Mobility parameters of Tribolium castaneum and Rhyzopertha dominica populations with different susceptibility to phosphine

Phosphine (PH₃) is the most commonly used fumigant to protect stored products from arthropod infestations worldwide. Our knowledge about the behavioral differences between phosphine-resistant and -susceptible stored product pest populations is limited. This study evaluated differences in mobility and behavior of populations of two major stored product insects, Tribolium castaneum (Herbst) and Rhyzopertha dominica (F.), which have different susceptibility to phosphine. In this regard, laboratory bioassays in Petri dish arenas were designed to determine if phosphine resistance has an impact on the walking and mobility behavior of adult beetles of both species. Results indicated that there were significant differences between resistant and susceptible populations for both species. Regarding velocity, R. dominica susceptible individuals moved faster than resistant ones. However, the resistant population showed reduced activity for several parameters tested compared to the susceptible population. Similar trends were also noted for T. castaneum. Knowledge of these parameters should be further utilized in management tactics, as resistant populations may behave in a different way in key management indicators such as trapping and sampling, as compared with susceptible ones.     

Field validation of phosphine efficacy on the first recorded resistant strains of Sitophilus granarius and Tribolium castaneum from the Czech Republic

This study first estimated the current state of phosphine sensitivity (using a knock-down/KT₁₀₀/Degesch kit) in Sitophilus granarius (23 strains) and Tribolium castaneum (8 strains) in Czech Republic grain stores. The resistance of S. granarius (21.7% resistant strains; coefficient of resistance KT₁₀₀ ranged from 0.5 to 2.3 among strains) was substantially lower and less frequent than that of T. castaneum (87.5% resistant strains; coefficient of resistance KT₁₀₀ ranged from 0.9 to 52.5 among strains). The phosphine efficacy of the laboratory and field (i.e., resistant) pest strains was validated during commercial fumigation when suboptimal tarpaulin sealing resulted in low-concentration phosphine exposure (Ct products ranged from 5.9 to 7.4 g*hr/m³). Although even low-dose fumigation led to 100% adult mortality of both laboratory and field strains of S. granarius and laboratory strains of T. castaneum, the mortality of the field strain of T. castaneum ranged from 47% to 95%. Larval emergence from the fumigated commodity samples with pest eggs was zero or near zero for laboratory strains, while 1.3–6.0 (S. granarius) and 63.7–80.00 (T. castaneum) field-strain larvae emerged per sample (100 g). This study shows that although a high proportion of the tested pest populations were still sensitive, several T. castaneum populations showed an elevated level of resistance that may decrease field fumigation efficacy, especially under suboptimal phosphine dosage conditions.     

Survival and reproduction of Tribolium castaneum (Herbst), Rhyzopertha dominica (F.) and Sitophilus oryzae (L.) following periods of starvation

This study determined the starvation tolerance of Tribolium castaneum (Herbst), Rhyzopertha dominica (F.) and Sitophilus oryzae (L.) in terms of both adult survival and reproduction, the impact of starvation on reproduction not having been studied before. Experiments were conducted at 30 °C and 55% or 70% r.h. using a laboratory strain and a field strain of each species. The number of progeny was a better indicator of the impact of starvation on a species than adult survival. Tribolium castaneum was the most tolerant species, requiring up to 35 d starvation before no progeny were produced. Rhyzopertha dominica and S. oryzae required up to 8 d starvation before no progeny were produced. The results suggest that hygiene will have a greater impact on populations of S. oryzae and R. dominica than T. castaneum.     

Developmental and population growth rates of phosphine-resistant and -susceptible populations of stored-product insect pests

Phosphine resistance positively contributes towards an individual's fitness under phosphine fumigation. However, phosphine resistance may place resistant individuals at a fitness disadvantage in the absence of this fumigant, which can be exploited to halt or slow down the spread of resistance. This study aimed to determine if there is a fitness cost associated with phosphine resistance in populations of the red flour beetle (Tribolium castaneum (Herbst)), the lesser grain borer (Rhyzopertha dominica (F.)) and the sawtoothed grain beetle (Oryzaephilus surinamensis (L.)). The developmental rate and population growth of phosphine-resistant and -susceptible populations of these three species of stored-product insects were therefore determined under phosphine-free environment. The majority of the phosphine-resistant populations exhibited lower developmental and population growth rates than the susceptible populations indicating that phosphine resistance is associated with fitness cost in all three species, which can potentially compromise the fixation and dispersal of the resistant genotypes. Nonetheless, some phosphine-resistant populations did not show a fitness cost. Therefore, resistance management strategies based on suppression of phosphine use aiming at eventual reestablishment of phosphine susceptibility and subsequent reintroduction of this fumigant will be useful only for insect populations exhibiting a fitness cost associated with phosphine resistance. Therefore recognition of the prevailing phosphine-resistant genotypes in a region is important to direct the management tactics to be adopted.     

Responses of phosphine susceptible and resistant strains of five stored-product insect species to chlorine dioxide

Adults of phosphine susceptible laboratory strains and phosphine resistant field strains of five stored-product insect species were exposed in vials with 0 or 10 g of wheat for different time periods to 0.54 g/m³ (200 ppm) of chlorine dioxide gas. After exposure, adult mortality was determined 5 d later at 28 °C and 65% r.h. The 5-d mortality was 100% in laboratory and field strains of the red flour beetle, Tribolium castaneum (Herbst); sawtoothed grain beetle, Oryzaephilus surinamensis (L.); lesser grain borer, Rhyzopertha dominica (F.); maize weevil, Sitophilus zeamais Motschulsky; and rice weevil, Sitophilus oryzae (L.) that were exposed in vials with 10 g of wheat to chlorine dioxide for 26, 16, 24–34, 18–24, and 15–18 h, respectively. Corresponding exposure durations for these species and strains in vials without wheat were 15, 3, 18–20, 7–15, and 5–7 h, respectively. Dosages of chlorine dioxide producing 99% mortality (LD₉₉) of T. castaneum, O. surinamensis, R. dominica, S. zeamais, and S. oryzae strains in vials with wheat ranged from 14.79−22.57, 8.20–8.41, 15.79–21.60, 10.66–14.53, and 7.67–12.20 g-h/m³, respectively. In vials without wheat, corresponding LD₉₉ values for T. castaneum, R. dominica, and S. zeamais strains were 6.51–8.66, 11.46–23.17, and 5.79–10.26 g-h/m³, respectively. LD₉₉ values for O. surinamensis and S. oryzae could not be computed, because of 100% mortality after a 3–5 h exposure to chlorine dioxide. No adult progeny production of T. castaneum and O. surinamensis was observed after 8 weeks in control and chlorine dioxide-exposed samples. Adult progeny production of Sitophilus spp. was found only in the control samples. The dosage for 99% adult progeny reduction relative to control for R. dominica strains ranged from 10.07 to 18.11 g-h/m³. Chlorine dioxide gas is effective in killing phosphine susceptible and resistant strains of five stored-product insect species and suppressing adult progeny production of three out of the five species.     

Distance and height of attraction by walking and flying beetles to traps with simultaneous use of the aggregation pheromones from Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) and Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae)

Adults of Tribolium castaneum (Herbst) and Rhyzopertha dominica (F.) biosynthesize aggregation pheromones 4,8-dimethyldecanal and dominicalure-1/dominicalure-2, respectively. These pheromones are commonly used independently, and their simultaneous use has not been adequately studied. Furthermore, information on trapping flying R. dominica in pheromone traps is minimum. Therefore, the objectives were to evaluate distance of attraction of T. castaneum and R. dominica adults to traps having both pheromones, and height of R. dominica adult attraction to traps with its pheromone lure alone. In first experiment, both pheromones were deployed simultaneously inside a commercial pitfall trap. One-month-old 20 T. castaneum and R. dominica adults were released every 30 cm from the pheromone trap. The adults trapped were recorded at 4 and 24 h following their release. Adults of both species released were captured in higher percentages at 24 h than 4 h. At 30 cm distance, these values were 45.5% for T. castaneum and 10–12% for R. dominica for 24 h whereas they were 40.5% for T. castaneum and 5–7.5% for R. dominica following 4 h exposure. The maximum trap capture was at 30 cm for T. castaneum and 30–60 cm for R. dominica. In second experiment, a trap with two rubber septa containing dominicalure-1 and dominicalure-2 was placed at different heights inside cage, and R. dominica adults were released at the bottom. In each experiment, four replicates were tested. After 24 h, flying R. dominica adults were captured in progressively lower percentages as trap height increased up to 40 cm above the bottom of cage. Trap heights above 10 cm exhibited decreased trap capture of R. dominica compared to those at 3 cm. We conclude that simultaneous use of both aggregation pheromones better facilitates trapping of walking T. castaneum and R. dominica. Traps above 10 cm show decreased captures of flying R. dominica.     

Detection of phosphine resistance in major stored-product insects in Greece and evaluation of a field resistance test kit

The use of phosphine has been effective against a wide range of stored-product pests in different types of commodities and facilities. However, its continuous and improper use has led to resistance development in -several major insect species. Although phosphine resistance has been reported from many countries across the globe, reports from Europe have been very limited. In the present study, we determined phosphine resistance in insect populations that had been collected from a range of storages across Greece, using two different diagnostic protocols. Apart from the traditional Food and Agriculture Organization (FAO) protocol, a field test kit (known as the Detia Degesch Tolerance Test Kit, DDTTPK) was utilized, for “same day” determination of the resistance status of field collected insects. In total, 53 populations belonging to Rhyzopertha dominica, Sitophilus oryzae, Sitophilus granarius, Cryptolestes ferrugineus, Tribolium confusum, Tribolium castaneum and Oryzaephilus surinamensis were tested. For the majority of the species and populations tested, both FAO and DDPTTK provided similar results, for the susceptibility to phosphine and thus, the quick test could be used with success for an initial same day screening of phosphine resistance. Among the tested species, the populations recorded with the most frequent survival at the FAO testing dose of phosphine was that of R. dominica. The dissimilar evaluation and characterization of resistance to phosphine between diagnostic protocols is particularly important, as it poses risks in the over or underestimation of the resistance status of a given population. Our data indicate that the DDPTTK could be used to determine resistance to phosphine in the field, before the initiation of fumigations to disinfest stored commodities.     

Testing the consistency of spatio-temporal patterns of flight activity in the stored grain beetles Tribolium castaneum (Herbst) and Rhyzopertha dominica (F.)

Many studies have been published on the flight activity of stored grain beetles in temperate regions, but few have focussed on tropical and sub-tropical regions. The spatio-temporal patterns of flight activity of Tribolium castaneum (Herbst) and Rhyzopertha dominica (F.) were quantified in a grain growing district on the Tropic of Capricorn in central Queensland, Australia. Nine traps baited with aggregation pheromone lures were monitored at 2-wk intervals (fortnightly) for 1 year along a 28.4 km linear transect that included sites at bulk grain depots and sites away from stored grain. Beetles of both species were trapped every fortnight during the study. The spatio-temporal patterns of flight activity differed greatly across the two species, as predicted from studies elsewhere. Rhyzopertha dominica was widespread across the landscape, as the mean trap catch of this species was equal in depot and non-depot traps. In contrast, T. castaneum was more frequently trapped in depot traps than non-depot traps during the colder months, but was much more widespread across the landscape during the summer months. Tribolium castaneum also showed a clear mid-summer peak in flight activity, whereas R. dominica flight activity was highly variable throughout the study. In general, our results reveal patterns that are consistent with those found for T. castaneum and R. dominica in southern Queensland. The contrasting spatio-temporal patterns of flight activity of R. dominica and T. castaneum show that species-specific approaches may be needed to manage these pests and that the spatio-temporal dynamics of resistance genes may differ across these species.     

Stored grain pest prevalence and insecticide resistance in Egyptian populations of the red flour beetle Tribolium castaneum (Herbst) and the rice weevil Sitophilus oryzae (L.)

As a consequence of the widespread use of insecticides against insect pests in grain warehouses and in the food industry, insecticide resistance has greatly increased among these species all over the world. Nonetheless, insecticide resistance in stored grain insects in Egypt was little studied. Therefore, our study aimed to forecast infestation risks and determine the insecticide resistance level in Egyptian populations of the main insect pests of stored grains. To achieve this goal, we surveyed the populations of stored-product insects in wheat in two different sites in the Alexandria governorate (Egypt) between June and August 2017. Moreover, insecticide resistance levels in Egyptian populations of Tribolium castaneum (Herbst) and Sitophilus oryzae (L.) collected from wheat storage facilities (granaries and flour mills) in Alexandria governorate were also determined. Three contact insecticides, namely malathion, pirimiphos-methyl and cypermethrin, were tested. Tarsal contact bioassays on filter paper were carried out on adults following FAO methods and concentration-mortality lines were estimated to determine the resistance ratios for each insecticide and population. The data from the monitoring of insects revealed that T. castaneum and S. oryzae were the most common species infesting wheat in both Alexandria granaries and mills. Populations of S. oryzae were generally more resistant to malathion than the populations of T. castaneum. However, T. castaneum populations were more resistant to pirimiphos-methyl than the S. oryzae populations. Both populations of T. castaneum and S. oryzae were susceptible to cypermethrin. The detected levels of insecticide resistance indicate that this phenomenon takes place and should be a concern in Egyptian populations of T. castaneum and S. oryzae requiring monitoring and design of resistance management practices.     

Bioactivity of Jatropha curcas L. to insect pests of stored products

The objective of this research was to assess insecticidal activity of seeds and derivatives of Jatropha curcas to insect pests of stored grains. Lethal exposure times were estimated for 50 and 95% (LT₅₀ and LT₉₅) of the adults of Sitophilus zeamais and Rhyzorpertha dominica to seeds of four genotypes of J. curcas: Gonçalo, Paraguaçu, Filomena and Bento. Toxicity of powders and aqueous extracts from seeds and pericarps of J. curcas were tested on S. zeamais, R. dominica, Tribolium castaneum and Oryzaephilus surimanensis. A uniform toxic response was observed among the four genotypes for S. zeamais and R. dominica. The powders and aqueous extracts from seeds and pericarps were lethal to S. zeamais, R. dominica, T. castaneum and O. surimanensis. However, insect mortality was higher in the treatments from seeds, compared to the treatments from pericarps.     

A rapid assay for the detection of resistance to phosphine in the lesser grain borer,Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae)

Resistance to the fumigant phosphine in stored product insect pests is a global problem. Diagnosis of resistance relies on a bioassay developed by the FAO that involves a mortality assessment after 20-h fumigation of a pest population at a discriminating concentration of gas, followed by a 14-day post fumigation assessment. This bioassay is impractical for monitoring and early detection of phosphine resistance in routine pest management. We utilized the procedure of a commercial resistance detection test kit for rapid detection in field populations of lesser grain borer, Rhyzopertha dominica (F.). We established a knockdown effect of either susceptible or resistant insects by exposing them to a high concentration of phosphine. We assessed the relationship between adult knockdown times and the FAO method for 18 beetle populations utilizing knockdown criteria for a single beetle in a chamber, or for 50% or 100% knockdown times for groups of beetles, exposed to 3000 ppm of phosphine. We also determined the most effective concentrations that would elicit the quickest knockdown while estimating the recovery times from exposure. Results suggest that a KT₁₀₀ test was better than the KT₅₀ and the KT_single tests. Based on the responses of susceptible populations, we established that a KT₁₀₀ of approximately 18 min can be used as a viable knockdown time to distinguish a susceptible from a resistant populations. Higher concentrations of phosphine significantly elicited a quicker recovery in strongly resistant populations compared to susceptible populations. These findings have potential for developing a robust commercial kit for practical phosphine resistance detection in populations of R. dominica by commercial fumigators, and could be incorporated in a resistance management program.     

Insecticidal activities of essential oils from leaves of Laurus nobilis L. from Tunisia,Algeria and Morocco,and comparative chemical composition

Laurus nobilis essential oils from Tunisia, Algeria and Morocco were analyzed for their chemical composition and assessed for their repellent and toxic activities against two major stored product pests: Rhyzopertha dominica and Tribolium castaneum. The three oils showed quantitative rather than qualitative differences in their chemical compositions. 1,8-cineole, linalool and isovaleraldehyde, were identified as the major common compounds whereas, α-pinene, α-terpineol, eugenylmethylether, β-pinene, spathulenol and β-myrcene were also well represented in all three oils. Results showed that L. nobilis essential oils were repellant and toxic to adults of R. dominica and T. castaneum. Repellent and fumigant toxicities were highly dependent upon insect species and oil origin.In filter paper tests, L. nobilis essential oil from Morocco was more effective compared to Tunisian and Algerian oils. RD₅₀ values were respectively 0.013 μl/cm², 0.036 μl/cm² and 0.033 μl/cm² for R. dominica versus 0.045 μl/cm², 0.139 μl/cm² and 0.096 μl/cm² for T. castaneum. Moreover, fumigant activity tests showed that both R. dominica and T. castaneum were more susceptible to L. nobilis essential oil from Morocco than that from Algeria or Tunisia. The corresponding LC₅₀ values were respectively 68, 99 and 113 μl/l air for R. dominica against 172, 194 and 217 μl/l air for T. castaneum. Our work clearly vindicates interest in the efficacy of essential oils from plants of Mediterranean origin both as insecticides and repellents against stored product pests.     

Laboratory assessment of insecticidal effectiveness of natural zeolite and diatomaceous earth formulations against three stored-product beetle pests

The insecticidal effectiveness of two natural zeolite formulations (Minazel plus and Minazel), applied to wheat at selected rates of 0.25, 0.50 and 0.75 g/kg, and a diatomaceous earth formulation (DE) (Protect-It™), applied at the recommended rates of 0.15 g/kg for Sitophilus oryzae, 0.20 g/kg for Rhyzopertha dominica and 0.30 g/kg for Tribolium castaneum, were tested under laboratory conditions (24 ± 1 °C temperature and 45 ± 5% relative humidity). The highest adult mortality was observed after the longest exposure period of 21 days and 7 days of recovery, when all three zeolite dosage rates and the recommended DE dosage caused 97-100% mortality of S. oryzae and 94-100% of T. castaneum. On the other hand, 100% mortality was not achieved in any test variant involving R. dominica; the highest (about 92%) was detected for DE, while 52% and 79% mortality was achieved with the zeolites at the highest rate of 0.75 g/kg. Progeny reduction by >90% was achieved after 21 days of contact of all three beetle pests with DE-treated wheat, while the same level of reduction was achieved for S. oryzae and T. castaenum only after contact with the highest rate of the zeolite product, Minazel. Thus the two zeolite formulations are comparable to diatomaceous earth in controlling adult S. oryzae, R. dominica and T. castaneum, but only the Minazel formulation could effectively protect wheat from attack by S. oryzae or T. castaneum, and only with a higher rate of application than for the DE formulation.     

A field demonstration of odour-based,host-food finding behaviour in several species of stored grain insects

In order to demonstrate odour-based, host-food finding behaviour in field populations of stored grain insects, traps were inserted into the fabric of PVC-covered, fully sealed, earth-walled storages at two sites in New South Wales, Australia. Cryptolestes ferrugineus, Rhyzopertha dominica and Tribolium castaneum were trapped relatively commonly, as were the economically less significant Carpophilus dimidiatus, Typhaea stercorea and Ahasverus advena. Trap odours from stored wheat contributed markedly to the attraction of all six species. Other species were taken also, mainly those associated with moist grain. These results are discussed in relation to the abundance of stored-product species in the trapping area.     

Fumigant toxicity of allyl isothiocyanate to populations of the red flour beetle Tribolium castaneum

The phasing out of methyl bromide as a fumigant, the phosphine resistance problems in stored product insect-pests, and the ever-growing concerns with human health and environmental safety have been guiding the search of alternative fumigants. Allyl isothiocyanate (AITC) is the main component of mustard oil with reported pesticide activity and potential as a fumigant of stored foodstuffs. The fumigant toxicity of AITC was assessed in adults of 18 populations of the red flour beetle Tribolium castaneum (Coleoptera: Tenebronidae). These populations were all susceptible to AITC with negligible variation among them. Two of these populations were further used to test the AITC susceptibility of eggs, larvae (early and late instars), and pupae of T. castaneum. All of the developmental stages of both populations were similarly susceptible to AITC. No cross-resistance between phosphine and AITC was observed. Despite the significant variation in body mass, respiration rate, and fitness among the populations of T. castaneum, they were not correlated with AITC susceptibility. Larvae and adult malformations were observed when larvae and pupae were exposed to AITC. These results show the potential of AICT as an alternative fumigant against stored product insects.     

Insect population dynamics in commercial grain elevators

Data were collected in 1998-2002 from wheat stored in commercial grain elevators in south-central Kansas. Bins at these elevators had concrete walls and were typically 6-9 m in diameter and 30-35 m tall. A vacuum-probe sampler was used to collect grain samples in the top 12 m of the wheat in each bin. The primary insect species found in the wheat samples were: Cryptolestes ferrugineus, Rhyzopertha dominica, and Tribolium castaneum. In the top 3.7 m of grain, R. dominica, C. ferrugineus, T. castaneum and Sitophilus oryzae made up 44, 36, 19 and 1% of the insects found in the samples, respectively. From 3.8 to 12.2 m, R. dominica, C. ferrugineus, T. castaneum and S. oryzae were present at 84, 8, 8, and 1%, respectively. The most prevalent species also changed over time. In June, the start of wheat harvesting and storage in Kansas, insect density was low in the bins. At this time, C. ferrugineus was the most common insect, and it was found mostly in the top grain sample (0-1.2 m). In September through November, C. ferrugineus and R. dominica were at similar densities; however, from February to March, R. dominica was more common.Generally, insect density was greatest at the top and decreased with grain depth. Very few insects were found in samples collected from greater than 12 m (most of the bins contained grain to depths of 24-36 m). Insect density for all species increased rapidly from June through October. During this period less than 20% of the bins had economically significant insect densities (>2 insects/kg). From October until February, the average insect density remained fairly constant but it was greatly reduced in April, May, and June. Bins that had insect densities >2 insects/kg tended to be located adjacent to other heavily infested bins.     

Toxicity and growth inhibitory activities of methanol extract and the β-carboline alkaloids of Peganum harmala L. against two coleopteran stored-grain pests

Methanol extract and the β-carboline alkaloids were extracted from the seeds of Peganum harmala (Zygophyllaceae). Their toxicity, growth inhibitory and effects on the progeny production of Tribolium castaneum and Rhyzopertha dominica was studied. To assess any additive effects among the extracted β-carbolines, they were tested as binary mixtures (1:1) or as a crude alkaloid fraction. All extracts exhibited a considerable adulticidal effect with increasing activities in response to increased exposure period. Using the contact toxicity bioassay, the crude β-carboline fraction was the most effective (LC_50’s were 20.1 and 36.7) μg/cm², 48 h post-treatment against R. dominica and T. castaneum, respectively. LC_50’s of (harmaline + harmine), (harmaline + harmane), and methanol extract were (31.2, 39.4), (33.7, 47.2), and (39.8, 65.2) μg/cm², 24 h post-treatment against R. dominica and T. castaneum, respectively. At 48 h post-treatment, LC₅₀ of (harmaline + harmine) reached 22.4 μg/cm² against R. dominica. When mixed with the insect’s diets, toxicity of all extracts were increased with the crude alkaloidal fraction the most toxic (LC_50’s were 7.8 and 14.7) mg/kg grains, 48 h post exposure against R. dominica and T. castaneum, respectively. When the 2nd instar larvae were fed sub-lethal doses-treated grains, development and F1 progeny of both insects were significantly affected (P ≤ 0.001). At 3.5 mg/kg grains of the crude alkaloidal extract, percentages of malformed larvae and pupae of T. castaneum were 19.7 and 33.4%, respectively. In this case, a total life span of 81.3 days was recorded for the treated individuals compared to 44.2 for the control. A reduction in the adult progeny of 56.9, 44.0 and 43.6% was obtained with 3.5 mg/kg of the crude alkaloids, (harmaline + harmine) and methanol extract, respectively. Meanwhile, the reduction in adult progeny of R. dominica reached 79.2% with the same concentration of the crude alkaloid extract.     

Interspecific associations among stored-grain beetles

Recent increases in prices of raw grain, including wheat, will reduce action thresholds for insect damage and therefore justify more research into management practices and understanding of pest ecology in stored grain. Compared to most other habitats, natural or man-made, a filled grain silo constitutes a unique and fairly homogeneous habitat in which food availability for many grain-feeding insects is unlimited. A fundamental aspect of stored-grain insect ecology is a better understanding of associations among common beetle species. We analyzed the densities of three important stored-grain beetle species, Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae), Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae), and Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) in wheat samples collected in 1999–2001 from 129 grain silos in Kansas. The beetles studied here are highly mobile, and the number of insects in each grain sample is a result of the beetles' preference for favorable micro-environmental conditions and possibly of intra- and interspecific associations. In general, the number of T. castaneum in a grain sample increased as the number of R. dominica increased, but the number of C. ferrugineus was not correlated with the number of R. dominica. The densities of both T. castaneum and R. dominica decreased as the number of C. ferrugineus increased. Cryptolestes ferrugineus and T. castaneum can be predators and the species composition of insects in a grain sample may be modified by predation. As T. castaneum populations increased, so did R. dominica but not C. ferrugineus. Our analysis of the species composition in grain samples is discussed in an ecological context.     

Active exclusion of phosphine as a mechanism of resistance in Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae)

A strain of Rhyzopertha dominica which is resistant to the fumigant phosphine, was found to absorb less radioactive phosphine than a susceptible strain. The non-absorption of phosphine was enhanced by increased temperature and carbon dioxide. Living resistant insects absorbed less toxin than dead insects, and metabolic detoxication did not appear to contribute to resistance. Since respiratory activity in the presence of the gas is known to be unaffected in the resistant insects, a resistance mechanism based on exclusion of toxic gas is proposed.