with ash (0.2 g/g). Mortality of P. truncatus in all ash with conidia treatments was significantly higher than untreated controls over a 4-week period, except the lowest dose of conidia at week 4. The three highest concentrations of conidia per gram of ash resulted in significantly (1% level) higher mortality of P. truncatus than ash alone.In an experiment to investigate the possible synergy of ash and conidia, there were no significant differences in mortality of P. truncatus between treatments with the same concentration of conidia regardless of whether the conidia were mixed with ash or applied alone. Larval populations of P. truncatus were observed in all treatments without ash, but only one larva was observed in one replicate of a treatment containing ash suggesting that ash has an important effect in preventing the establishment of a second generation of insects.     

Prey-specific contact kairomones exploited by adult and larval Teretrius nigrescens: A behavioural comparison across different stored-product pests and different pest substrates

Prostephanus truncatus is a serious exotic beetle pest of stored maize and cassava in Africa. Teretrius nigrescens is a classical biological control predator of this pest that was released into Africa in 1991. In previous work it was shown that adult T. nigrescens are arrested in the presence of dust and frass produced by P. truncatus feeding on maize, and both adult and larval T. nigrescens are arrested by solvent extracts of this dust/frass when presented on filter paper. The current study used these two complementary bioassay approaches to test crude dust/frass and then solvent extracts of the same materials. Results demonstrate that T. nigrescens adults are arrested more strongly by the dust/frass of P. truncatus than the dust/frass of six other beetle pests of stored grain. Similar behaviours are observed for responses of both adults and larvae to solvent extracts. However, extract of dust/frass from Sitophilus zeamais is repellent to adults and larvae of T. nigrescens. Collections of dust/frass from P. truncatus cultured on maize, cassava and an artificial maize substrate previously stripped of all hexane extractable compounds, are all shown to arrest adult T. nigrescens when presented in their crude form and to arrest both adults and larvae when presented as solvent extracts. These experiments demonstrate that P. truncatus produces a species-specific kairomone that is independent of the food or tunnelling substrate. Findings are discussed in the context of prey location, particularly in natural habitats.     

Analysis of free fatty acids in food substrates and in the dust and frass of stored-product pests: Potential for species discrimination?

The larger grain borer, Prostephanus truncatus, is a serious beetle pest that tunnels extensively to produce large quantities of dust and frass. The natural enemy Teretrius nigrescens is an important biological control beetle which is known to exploit at close-range solvent-extractable chemical cues in the dust and frass. The objective of the current study was to analyse quantitatively and qualitatively, the free fatty acid mixtures in different food-substrate materials both before and after insect attack by a range of stored-product pests in order to ascertain whether differences in these mixtures could explain the T. nigrescens selectivity to P. truncatus dust/frass over that of other species irrespective of food substrate. By TLC, GC and GC-MS we found triglyceride and five free fatty acids were the most abundant chemicals in dust/frass (palmitic acid (C16:0), stearic acid (C18:0), oleic acid (C18:1), linoleic acid (C18:2) and linolenic acid (C18:3)). In maize flour, Sitophilus species did not significantly change free fatty acid concentrations whereas with P. truncatus, Rhyzopertha dominica and Dinoderus minutus there were 4-6-fold increases, and, for Tribolium species there were over 20-fold increases. These differences provide interesting insights to tunnelling/feeding habits and are correlated with known feeding preferences within grain. Principal component analysis (PCA) demonstrated that free fatty acid ratios in dust/frass of different species are most linked to the food substrate and confer little discriminatory information that could be used to distinguish between the different species. Although increases in free fatty acid concentrations are good indicators of pest infestation and this may contribute behaviourally in an additive or synergistic way, we conclude that other chemical(s) are present and are key to T. nigrescens recognition of P. truncatus on different substrates.     

Physical factors in the hermetic SuperGrainBag® and effect on the larger grain borer [Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae)] and aflatoxin production by Aspergillus flavus during the storage of ‘Obatanpa’ maize (Zea mays L.) variety

Controlling invasive pests and aflatoxin production by moulds in stored grains is a global challenge to food security and public health, particularly in Africa. Food storage systems are designed to provide constraints to spoilage organisms by presenting mechanical barrier or unfavourable atmospheric conditions to their growth and productivity. This study examined the physical factors generated in hermetic SuperGrainBags^® during the storage of ‘Obatanpa’ variety of maize (Zea mays L.) and the effect on growth of the larger grain borer Prostephanus truncatus (Horn) and aflatoxin contamination by Aspergillus flavus. Cultured P. truncatus or A. flavus was introduced into 1.5 kg of the dried maize and stored in either hermetic SuperGrainBags^® or non-hermetic polypropylene bags. The hermetic conditions elicited an increase in the interstitial temperature (ca. 27 °C) but a decrease in the relative humidity (<70%), oxygen concentration (<6.4%) and the grain moisture content (<13.7%), the combined effects of which inhibited growth of the insects and aflatoxin production by the moulds. Total mortality of P. truncatus occurred after 52 d of storage in the SuperGrainBags^® while aflatoxins concentration remained within safe limits for human consumption. In contrast, there was proliferous growth of P. truncatus and significant increase in aflatoxin concentration to lethargic levels within the polypropylene bags where temperature, relative humidity and grain moisture increased significantly. Accordingly, grain damage and weight loss percentages were significantly high in the polypropylene bags while that in the SuperGrainBags^® were negligible. Altogether, the SuperGrainBags^® better preserved the maize grain quality and safeguarded it against health risks than the polypropylene bags.     

Partial characterization of a chymotrypsin-like protease in the larger grain borer (Prostephanus truncatus (Horn)) in relation to activity of Hyptis suaveolens (L.) trypsin inhibitor

Hyptis suaveolens trypsin inhibitor (HSTI) inhibits, in vitro, most of the trypsin-like serine proteases present in the gut of Prostephanus truncatus (Horn), the larger grain borer. In order to measure its in vivo effectiveness, different doses of the purified HSTI were incorporated into artificial seeds to measure their effects on the growth of P. truncatus larvae and pupae. Results showed that this protease inhibitor had no significant effect in vivo. Consequently, we investigated, identified, and partially characterized an inhibitor-insensitive protease that could explain the growth of P. truncatus on diet containing HSTI. This protease had an apparent molecular mass of 31 kDa, an optimum pH of 7.0, and an activity range extending from pH 6.0 to 8.0, which would allow activity in the gut of P. truncatus. This enzyme could be one of the physiological mechanisms in P. truncatus that allow the species to avoid the adverse effects of trypsin-like protease inhibitors present in the normal diet.     

A simple curve describing the development pattern of some beetles breeding on stored products

The curve representing the emergence pattern of a generation of Sitophilus zeamais Motsch. developing upon maize was investigated, and it was found that the relationship between the cumulative number of emerged insects (y) and time (t) could be described by the asymptotic model log_ey = a ? bp^t. The model also satisfactorily fitted data for the emergence of S. zeamais breeding on triticale and on barley, Callosobruchus maculatus (F.) breeding on cowpeas and Prostephanus truncatus (Horn) breeding on maize.     

Environmental factors influencing the development and rate of increase of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) on stored maize

Temperature, relative humidity (r.h.), food density, and maize (corn) cultivar (CV) influenced oviposition, development and intrinsic rate of increase of Prostephanus truncatus (Horn). The lower and upper limits for complete development were 25 and 32°C at 40% r.h.; 18 and 37°C, at 70% r.h.; and 20 and 32°C at 90% r.h. The shortest developmental periods were 25.4 ± 0.2 days in ground maize, CV Golden Beauty, at 32°C, 70% r.h., and 24.1 ± 0.2 days in whole kernel maize, CV Pride 1108, at 32°C, 80% r.h. Ground maize at a density of 750 mg/cc promoted rapid development of larvae accompanied by lower mortality and fewer malformed adults than ground maize at a density of 450 mg/cc. P. truncatus laid 430 eggs per female in blocks of maize consisting of 6-cemented kernels during 22 weeks compared with 205 eggs per female in ground maize during 15 weeks, and 36 eggs in loose maize kernels during 4 weeks. Viable eggs were laid at 18°C, 70% r.h., and 32°C, 70–80% r.h. The rate of self-multiplication and the intrinsic rate of increase per week, were: 1.94–2.24 and 0.665–0.806, respectively, at 30–32°C, 70–80% r.h. The highest rate was on field maize, CV Pioneer 3993, at 30°C, 70% r.h. The wide range of temperatures and relative humidities at which P. truncatus can develop in maize favor its establishment in tropical and subtropical regions where maize is grown and stored.     

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.     

Predicting Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) populations and associated grain damage in smallholder farmers’ maize stores: A machine learning approach

Prostephanus truncatus is a notorious pest of stored-maize grain and its spread throughout sub-Saharan Africa has led to increased levels of grain storage losses. The current study developed models to predict the level of P. truncatus infestation and associated damage of maize grain in smallholder farmer stores. Data were gathered from grain storage trials conducted in Hwedza and Mbire districts of Zimbabwe and correlated with weather data for each site. Insect counts of P. truncatus and other common stored grain insect pests had a strong correlation with time of year with highest recorded numbers from January to May. Correlation analysis showed insect-generated grain dust from boring and feeding activity to be the best indicator of P. truncatus presence in stores (r = 0.70), while a moderate correlation (r = 0.48) was found between P. truncatus numbers and storage insect parasitic wasps, and grain damage levels significantly correlated with the presence of Tribolium castaneum (r = 0.60). Models were developed for predicting P. truncatus infestation and grain damage using parameter selection algorithms and decision-tree machine learning algorithms with 10-fold cross-validation. The P. truncatus population size prediction model performance was weak (r = 0.43) due to the complicated sampling and detection of the pest and eight-week long period between sampling events. The grain damage prediction model had a stronger correlation coefficient (r = 0.93) and is a good estimator for in situ stored grain insect damage. The models were developed for use under southern African climatic conditions and can be improved with more input data to create more precise models for building decision-support tools for smallholder maize-based production systems.     

Immediate and delayed mortality of the larger grain borer,Prostephanus truncatus (Horn), on different surfaces treated with thiamethoxam and alpha-cypermethrin

In this study, we evaluated efficacy of alpha-cypermethrin and thiamethoxam on concrete, linen and plastic surfaces, against adults of the larger grain borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrychidae), which is an important pest of stored maize in the tropics. Insect mortality was recorded after 1, 3 and 7 days of exposure. Then, separately for each initial exposure, the surviving individuals were transferred to untreated surfaces and delayed mortality was assessed again 1, 3 and 7 days later. Adult mortality on both insecticide treated linen surface was significantly lower than those on the other surfaces. Regarding delayed mortality, for adults that had been initially exposed for 7 d, complete control was detected 7 d later on plastic and concrete surfaces treated with thiamethoxam, while for alpha-cypermethrin complete mortality was detected only on plastic surface. Overall, there was a noticeable delayed mortality of P. truncatus adults, for all exposures and surfaces, which indicates that even if the insects are removed from the substrate that with either alpha-cypermethrin or thiamethoxam, delayed mortality is more likely to occur than recovery.     

Infestation of dried cassava (Manihot esculenta Crantz) by Prostephanus truncatus (Horn( (Coleoptera: Bostrichidae)

Stored dried cassava is known to become heavily infested by Prostephanus truncatus (Horn). A field study was undertaken in Tanzania to determine the extent of losses that this pest could cause in fermented and unfermented dried cassava roots stored over a period of about 4 months. In fermented roots, the mean weight loss (±SD) rose to 73.6 ± 25.9% over this period compared with 52.3 ± 12.0% in unfermented roots. At each time interval that roots were examined, the weight loss in fermented roots was significantly higher. Subsequent laboratory studies, undertaken to identify some of the factors responsible for this difference between the two types of cassava comprised (1) a determination of the length of larval and pupal development at 27°C and 50 or 70% r.h., (2) adult preference for the two types, and (3) their susceptibility to adult boring. P. truncatus developed at a similar rate in both fermented and unfermented roots, but the adults appeared to prefer the fermented cassava possibly because it was easier to bore into. This was certainly one factor making fermented roots more susceptible to damage by adults. It is concluded that although P. truncatus caused lower weight loss in the unfermented compared with the fermented roots, both were so heavily damaged that it is not worth recommending the storage of one rather than the other. The role of cassava as an intermediate host for P. truncatus is discussed and consideration is given to the need to control the beetle in cassava in order to reduce cross-infestation to maize.     

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.     

Quality of maize for sale in markets in Benin and Niger

A follow-up study on the quality of maize for sale in West African public markets was carried out in Benin and Niger from August 15–28, 2013. Complementing the earlier study, this present assessment included not only retailers but also wholesalers and maize producers. Samples were evaluated for parameters related to the physical quality of the maize and for aflatoxin contamination. Most maize value chain actors process their offered grain using traditional methods for threshing, winnowing and drying. Maize for sale in the markets surveyed had an average moisture content ranging between 12 and 14%. Non-grain impurities amounted to 0–2.3% while mouldy grains ranged between 0.2 and 0.8%. The impurity level in grain was three times higher among wholesalers compared to retailers and producers. An insect pest, the Larger Grain Borer (Prostephanus truncatus (Horn) was found only in Benin but Sitophilus zeamais Motschulsky, Cryptolestes ferrugineus Stephens, and Tribolium castaneum Herbst, were present in maize for sale in the markets in of both countries. Insect pest frequency was 16 times higher in wholesalers' grain compared to that of retailers and producers. Aflatoxin levels exceeding the accepted standard of 20 ppb were noted in markets in both countries. The highest proportion of aflatoxin-contaminated maize was in wholesalers’ grain in Malanville market.     

Efficacy of diatomaceous earths and their low-dose combinations with spinosad or deltamethrin against three beetle pests of stored-maize

A key element in postharvest IPM is the reduction of chemical residues in food through the use of reduced dosages of less toxic grain protectants. Two laboratory experiments were conducted: Experiment I determined the efficacies of straight diatomaceous earths (DEs) – “Chemutsi” (African raw DE), MN51 (new formulation) and Protect-It^® (enhanced DE), and two new food grade DE-based formulations (A2 and A3) against adult Prostephanus truncatus (Horn), Sitophilus zeamais (Motschulsky) and Tribolium castaneum (Herbst) admixed with shelled maize. In Experiment II, Chemutsi and Protect-It^® were further tested in varying combinations with low-dose deltamethrin and spinosad. At 21 days post-exposure, MN51 800 ppm and 1000 ppm, Chemutsi 1000 ppm, Protect-It^® 600 ppm and food grade A3 150 ppm caused S. zeamais mortalities that were not significantly different from the positive control (Protect-It^® 1000 ppm). However, after the same exposure period, all the straight DEs (applied at ≤ 1000 ppm) and the DE-based food grade formulations were not effective on P. truncatus and T. castaneum. In low dose combinations, 7 day mortalities showed high S. zeamais susceptibility to both DE-spinosad and DE-deltamethrin while P. truncatus was more susceptible only to DE-spinosad and T. castaneum to Protect-It^®-deltamethrin only. At 21 days, all DE-spinosad and DE-deltamethrin treatments were effective and not significantly different from the commercial grain protectant (fenitrothion 1.0% w/w (10000 ppm) + deltamethrin 0.13% w/w (130 ppm)) on all test species. DE-spinosad and DE-deltamethrin combinations significantly suppressed (P < 0.001) F₁ progeny for the three test species whereas straight DEs and DE-based food grade formulations did not. Our results showed that at half the label rates or lower, DE-spinosad and DE-deltamethrin combinations were effective alternative grain protectants that are safer and possibly cheaper. We also give the first report on the effectiveness of Chemutsi in combination with spinosad or deltamethrin on maize grain.     

Residual toxicity of a commercial cypermethrin formulation on grains against four major storage beetles

In the present study, we investigated the residual insecticidal efficacy of a commercial cypermethrin formulation applied on grains (wheat, maize) against adults of Sitophilus oryzae (L.), Oryzaephilus surinamensis (L.), Rhyzopertha dominica (F.) and Prostephanus truncatus (Horn) in laboratory bioassays. The insecticide was applied in its label dose on the grains and a bioassay was initiated at the day of application and every month for four consecutive months (5 bioassays in total). In each bioassay, adult mortality levels were determined after exposure for 7, 14 and 21 d. After the final evaluation, all beetles (dead and alive) were removed and the number of offspring was assessed after an additional interval of 65 d. Based on our results, the application of the tested cypermethrin formulation as a grain protectant provided long-term protection against R. dominica and P. truncatus adults, as in most cases complete control was achieved for these two species, even after four months of storage of the treated commodities. In the case of S. oryzae and O. surinamensis, adult mortality did not reach 100% in any of the cases, moreover, a gradual decline of mortality levels was recorded over the 4-month period of storage. Finally, for S. oryzae and P. truncatus progeny production was notably suppressed by cypermethrin. Results show that the cypermethrin formulation tested could be a valuable tool for stored grain protection, however, factors such as the target species and the storage interval should be taken into consideration.     

What does global warming mean for stored-grain protection? Options for Prostephanus truncatus (Horn) control at increased temperatures

Global climate change is expected to accelerate reproduction, development and activity of stored-product insect pests and degradation of grain protectants hence compromising efficacy of available storage pest management technologies. However, there is little information on these effects. The current laboratory study examined the effect of increasing temperatures on the efficacy of stored maize grain protectants and hermetic containers in controlling Prostephanus truncatus (Horn). In Experiment I, three commercial synthetic grain pesticides (cocktails of an organophosphate and a pyrethroid or a neonicotinoid) and two farmer-practices (neem leaf powder and wood ash) were tested on shelled maize grain. In Experiment II, four storage containers, viz Purdue Improved Crop Storage (PICS) bag, Super Grain bag (SGB), metal silo (MS) and polypropylene bag (PP) (all containing untreated maize) were tested. Both experiments were conducted for 12 weeks at 32 °C, 38 °C and mean ambient temperature of 26 °C; with three replicates per treatment. All treatments were artificially infested with laboratory-reared adult P. truncatus. Sampling was at baseline (0 weeks) and 4-weekly intervals. Overall, results showed significant differences in grain damage and weight losses between non-synthetic and synthetic grain protectants in all treatments at all tested conditions. The hermetic storage containers kept mean insect grain damage below 6.4% compared to 24.5% in the untreated control at all the experimental conditions. These results indicate that the use of synthetic grain protectants and hermetic storage containers (SGB, PICS and MS) in the management of P. truncatus may not be negatively affected by projected warmer temperatures of 32 °C or 38 °C; suggesting these storage technologies will remain efficacious under sub-Saharan Africa’s warming climates.     

Frequency and time pattern differences in acoustic signals produced by Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) and Sitophilus zeamais (Motschulsky) (Coleoptera: Curculionidae) in stored maize

Frequency spectra and timing patterns of brief, 1–10 ms broadband sound impulses produced by movement and feeding activities of Prostephanus truncatus and Sitophilus zeamais last instars and adults in maize were investigated to find spectral and temporal pattern information useful for distinguishing among these species and stages. The impulse spectra were categorized into five different types of frequency patterns (profiles), designated Broadband, HighF, MidF1, MidF2 and LowF to indicate differences in their peak energies and broadness of frequency range. Groups (trains) of three or more closely spaced impulses, termed bursts, were observed to occur frequently in all recordings, as has been reported for sounds produced by other insects. Mean rates of bursts, mean counts of impulses per burst, and mean rates of impulses in bursts were calculated and compared among the two species and stages. The counts of broadband and MidF2 impulses per burst and the rates of broadband and MidF2 impulses in bursts were significantly different for adult than for 4th instar S. zeamais and either stage of P. truncatus. These findings can be useful in developing an acoustic sensor system for automated detection of hidden insects including P. truncatus and S. zeamais in bulk storage warehouses. The findings are discussed in relation to different movement and feeding behavior patterns that have been identified in these important pests.