Pergamon J. stored Prod. Res . Vol. 33, No . 2. pp. 157-166, 1997

© 1997 Published by Elsevier Science Ltd All rights reserved. Printed in Great Britain

PIP.'150022-474X(96)00043-4 0022-474X/97 $17.00 + o.oo

Immigration of Insects into On-farm Stored Wheat and Relationships Among Trapping

Methods E. L. VELA-COIFFIER, 1 W. S. FARG0, 1* E. L. BONJOUR, 1t

G. W. CUPERUS1 and W. D. WARDE2

1Department of Entomology, Oklahoma State University , Stillwater , OK 74078, U.S.A. and 2Department of Statistics, Oklahoma State University, Stillwater,

OK 74078, U.S.A .

(Accepted 7 October 1996)

Abstract- Relationships among numbers of insects trapped by unbaited flight and probe traps, and grain trier and deep bin cup samples were studied in 1991 in three farm bins in North Central Oklahoma. Placement of flight traps at the eaves of bins resulted in higher insect catches than at other positions. In the grain mass, insects were more likely to be in the central core during the first weeks of storage. Insects favored the center of the grain mass, but no significant differences were found among temperatures and moisture contents at the different regions sampled.

Increasing numbers of insects with depth and length of storage time combined with the capture pattern of grain insects by flight traps indicated that infestation occurred after grain binning. The most abundant species were Cryptolestes spp., Tribolium spp., Rhyzopertha dominica (F.), Ahasverus advena (Waltl) and Typhaea stercorea (L.). Action thresholds (the insect densities at which managers need to control insect populations) for the number of insects in weekly probe trap catches were 7.35 for R. dominica, 720.14 for Crypto/estes spp., and 101.34 for Tribo/ium spp. when compared with estimates from grain trier samples, and 8.04, 749.03, and 68.14 insects per probe trap, respectively, when compared with estimates from cup samples. © 1997 Published by Elsevier Science Ltd

Key words-stored wheat , trapping and sampling methods, immigration

INTRODUCTION

High temperature and early harvest dates in late spring and early summer make the Southern High Plains a high risk management area for insect pests of stored wheat. In Oklahoma, insects are considered the major cause of grain spoilage followed by moisture and molds (Noyes et al., 1988, 1995). The lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae), is the most important pest, followed by Cryptolestes spp. (Coleoptera: Cucujidae), Tribolium spp. (Coleoptera: Tenebrionidae), and the Indianmeal moth, Plodia interpunctella (Hubner) (Lepidoptera: Pyralidae) (Cuperus et al., 1990b).

The primary objective of insect sampling in stored grain is early detection of infestations. Insect detection permits control measures to be implemented before extensive damage occurs (Wilkin,

*Deceased, 9 September 1995. tTo whom reprint requests should be addressed.

157

158 E. L. Vela-Coiffier, et al.

1990; Hagstrum et al., 1995). Insett traps detect populations sooner than grain sampling devices and allow for earlier monito~ing of potential insect problems (Wright and Hagstrum, 1990).

Sticky traps are used to detect flying stored product insects. In these traps, insects are entrapped in contact adhesives by random encounter or after being attracted into the trap by pheromone lures (Barak et al ., 1990). Probe traps, specifically designed for use within bulk grain, are perforated metal or plastic probes inserted into the grain"mass. Insects crawl through the holes and fall into a removable cup or collection fube (Barak et al. , 1990). Insect detection by probe traps is more effective than by grain triers or deep bin cups because probe traps can be left in the grain for long periods of time (Loschiavo and Atkinson, 1967; Fargo et al. , 1989). Grain triers and deep bin cups are specifically designed to sample grain. They are inserted and immediately withdrawn from the grain, so the probability of collecting insects is low, especially in lightly infested grain (Loschiavo and Atkinson, 1973). The number of insects collected in grain samples depends on the condition of the grain at the time of sampling and the area of the grain mass sampled (White et al ., 1990).

Sticky traps and probe traps depend on insect movement, and any factor that influences movement may also influence reliability of trapping (Cuperus et al., l 990a). Un baited sticky traps capture randomly flying insects by interception. Factors such as light, wind direction and speed, the spatial location of the population to be sampled, trap placement and insect behaviour affect flight trap catches. Temperature and moisture gradients are a common feature of stored grain, and insect populations are rarely dispersed uniformly in the grain mass (Surtees, 1965). Five of the most important variables affecting probe trap catch are insect species, trapping duration, grain temperature, grain type and condition, and trap placement (Cuperus et al., 1990a).

As the use of traps for monitoring insect populations increases, proper interpretation of trap catch is essential. Wheat is considered infested in the USA when a density of two or more live insects are found in a representative 1 kg sample (Anonymous, 1994). This density represents the action threshold at which managers need to control insect populations. Grain triers and deep bin cups sample grain as well as insects, which permits the direct calculation of insect density. However, probe traps sample only insects and a direct conversion to density is not possible. The relationship between trap catches and insect densities obtained from grain samples needs to be determined so that trap information can be utilized in integrated pest management programs.

Stored grain insects are generally not found in newly harvested wheat. Hagstrum (1989) determined that most of the infestation in on-farm storage occurred after the wheat was loaded into the bin instead of before or during loading. Chao et al. (1953) determined that only 13 of 431 samples taken from trucks of the first-delivered wheat from the field were contaminated with insects. They also stated that no stored grain insects were recovered from sweep samples collected from 100 fields of rip~ned standing grain. In 1992, Oklahoma State University personnel collected 10 samples each from 353 trucks delivering newly harvested wheat to one elevator in Grant County. Samples were processed on an inclined sieve and no insects were found. A similar study was conducted in 1995 in which a total of 1314 samples were collected from 16 sites from southern to northern Oklahoma. These samples were officially graded and no insects were reported.

A better understanding of the initiation and progression of stored grain insect infestations would allow managers to take preventive measures to reduce the probability of infestations occurring. Also, an improved understanding of insect spatial distribution in the grain mass would enhance sampling strategies to allow for early detection of stored pests.

In this study, the number of insects captured in unbaited flight traps, probe traps, and grain trier and cup samples were examined: (1) to determine the optimum location of flight traps for monitoring insect populations; (2) to determine the vertical and lateral progression and distribution of insects in the upper grain mass region; and (3) to interpret probe trap catches in relation to the density of insects collected in grain trier and cup samples.

MATERIALS AND METHODS

Three steel farm bins located in North Central Oklahoma, with capacities of 141.52 metric tons (5200 bu), 68.04 metric tons (2500 bu filled to one-third capacity), and 136.08 metric tons (4500 bu) and filled with hard red winter wheat (Triticum aestivum L.) were used for this study in 1991.

Immigration and sampling method relationships 159

On May 17, before grain binning, the bins-were swept to remove grain residue and fumigated with chloropicrin.

Before wheat harvest (May 24), 1'6 unbaited flight traps (Pherocon II traps; Trece Inc. , Salinas, California) were fastened to ropes on the outside of the grain bins in the four cardinal directions (N, S, E, and W). The traps were placed at four heights: ground level, one-third bin height, two-thirds bin height, and at the outside eaves. The r-0pes passed through small pulleys at the eaves so that the traps could be lowered for retrieval. After wheat binning (June 17, July 1, and July 8 on the three farms) , four additional traps were placed inside the eaves of each bin in cardinal directions.

Plastic probe traps (WB Probe II traps; Trece Inc., Salinas, California) were placed 2.54 cm below the grain surface. Two probe traps were placed per cardinal direction at 30.5 cm from the bin wall and at one-half the bin radius. Additionally, one probe trap was placed in the center of each bin.

Samples were taken using a 1.6 m brass non-partitioned grain trier (650 g capacity; Seedburo Equipment Co. , Chicago, Illinois) beside the nine probe trap locations (starting on July 16, July 8, and July 16). Cup samples (38.1 cm deep bin cup, 265 g capacity; Seedburo Equipment Co. , Chicago, Illinois) were drawn from the grain mass at the same locations at three depths; 91.4, 61.0, and 30.5 cm, and at the surface, resulting in 36 samples per bin.

Flight traps were replaced weekly and taken to the laboratory for insect identification. At the same interval, samples taken with the grain trier and cup, and insects collected from probe traps were placed in individual plastic bags and taken to the laboratory for processing.

An inclined sieve similar to that described by White (1983) was used to separate insects from grain collected by grain trier and cup samples. Each sample was passed over the sieve three times. Insects were then identified and counted. Weight and moisture content of the grain were determined using an electronic balance (Ohaus Lume-0-Gram balance; Ohaus Scale Corp. , Florham Park New Jersey) and an hygrometer (Agromatic WK II; ASIDIC Ltd, Clear Lake, Iowa), respectively. The number of insects in trier and cup wheat samples was expressed on a per 1/2 kg (500 g) basis.

When the number of insects in trier samples averaged two R. dominica per kg in one of the bins, that bin was fumigated (August 31). The other two bins were sampled until September 27.

Thermocouples were used to determine grain temperature at the time of sample collection. Temperatures were recorded at the center of the bin and at one-half the bin radius in cardinal directions at the depths corresponding to cup samples.

Differences among species collected by the different sampling methods and trap and sampling locations were determined by Fisher's least significant difference (LSD) multiple comparison technique with a = 0.05 using SAS General Linear Models (GLM) (SAS Institute, 1988). A log transformation (log10(X + 1)) was used on insect counts before analysis to normalize data and make variances homogeneous (Little and Hills, 1978). GLM with LSD multiple comparisons were also used to analyze mean temperatures and moisture contents among locations sampled in the grain mass.

Sets of independent contrasts (SAS Institute, 1988) were computed to compare the mean number of insects sampled in cardinal directions in the grain mass. Paired !-tests were used to compare the number of insects detected at different depths throughout the sampling period. Pearson product-moment correlations were computed to determine the degree oflinear relationship between temperature and moisture content with the number of insects detected in the grain mass. Moisture contents of cup samples at the surface and at depths of 30.5 and 61.0 cm were used for correlations with probe trap catches. Probe trap action thresholds were determined by regressing the number of insects in probe traps versus the number of insects in grain trier and cup samples (PROC REG, SAS Institute, 1988).

RESULTS

Occurrence of insect species

Most of the insects collected belonged to the order Coleoptera. The most commonly occurring species in flight traps, in order of abundance, were the hairy fungus beetle, Typhaea stercorea (L.) (Mycetophagidae); Cryptolestes spp. (Cucujidae); foreign grain beetle, Ahasverus advena (Waltl) (Cucujidae); Indianmeal moth, P . interpunctella (Lepidoptera:Pyralidae); lesser grain borer, R .

160 E. L. Vela-Coiffier, et al.

dominica (Bostrichidae); and Tribolium spp. (Tenebrionidae) (Table 1). Species collected in probe traps, in order of abundance, were Cryptolestes spp., A. advena, Tribolium spp., T . stercorea, and R. dominica. In grain trier arid cup samples, the order of abundance was similar to probe traps except that R . dominica was the second most abundant species and A. advena was the fifth most abundant species.

Species collected in much lower numbers were the saw-toothed grain beetle, Oryzaephilus surinamensis (L.) (Cucujidae); rice weevil, Sitophilus oryzae (L.) (Curculionidae); larger black flour beetle, Cynaeus angustus (Le Conte) (Tenebrionidae); Corticaria sp. (Lathridiidae) and other insects of the families Derrnestidae and Anthicidae that were not further identified. Mites were also detected but no effort was made to distinguish flour or grain mites from predatory mites and they were not quantified. In addition, parasitic wasps (Hymenoptera) were collected and recorded as an order but were not identified.

Flight traps

Insects were collected in flight traps before wheat harvest. As soon as the harvest started (June 17) the average number of insects captured by flight traps increased (Fig. 1 ); greater numbers of insects occurred at all directions and heights. Traps placed on the north sides of the bins trapped more insects than those placed elsewhere (Table 2). However, during the first and second weeks of sampling, insects were captured only by flight traps on the south and west sides of the bins, respectively.

More insects were captured by fl ight traps at the eaves than at the other heights (Table 2). Before grain binning, insects were detected only in traps placed at the outside eaves. Independent contrasts indicated that the average number of insects captured by traps placed outside the bins was greater than the average number of insects found inside traps in the bins (F = 17.63; df = 1, 92; P = 0.0001). No difference occurred in numbers when comparing traps at the eaves (inside and outside) (F = 0.08; df = 1, 792; P = 0.7731). No significant interaction was found between cardinal directions and heights (F = 1.02; df = 12, 792; P = 0.4310).

Probe traps

Insects were collected from probe traps during the first week after binning (June 24). Because some grain had to be removed from the bins to facilitate sampling, probe traps were not replaced until July 1. After the fourth week of sampling (July 23) the number of insects found in probe traps increased, reaching a maximum by the beginning of September (Fig. 1).

More insects were collected by probe traps in the center of the bins than all other eight trap positions (Table 3). Sets of independent contrasts indicated no difference among the average number of insects captured when cardinal directions were compared. The average number of insects captured by all traps placed by the wall did not differ from the average number of insects found in traps at one-half the bin radius.

Table l . Mean number of insects collected in unbaited flight traps, probe traps, and grain trier and cup samples

Flight traps Probe traps Grain trier Cup Species (n = 812) (n = 270) (n = 270) (n = 1044)

Cryptolestes spp. 0.843b 210. l 70a 0.780a 0.509a ( - 0.230, 3.408) (27.278, 1575.921 ) ( - 0.106, 2.541) ( - 0.292, 2.213)

R. dominica 0.122de l.445d 0.424b 0.307b ( - 0.195, 0.538) ( - 0.393, 8.851) ( - 0.366, 2.197) ( - 0.398, 1.839)

Tribolium spp. 0.05le 5.542b 0.136c 0.206c ( - 0.169, 0.329) ( - 0.010, 46.554) ( - 0.183, 0.580) ( - 0.278, l.013)

T . stercorea l.090a 2.500c 0.025d 0.049d ( - 0.169, 4.256) ( - 0.098, 12.580) ( - 0.092, 0.158) ( - 0.178, 0.337)

A . advena 0.422c 6.814b 0.006d 0.006d ( - 0.232, l.630) (1.559, 22.860) ( - 0.053, 0.069) ( - 0.075, 0.095)

P. interpunctella 0.136d ( - 0.222, 0.660)

F 188.65 342.82 66.04 100.26 df 5, 4752 4, 1305 4, 1305 4, 5040 p 0.0001 0.0001 0.0001 0.0001

Means within columns with different letters are significantly different (Fisher's least significant difference [SAS Institute, 1988)). Means are geometric means (antilog (log,.,x) - I) transformed back to the original unit of measure. Standard deviation ranges are the geometric standard deviations about the geometric means. Lower SD = antilog (log,.,x - log10SD) - I and upper SD = antilog (log1.,x + log 10SD) - I.

(/') 1-u w (/')

z u... 0

y1

25

20

15

[:i 1 0 ID ::E ::::> z

5

Immigration and sampling method relationships

+ PROBE TRAP (y2) 0 FLIGHT TRAP (y1)

e GRAIN TRIER (y1) v DEEP BIN CUP (y1)

DATE

y2

2500

2000 ·z c !!!: CD

1500 rr1 :;u

0 .,., 1000 z

en rr1 C"') -i en

500

Fig. I. Mean total number of insects collected in flight and probe traps and trier and cup samples throughout the sampling period.

Grain triers

161

Insects were found in grain trier samples beginning the first week that samples were taken with this device (July 8). The average number of insects collected increased throughout the sampling period reaching a maximum at the end of August (Fig. 1). The reduction in the average number of insects after this date can be attributed in part to the fumigation of one heavily infested bin.

Higher numbers of insects were found in the center of the bins than in any of the other eight regions (Table 3). The average number of insects compared among cardinal directions did not differ. No significant difference was found between the average number of insects collected in traps by the wall and at one-half the bin radius.

Deep bin cups

Insects were collected in cup samples during the first week of use for this device (July 16). The

SPR 33/2- C

Table 2. Mean number of insects captured in flight traps in various directions and at various heights

Direction Height

North 5.26a Eaves (inside) 4.89a (0.88, 19.79) (1.66, 12.01)

West 3.05b Eaves (outside) 4.67a (0.30, l 1.59) (0.56, 19.55)

East 2.76bc 2/3 height 2.78b (0.34, 9.53) (0.15, 11.47)

South 2.14c 1/3 height 2.36b (0.18, 7.40) (0.13, 9 .00)

Ground 2.18b (0.21, 7 .32)

F 15.60 F 12.86 df 3, 792 df 4, 792 p 0.0001 p 0.0001

Means within columns with different letters are significantly different (Fisher's least significant difference [SAS Institute, 1988]). Means are geometric means (antilog (log10x) - I) transformed back to the original unit of measure. Standard deviation ranges are the geometric standard deviations about the geometric means. Lower SD = antilog (log1.x - log,.SD) - I and upper SD = antilog (log,.x + log 10SD) - I.

162 E. L. Vela-Coiffier, et al.

Table 3. Spatial pattern of mean numb~r of insects in the grain mass sampled with probe traps, grain trier, and cup samples

Probe trap Trier Cup

Center l47l.46a 4.05a 3.57a (347.88, 6213 .56) (0.63, 14.69) (0.29, 15.18)

North wall 2!9.95b 1.32b 1.30b (55. l 9, 867.76) (0.14, 3.70) ( - 0.19, 5.53)

North 1/2 radius 290.69b 1.35b 0.65b (77. 52, 1082.65) (0, 4.51) ( - 0.30, 2.87)

East wall 265 .16b l.67b l.02b (69.85, 998.84) ( - 0.o7, 6.62) ( - 0.18, 3.97)

East 1/2 radius 280.28b l.54b l.14b (61.47, 1265.44) (0, 5.43) ( - 0.33, 5.86)

South wall 254.6lb l.04b 0.89b (46.33, 1379.41) (0.01 , 3.13) ( - 0.20, 3.47)

South 1/2 radius 265.03b 0.90b 0.74b (58.43, 1189.94) ( - 0.19, 3.47) ( - 0.24, 2.97)

West wall 278.78b l.28b l.19b (46.43 , 1649.34) ( - 0.04,4.40) ( - 0.09. , 4.29)

West 1/2 radius 292.26b l .85b l.41b (40.24, 2084.26) (0.13, 6.22) ( - 0.19,6.22)

F 4.08 2.99 10.40 df 8, 261 8, 261 8, 1008 p 0.0001 0.0032 0.0001

F p F p F p North vs east 0.07 0.7922 0.43 0.5130 0.56 0.455 l North vs south 0.01 0.9248 l.07 0.3016 0.57 0.4505 North vs west 0.18 0.671 l 0.29 0.5897 3.30 0.0695 East vs south 0.03 0.8657 2.86 0.0922 2.26 0.1334 East vs west 0.03 0.8720 0.01 0.9084 l.14 0.2849 South vs west 0.11 0.7412 2.48 0.1165 6.62 0.0103 Wall vs 1/2 radius 0.27 0.6014 0.06 0.7999 0.98 0.3231

Means within columns with different letters are significantly different (P ~ 0.05; Fisher's least significant difference [SAS Institute. 1988]). Means are geometric means (antilog (log,0x) - 1) transformed back to the original unit of measure. Standard deviation ranges are the geometric standard deviations about the geometric means. Lower SD = antilog (log ,,,x - log ,,,SD) - I and upper SD = antilog (log ,,,x + log10SD) - l. For contrasts. df= I, 261 for probe trap and trier: df = I. I 008 for cup.

average number of insects in cup samples increased throughout the sampling period until August 28 (Fig. I). Again, the reduction in the number of insects collected after this date can be attributed in part to the fumigation of the heavily infested bin .

The average number of insects detected by cup samples in the center of bins was significantly higher than the other eight locations (Table 3). The number of insects at different depths did not differ significantly (F = 2.00; dl = 3, I 008; P = 0.1127). No significant interaction occurred for numbers of insects between sample positions and at different depths within the grain mass (F = 0.88; dl = 24, I 008; P = 0.6372).

Comparison of the number of insects in the cardinal directions showed differences only between the south and the west (Table 3). No difference was found between the mean number of insects captured at the wall and at one-half the bin radius.

Temperature and moisture content

The mean moisture contents for grain trier and cup samples were 10.83 ± 1.0% and I 0.70 ± 1.87%, respectively. No significant differences were found among the moisture contents of grain trier samples (F = 0.75; df = 648; P = 0.9994).

Grain temperatures did not differ among the five regions (center and four cardinal directions) with a mean temperature of 31.83 ± 2.53°C (F = 1.62; df = 4, 332; P = 0.1677). However, mean grain temperatures at each depth were significantly different from one another (F = 195.88; df = 3, 332; P = 0.0001). Grain temperatures averaged 27.39 ± 4.17, 32.54 ± 1.84, 33.39 ± 2.76 and 34.0 ± 2.89°C at the surface and at depths 30.5, 61.0 and 91.4 cm, respectively. No significant interaction was found between region and sample depth.

Efficiencies

Probe traps were more efficient than the grain trier and cup methods in detecting insects in stored wheat. Probe traps collected A. advena more often than grain trier and cup samples as shown by the frequency of detection (Table 4). The smallest ratios were observed for R. dominica and

Immigration and sampling method relationships 163

Table 4. Frequency of detection .g__ ·nsects in_probe traps, and grain trier and cup samples

Ratios

Species Trap: trier Trap: cup Trier: cup

Cryptolestes spp. 1.9: I 4.0: I 2.0: I A. advena 84.3: I 15 l.8: I l.8: I Tribolium spp. 4.1: I 5.2: I 1.3: I T. stercorea 15.3: I 16.7: I I.I: I R. dominica l.6: I 3.1: I l.9: I

Table 5. Ratios relating probe trap catches and numbers of insects in grain trier and cup samples

Ratios

Species Trap: trier Trap: cup Trier: cup

Cryptolestes spp. 630.8 : I 655.2: l 1.0: I A. advena l ,293 .0: I 1,293 .0: I 1.0: I Tribolium spp. 187.6: l 88 .1: I 0.5: I T . stercorea 342.0: I 102.6: I 0.3: I R. dominica 4.9: I 6.1 : I l.2: I

Cryptolestes spp. The five most abundant species were also more frequently collected in grain trier samples than in cup samples.

Ratios of probe trap catch to numbers of insects in grain trier and cup samples were largest for A . advena and Cryptolestes spp. (Table 5). The smallest ratios were obtained for R . dominica . Intermediate ratios were observed for Tribolium spp. and T. stercorea . The efficiencies of grain trier and cup samples in collecting insects varied according to species. The numbers of Tribolium spp. and T. stercorea were higher in cup samples than in trier samples, while the number of R . dominica was higher in grain trier samples. Numbers of A . advena and Cryptolestes spp. were equally collected with these two methods.

Probe trap interpretation

A linear relationship was detected between probe trap catches and numbers of insects in grain trier samples for R . dominica , Cryptolestes spp. and Tribolium spp. (Table 6). However, the models explained only 66, 25 and 29% of the variability, respectively, between these two methods. Simple linear regression between numbers of these species in probe traps and cup samples also indicated a linear relationship. In this case, the models explained 48, 34, and 28% of the variability between estimates of these two meth~ds for R . dominica , Cryptolestes spp. and Tribolium spp. , respectively. Regression coefficients for R . dominica and Tribolium spp. were higher from grain trier samples than from cup samples.

Estimates of action thresholds for probe traps using standard infestation thresholds of two insects per kg of wheat (Anonymous, 1994) are presented in Table 6. As our samples were standardized to 1/2 kg, one live insect per grain sample was used as the threshold value. The action threshold for R. dominica was 7.35 when estimated from grain trier samples and 8.04 insects per trap when estimated from cup samples. Action thresholds were higher for Cryptolestes spp. with

Table 6. Estimates of action thresholds for probe traps using simple linear regressions between probe trap catches and number of insects collected in grain trier and cup samples

Species /30 /31 r' Threshold Root MSE

Grain trier (n = 270) R . dominica 4.61 2.74 0.66 7.35 (4.76, 9.93) 21.46 Crypto/estes spp. 355.65 364.49 0.25 720.14 (558.7, 881.6) 1,329.61 Tribolium spp. 26.21 75.13 0.29 101.34 (86.56, 116.1) 82.06 All species 647.09 78.02 0.31 725.11 (562.5, 887.7) 1,317.73

Cup (n = 261) R. dominica 4.25 3.79 0.48 8.04 (4.74, 11.35) 27.00 Cryptolestes spp. 322.66 426.37 0.34 749.03 (594.6, 903.5) 1,255.55 Tribolium spp. 22.06 46.08 0.28 68.14 (56.98, 79.30) 83.48

For {30 P < 0.001 for grain trier and P < 0.02 for cup. For fJ , P = 0.0001. Threshold values are represented with a 95% confidence interval about the mean.

164 E. L. Vela-Coiffier, er al.

720.14 and 749 .03 insects per trap fol'"'gTain trier and cup samples, respectively. Tribolium spp. had action thresholds of 101.34 insects per trap from grain trier estimates and 68.14 from cup sample estimates. The intercepts in all° regressions performed were significant which indicates that when no insects of these species are found in grain trier or cup samples, probe trap catches would correspond to the value of the intercept.

Most sampling is done using the grain trier and 'infested' effectively means ~ 2 insects of any species occurring in the representative sample. Due to these considerations, all insect species were summed and the final regression under the "Grain Trier" section in Table 6 was included. This result means that if, in a series of probe traps under the conditions of our study, the average total number of insects collected approached 725 per week, it would be cause to fumigate the grain mass.

DISCUSSION

In a 4-year study of insect populations in Oklahoma stored wheat, Cuperus et al. (1986) found that R. dominica, S . oryzae, Cryptolestes spp., Tribolium spp., and 0. surinamensis were the species most frequently collected using the grain trier and deep bin cup. In contrast, our results indicated that A. advena and T. stercorea were more common than S. oryzae and 0 . surinamensis. Explanation for these differences might be attributed to variations in environmental factors and additional sampling methods used in our study. Numbers of T. stercorea increased after periods of rain and were collected in flight traps in large numbers outside the bins.

Hagstrum (1989) stated that the origin of infestations was from insects coming from outside the storage structure. Our results indicated that flight traps started detecting insects before grain harvest (Fig. 1) and that these insects were detected only by traps placed at the outside eaves. The number of insects captured increased after grain binning indicating that insects were attracted to the stored grain.

Significant differences occurred in the number of insects captured in flight traps in the various directions. These differences may have been caused by variations in wind direction and speed. However, wind data were not collected at any of the sites.

Flight traps placed inside and outside the eaves captured more insects than those at other heights. Before grain harvest, only traps placed at the outside eaves on the south and west sides captured insects. Since the bins contained no grain at this time, these insects might have landed at random. Why insects were detected at the outside eaves and not at the other trap heights outside the bins is not known. One week after binning, flight traps inside had collected insects, which indicated movement of insects to the inside of the storage structures. In contrast to our findings, Cogburn et al. (1984) found that trap catches of R. dominica were inversely proportional to the height of traps around bins containing infested rice.

Probe traps and grain trier samples collected a total of 568 and four insects, respectively, on the first date of data collection. The time lag between grain binning and the first day of insect collection may explain the existence of insects in the grain when the first samples were collected.

Wright et al. (1988) stated that, before wheat harvest, flight traps detected moderate to high numbers of insects at various locations on farms. One month after harvest, probe traps captured large numbers of insects in newly harvested grain. By September, the number of insects trapped in flight traps had decreased and populations in bins had increased. Insect catches with the different sampling methods used in our study followed similar patterns (Fig. 1).

Following the vertical distribution pattern of C. ferrugineus, Hagstrum (1989) concluded that the top layer of grain was infested first because the insect population increased earlier in this layer than in the middle layers, and there was a logarithmic decrease in the number of insects with increased distance from the grain surface. Therefore, grain infestation occurred after grain was stored. In our study, cup samples taken at the grain surface contained higher densities of insects than those taken at 61.0 and 91.4 cm during the first week and higher densities than those taken at 30.5 cm during the second week. By August, the density of insects at the surface was lower than the density at the other depths. This indicated that infestation occurred after grain binning. However, a definite pattern did not exist. Because of the time lapse between grain binning and sampling, the first phase of insect infestation was not observed.

Numbers of insects collected by the three sampling methods used in the grain mass indicated

Immigration and sampling method relationships 165

that insects accumulated in the center of the bins. Temperature and moisture content in the central core did not differ from other regioI?-s in the grain mass. Therefore, the higher number of insects in the center cannot be attributed to these two factors. Hagstrum (1987) also found that during the first 12 week:s of storage, neither the temperature nor the moisture content was sufficiently different to explain why insects favored the center of bins. He concluded that the level of fines might explain the concentration of insects in the center early in the storage period. The central core is known to have the highest level of dockage (Noyes et al., 1988).

Probe traps were most efficient in detecting A. advena and Cryptolestes spp. Greater probe trap efficiencies for these species might occur because they are active species. In a laboratory study, Fargo et al. (1989) found that significantly more C. ferrugineus were caught in probe traps than T. castaneum, R. dominica, and S. oryzae. In contrast, grain trier and cup samples were more efficient in detecting species such as R. dominica , which represents the second most abundant species sampled using these methods. Subramanyam and Harein (1989) stated that probe traps were less likely to capture insect species that are less mobile in grain, release aggregation pheromones (e.g. R. dominica), or feed internally on kernels (e.g. R . dominica and Sitophilus spp.).

Probe trap action thresholds were similar using numbers of insects in grain trier and cup samples for Cryptolestes spp. and R. dominica (Table 6). However, large differences were found for Tribolium spp. These differences may be explained by differences in the two grain sampling techniques: the grain trier sampled a continuous column of wheat whereas the cup sampled at four depths per location and was more representative of an individual population. Although a linear relationship existed between probe trap catches and numbers of insects in grain trier and cup samples for these species, the values of r2 indicated that trap catch is a poor indicator of population density as estimated by grain trier and cup samples. Similarly, Reed et al. (1991) found that correlation coefficients described less than half of the variability between insects in probe traps and grain samples, taken with a vacuum probe before November.

The numbers of insects collected in traps and from grain samples varied according to the sampling method used and according to the site of collection. Knowledge of the relationships between trapping methods aids in interpreting counts of insects and provides valuable information in determining an appropriate pest management strategy.

Acknowledgements- Special thanks are extended to John Snyder, Ron Schultz and Ed Long for their permission and cooperation to sample their stored wheat. We also thank R. C. Berberet and A. K. Dowdy for carefully reviewing an earlier draft of this manuscript. This article was approved for publication by the Director, Oklahoma Agricultural Experiment Station. This research was supported under project H 1825.

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