Potential of three enhanced diatomaceous earths against Sitophilus zeamais Motschulsky and Prostephanus truncantus (Horn) on stored maize grain

Global health and environmental concerns against synthetic pesticides in stored-grain coupled with high cost, unavailability and inaccessibility of modern non-chemical grain treatment technologies in Africa are driving the need for low-cost and low-dose combination of readily available alternatives. Laboratory studies were conducted to determine the efficacy of non-registered low-dose food grade diatomaceous earths enhanced with natural grain protectants: IBS at 50, 75 and 100 ppm; IPE1 at 200, 300 and 400 ppm; IPE2 at 100, 150 and 200 ppm against Prostephanus truncatus and Sitophilus zeamais adult insects on maize. Adult insect mortality was assessed at 7, 14 and 21 days after treatment (DAT) while progeny emergence was assessed at 49 and 70 DAT for S. zeamais and P. truncatus, respectively. IBS caused significant (p < 0.05) P. truncatus and S. zeamais adult mortality at 14 and 21 DAT, respectively. Except for IPE1 at 400ppm, lower doses of IPE1 and IPE2 showed significantly low adult mortality (p < 0.05) (54-74%) on Sitophilus zeamais compared to IBS and the positive control. All rates of IPE1 and IPE2 were also not effective on P. truncatus adults, with significantly low mortality (p < 0.05) (18–38% compared to positive control). IBS at 75 and 100 ppm were effective on progeny suppression on both S. zeamais and P. truncatus but IPE1 at 400 ppm was effective on S. zeamais only. These results suggest the potential of IBS and IPE1 (at 400 ppm) as effective sustainable alternatives to synthetic grain protectants for small-scale farmers in sub-Saharan Africa who may not afford the modern non-chemical grain protection technologies. Field efficacy of these materials under typical farmer conditions is required.     

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.     

Resistance of selected Mozambican local and improved maize genotypes to maize weevil,Sitophilus zeamais (Motschulsky)

Maize weevil Sitophilus zeamais (Motschulsky) is one of the most important pests of on-farm stored maize in thetropics, particularly where there is no control of moisture content and application of pesticides. This study aimed todetermine the level of resistance among Mozambican maize genotypes against S. zeamais. A total of 17 genotypes,composed of seven experimental hybrids developed from S5 inbred lines from five populations, one released hybrid,two improved open pollinated varieties (OPV), three landraces from Mozambique, two resistant and two susceptiblechecks from Kenya were screened for their resistance to S. zeamais. Four hybrids were screened at their F1 and F2generations. The genotypes were evaluated in the post-harvest laboratory, Kiboko, Kenya in a completely randomizeddesign, replicated 4 times. Data was collected on the number of emerged insects, seed weight loss, seed damage,median development period; and protein, starch and oil contents. The Dobie index of susceptibility was used to groupthe genotypes. Six of the of the eight hybrids at the F1 generation were resistant while two were moderately resistant.Three of the four hybrids evaluated at the F2 generation were moderately resistant while one was susceptible. Two ofthe five OPVs evaluated (EV8430DMRSR and landrace Kandjerendjere) were resistant. Genotypes with high proteincontent showed resistance while those with high starch contents showed susceptibility. Weevils fed on the resistantgenotype produced low number of F1 progeny, had a high median developmental time, caused low seed damage andlow seed weight loss. The resistant genotypes can be either used as cultivars or as sources of resistance in a breedingprogram for weevil resistance.     

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.     

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.     

Susceptibility of Brazilian maize landraces to the attack of Sitophilus zeamais (Coleoptera: Curculionidae)

Landraces are used in breeding programs as a source of resistance and compose a small group of genetic materials accepted under organic seed production. The goal of this study was to evaluate the susceptibility of Brazilian landraces grains to the attack of Sitophilus zeamais. Ten different landraces were subjected to the attack of S. zeamais in choice and no-choice experiments. We evaluated the number of emerged insects and the weight reduction of the damaged grains. The Presidente Amarelo landrace received 4.4 and 5.4 times less insects than Colorado and Cateto in the choice experiment. Also, the Ligeirinho and Presidente Amarelo landrace provided the emergence of 4.2 times less adults than Cateto. The landraces presented different results when submitted to choice and no-choice experiments. However, Cateto grains presented the highest susceptibility to attack in both experiments. Presidente Amarelo, on the other hand, presented the highest resistance to the attack of S. zeamais and so might be considered to be used as a source of resistance in breeding programs.     

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.     

Deleterious effects of Myracrodruon urundeuva leaf extract and lectin on the maize weevil,Sitophilus zeamais (Coleoptera,Curculionidae)

Deterioration and degradation of grains by storage insect pests lead to economic losses of several billion dollars and affect food security. Sitophilus zeamais is responsible for pre- and post-harvest damages to maize. The high toxicity of synthetic insecticides and the development of resistance by insects to the chemicals currently used stimulate the investigation of plant-derived insecticides as new alternatives for pest control. In this study, we report the effects of diets containing Myracrodruon urundeuva leaf extract (10–150 mg/g) and lectin (MuLL; 3–150 mg/g) on the survival, feeding, and nutrition of the storage pest S. zeamais. The digestive enzyme activity in gut extracts from the insects reared on the leaf extract (25 mg/g) or MuLL (15 mg/g) diets was also evaluated. The leaf extract induced mortality (LC₅₀: 72.4 mg/g), while MuLL (30–150 mg/g) exerted strong feeding deterrence. The leaf extract and MuLL promoted the loss of biomass, as reflected in the negative values for relative biomass gain rates and efficiencies in converting ingested food. Protease, trypsin-like, acid phosphatase, and amylase activities in the insects reared on leaf extract or MuLL diets were significantly (P < 0.05) lower than those in the control insects. MuLL ingestion also significantly reduced (P < 0.05) endoglucanase and alkaline phosphatase activities. In conclusion, the leaf extract and MuLL have the potential for S. zeamais control by killing adults and preventing the use of a food source, respectively. The deleterious effects of the extract and lectin on S. zeamais may be linked to enzyme inhibition and consequent suppression of digestive processes.     

Effects of short-term feeding by adults of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) on shelled maize

Laboratory tests were performed to determine the damage caused by adults of Prostephanus truncatus (Horn) feeding on shelled maize. Adults when fed at densities of 25, 50, or 100 on 200 maize kernels for 10–40 days did not preferentially feed on the endosperm or the germ and damaged both. Fifty or 100 adults of P. truncatus feeding on 100 g of maize for 10–40 days caused substantial damage resulting in decreased weight and kernel germination. Kernel damage, grain weight loss, and germination after 40 days of adult feeding was about 5–8, 6–8, and 68–72%, respectively. Adult survival during the 40-day period decreased linearly with time at the rate of 1.5 and 1.3% of adults/day at adult densities of 50 and 100, respectively.Fifty adults of P. truncatus when fed on 100 g of maize for 20 days at 75% r.h. caused significantly greater kernel damage and grain weight loss at 30°C than at 25 or 35°C. However, adult survival at the three temperatures was similar. Though P. truncatus develops less effectively on shelled maize than maize on the cob, our results suggest that a short period of adult feeding could result in substantial grain damage.     

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.     

基于线粒体COI和COII基因的米象与玉米象的分子鉴定

为建立基于线粒体COI和COII基因序列的米象和玉米象的分子鉴定方法。采自全国6个地理种群的米象和2个地理种群的玉米象,PCR扩增试虫线粒体COI和COII基因,将获得的基因序列在GenBank中进行BLAST比对,分析基因序列相似性,分别计算基于COI和COII基因序列的遗传距离,并构建系统发育树。结果表明在本研究所分析的2种象虫样本中,米象COI基因的种内相似度在98.17% 以上,米象和玉米象COI基因的种间相似度为85.84%~86.92%;米象COII基因的种内相似度在98.37% 以上,玉米象COII基因的种内相似度在97.77% 以上,2种象虫COII基因的种间相似度为86.27%~88.31%。米象的种内遗传距离为0.002~0.020(COI)和0.011~0.015(COII),玉米象的种内遗传距离为0.001~0.023(COII)。两种象虫的种间遗传距离为0.145~0.185(COI)和0.128~0.154(COII),基于COI和COII基因的种间遗传距离显著高于种内遗传距离。系统发育树结果显示,在2个基因的系统发育树上,米象、玉米象的基因序列分别位于不同的进化支,同种象虫的基因序列位于相同的进化枝。根据本研究所用的线粒体COI和COII基因序列的差异性可以进行米象和玉米象的分子鉴定。     

Efficacy of the furanosesquiterpene isofuranodiene against the stored-product insects Prostephanus truncatus (Coleoptera: Bostrychidae) and Trogoderma granarium (Coleoptera: Dermestidae)

In plant and animal kingdoms, isofuranodiene is a widespread, lipophilic and volatile furanosesquiterpene endowed with insecticidal properties. In the current study, we evaluated its toxicity against the larger grain borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrychidae) adults and the khapra beetle, Trogoderma granarium Everts (Coleoptera: Dermestidae) adults and larvae at 500 and 1000 ppm. Results showed that the overall mortality of P. truncatus adults exposed to 500 ppm was low, while at the higher dose tested it increased to 56.7% after 7 days of exposure. Notably, isofuranodiene killed 95.6 and 96.7% of the exposed T. granarium adults 5 and 7 days post-exposure at 1000 ppm. On the other side, T. granarium larval mortality did not exceed 24.4 and 37.8% at 500 and 1000 ppm respectively, after 7 days of exposure. Isofuranodiene could be considered further to develop novel green tools against T. granarium adults, since there is a lack of effective grain protectants against this noxious species.     

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.     

Efficacy of alpha-cypermethrin,chlorfenapyr and pirimiphos-methyl applied on polypropylene bags for the control of Prostephanus truncatus (Horn), Rhyzopertha dominica (F.) and Sitophilus oryzae (L.)

We examined the immediate and delayed mortality of adults of the larger grain borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrychidae), the lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae) and the rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae) on surfaces of polypropylene storage bags under different treatment scenarios with alpha-cypermethrin, chlorfenapyr and pirimiphos-methyl. These scenarios were: only one surface of the bag was sprayed and insects were exposed on sprayed or unsprayed surface or both surfaces were sprayed. We evaluated the immediate mortality for 1, 3 and 5 days (d) of exposure. Then, we transferred the surviving adults to untreated surfaces of the same type of storage bags and measured the delayed mortality after 7 d of exposure. We also evaluated the effect of insecticidal treatments on the numbers of holes/bites made by the activity of P. truncatus and R. dominica on storage bags. In terms of immediate mortality, chlorfenapyr and pirimiphos-methyl were very effective against all three species and alpha-cypermethrin against P. truncatus. We noticed high mortality values of all species to chlorfenapyr and pirimiphos-methyl, even after 3 d of exposure, reaching 100% in many of the cases examined. For the majority of exposure intervals, insecticides and insects, we did not find significant differences in mortality counts between exposures on sprayed surfaces of the bag. Immediate mortalities of insects exposed on the untreated surface of the bag did not differ significantly with mortalities at the surface single treated with alpha-cypermethrin and pirimiphos-methyl. Concerning delayed mortality, all S. oryzae adults were found dead 7 d after their transfer to untreated bags irrespectively of the treatment. The delayed mortality of P. truncatus and R. dominica adults was either complete (after exposure to chlorfenapyr) or almost complete (after exposure to alpha-cypermethrin). The exposure of P. truncatus and R. dominica to alpha-cypermethrin almost completely suppressed the numbers of holes/bites. Our study indicates that the application of insecticides on polypropylene storage bags can be considered as an effective method for the control of stored-product insects.     

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

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

The 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.     

电子鼻检测玉米象不同虫态的技术研究

针对粮堆中储粮害虫玉米象检测技术落后,检测结果不可靠等问题,利用电子鼻对玉米象不同虫态及虫态组合进行检测,并采用主成分分析法(PCA)和判别因子分析法(DFA)对检测数据进行分析,结果表明:电子鼻可以对不同密度的玉米象成虫进行有效的识别,但当虫口密度高于20头/瓶时,无法进行区分;使用电子鼻检测玉米象不同虫态时,PCA分析法的重复间数据较分散,区分度不高,而DFA分析法可将各虫态有效区分;电子鼻检测玉米象混合虫态时,DFA分析法可将不同组合有效区分,而PCA分析法无法区分,因此将电子鼻检测结合DFA分析法用于玉米象不同虫态及虫态组合的检测是可行的。     

Field monitoring Sitophilus zeamais and Sitophilus oryzae (Coleoptera: Curculionidae) using refuge and flight traps baited with synthetic pheromone and cracked wheat

Flight traps with open vertical baffles and refuge traps in the form of rectangular cardboard boxes were used to catch Sitophilus zeamais and S. oryzae in Kenya. Traps baited with the aggregation pheromone (Sitophilure), and for the first time traps baited with pheromone and food bait (cracked wheat), were field tested. Both traps types were effective in capturing Sitophilus spp and gave higher weevil catch when baited with pheromone and cracked wheat combined than with pheromone or cracked wheat alone. The combination of pheromone and cracked wheat had an additive effect on trap catch and not a synergistic effect as reported in earlier laboratory trials. Different pheromone loadings of 0.5, 1 and 2 mg were tested. The catch of S. zeamais in flight traps rose significantly with increasing dosage. For refuge traps, the same effect was observed although the difference between 1 mg and 2 mg was not statistically significant. The trend was for increasing S. oryzae catch with dosage, although total catches were very much lower than those for S. zeamais, and differences did not prove statistically significant. Traps placed on the floor near infested maize cribs captured significantly more Sitophilus spp than those actually placed in the cribs. Adult Sitophilus captured were sexed and in both flight and refuge traps the sex ratio was biased in favour of females even though the sex ratio of weevils from infested maize cobs, which were the source of the captured weevils, was approximately even.     

A comparison of two methods of assessment of maize varietal resistance to the maize weevil, Sitophilus zeamais Motschulsky, and the influence of kernel hardness and size on susceptibility

Fifty-two maize varieties were screened for resistance to infestation by the maize weevil, Sitophilus zeamais, using assessment methods proposed by Dobie (J. Stored Products Res. 10 (1974) 183-197) and Urrelo et al. (J. Stored Products Res. 26 (1990) 100). The two methods gave similar assessments of maize susceptibility to S. zeamais. The Dobie method is preferred due to the lower total time required for assessment of relative susceptibility of maize varieties. The greatest disadvantage of the Urrelo method is the intensive labour requirements in the early stages of a trial, when numbers of eggs have to be counted, although it has the advantage that the assessment may be terminated upon emergence of the first F(1) adult. Two explanatory variables, kernel size and hardness, were investigated to determine whether they may be used as indicators of resistance. Results suggested that kernel size is the more important in determining resistance to attack by S. zeamais, with large kernels appearing to show greater resistance than small ones. Contrary to expectations, of the varieties tested, including local, hybrid and improved open pollinated (IOP) varieties, the local varieties were generally more susceptible. This may be related to kernel size, as all IOPs and hybrids tested had large kernels, whereas the majority of the local varieties had small ones. No clear relationship between weevil susceptibility and kernel hardness could be detected, although there was an indication that differences associated with kernel size varied depending on kernel hardness. None of the varieties tested showed high levels of resistance to attack by S. zeamais.