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Journal of Stored Products Research 42 (2006) 226239Insect populations in grain residues associated withcommercial Kansas grain elevators$Frank H. Arthura,?, David W. Hagstruma, Paul W. Flinna,Carl R. Reedb, Thomas W. PhillipscaGrain Marketing and Production Research Center, USDA-ARS, 1515 College Avenue, Manhattan, KS 66502, USAbDepartment of Grain Science and Industry, Kansas State University, Manhattan, KS, USAcDepartment of Entomology, Oklahoma State University, Stillwater, OK, USAAccepted 18 February 2005AbstractGrain residues in nine commercial elevators in Kansas were sampled monthly for insects in grain residuesduring 1999 and 2000. Five locations per elevator were observed; the boot pit, dump pit, headhouse, railarea, and tunnel. When a grain residue was found, the quantity was estimated and a sample taken. Adultgrain pest insects and beneficial insects were removed and identified. Cryptolestes spp. and Sitophilus prised about 80% of the pest insects collected. The density of Cryptolestes spp. appeared to rapidlyincrease in spring but remained low at other times of the year. The density of Sitophilus spp. in the residuesincreased consistently through the warm months, peaked immediately after the warmest month, anddeclined gradually as ambient temperatures cooled. Pest insects were observed in 41.7% of the 1575samples examined, and beneficial insects were collected from 5.1% of the samples. Residue samples takenfrom the elevator boot pit and tunnel areas contained a greater density of pest insects (all species combined)than other locations. About 42% of the residues were estimated to be smaller than 1.5kg, and samples fromthese residues contained more insects per sample than did samples from larger residues. Anisopteromaluscalandrae comprised 88.9% of the total number of beneficial insects found. Beneficial insects were observedinfrequently, and mean populations exceeded 1insect/kg of residue in any month at only two of the nineelevators. Results from our study showed that grain residues within the elevator often contain pest insectsARTICLE IN PRESS/locate/jspr0022-474X/$-see front matter r 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.jspr.2005.02.003$This study reports the results from research only. No recommendations are given or implied by the US Departmentof Agriculture, Kansas State University, or Oklahoma State University.?Corresponding author. Tel.: +17857762783; fax: +17857762792.E-mail address: arthur (F.H. Arthur).and could provide food and harborage when the bins are empty, serving as sources of insect infestation fornew grain.r 2005 Elsevier Ltd. All rights reserved.Keywords: Sanitation; Cleaning; Elevators; Grain; Insects1. IntroductionIn the central plains region of the United States, stored-grain insects can quickly infest newlyharvested grain in farm bulk bins (Hagstrum, 1987, 1989; Reed et al., 1991; Dowdy andMcGaughey, 1994; Hagstrum et al., 1994; Vela-Coiffier et al., 1997; Hagstrum, 2001). A likelysource of this infestation is residual grain inside the bins, which can be heavily infested withstored-grain insects (Ingemansen et al., 1986; Loschiavo and Smith, 1986; Barker and Smith,1987, 1990). Sanitation practices such as removing grain residues from inside bins and applyingbin sprays are often recommended for farm-stored hard red winter wheat in the south-central US(Reed and Pedersen, 1987; Reed and Harner, 1998; Reed et al., 1990; Cuperus et al., 1990).Nevertheless, Herron et al. (1996) found little correlation between hygiene practices andinfestations inside farm bins in Australia. Furthermore, Reed et al. (1990) showed that there wasno economic incentive for using insecticides as pre-binning sprays to eliminate residualpopulations inside empty bins, even when these sprays appeared to be effective.Grain in commercial elevators can become infested with insects quickly, as illustrated by astudy of 13 elevator sites in Kansas (Reed et al., 2001). Pitfall traps placed in wheat immediatelyafter storage in these elevators detected insects in half the silos within two weeks. In warmclimates, stored-grain insects are known to infest crops in the field (summarized by Reed et al.,2003). However, in climatic zones similar to those of the present study, sampling of new graindelivered directly from the field has given little indication of infestation (Chao et al., 1953;Hagstrum et al., 1995; Vela-Coiffier et al., 1997).In elevators, it appears likely that a major source of the insects that infest new grain ispreviously infested grain present when the new crop is received. Other likely sources are infestedgrain residues in empty bins, spills and other related debris in and around elevators. It is alsopossible that trucks and railcars used to transport grain to commercial elevators may be infested.Dowdy and McGaughey (1996) surveyed four elevators in Kansas and consistently detectedinsects in the elevator base areas, dump pits, head houses, on the top of the elevators, silo headspaces, and outside areas. They also sampled residual material containing grain dust and did notfind insects upon initial examination, but did collect some insects (primarily dermestids) after a60-day incubation period.In a recent survey of insects collected from grain in the bottom of elevator silos and fromdischarge spouts (Reed et al., 2003), Cryptolestes spp., Rhyzopertha dominica (F.), Oryzaephilusspp., Tribolium spp., and Sitophilus spp. weevils were observed consistently over a 2.5-year period.In the discharge spouts, Cryptolestes spp. comprised 65% or more of the insects collected in fourof the five sampling periods (6 months each). Rhyzopertha dominica comprised about 10% ofthe population most of the time, and Sitophilus spp. comprised from 3.9 to 23.3% of theinsects collected, depending on the sampling period. In the grain remaining inside empty bins,ARTICLE IN PRESSF.H. Arthur et al. / Journal of Stored Products Research 42 (2006) 226239227Cryptolestes spp. comprised 45%, Sitophilus spp. represented 32.4%, and R. dominica comprised9% of the insects collected. Two of the grain elevators reported in Reed et al. (2003) were sampledin the present study. Most research on insects in grain residues found outside of storage bins hasbeen conducted at farm sites, and there is comparatively little research in large commercial grainelevators.Spilled grain and other residual grain materials in and around elevators but outside the storagebins may also contain resident insect populations, and represent an important source ofinfestation for new grain. There are few published data regarding the composition and abundanceof insects in grain residues found inside elevators. The purpose of this study was to determine: (1)species composition of insects collected from grain residues in commercial elevators but outsidethe storage bins; and (2) distributional and seasonal density patterns of these populations.2. Materials and methodsNine elevators in Kansas with concrete silos were visited monthly in 1999 and 2000 to inspectfor accumulations of spilled grain or grain residue, including broken grain, grain dust, and planttrash. A few visits (3% of samples) were made in January and February of 2001. Although visitswere made monthly, the actual amount of grain varied depending on the time of year andfrequency of grain movement through the elevator. Similar difficulties were noted in an earlierstudy by Reed et al. (2003). Grain residues were recorded as being in or close to one of thefollowing locations: elevator boot, tunnel, truck dump, rail line, ground-level areas of theheadhouse, or bin-deck level of the headhouse or annex (Fig. 1). The elevator boot is the enclosedbase of the elevator leg, and is located in the basement or in a sub-basement pit. The tunnel housesthe reclaim conveyor beneath the annex bins. The truck dump is the area above the dump pitARTICLE IN PRESSAnnex Dump PitElevator BootTunnelElevatorLegHeadhouseFig. 1. Illustration of the inspection and sampling locations in grain elevators.F.H. Arthur et al. / Journal of Stored Products Research 42 (2006) 226239228where grain enters the elevator from trucks. The rail load area usually is a covered part of therailroad track that passes close to the elevator. Samples collected on the rail line, including thosecollected in the rail load area, were designated as rail line samples. The main elevator legs arehoused in the headhouse. The roof of the grain storage bins forms the floor of the bin-deck level ofthe headhouse and annex(es). The bin-deck level contains spouts that transport the grain from theelevator leg to the bins or to lateral conveyors.Elevator boot areas and boot pits are often damp and the temperature in them is moderated bythe subterranean location. Spills and residues in this area result from clean-out of the elevatorboot (rare) or spills from worn spouts. Spills in the tunnel area are often exposed to water fromleaks in the wall or floor, and the temperature in the tunnel is moderated by its subterraneanlocation. The pit of the truck dump is nearly self-cleaning because most residues from a previousload are flushed when new grain is subsequently moved through the pit. However, spills oftenoccur around the dump pits; these residues are exposed to ambient air. The railroad line is outsidethe facility and is usually covered by a roof where it passes over the dump pit. Spills on the rail lineare often the result of railcar clean-out. Residues in the headhouse ground level consisted of spillsfrom worn grain spouts. Spills on the bin-deck level may be due to worn spouts or spills from theconveyor belt. Typically, they are exposed to ambient conditions.The amount of grain residue was recorded monthly for each elevator. Quantities of residueswere estimated by the sampler as either p1.5kg, 41.5kg p27kg (about one bushel),1.5kgp27kg (5bu), and 4135kg. Samples of the residues were collected as randomly aspossible. When the quantity of residue was small, it was brushed into a dustpan and placed in asample bag. When a large amount of residue or spillage was present, several kg samples werecollected from different locations and placed in labeled sample bags. Samples were transported tothe laboratory, where they were weighed and characterized as to grain type. Extremes of heat orcold were avoided during sample transport and the samples were stored at room temperature for amaximum of five days before analysis.To facilitate accurate identification of insects, each sample was sieved through various wire-mesh screens to separate as much as possible the live insects from the grain and trash. Adults ofTyphaea stercorea (L.), hairy fungus beetle; R. dominica; and Ahasverus advena (Walt.), foreigngrain beetle were identified to species. Adults of the following were identified to genera becauseclosely related species are sometimes found in stored grain: Sitophilus, Cryptolestes, Tribolium,and Oryzaephilus. Adults of the four parasitoid wasps Habrobracon ( Bracon) hebetor (Say),Anisopteromalus calandrae (Howard), Theocolax ( Choetospila) elegans (Westwood), andCephalonomia waterstoni (Gahan) also were identified to species.Data for the two years in which these samples were collected were combined and the StatisticalAnalysis System (SAS Institute, 2001) was used to analyze these data. Raw data were transformedusing the square root in an attempt to normalize variances for purposes of means-testing, but non-transformed means and other statistics are reported here to facilitate intuitive interpretation.Insect density is reported as the number of live adult insects per kg of residue. The general linearmodels (GLM) procedure was used to determine the significance of various factors with respect toinsect distribution. When a significant model effect was observed, means were separated using theWaller/Duncan k-ratio t-test. The correlation procedure (CORR) was used to correlate numbersof beneficial insect species with those of their primary host among the pest insect species. Thefrequency procedure (FREQ) was used for categorical analysis and w2tests.ARTICLE IN PRESSF.H. Arthur et al. / Journal of Stored Products Research 42 (2006) 2262392293. ResultsIn total, 46,725 pest insects and 933 beneficial insects were collected and identified from 1575grain residue samples. The mean sample size was 559.6712.5g (SE) of grain residue. The meaninsect population density varied from 16.5 to 66.2/kg between individual grain elevators (Table 1),but means were not significantly different because variability was great. Because there was nosignificant relationship (r ?0:28, P 0:42) between the storage capacity of the elevator and theinsect density (all species combined), data from all elevators were combined for further analysis.The predominant insect species in all residue samples were Sitophilus spp. and Cryptolestes spp.,comprising 50.3% and 30.7%, respectively, of the total number of insects collected. In themajority of the residue samples (58.3%) no pest insects were found. Of the samples in which pestinsects were found, 3.2% contained p1 insect/kg, 44.7% contained 41 insects p10/kg, 39.6%contained 410 p100 insects/kg, and 12.5% contained 4100 insects/kg.The highest density of pest insects (all species) and of Sitophilus spp. in particular was found inthe smallest residues (o1.5kg) compared with the other three size categories (Table 2). Thedensity of the other insect species did not differ significantly by estimated size of the residue.Samples from residues found in or near the elevator boot pit and tunnel contained higher insectdensities than did residues from the dump pit, headhouse, or rail line (Table 3). This relationshipwas true for each of the dominant species individually. For example, the density of both Sitophilusspp. and Cryptolestes spp. was more than four times greater in samples from or near the boot pitthan in samples from the dump pit, headhouse areas or on the rail line. Density of Tribolium spp.in samples from the boot pits and tunnels was at least twice as great as from any of the otherlocations.Samples identified as wheat were the most common type (42.8%). Other residue samples wereidentified as maize (17.7%), sorghum (14.0%), a mixture of wheat and sorghum (3.4%) or amixture of maize, wheat, and sorghum (14.1%). About 8.4% were called other because thematerial consisted of uncommon mixtures of grains and dust. Mixtures of wheat and sorghumhad a greater density of Sitophilus spp. and all species combined than did samples of eithersorghum or wheat individually, and the population density of Tribolium spp. was lowestin residues that contained only sorghum (Table 4). The density of Sitophilus spp. was markedlydifferent in residues of different grains, with an average of nearly 31/kg in wheatsorghummixtures but less than 7/kg in sorghum residues. The density of Tribolium spp. alsowas significantly affected by the composition of the residue. Of the species present in highnumbers, only the density of Cryptolestes spp. was not affected by type of grain in the residuesample (Table 4). Of the insects found only in low numbers, a significantly higher density ofT. stercorea was found in the samples classified as a mixture of wheat, maize, and sorghum thanany other type of sample.The mean density of T. stercorea, R. dominica, A. advena, and Oryzaephilus spp. in the residuesamples did not exceed 2 insects/kg regardless of the time of year (Fig. 2AD). Numbers ofT. stercorea peaked in April and September, the beginning and the end of the warm season, andwere low at other periods. Densities of Oryzaephilus spp. also showed two peaks, in June andOctober. In contrast, the greatest densities of R. dominica were observed in three consecutivewinter months. The density of Sitophilus spp. was 45 insects/kg in the residue samplesthroughout the year except for the coldest winter months, which suggests that low temperaturesARTICLE IN PRESSF.H. Arthur et al. / Journal of Stored Products Research 42 (2006) 226239230ARTICLE IN PRESSTable 1Density of live adult pest insects per kilogram (mean7SE) from residues collected over all sampling times, percentage of residue samples infestedwith pest insects, and total number of samples and adult live beetles of each species, by elevator (Elev.)Elev.naSitophilusCryptolestesTriboliumTyphaeastercoreaRhyzoperthadominicaOryzaephilus AhasverusadvenaTotal%InfestedA645.9474.0216.5279.281.6671.190.0070.000.2770.160.0870.080.0070.0024.47711.6843.8B9417.0576.870.8770.400.8670.520.0770.070.3270.170.3270.230.1970.1119.6877.0930.8C12431.28712.596.2271.882.5170.870.0470.030.0970.040.2870.100.0570.0540.45713.8647.6D15213.4573.3651.33733.650.5370.200.2570.090.5070.190.0370.020.0470.0366.15735.7146.7E1927.9173.2317.3178.282.5871.990.1270.080.9770.470.1470.100.3370.3329.3579.8541.7F17412.0473.785.3872.112.2270.890.1770.090.0970.080.2470.100.0370.0220.1875.7444.8G18629.1577.256.0471.6214.60710.74 0.1370.100.0470.030.2870.130.1470.1050.37713.8554.8H3114.7971.885.2071.754.1772.380.9270.640.0870.520.5570.420.0470.0416.4874.9937.3I27811.2973.440.8970.293.2872.320.0570.030.1770.110.8170.570.0370.0216.5274.3333.8Total157521,57016,960638442864459114446,725an number of samples.Table 2Number of live adult insectsaper kg (mean7SE) and percentage of residue samples infested with pest insects, by estimated size of residueResiduesize (Kg)SitophilusCryptolestesTriboliumTyphaeastercoreaRhyzoperthadominicaOryzaephilusAhasverusadvenaTotal% Infestedo1.519.3673.31a16.4677.34a6.4073.06a0.3070.18a0.3970.12a0.4670.22a0.1570.09a43.5279.17a44.01.5279.8372.15b3.8370.89a3.0771.44a0.3870.33a0.5370.33a0.4770.30a0.0770.04a18.1673.23b40.52713510.1973.11b9.4675.23a0.7470.25a0.0770.04a0.2070.11a0.1470.07a0.0370.02a20.8477.44b39.141355.3371.79b8.4276.23a1.3970.69a0.1070.06a0.4670.30a0.0670.04a0.0070.00a15.7877.28b39.1Data combined over all elevators and sampling visits, n 705, 482, 230, and 158 for each size category, from smallest to largest.Means within columns followed by the same letter are not significantly different (PX0:05, WallerDuncan k-ratio t-test, SAS Institute).aANOVA model significant with respect to total number of insects (PROC GLM, SAS Institute, F 6:2, df 3,1571, Po0:01).F.H. Arthur et al. / Journal of Stored Products Research 42 (2006) 226239231ARTICLE IN PRESSTable 3Number of live adult insectsaper kg (mean7SE) and percentage residue samples infested with pest insects, by inspection locationLocationSitophilusCryptolestesTriboliumTyphaeastercoreaRhyzoperthadominicaOryzaephilusAhasverusadvenaTotal% InfestedBoot pit40.0877.90a9.3172.80ab 10.9475.73a0.1470.06a0.7070.35a1.2370.56a0.1170.08ab62.54712.92a71.1Dump pit8.5271.54b6.5071.66bc1.1170.27b0.4370.29a0.4470.19a0.1470.04b0.0270.02b17.1672.56b45.3Headhouse3.3871.44c6.8872.74bc1.4770.48b0.0970.04a0.2370.08a0.0770.04b0.0170.01b12.1073.48b29.7Rail line8.1672.41b2.6171.42c4.7674.30b1.1771.07a0.1470.10a1.0170.95b0.0570.04b17.8975.96b35.4Tunnel37.0978.49a33.97720.94a 11.3178.20a0.0170.01a0.8070.65a0.6770.54b0.4370.27a84.28725.24a53.1Data combined for all elevators and sampling visits n 142, 417, 626, 147, and 243 for each location from top to bottom.Means within columns followed by the same letter are not significantly different (PX0:05, WallerDuncan k-ratio t-test, SAS Institute).aANOVA model significant with respect to total number of insects (PROC GLM, SAS Institute, F 29:3, df 4,1570, Po0:01).Table 4Number of live adult insectsaper kg (mean7SE) and percentage residue samples (all elevators and visits) infested by pest insect, by grain type in theresidue, including wheat (W), maize (M), sorghum (S), wheat+sorghum (WS), wheat+sorghum+maize (WSM), and other (O)GrainnbSitophilusCryptolestesTriboliumTyphaeastercoreaRhyzoperthadominicaOryzaephilusAhasverusadvenaTotal%InfestedW67211.0571.93bc16.9077.94a2.2270.98b0.1370.06ab 0.7570.28a0.1870.11a0.0270.01a31.2778.66bc41.4M28116.7975.73bc5.5871.96a8.4477.06ab 0.0770.05ab 0.0470.02a0.1570.07a0.1270.07a31.2079.99bc39.2S2216.7172.77c5.8373.34a0.6570.25b0.0270.02b0.1570.06a0.8270.64a0.0170.01a14.1274.50c32.1WS5430.9979.40a8.1374.14a5.0474.39ab 0.0470.04b0.3370.23a0.0570.03a0.1870.13a44.77712.81a46.3WSM22223.8076.40b7.3971.83a8.9173.80ba 1.3970.90a0.2470.11a0.9670.60a0.0570.04a42.7679.86ab50.5Other1257.9073.51c5.2972.26a1.0070.34b0.0370.03b0.1870.13a0.2370.23a0.5570.50a15.1775.92bc48.8Means within columns followed by the same letter are not significantly different (PX0:05, WallerDuncan k-ratio t-test, SAS Institute).aANOVA model significant with respect to total number of insects (PROC GLM, SAS Institute, F 3:6, df 5,1569, Po0:01).bn number of samples.F.H. Arthur et al. / Journal of Stored Products Research 42 (2006) 226239232suppressed insect populations in the residues (Fig. 3A). In contrast, Cryptolestes spp. densityin grain residues increased markedly during the warm spring months of April, May, and June(Fig. 3B). Peak Tribolium spp. population density was observed in May, with considerablevariability in density in other months (Fig. 3C). The pattern of insect density for all speciescombined reflected the temporal density distribution of Cryptolestes spp. which peaked during thespring and Sitophilus spp, which peaked in early fall (Fig. 3D).The most abundant beneficial insect species collected in the residue samples was A. calandrae, aparasite of Sitophilus spp. Anisopteromalus calandrae comprised 89% of the total number ofbeneficial insects observed (Table 5). However, in comparison to the host density, the density ofthe parasitoid wasps was low, the mean density of these beneficial insects (all species) ranged from0.0670.03 to 1.5070.72 insects per kg, and exceeded 1 per kg at only two elevators (Table 5).Habrobracon hebetor, a parasite of the Indianmeal moth (Plodia interpunctella Hu bner), wasfound at only two elevators, and only at low densities. Theocolax elegans, a parasite of Sitophilusspp. and R. dominica, was found in low numbers at most of the elevators, as was C. waterstoni, aparasite of Cryptolestes spp. Beneficial insects were recovered from only 5.1% of the samples. Ofthe samples from which beneficial insects were recovered, 8.6% contained o1 insect/kg, 21.0%contained X1 insectso2/kg, 18.5% contained X2 insects o3/kg, 3.7% X3 insects o4/kg, 7.4%contained X4 insectso5/kg, and 40.7% contained X5 insects/kg.There were no differences in mean density for any of the beneficial insects with respect tosample size (Table 6); however, more A. calandrae were found in the samples from or near theboot pit and rail line compared to the other three locations (Table 7). Residues comprised ofARTICLE IN PRESS0123 T. stercorea0123 R. dominica0123 Oryzaephilus0123 Ahasverus advena Sample MonthDensity of non-dominant pest insects (#/kg)MJSDSample MonthMJSD(A)(B)(C)(D)Fig. 2. (AD) Population density trends by month (mean7SE) for each of the four non-dominant insect speciescollected in grain residue samples. Data combined for all visits in 1999 and 2000.F.H. Arthur et al. / Journal of Stored Products Research 42 (2006) 226239233wheat contained the greatest density of A. calandrae and beneficial insects (all species) whereas thelowest density of A. calandrae and beneficial insects (all species) was found in residues of maize(Table 8).ARTICLE IN PRESS0102030405060 Sitophilus 0102030405060 Cryptolestes0102030405060 Tribolium020406080100Total InsectsSample MonthDensity of dominant pest insects and all species combined (#/kg)MJSDSample MonthMJSD(A)(B)(C)(D)Fig. 3. (AD) Population trends for each of the three dominant insect species in grain residue samples and all pestspecies combined (mean7SE). Data combined for all visits in 1999 and 2000.Table 5Number of beneficial insectsaper kilogram (mean7SE) over all visits, percentage of residue samples in which beneficialinsects were observed, and number of adult beneficial insects, by elevator (Elev.)Elev.nbAnisopteromalus calandraeTheocolax elegansCephalonomia waterstoniTotal%cA640.1370.080.0070.000.2470.150.3670.187.2B940.8070.570.0070.000.0270.020.8270.585.3C1240.6670.510.0070.000.0370.010.7070.518.9D1521.3070.580.1770.170.0170.011.4870.647.2E1920.0570.030.0170.010.0070.000.0670.032.1F1740.0770.060.0270.020.0770.050.1770.122.9G1860.0670.040.0070.000.0070.000.0670.041.6H3110.1070.080.0270.010.0370.030.1870.103.5I2781.4570.720.0070.000.0570.031.5070.729.4Total15758303657933aTen Habrobracon hebetor collected, 0.0170.01/kg and 0.0370.03/kg at elevators F and G, respectively.bn number of samples.c% of samples which contained one or more of these species.F.H. Arthur et al. / Journal of Stored Products Research 42 (2006) 226239234A significant relationship (Po0:01) between the presence or absence of the pest and beneficialinsects was demonstrated by w2analysis. Parasitoids were found in 10.2% of samples infested withpest insects, but in only 1.5% of samples in which no pest insects were recovered. However,correlation analysis did not show a significant relationship between the density of any pest insectand the density of the corresponding parasitoid.4. DiscussionThis study demonstrated that grain residues in commercial elevators in Kansas often containinfestations of pest insects. In the nine elevators studied, the frequency of samples infested withARTICLE IN PRESSTable 7Number of beneficial insectsaper kilogram (mean7SE) over all visits and percentage residue samples in whichbeneficial insects were found, by sampling locationLocationnbAnisopteromaluscalandraeTheocolax elegansCephalonomiawaterstoniTotal%cBoot pit1420.8770.46a0.0270.02a0.0870.06a0.9770.47ab 11.3Dump pit4170.4670.21b0.0670.06a0.0470.03a0.5670.24bc5.3Headhouse6260.2070.10b0.0170.01a0.0170.01a0.2270.10c2.6Rail line1471.8971.30a0.0170.01a0.0270.02a1.9871.30a9.5Tunnel2430.4570.24b0.0070.00a0.0770.04a0.5370.24bc5.4Means within columns followed by the same letter are not significantly different (PX0:05, WallerDuncan k-ratio t-test,SAS Institute).aH. hebetor collected at 0.0470.03 and 0.0170.01 per kg from the rail line and tunnel, respectively. ANOVA modelsignificant with respect to total number of insects (PROC GLM, SAS Institute, F 4:7, df 4,1570, Po0:01).bn number of samples.c% of samples which contained one or more of these species.Table 6Number of beneficial insectsaper kilogram (mean7SE) over all visits and percentage residue samples in whichbeneficial insects were found, by estimated size of residueResiduesize (kg)nbAnisopteromaluscalandraeTheocolax elegansCephalonomiawaterstoniTotal%co1.57050.3470.140.0470.030.0370.020.4274820.9370.430.0170.010.0170.010.9570.446.0271352300.4770.250.0070.000.0870.050.5770.265.741351580.1970.100.0170.010.0870.050.2970.117.6aH. hebetor collected at 0.0270.02 and 0.0170.01 for 27135kg and 4135kg, respectively. ANOVA model notsignificant with respect to total number of insects (PROC GLM, SAS Institute, F 0:9, df 3,1571, P 0:44).bn number of samples.c% of samples which contained one or more of these species.F.H. Arthur et al. / Journal of Stored Products Research 42 (2006) 226239235pest insects ranged from about one-third to more than one-half. Furthermore, the infestation wasdense in the residues, with the mean of all pest species combined over all samples being greaterthan 53 adult insects per kg. Only about 3% of the infested samples contained less than 1 insect/kg, and more than 50% contained 410 insects/kg. These results are consistent with the hypothesisthat grain residues were likely to be either densely infested with pest insects or not yet infested.In our study, Sitophilus spp. were the most prevalent insects collected from the residue samples,comprising 46.2% of all insects collected and being found in 27.1% of all samples. Reed et al.(2003), reporting data collected in the same elevators, showed that Sitophilus spp. usually were aminor component of the insect population in the stored grain. Nevertheless, these species wereobserved often in grain left inside the bins, comprising about one-third of the pest insects andbeing present in 37.8% of all in-bin residues (Reed et al., 2003). These facts may indicate thatresidues accessible to insects in these elevators were likely to contain Sitophilus spp. even wherethe weevils comprised a minority of the insect populations in the stored grain.Cryptolestes spp. were recovered from 23.3% of grain residues in the present study. Overall, themean density of Cryptolestes spp. in samples was high (19.2/kg). However, as seen in Fig. 2, mostof these insects were collected during April, May, and June. Because most bins contained nostored grain at this time of year, the insects tended to be found in the grain residues. When newwheat arrived in late June and early July, adult insects rapidly dispersed from the grain residuesinto the more attractive new wheat. This exodus reduced the populations in the residues. Thishypothesis appears to be consistent with the results of Reed et al. (2003). They found Cryptolestesspp. frequently in in-bin residues and showed that these species rapidly infested the recentlyharvested wheat in storage.The annual pattern of density in grain residues of both the fungivore A. advena and the generalscavenger Oryzaephilus spp. appears to be related to season and harvest time. Ahasverus advena isARTICLE IN PRESSTable 8Number of beneficial insectsaper kg (mean7SE) over all visits and percentage of residue samples in which beneficialinsects were found, by composition of the residue, either wheat (W), maize (M), sorghum (S), wheat+sorghum (WS),wheat+sorghum+maize (WSM), or other (O)GrainnbHabrobraconhebetorAnisopteromaluscalandraeTheocolaxelegansCephalonomiawaterstoniTotal%cW6720.0170.01a1.0270.34a0.0470.04a0.0470.02a1.1170.34a6.9M2810.0170.01a0.0370.02c0.0170.01a0.0070.00a0.0570.03c1.4S2210.0070.00a0.2770.23bc0.0070.00a0.0170.01a0.2870.23bc 2.3WS540.0070.00a0.7070.44ab0.0070.00a0.1570.15a0.8570.46ab13.0WSM2220.0170.01a0.1070.04bc0.0170.01a0.0370.02a0.1570.04bc 6.3Other1250.0170.01a0.1070.08bc0.0370.03a0.1270.08a0.2570.17bc 4.0Means within columns followed by the same letter are not significantly different (PX0.05, WallerDuncan k-ratio t-test, SAS Institute).aH. hebetor collected at 0.0270.02 and 0.0170.01 for 27135kg and 4135kg, respectively. ANOVA modelsignificant with respect to total number of insects (PROC GLM, SAS Institute, F 3:9, df 4,1570, Po0:01).bn number of samples.c% of samples which contained one or more of these species.F.H. Arthur et al. / Journal of Stored Products Research 42 (2006) 226239236often found in wheat immediately after harvest in Kansas (Dowdy and McGaughey, 1994; Reedet al., 1991), although the density in the stored grain gradually decreases in the months afterharvest. The present study showed peak densities in grain residues in the elevators about a monthprior to the summer harvest of wheat and immediately prior to the main fall harvest of maize andsorghum. The Oryzaephilus spp. density peaks occurred immediately prior to wheat harvest (June)and during the fall harvest (October).In contrast to the erratic pattern of the other insects, the seasonal pattern of Sitophilus spp.density in grain residues showed a steady increase throughout the warm months, followed by agradual decline. The greatest density occurred immediately after the warmest month and thelowest mean density corresponded with the coldest month. The general pattern of gradual build-up of Sitophilus spp. populations in residues is similar to that seen within stored-grain masses inelevators (Reed et al., 2003), and appears to indicate that weevils are consistent and ubiquitousresidents of grain elevators. In contrast, they are seldom observed in farm-stored wheat in Kansas(Reed et al., 1991; Dowdy and McGaughey, 1994). For Sitophilus species, the grain elevator mayprovide a more constant supply of food and harborage that is more stable relative to temperatureand duration than does the farm habitat.The analysis by location within the elevator suggests that the more stable elevator environmentfavors Sitophilus spp. The density of these species was at least four times greater in samples fromresidues in or near the boot pit and tunnel than in samples from other locations. Weevils werefound in over 70% of samples from or near the boot pit and more than half those from the tunnel,but were found in less than one-half of the samples from other locations. The boot pit and tunnelare below-ground locations that are less exposed to the variations of the ambient environmentthan the other locations.In this study, the large variation in insect presence and density in grain residues complicated theanalysis and interpretation of results. One important unknown was the age of the residue, i.e.,how long a given residue had been in place before a sample was taken. It seems logical thatresidues present for a longer time would have greater insect populations if other factors wereequal. The fact that residues characterized as consisting of a single grain type had fewer insects (allspecies combined) than residues comprised of mixtures of grains may be significant in this regard.Because only one grain is handled at a time in the elevator transport equipment, spills and otherresidues that contain more than one grain were by definition the result of at least two periods ofgrain handling. Therefore, residues comprised of mixtures of grains were likely to have remainedin the same place, without clean-up, longer than residues containing only one grain.Our study showed that beneficial insects were at least occasionally present in grain residues ingrain elevators. The same species were also observed in the masses of stored grain, in grain fromthe discharge spouts, and in the grain residues in empty bins (Reed et al., 2003). The large numberof samples taken and the large number of insects collected in this study allowed us to demonstratethrough frequency analysis that the presence of beneficial insects and the presence of host specieswere related. The known deficiencies of the sampling method for the tiny parasitoid wasps (Reedet al., 2003) probably caused the frequency and density of the beneficial insects to be substantiallyunderestimated.Laboratory studies have documented that certain parasitoids observed in this study are capableof suppressing the population growth of associated host pest species (Flinn et al., 1996). Studiesby Cline et al. (1985) and Press (1984) also show field suppression of S. oryzae by A. calandrae,ARTICLE IN PRESSF.H. Arthur et al. / Journal of Stored Products Research 42 (2006) 226239237and field suppression of Sitophilus zeamais (Motschulsky), the maize weevil, by A. calandrae(Williams and Floyd, 1971). In our study, the occurrence of beneficial insects in theresidue samples was sporadic and not always correlated with occurrence of the host species.Few C. waterstoni were found in our study even though Cryptolestes spp. were abundant in theresidue samples. Although the density of A. calandrae and that of Sitophilus spp. were significantlycorrelated in data sets from certain individual elevators in our study (statistics not shown), mostwere not. Furthermore, the elevators with the fewest A. calandrae had high populations ofSitophilus spp. Also, more Sitophilus were found in or near the elevator boot pit and dump pitcompared to the other three areas, yet the greatest numbers of A. calandrae were found in thesamples from the rail line. Thus, correlation analysis provided only weak evidence of a significanthost-parasitoid relationship. This was expected because we determined the density of both hostand parasitoid at a single point in time whereas, theoretically, the population density trends of theparasitoid would either increase or decrease as that of the host changed with a lag of several daysor weeks. Arbogast and Mullen (1988, 1990) studied interactions of A. calandrae and S. zeamaisduring a 7-year period in stored maize, and also found a delayed density-dependent relationshipbetween the parasite and the weevil host.Our study showed that spilled grain and grain residues within the elevator environment wereimportant sources of insect infestation. These residues could serve as harborage and dispersal siteseven when bins are empty, thus playing a critical role in maintaining pest insect populations.Regular sampling and maintenance could alleviate some of the problems associated with insectinfestations in commercial elevators.AcknowledgmentsThe authors thank the various grain companies and elevator managers who participated in thisstudy.ReferencesArbogast, R.T., Mullen, M.A., 1988. Insect succession in a stored-corn ecosystem in southeast Georgia. Annals of theEntomological Society of America 81, 899912.Arbogast, R.T., Mullen, M.A., 1990. Interaction of maize weevil (Coleoptera: Curculionidae) and parasitoidAnisopteromalus calandrae (Hymenoptera: Pteromalidae) in a small bulk of stored corn. Journal of EconomicEntomology 83, 24622468.Barker, P.S., Smith, L.B., 1987. Spatial distribution of insect species in granary residues in the prairie provinces.Canadian Entomologist 119, 11231130.Barker, P.S., Smith, L.B., 1990. Influence of granary type and farm practices on the relative abundance of insects ingranary residues. Canadian Entomologist 122, 393400.Chao, Y., Simkover, H.G., Telford, H.S., Stallcop, P., 1953. Field infestation of stored grain insects in easternWashington. Journal of Economic Entomology 46, 905907.Cline, L.D., Press, J.W., Flaherty, B.R., 1985. Suppression of the rice weevil, Sitophilus oryzae (Coleoptera:Curculionidae) inside and outside of burlap, woven polypropylene, and cotton bags by the parasitic wasp,Anisopteromalus calandrae (Hymenoptera: Pteromalidae). Journal of Economic Entomology 78, 835838.Cuperus, G.W., Noyes, R.T., Fargo, W.S., Clary, B.L., Arnold, D.C., Anderson, K., 1990. Management practices in ahigh-risk stored-wheat system in Oklahoma. American Entomologist 36, 129134.ARTICLE IN PRESSF.H. Arthur et al. / Journal of Stored Products Research 42 (2006) 226239238Dowdy, A.K., McGaughey, W.H., 199
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