Susceptibility of Eight US Wheat Cultivars to Infestation by ...

POPULATION ECOLOGY

Susceptibility of Eight U.S. Wheat Cultivars to Infestation by Rhyzopertha dominica (Coleoptera: Bostrichidae) MICHAEL D. TOEWS, GERRIT W. CUPERUS,

AND

THOMAS W. PHILLIPS

Department of Entomology and Plant Pathology, Oklahoma State University, 127 Noble Research Center, Stillwater, OK 74078

Environ. Entomol. 29(2): 250Ð255 (2000)

ABSTRACT Cultivars of wheat, Triticum aestivum L., were assessed to determine their respective level of resistance to lesser grain borers, Rhyzopertha dominica (F.), in postharvest storage. Cultivars were representative of hard red winter, soft red winter, white spring, and durum wheat classes currently grown in the United States. Samples of each cultivar were maintained at 30.0⬚C and 70% RH and infested with 2- to 3-wk-old adult beetles for 1 wk. Adult progeny were counted at the end of one life cycle. Two temperatures, 27.0 and 34.0⬚C, were studied to examine the role of temperature (calculated in degree-days) in development. This experiment was conducted three times under similar conditions. Cultivars harboring a large quantity of progeny were considered more susceptible than those cultivars in which fewer progeny completed their life cycle. Each cultivar was analyzed for single kernel properties such as hardness, protein, and diameter. Wheat cultivar had a signiÞcant inßuence on quantity of progeny in all experiments. There were no signiÞcant effects on survivorship of progeny as a result of temperature when calculated in degree-days. Cultivars with smaller kernels were more susceptible to development of larger generation sizes in experiment 1 but not in the other two experiments. A kernel size experiment using large and small kernels from the same cultivar suggested that larger quantities of progeny are produced on small kernels compared with large kernels. Individual beetle weights were not inßuenced by temperature or cultivar. These results imply that stored grain managers should be aware of potential differences in susceptibility, attributable to wheat cultivar, to lesser grain borer infestations. KEY WORDS Triticum aestivum, lesser grain borer, stored-products, susceptibility, temperature

THE UNITED STATES is a major grower of hard red winter and spring wheat, Triticum aestivum L., and stores one billion bushels (Kenkel et al. 1994) of the estimated 2.1 billion bushels harvested each year. The primary purpose of grain storage is to increase the net value of the crop by holding grain until prices are more favorable (Anderson et al. 1995). However, storing grain can also cause the overall quality of the commodity to decrease, thereby offsetting positive economic returns. Common storage problems include mold and insect infestations (Cuperus et al. 1990). A survey in 1987 and 1993 of elevator managers determined that insects were perceived as their worst problem (Cuperus et al. 1990, Kenkel et al. 1994). Wheat cultivars are developed for a wide variety of locations and applications in crop production. Improved cultivars are bred to increase resistance to diseases and insects, adapt to new environments, increase quantities of certain nutrients, change growth habits, and to increase productivity (Martin et al. 1976). Although increased yield is often the target of crop improvement, it is important that the commodity not be highly susceptible to pathogens or grain degrading components capable of negating any increases to net returns. Wheat cultivars have different levels of resistance to stored product insects (Singh and Mathew 1973, Phadke and Bhatia 1975, Amos et al.

1986, Sinha et al. 1988, McGaughey et al. 1990, CortezRocha et al. 1993). The lesser grain borer, Rhyzopertha dominica (F.), is a serious pest of stored wheat and is cosmopolitan in distribution (Potter 1935). The female lays eggs singly or in small groups exterior to the grain; eggs can be attached to a kernel or laid loosely in the grain. After hatching, the Þrst instar immediately bores into the kernel where it completes Þve instars. Lesser grain borers pupate inside the kernel before adults emerge and chew their way out of the kernels. This insect is extremely damaging because it feeds exclusively on the grain as both a larva and adult. Total developmental time averages 25 d at 34.0⬚C and 70% RH (Birch 1945a). Optimum temperature for development is 35.0⬚C (Birch 1945b). Temperature in a grain storage facility is often unregulated unless aeration is used to cool the grain as part of an integrated pest management program. Aeration is done by forcing cooler outside air through a grain mass using electric fans. The reduction in grain temperature can directly affect the ability of pest insects to reproduce in the grain mass (Cuperus et al. 1986, Kenkel et al. 1994). This study examines how grain temperature, inßuenced by the management practice of aeration, could interact with grain cultivar to affect pest population development.

0046-225X/00/0250Ð0255$02.00/0 䉷 2000 Entomological Society of America

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TOEWS ET AL.: SUSCEPTIBILITY OF WHEAT CULTIVARS TO R. dominica

For host plant resistance studies in postharvest research, the plant is actually represented by the dried kernel of grain instead of a green plant. Host resistance in this research relates to qualities of kernels that reduce infestation or damage by insects. This study was conducted to determine relative levels of host resistance to infestation by the lesser grain borer among eight cultivars of wheat when stored at two temperatures. The speciÞc objectives were to compare yield of adult lesser grain borer progeny in eight wheat cultivars, compare the effects of temperature on the survivorship of lesser grain borer progeny, correlate levels of progeny production by lesser grain borers with physical and chemical characteristics of wheat cultivars, and assess the effect of wheat cultivar on dry weights of adult beetles. Materials and Methods Cultivar Resistance Experiments. Eight wheat cultivars were procured from commercial seed producers and various foundation seed stocks across the United States. The cultivars selected represented divergent genetic lineages and were selected from Þve major classes of wheat. Cultivars included the hard red winter selections ÔTriumph 64Õ and Ô2180Õ (Oklahoma Foundation Seed Stocks, Stillwater, OK), soft red winter selections included ÔMadisonÕ (Arkansas Agricultural Experiment Station, Fayetteville, AR) and ÔCoker 916Õ (Novartis Seeds, Bay, AR), durum cultivars were ÔMunichÕ and ÔMonroeÕ (North Dakota Seedstocks Project, Fargo, ND), the hard red spring selection was ÔNewanaÕ (Montana Foundation Seedstocks, Bozeman, MT), and the white spring cultivar was ÔWawawaiÕ (Washington State Crop Improvement Association, Pullman, WA). Wheat samples were frozen at ⫺20.0⬚C for 1 wk after receipt to eliminate the possibility of previous infestations. Seed was then cleaned with multiple passages over a seed cleaner (Clipper model M2B, Bluffton Agricultural Industrial, Bluffton, IN). All experiments were conducted using subsamples of a single lot of seed. The bulk seed was stored in plastic bags at 10.0⬚C until used in experiments. Before each experiment the moisture content of the wheat was adjusted to 13 ⫾ 0.5% by adding water or allowing ambient drying. Moisture contents were determined using the dry-weight method ASAE standard (Society of Agricultural Engineers 1996). Before starting experiments, subsamples of each cultivar were analyzed at the Oklahoma State University Wheat Quality Laboratory for physical and chemical properties. Protein content was determined using the Kjeldahl method (AACC 1983). Hardness measures were determined using both near-infrared reßectance (Technicon InfraAlyzer model 400, Bran ⫹ Luebbe GmbH. Norderstedt, Germany) and a single kernel hardness tester (model 594, Pertain, SpringÞeld, IL). Average kernel weight, peak force required for crush, conductance, total force required for crush, time required to completely crush, and diameter were obtained using the single kernel hardness tester. All measurements given by the hardness tester, except

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average weight and diameter, are unitless and therefore are intended solely for comparison among cultivars within that parameter. Experiments were conducted with 100 ⫾ 1.0 g of wheat in 0.24 liter glass canning jars. Fifty 2- to 3-wkold unsexed lesser grain borer adults from stock cultures reared on hard red winter wheat were introduced into each jar. Ventilation was permitted through Þlter paper Þtted into lids. All jars were initially held at 30.0 ⫾ 0.5⬚C in one chamber for 1 wk to allow a consistent opportunity for oviposition by adults in each jar. Adult insects were removed after the ovipositional period by gently sieving the grain over a 1.4-mm standard sieve. Beetles were collected below on a 0.425-mm sieve, while the dust fell through both sieves to be collected in a bottom pan. All grain and dust were returned to the jars. Equal numbers of jars of each cultivar were then placed in each of four upright environmental chambers (model I-35 LVL, Percival ScientiÞc, Boone, IA). Two chambers were set at 27.0 ⫾ 0.5⬚C and two at 34.0 ⫾ 0.5⬚C, all with 70 ⫾ 5% RH. Hygrothermographs were maintained inside each chamber to monitor and record temperature and humidity for the duration of all experiments. Completely dark conditions were used throughout all experiments. A degree-day model (Subramanyam et al. 1990) was used to assure similarity in experimental protocol for measuring survivorship at the different temperatures. In this study, insects were allowed a total of 900 DD for development; adult progeny were collected at four intervals within the 900 DD. Data analysis was performed only on the total number of progeny collected and not broken down by individual collection interval. The experimental design was a split plot arrangement in which the main unit effect was temperature and the subunit effect was cultivar. Each chamber represented one application of temperature. To increase the number of observations at the subplot level, cultivar was replicated four times within each main plot. Therefore, each experiment included two replications of temperature at each temperature and 16 replications of cultivar (4 replications within each of the four chambers). This experiment was conducted three times over the course of 8 mo, resulting in experiments 1, 2, and 3. The inßuence of cultivar on beetle size was determined by weighing beetles that emerged from experiment 2 that were ⱕ1 wk old. Specimens were killed by freezing and cleaned for 5 min in an ultrasonic cleaner (model FS-30, Fisher, Pittsburgh, PA) Þlled with water. Clean and intact beetles from each jar were placed in groups of 20 into 3-ml shell vials. Vials were placed in a laboratory oven (model 18A, Blue M, Blue Island, IL) at 130.0⬚C for 24 h. Immediately before vials were removed from the oven, lids were placed on each vial. Vials were placed in a dessicator with aluminum oxide for 30 min before weighing on a microbalance (model M3P, Sartorius, Bohemia, NY). Kernel Size Experiment. Because experiment 1 indicated that quantity of lesser grain borer progeny was related to kernel size, a separate experiment was con-

NIR, near-infrared reßectance, a relative measure of hardness, machine does not provide standard deviation; SK, single kernel hardness, unitless. a n for each cultivar ⫽ 300 kernels, except NIR is based on an ⬇4-g sample and percentage protein is based on a 1-g sample. Numbers in parentheses are standard deviations generated by the machine. b Average kernel weight of each kernel. c Average peak force required to crush each kernel, unitless. d Conductance of each kernel, unitless. e Average area under the load proÞle, unitless. f Length of time required to complete crush of each kernel, unitless. g Average diameter of each kernel.

41.0 10.3 (14.7) 12.9 33.5 (7.9) 564.0 (196.3) 1,630.5 (164.2) 144,920 (48,600) 389.6 (40.8) 2.3 (0.4)

Madison Coker 916

26.3 7.2 (13.4) 10.6 29.3 (6.4) 482.5 (146.7) 1,846.5 (146.5) 118,232 (36,100) 375.1 (39.4) 2.1 (0.4) 75.3 66.0 (16.6) 16.7 21.2 (5.5) 590.8 (198.1) 1,751.6 (184.6) 122,356 (42,800) 341.0 (36.3) 1.8 (0.3)

Truimph 64 Munich

179.8 93.6 (13.2) 14.9 39.4 (11.0) 1,629.1 (507.4) 1,082.6 (163.2) 339,830 (125,000) 417.0 (49.3) 2.5 (0.5)

Monroe

140.1 94.4 (17.0) 15.0 36.3 (12.8) 1,516.0 (594.9) 985.2 (212.3) 316,556 (150,000) 404.5 (58.5) 2.4 (0.6)

Newana

101.9 58.6 (14.0) 11.5 36.6 (7.7) 1,070.1 (319.7) 1,605.6 (136.1) 269,260 (85,500) 417.2 (43.9) 2.5 (0.4) 130.8 79.5 (30.6) 11.8 30.6 (6.5) 997.7 (307.4) 1,224.3 (146.6) 230,589 (74,000) 383.9 (40.3) 2.2 (0.4)

2180 Parameter

NIR hardness Avg SK hardness % protein Avg wt, mgb Avg peak forcec Avg conductanced Avg areae Avg lengthf Avg diam, mmg

a

Physical and chemical parameters of each wheat cultivar Table 1.

42.0 26.0 (15.0) 11.1 42.2 (11.5) 919.8 (328.5) 1,409.4 (191.7) 242,000 (96,600) 417.5 (52.4) 2.5 (0.5)

ENVIRONMENTAL ENTOMOLOGY

Wawawai

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Vol. 29, no. 2

ducted to assess the role of kernel size within a single cultivar. A hard red winter wheat cultivar, Ô7853Õ, was obtained directly from a local grower. Individual kernels were separated into two size groups using slotted hand sieves; slot length of each sieve was 19.05 mm. Large kernels were collected between a 3.18-mm and a 2.78-mm sieve; small kernels were collected between a 2.38-mm and a 1.98-mm sieve. Equal quantities by weight of the two kernel sizes were the treatments for this experiment. Both kernel sizes were then adjusted to 13.0% moisture content by adding water or drying at room temperature. The experimental design was a randomized complete block with a total of 36 replications per treatment. The entire experiment was located on a centered shelf in an upright environmental chamber. Throughout the experiment, environmental conditions were held constant at 34.0 ⫾ 0.5⬚C and 70 ⫾ 5% RH. Forty unsexed adult lesser grain borers, 2Ð3 wk old, were placed on 75 ⫾ 1.0 g of wheat. After a 4-d ovipositional period, adults were sieved out of the grain and the beetle progeny were allowed to mature for 6 wk at which time adult progeny were collected, counted, and analyzed. Data Analysis. Data for all experiments were analyzed using PC-SAS version 6.11 (SAS Institute 1994). All progeny counts were transformed using a squareroot transformation (Zar 1984) to correct for heteroscedastic data. Treatment means were analyzed using PROC GLM (SAS Institute 1994) and separated using the least signiÞcant difference (LSD) method (Steel and Torrie 1980) at the ␣ ⫽ 0.05 level. Data on mean kernel characteristics and mean progeny production were analyzed via regression using PROC REG (SAS Institute 1994). Results Cultivar Resistance Experiments. Raw data from the analyses of individual wheat cultivar parameters are summarized in Table 1. Hardness values, obtained by both the near-infared reßectance and single kernal hardness methods, varied widely among cultivars. Coker 916 had a near-infared reßectance value of 26.3, whereas Munich had a value of 179.8. Percentage protein, the only chemical property measured, varied from 10.6% (Coker 916) to 16.7% (Triumph 64). The values for kernel diameter ranged from 1.8 mm in ÔTriumph 64Õ to 2.5 mm in Newana, Munich, and Wawawai. Temperature did not contribute signiÞcantly to differences in survivorship in experiment 1 (F ⫽ 3.57; df ⫽ 1, 2; P ⫽ 0.199). However, the subplot comparison, cultivar, was signiÞcant (F ⫽ 3.01; df ⫽ 7, 98; P ⫽ 0.007) (Table 2). Triumph 64 and Coker 916 harbored more progeny than the remaining cultivars except Monroe. The difference between the cultivar Triumph 64, in which the most beetles were produced, and that with the least, Wawawai, was ⬎1.5 times. Cultivar parameters found to be signiÞcant predictors of quantity of progeny in experiment 1 were average kernel weight, average length, and average

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TOEWS ET AL.: SUSCEPTIBILITY OF WHEAT CULTIVARS TO R. dominica

Table 2. Number of lesser grain borer progeny per cultivar during experiment 1 No. of beetles

Cultivar

Mean

⫾

SE

939.7 866.3 700.8 645.3 624.3 570.0 569.6 568.0

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

134.2a 77.1a 76.9ab 104.5b 84.5b 69.9b 53.4b 66.8b

Triumph 64 Coker 916 Monroe Munich 2180 Madison Newana Wawawai

253

Table 4. Number of lesser grain borer progeny per cultivar and dried weights of 20 beetles during experiment 2 Cultivar

Mean ⫾ SE no. of beetlesa

Mean ⫾ SE wt, mgb

Coker 916 Monroe Munich 2180 Wawawai Triumph 64 Madison Newana

349.1 ⫾ 46.2a 312.1 ⫾ 26.0ab 284.6 ⫾ 50.3abc 277.6 ⫾ 56.1bc 231.6 ⫾ 29.9cd 218.9 ⫾ 32.2cd 176.3 ⫾ 21.1d 122.8 ⫾ 23.5e

7.96 ⫾ 0.17 7.98 ⫾ 0.11 8.03 ⫾ 0.17 8.37 ⫾ 0.12 8.45 ⫾ 0.21 8.46 ⫾ 0.12 8.48 ⫾ 0.17 8.49 ⫾ 0.20

Means followed by the same letter are not signiÞcantly different (n ⫽ 16, P ⬎ 0.05, LSD test).

Means followed by the same letter are not signiÞcantly different (n ⫽ 16, P ⬎ 0.05, LSD test). n ⫽ 16.

diameter (Table 3). However, further investigation revealed that average weight was highly correlated with average diameter (F ⫽ 110.15; df ⫽ 1, 6; P ⬍ 0.001; r2 ⫽ 0.948; n ⫽ 8), average area (F ⫽ 6.84; df ⫽ 1, 6; P ⫽ 0.040; r2 ⫽ 0.533; n ⫽ 8), and length of crush (F ⫽ 107.32; df ⫽ 1, 6; P ⬍ 0.001; r2 ⫽ 0.947; n ⫽ 8). These characteristics all describe the physical size of the kernel. Regression analysis found that all other kernel characteristics were not signiÞcantly related to number of lesser grain borer progeny. Quantities of adult progeny were greater in experiment 1 compared with the other experiments. Temperature was not a signiÞcant factor (F ⫽ 1.57; df ⫽ 1, 2; P ⫽ 0.337) in the survivorship of progeny during experiment 2, but cultivar was a signiÞcant factor (F ⫽ 11.79; df ⫽ 7, 98; P ⬍ 0.001). The cultivars Coker 916 and Monroe harbored the greatest number of progeny; less progeny emerged from Newana than any other cultivar (Table 4). No signiÞcant relationships were detected with regression analysis between any of the single kernel characteristics or cultivar parameters and number of lesser grain borer progeny. Weights of adult dried beetles from experiment 2 were not signiÞcantly different between temperatures (F ⫽ 0.62; df ⫽ 1, 2; P ⫽ 0.515) or among cultivars (F ⫽ 0.96; df ⫽ 7, 84; P ⫽ 0.463). Mean weights for 20 beetles ranged from 7.96 mg in Coker 916 Ð 8.49 mg in Newana (Table 4). Total progeny production from experiment 3 was similar in magnitude to experiment 2. In experiment 3, temperature was not a contributing factor to survivorship in quantity of progeny (F ⫽ 0.17; df ⫽ 1, 2; P ⫽

0.719). There were signiÞcant differences in progeny numbers attributable to cultivars (F ⫽ 4.19; df ⫽ 7, 98; P ⬍ 0.001). Munich, Monroe, and Coker 916 harbored the most progeny, whereas Madison and Newana harbored the fewest (Table 5). Similar to experiment 2, no signiÞcant relationships were detected with regression analysis between any of the single kernel characteristics or cultivar parameters and quantity of progeny. Kernel Size Experiment. Results from the test on kernel size using Ô7853Õ wheat indicated that more lesser grain borer progeny were recovered from small kernels than large kernels although there was no statistical difference (F ⫽ 4.11; df ⫽ 1, 8; P ⫽ 0.077). The number of progeny emerged from small kernels was 266.8 ⫾ 17.8 (mean ⫾ SE), whereas the number emerged from large kernels was only 219.9 ⫾ 17.3. The average weight of small kernels was 23.31 ⫾ 0.22 mg, whereas larger kernels had a mean weight of 40.76 ⫾ 0.27 mg (n ⫽ 300).

Table 3. Significant linear regressions between mean physical wheat cultivar parameters and mean number of lesser grain borer progeny during experiment 1 Parameter

r2

F

P

Slope (SE)

y-intercept (SE)

Avg wta Avg lengthb Avg diamc

0.609 0.631 0.623

9.348 10.265 9.931

0.022 0.019 0.020

⫺0.302 (0.099) ⫺0.076 (0.024) ⫺7.831 (2.485)

35.43 (3.37) 55.09 (9.32) 43.32 (5.75)

For each regression df ⫽ 1, 6; n ⫽ 8. Kernel weight in milligrams. Length of time required to complete crush of each kernel, unitless. c Average diameter of each kernel in millimeters. a

b

Discussion Differences in total number of progeny in experiment 1 (Table 2) versus experiments 2 and 3 (Tables 4 and 5) were possibly the result of environmental factors. Experiment 1 was conducted during the fall season of the year, and experiments 2 and 3 were conducted during the winter and spring, respectively. Although the insects were from a laboratory colony and the experiments were conducted in growth chamTable 5. Number of lesser grain borer progeny per cultivar during experiment 3 Cultivar Munich Monroe Coker 916 Triumph 64 2180 Wawawai Madison Newana

No. of beetles Mean 370.1 368.3 356.9 328.1 290.6 284.0 239.9 214.3

SE ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

57.2a 54.9a 45.2a 51.0ab 32.2ab 38.9ab 33.4bc 46.3c

Means followed by the same letter are not signiÞcantly different (n ⫽ 16, P ⬎ 0.05, LSD test).

254

ENVIRONMENTAL ENTOMOLOGY

bers, the laboratory population may still exhibit physiological phenomena characteristic of wild populations such as variation in reproduction across seasons. Blake (1958, 1959) found that Anthrenus verbasci (L.) (Coleoptera: Dermestidae) larvae possess an endogenous circannual rhythm. This circannual rhythm dictates that the larvae always pass through a dormant period even when bred under constant conditions in the laboratory. Another possibility is that P1 adults used in experiment 1 may have been reared in different media than experiments 2 and 3. This study consistently showed that temperature, at least between 27.0 and 34.0⬚C, does not affect survivorship of lesser grain borer progeny when incubated for the same number of degree-days. However, based on calendar time there was an obvious difference in rate of development between the two temperatures. It took 17 d longer for 50% of the progeny to have emerged at 27.0⬚C than at 34.0⬚C. Obviously, grain temperature should be considered an important management tool because cooler temperatures reduce population buildup in calendar time. This study clearly showed that cultivars of wheat vary signiÞcantly in their susceptibility to infestation by lesser grain borers (Tables 2, 4, and 5). In all three experiments Newana, Madison, and Wawawai were grouped similarly among those producing the least progeny. No single cultivar demonstrated complete resistance to lesser grain borer. Coker 916 and Monroe generally harbored large numbers of progeny, whereas progeny numbers from Triumph 64 were inconsistent. Although new wheat cultivars are bred for resistance to insects and disease problems in Þeld settings, very limited consideration has been given to postharvest insect issues. As evidenced by the wide gradient of susceptibility among cultivars in this research, cultivars can have a signiÞcant inßuence on the apparent resistance or lack thereof to lesser grain borer (Bhatia and Gupta 1969). Experiment 1 and the kernel size experiment revealed that physical kernel size was responsible for limiting progeny numbers. Kernel size is a result of positioning of the kernel on the plant while still in the Þeld (Kirby 1974, Simmons and Moss 1978); however, chemical composition within kernels of the same cultivar is similar. If each kernel represents an ovipositional or larval infestation site, then fewer eggs may be laid in collections of large kernels because fewer ovipositional sites are available in the same jar. Crombie (1944) documented intraspeciÞc competition, resulting in mortality of one individual, between lesser grain borers when tunneling in the same kernel. Because there are fewer kernels in a given weight of a largekernel cultivar compared with the same weight of a small-kernel cultivar, there are potentially fewer total sites for larval development. In experiments 2 and 3, physical kernel size did not relate well with quantity of progeny. The low numbers of progeny produced in experiments 2 and 3 may have contributed to the lack of a signiÞcant relationship with kernel characteristics because there were less opportunities for signiÞcant differences. Previously,

Vol. 29, no. 2

Amos et al. (1986) found that grain weight was a signiÞcant factor in reproduction by lesser grain borers; however, McGaughey et al. (1990) found the contrary. The inconsistency noticed among experiments in this project could be similar to conßicting conclusions in the literature. Hardness is a widely measured parameter of kernels that is also related to different functional properties of each wheat class (Pomeranz et al. 1988). Hardness can strongly inßuence the ability of other stored product insects, such as Sitophilus oryzae (L.), to reproduce in stored wheat (McGaughey et al. 1990). In this study, hardness was not signiÞcantly related with reproduction of lesser grain borers. This Þnding supports the conclusion of numerous other authors studying lesser grain borer (Bhatia and Gupta 1969, Amos et al. 1986, Sinha et al. 1988, McGaughey et al. 1990). Amos et al. (1986) found protein content helped to predict quantity of lesser grain borer progeny. None of our three experiments support this hypothesis. Because testing of physical characteristics and protein content did not relate to progeny potential, other factors need to be evaluated. A good starting place for such research might include other nutrients or allelochemicals. Arnason et al. (1992) found that high phenolic content of maize, Zea mays L., grain was strongly correlated with resistance to Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae). Biochemical resistance to lesser grain borer could possibly be bred into wheat cultivars. Cinco et al. (1991) were successful at demonstrating that water extracts of resistant wheat cultivars inhibited the amylase activity of lesser grain borers in vitro. Molecular genetic techniques have proven successful for other pest species in postharvest storage. Pueyo et al. (1995) demonstrated the ability of proteinaceous inhibitors from Phaseolus vulgaris L. (Leguminosae) to be effective in vitro against ␣-amylase of the red four beetle, Tribolium castaneum (Herbst). They also demonstrated the technique by adding 1% of the inhibitors to a diet consisting of wheat ßour plus germ and slowed the development of T. castaneum larvae. Zhu et al. (1996) demonstrated that a gene speciÞc to N-acetylglucosamine from Griffonia simplicifolia Baillon (Leguminosae) had insecticidal properties to the cowpea weevil, Callosobruchus maculatus (F.). However, additional factors that contribute to lesser grain borer progeny production and survivorship should be determined before undertaking breeding projects to attempt to inßuence population dynamics.

Acknowledgments We are grateful to Edmond Bonjour (Department of Entomology and Plant Pathology, Oklahoma State University) for technical support and helpful advice during the course of this project. We also thank Mark Payton (Department of Statistics, Oklahoma State University) for help with the experimental design and analysis. Richard Berberet and Phillip Mulder (Department of Entomology and Plant Pathology, Oklahoma State University) provided helpful comments and

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TOEWS ET AL.: SUSCEPTIBILITY OF WHEAT CULTIVARS TO R. dominica

criticism of the manuscript. This work was supported by the Oklahoma Agricultural Experiment Station, Stillwater, OK.

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