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.     

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 simple technique to assess compounds that are repellent or attractive to stored-product insects

We have developed a simple and rapid technique that mimics storage conditions, and determines if products are repellent or attractive to stored-product insects. The technique determines the response of insects to potential repellents and attractants by measuring their movement from grain. The technique used a device consisting of a perforated cup (2 mm perforations) that holds 200 g of wheat. A Petri dish and cup collected the insects as they left the wheat. Several natural products were tested for repellency: diatomaceous earth (DE), ground peas (Pisum sativum), protein-rich pea flour, pea starch, and pea fibre. Adult insects of three species were tested: the rice weevil, Sitophilus oryzae, the red flour beetle, Tribolium castaneum, and the rusty grain beetle, Cryptolestes ferrugineus. DE at 0.01% was repellent to all insects tested. Pea fibre, pea protein, and ground pea at 1% caused increased emigration of C. ferrugineus from the wheat. Pea starch did not affect movement out of the grain for all three insects. Only pea fibre and ground pea increased the movement of T. castaneum out of the grain. For S. oryzae, there were no differences after 1 h, but after 24 h both pea protein and ground pea increased movement out of the grain. Several potential attractants were placed outside the grain and the emigration out of the grain noted. For R. dominica, the commercial R. dominica pheromone increased the emigration of insects from the grain; R. dominica adults on broken grain enclosed in a ventilated vial in the collection jar also increased emigration, but not as much as the synthetic pheromone. The commercial Tribolium pheromone did increase movement out of the grain for T. castaneum, but the other treatments were no different from the control. None of the potential attractants increased the movement of S. oryzae from the grain. The implications of this work are discussed with reference to controlling and sampling stored-product insect pests.     

Susceptibility of stored-product insects to enhanced diatomaceous earth

Mortality of adult Rhyzopertha dominica (F.), Sitophilus oryzae (L.) and Tribolium castaneum (Herbst) was recorded after exposure for different times to application rates of 0, 0.5, 1, 1.5 or 2 g/m² of the diatomaceous earth (DE) DEBBM-P/WP and to 0, 3, 4, 5 or 6 g/m² of the DE Protect-It^®. Mortality of all insects increased with increasing exposure interval; dry dusts were more effective than slurries, and overall mortality was greater for DEBBM-P/WP than Protect-It^®.     

Simulation model of Rhyzopertha dominica population dynamics in concrete grain bins

Rhyzopertha dominica is one of the most damaging insect pests in grain elevators and causes millions of dollars worth of stored grain losses annually in the USA. A simulation model was developed for predicting R. dominica population dynamics in concrete grain bins. The model used a two-dimensional representation of a cylindrical concrete bin (33 m tall×6.4 m wide), and used hourly weather data to predict changes in grain temperature. Output from the grain bin temperature and moisture module was used by the insect module to predict changes in insect density in 32 different bin regions. When compared to validation data from nine grain bins, the model accurately predicted insect vertical distribution and insect density. In December, the highest insect density was in the top center of the grain mass, and decreased steadily with increasing depth and towards the periphery of the grain mass. R. dominica attains this spatial distribution because immigration is primarily through the top of the bin, and higher populations occur in the interior of the grain mass because of warmer temperatures there. Initially, the model underestimated actual insect density in the grain bins. We increased the immigration rate by 50% and this resulted in a much better prediction of R. dominica density by the model. From 20 September to 14 December, populations of R. dominica increased from 0.1 to 3.5 insects per kg of wheat.     

Toxicity of ethyl formate to adult Sitophilus oryzae (L.), Tribolium castaneum (herbst) and Rhyzopertha dominica (F.)

Fumigations were conducted using a continuous flow-through laboratory process to maintain constant concentrations of ethyl formate and low levels (<0.8%) of respiratory carbon dioxide. The procedure minimised the effects of sorption by exposing test insects without media and minimised the effect of carbon dioxide by use of continuous flow. The concentration×time (Ct) products of ethyl formate for adult Sitophilus oryzae, Tribolium castaneum and Rhyzopertha dominica at 25 °C and 70% relative humidity for the 6 h exposure were, respectively: (1) LD₅₀ 107.8, 108.8 and 72.8 mg h L⁻¹ and (2) LD_99.5 207.4, 167.1 and 122.2 mg h L⁻¹. Endpoint mortality was reached within 24 h of initial exposure.     

Efficacy of diflubenzuron plus methoprene against Sitophilus oryzae and Rhyzopertha dominica in stored sorghum

The efficacy of diflubenzuron (1 mg kg⁻¹)+methoprene (1 mg kg⁻¹) against Sitophilus oryzae (L.) and Rhyzopertha dominica (F.) in sorghum was evaluated in a silo-scale trial in southeast Queensland, Australia. Sorghum is normally protected from a wide range of insects by mixtures of grain protectants. The chitin synthesis inhibitor diflubenzuron was evaluated as a potential new protectant for S. oryzae in combination with the juvenile hormone analogue methoprene, which is already registered for control of R. dominica. Sorghum (ca 200 t) was treated after harvest in 2000 and assessed for treatment efficacy and residue decline during 6.5 months storage. The reproductive capacity of S. oryzae and R. dominica was greatly reduced in bioassays of treated sorghum throughout the trial, and efficacy remained relatively stable during the trial. An initial exposure of S. oryzae adults to treated sorghum for 2 weeks reduced F₁ progeny production of all strains by 80.8-98.8%, but a second exposure of 4 weeks reduced F₁ progeny production by 98.5-100%. In addition, the reproductive capacity of any S. oryzae progeny produced was greatly reduced. Exposure of R. dominica adults to treated sorghum for 2 weeks reduced F₁ progeny production of all strains by 99.6-100%, including a methoprene-resistant strain. The results indicate that S. oryzae or R. dominica adults invading sorghum treated with diflubenzuron (1 mg kg⁻¹)+methoprene (1 mg kg⁻¹) would be incapable of producing sustainable populations.     

Feeding bioassay for stored-product insect pests using an encapsulated food source

A procedure that was developed to encapsulate liquid and semiliquid diets was used to encapsulate dry diet for use in a feeding bioassay for beetles that are pests of stored products. Vacuum was used to form Parafilm^® into numerous 6 mm diameter wells. The wells were filled with clean sand (control) or ground dry dog food (test), and the Parafilm^® sealed to produce individual pellets. A single pellet was then placed in the center of a 9 cm diameter Petri dish and feeding activity of groups of ten adults of Sitophilus oryzae (L.), the rice weevil; Oryzaephilus surinamensis (L.), the sawtoothed grain beetle; and Tribolium castaneum (Herbst), the red flour beetle, was tested. The number of insects on the encapsulated pellet and the amount of food or sand scattered were checked hourly for the first 8 h of the study and after 24 h. Few insects were observed on pellets containing sand and little or no sand was observed scattered outside of the pellet, so the presence of dog food in the pellet was needed for insects to feed on the pellet. In tests with encapsulated dog food, the amount of food scattered provided a better quantitative measure of feeding than the number of insects on the pellet. Insects that were starved for 48 h caused greater amounts of food scatter than insects starved for 24 h prior to the test. In direct comparisons among all three species, T. castaneum responded the most slowly and the bioassay may be improved by increasing the amount of time starved. Sitophilus oryzae responded very quickly and the entire pellet was essentially consumed within the first 4-5 h of the study. The encapsulated diets provide a promising method to evaluate feeding behavior of stored-product insect pests.     

Integrated management of insect vectors of Aspergillus flavus in stored maize, using synthetic antioxidants and natural phytochemicals

The purpose of this study was to investigate the insecticidal activity of two benzoic acids 2(3)-tert-butyl-4 hydroxyanisole (BHA) and 2,6-di(tert-butyl)-p-cresol (BHT); two phenolic acids 3-phenyl-2-propenoic acid (CA) and trans-4-hydroxy-3-methoxycinnamic acid (FA) and two essential oils of Eugenia caryophyllata (clove tree) and Thymus vulgaris (thyme) against Sitophilus zeamais, Tribolium confusum and Rhyzopertha dominica, vector carriers of aflatoxigenic fungi in stored maize. The susceptibility of insects, the frequency of isolation of Aspergillus section Flavi in insects and maize, and the analysis of aflatoxin B₁ in maize were determined. BHA, BHT, BHA/BHT mixture and the natural phytochemicals AF and AF/AC mixture showed the highest insecticidal activity against S. zeamais, T. confusum and R. dominica after 120 days of incubation. The insecticidal efficacy of the volatile fraction of essential oils of clove and thyme showed less inhibition. There was no contamination of Aspergillus section Flavi in dead and live insects collected from maize treated with BHA. No aflatoxin B₁ accumulation was detected in the control and treatments. The information obtained shows that these substances have the potential to control pest insect vectors of aflatoxigenic fungi in stored maize in microcosms during 120 days.     

Mortality of Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae) at grain temperatures ranging from 50°C to 60°C obtained at different rates of heating in a spouted bed

Heat disinfestation of grain has the potential for high market acceptance. However, the technology would be more affordable if less energy was used during the process. The paper explores two interrelated ways in which this could be achieved. The first is to decrease typical grain treatment temperatures, but hold those temperatures without the addition of subsequent energy for a time sufficient for disinfestation. The second is to increase the rate of heating to induce physiological heat shock and thereby bring about faster insect mortality. Using a spouted bed, rates of mortality of the heat-tolerant species Rhyzopertha dominica were determined over a range of grain temperatures from 50°C to 60°C at 12% m.c. The grain temperatures were gained at four rates of heating obtained by using different air inlet temperatures from 80°C to 200°C. From the data, a probit model was developed to predict LT values for five arbitrary immature development stages. At LT 99.9 and above, immature stage 2 was generally found to be the most heat-tolerant stage, while stage 5 was generally the most susceptible. When the initial rate of heating was increased by increasing air inlet temperature from 80°C to 100°C, the time required for a given level of mortality was significantly decreased. While subsequent rate increases had limited further impact on increasing rates of mortality, there was an increase in the efficiency of heating. Furthermore, decreasing the target grain temperature and increasing the treatment period accounted for additional cost savings. For example, at the most rapid rate of heating, grain that reached 60°C required 0.73 min of heat soak for 99.9% mortality and cost a theoretical $2.72/t, while grain that reached 55°C required 23.62 min for the same level of mortality and cost $1.87/t. By 50°C, some 22 h were required, but the theoretical running cost was reduced to $1.25/t. Thus, within practical boundaries, increasing the air inlet temperature or rate of heating, and decreasing the grain target temperature, increases the efficiency of the system and reduces operation costs. With the system used here, obtaining grain at 50°C using air at 200°C was 75% efficient and 80% cheaper than obtaining grain at 60°C using air at 80°C, which was only 18% efficient. An assessment of possible impact on grain quality due to the various heat treatments was also carried out, both initially and after 6 months storage. Using several criteria of germination measurement as a quality indicator, no adverse effects were observed.     

Calorimetric evaluation of responses of Sitophilus oryzae and Tribolium confusum to elevated temperatures and controlled atmospheres

Metabolic heat rates, determined by microcalorimetry, were used to measure the effect of controlled atmospheres (CAs) and elevated temperatures on the stored-product insects Sitophilus oryzae (rice weevil) and Tribolium confusum (confused flour beetle). Results for larval and adult stages in air, and in a range of low O₂ and/or high CO₂ CAs, at temperatures from 15 to 45 °C, showed the general effectiveness of such atmospheres in lowering the lethal temperatures relative to those in air. Effects on adult S. oryzae at 25 °C were explored in more detail in experiments using the following conditions: exposure to anoxic CAs for extended times; exposure to hypoxic CAs; and simulated hermetic storage. A simple scanning calorimetric method was developed for determining lethal temperatures and a combined thermo-gravimetric and differential thermal-analysis method was used to interpret the thermal events, due to loss of water, occurring at and above these temperatures.     

Grain loss caused by Tribolium castaneum, Sitophilus oryzae and Acanthoscelides obtectus in stored durum wheat and beans treated with Beauveria bassiana

The effect of the fungal entomopathogen Beauveria bassiana (Balsamo) Vuillemin (Hyphomycete) on the losses caused to durum wheat and beans by storage insects was investigated. Grains were infested with Tribolium castaneum (Herbst), Sitophilus oryzae (L.) and Acanthoscelides obtectus (Say). Beauveria bassiana was produced in inoculated autoclaved rice. The spore formulation (ground rice and B. bassiana) was applied to grain (wheat or bean) and shaken to evenly cover the grain. Adults of T. castaneum or S. oryzae were added to wheat and adults of A. obtectus to bean. Five replicates were set up for each treatment and controls (milled rice but no conidia). The insecticidal effect of B. bassiana was tested by measuring the fresh weight and weight loss of grains after four months of storage. Wheat grains infested with S. oryzae without the conidia was significantly more damaged by weevils than grains treated with B. bassiana. The mean fresh weight of grains with the conidia was significantly greater (18.4%) than the corresponding mean without the fungus when S. oryzae were present. Percentage weight loss decreased by 81.5% and was significantly smaller than the loss from the untreated grain. Significant differences were not found in the fresh weight of seed infested with T. castaneum or A. obtectus in treated or untreated grain nor in the percentage weight loss of grains infested with these insects, with and without B. bassiana.     

Temperature variation in stored maize and its effect on capture of beetles in grain probe traps

Seasonal and spatial distribution of Typhaea stercorea (L.) and Tribolium castaneum (Herbst) infesting farm-stored maize in South Carolina were studied by trapping with grain probe traps. Trap catch (numbers captured per week) and weekly mean grain temperature for each trap site were plotted against time and fitted to cubic polynomials, which adequately described seasonal trends. Spatial distribution of trap catch was examined by calculating the coefficient of variation, or dispersion (s²/m) and by constructing a contour map of trap catch values for each week. Seasonal trends in trap catch varied with species, farm, and storage season, and tended to parallel temperature trends, so that trap catch and temperature were positively correlated. Trap catch was highly aggregated except for weeks when few insects were captured. Residuals (observed-predicted values), calculated from the fitted trend curves, represent the spatial component of variation. When residuals were analyzed, the positive correlation between temperature and trap catch was usually reduced or abolished, indicating that spatial variability within any given week reflected the distribution of the population itself, rather than spatial variability in activity produced by temperature gradients. The number of insects captured by a trap is determined mainly by insect activity and the numbers present, but there are no rigorous methods for separating the immediate effect of temperature on activity from the delayed effect on population growth. However, by comparing trends in trap catch with those in temperature, it is sometimes possible to make inferences about changes in population density.     

Detection of Rhyzopertha dominica larvae in stored wheat using ELISA: The impact of myosin degradation following fumigation

Hard red winter wheat kernels were infested with eggs of Rhyzopertha dominica. After 20 d, when the larvae reached the fourth instar, they were killed by exposing the infested kernels to phosphine gas for 24 h. The infested kernels were then divided into four portions and treated as follows: one portion was immediately frozen at −80 °C to avoid myosin degradation; the other three portions were kept at 32 °C and 65% relative humidity, and then frozen at −80 °C after 14, 28, and 56 d post-fumigation, respectively. Each treatment was replicated five times. Myosin was measured using a commercial enzyme linked immunosorbent assay (ELISA) method that specifically detects this protein (Biotect^®, Austin, TX). Myosin degradation was most rapid in the first 2 weeks after the larvae were killed, decreasing from 1.672 to 0.695 ng/well during this period (a 58.4% reduction). There were no significant differences in myosin degradation between samples that were 14, 28, and 56 d post-fumigation. Grain is often fumigated to control insects. Frequently, this occurs many weeks before the grain is milled and may be repeated during the storage period. Therefore, estimates using the ELISA test may underestimate internal insect infestation because of myosin degradation. Insect fragment estimates for previously fumigated grain could be underestimated by as much as 58%.     

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.     

Initial and delayed mortality of late-instar larvae, pupae, and adults of Tribolium castaneum and Tribolium confusum (Coleoptera: Tenebrionidae) exposed at variable temperatures and time intervals

Late-instar larvae, pupae, and adults of Tribolium castaneum (Herbst), the red flour beetle, or Tribolium confusum (DuVal), the confused flour beetle, were exposed for variable durations at 36-54 °C. Beetles were placed in laboratory ovens set at a baseline of 27 °C, the temperature was increased by 0.1 °C per minute until the target temperature was achieved, and beetles were then held for specified exposure durations. There was no mortality after initial exposure or after a 1-week holding period of any life stage of T. castaneum or T. confusum exposed for 32 h to 36, 39, or 42 °C. At 45 °C, there was no initial mortality of either species exposed for different time intervals except for those exposed for 28 h. However, there was a significant increase in mortality after the 1-week holding period of those beetles exposed initially for at least 16 h to 45 °C. There was a sharp increase in mortality after the initial exposures of 4 h at 48 °C; mortality of T. confusum larvae was 90.0±5.7% but was only 10.0±10.0% for larvae of T. castaneum, and no pupae of either species were dead. All life stages of both species were killed after the initial exposure of 12 h, and 1-week mortality of beetles exposed for 4 and 8 h was generally greater than initial mortality. At 51 and 54 °C, 2- and 1-h exposures, respectively, killed all life stages of each species. Mortality in conditions of gradual temperature increase was less than previous studies with sudden temperature increases.     

Response of mixed-age cultures of phosphine-resistant and susceptible strains of lesser grain borer, Rhyzopertha dominica, to phosphine at a range of concentrations and exposure periods

Mixed-age cultures, containing all life stages, of a highly resistant strain (Strong-R) of lesser grain borer, Rhyzopertha dominica (F.), were exposed to a series of fixed concentrations of phosphine at a range of exposure periods at 25 °C. A susceptible strain and a less-resistant strain (Weak-R) were also tested. The aim was to characterise the resistant strain and determine if it could be controlled with phosphine. Times to population extinction (TPE) were recorded and lethal time (LT_99.9) values calculated. The relationship between exposure period t and phosphine concentration C for the resistant strains were: for Strong-R LT_99.9C^0.5457t=3.852 and TPE C^0.6105t=4.0404 and for Weak-R LT_99.9C^0.3553t=3.6521 and TPE C^0.4507t=3.4833. The results were used to define a range of minimum exposure period ×concentration protocols for control of the Strong-R populations. For example, at 1.0, 0.3, and 0.2 mg l⁻¹ complete control can be expected in 5, 10 and 14 days, respectively. This information will be used to recommend phosphine rates for field trial and eventual registration.     

Susceptibility of stored product insects to high concentrations of ozone at different exposure intervals

Ozone is a highly reactive gas with insecticidal activity. Past studies have indicated that ozone technology has potential as a management tool to control insect pests in bulk grain storage facilities. The objective of this study was to determine the efficacy of short periods of exposure to high ozone concentrations to kill all life stages of red flour beetle (Tribolium castaneum (Herbst)) (Coleoptera: Tenebrionidae), and Indianmeal moth (Plodia interpunctella (Hübner)) (Lepidoptera: Pyralidae), adult maize weevil (Sitophilus zeamais (Motsch.)) (Coleoptera: Curculionidae) and adult rice weevil (S. oryzae (L)) (Coleoptera: Curculionidae). Insects were treated with six ozone concentrations between 50 and 1800 ppm. The specific objective was to determine minimal time needed to attain 100% mortality. The most ozone-tolerant stages of T. castaneum were pupae and eggs, which required a treatment of 180 min at 1800 ppm ozone to reach 100% mortality. Eggs of P. interpunctella also required 180 min at 1800 ppm ozone to reach 100% mortality. Ozone treatments of 1800 ppm for 120 min and 1800 ppm for 60 min were required to kill all adult S. zeamais and adult S. oryzae, respectively. The results indicate that high ozone concentrations reduce the treatment times significantly over previously described results. Our results also provide new baseline information about insect tolerance to ozone treatment.