The Innate Rates of Increase in Two Common Stored Grain Insects under Different Grain Storage Conditions and Times.

The determination of innate rate of increase (r) values under different grain storage conditions is critical for insect population predictions. The r values for Cryptolestes ferrugineus (Stephens) and Tribolium castaneum (Herbst) were calculated by using a new suggested method (continuous time analysis) and data from the literature, and these calculated r values were compared to identify the r values and carrying capacities under real grain storage conditions and times. The insects were reared in small glass vials (0.3 kg wheat), small PVC columns (2 kg wheat), large PVC columns (14 kg wheat), and shallow containers (14 kg wheat or wheat + cracked wheat). The wheat or cracked wheat had 13.8 to 14.5% moisture contents at different constant temperatures (17.5 to 42.5 °C) and fluctuating temperatures. The r values at the beginning of the population were the highest. Before r became negative, it gradually decreased with increasing time. After the r value became negative, it sometimes increased to positive; however, the rebounded r was much less than the initial r and gradually tended to stabilize within an up-and-down range. This up-and-down r was related to the carrying capacity. The larger the grain bulk, the higher the innate rate was for both species. The r values associated with 14 kg of wheat could be used to predict the insect population dynamics in stored grain bins.

Synergistic and antagonistic effects of temperature and moisture differences on movement and distribution of Cryptolestes ferrugineus (Coleoptera: Laemophloeidae) adults in horizontal columns of wheat.

The distribution of insects in stored grain bulks is significantly influenced by temperature and moisture, or their gradients or differences. This study examined the movement and distribution of Cryptolestes ferrugineus (Stephens) adults under different combinations of temperature (5 or 10°C) and moisture differences (2.5 or 5 percentage point difference) in horizontal 1 m wheat columns in 24 h. Adults showed a nonoriented distribution in dry or damp wheat (less than 15% moisture content), while the distribution was partially biased in wet wheat (17.5% moisture content) due to slightly increased temperature or spoilage of the wet wheat in 1 replicate. Adults showed a positive response to warm and damp or wet wheat. Under any levels of temperature (5 or 10°C) and moisture differences (2.5 or 5 percentage points) in 24 h, about 75% of adults were recovered from moist wheat where insects were introduced. Adults equally preferred both moist cool grain and dry warm grain located at ±â€…0.25 m. However, the preference for dry warm grain was stronger than moist cool grain when the movement distance was 0.45 m. The sensing ability of adults and their preferences were not only determined by movement distance but also by the magnitude of temperature and moisture differences. Thus, the findings of the present study will help in better understanding adult response to realistic temperature and moisture distributions that commonly occur in storage structures and to develop stored grain ecosystems mathematical models.

Population dynamics of Tribolium castaneum (Coleoptera: Tenebrionidae) in wheat and in wheat mixed with cracked wheat held in different types of containers.

The population dynamics of Tribolium castaneum (Herbst), red flour beetle, was studied at 30 °C using long vertical columns (LVCs) (150 mm diameter and 1,020 mm long) and shallow containers (SCs) (460 mm long, 660 mm wide, and 150 mm high), containing 14 kg of whole wheat or a diet made of whole wheat and cracked wheat in 19:1 ratio by mass. The moisture content of the wheat or mixed diet was 14.5% (wb). Every 4 wk and up to 24 wk the live and dead adults were counted in the SCs or in each section of the LVCs. Each LVC was separated into 10 equal sections before removing grain from the LVC. After counting, the grains were incubated at 30 °C and 70% RH for 4 wk, and emerged adults after re-incubation were counted as offspring. The adults and offspring were mainly concentrated in the top section of the LVCs, which could be due to higher mortality in the lower sections and preference of T. castaneum for the surface of grain bulk. The diet influenced the population, and the insects developed better in the cracked wheat-based diet. Greater surface area of the container increased the multiplication and/ or survival of T. castaneum and insects inside SCs with larger surface area and with cracked wheat-based diet, had quicker population increase rate and larger carrying capacity than LVCs.

Identification of cuticular protein genes and analysis of their roles in phosphine resistance of the rusty grain beetle Cryptolestes ferrugineus.

The rusty grain beetle, Cryptolestes ferrugineus (Stephens) is one of the most economically important stored grain pests, and it has evolved the high resistance to phosphine. Cuticular proteins (CPs) are the major structural components of insect cuticle, and previous studies have confirmed that CPs were involved in insecticide resistance. However, the CPs of C. ferrugineus are still poorly characterized, and thus we conducted transcriptome-wide identification of CP genes and analyze their possible relationships with phosphine resistance in this pest. In this study, a total of 122 putative CPs were annotated in the C. ferrugineus transcriptome data by blasting with the known CPs of Tribolium castaneum. The analysis of conserved motifs revealed these CPs of C. ferrugineus belonging to 9 different families, including 87 CPR, 13 CPAP1, 7 CPAP3, 3 Tweedle, 1 CPLCA, 1 CPLCG, 5 CPLCP, 2 CPCFC, and 3 CPFL proteins. The further phylogenetic analysis showed the different evolutionary patterns of CPs. Namely, we found some CPs (CPR family) formed species-specific protein clusters, indicating these CPs might occur independently among insect taxa, and while some other CPs (CPAP1 and CPAP3 family) shared a closer correlation based on the architecture of protein domains. Subsequently, the previous RNA-seq data were applied to establish the expression profiles of CPs in a phosphine susceptible and resistant populations of C. ferrugineus, and a large amount of CP genes were found to be over-expressed in resistant insects. Lastly, an up-regulated CP gene (CPR family) was selected for the further functional analysis, and after this gene was silenced via RNA interference (RNAi), the sensitivity to phosphine was significantly enhanced in C. ferrugineus. In conclusion, the present results provided us an overview of C. ferrugineus CPs, and which suggested that the CPs might play the critical roles in phosphine resistance.

Response of Cryptolestes ferrugineus (Coleoptera: Laemophloeidae) adults to small temperature (0-6 °C) and moisture (1%) differences in wheat.

Insects respond to temperature and moisture and their differences or gradients in grain bulks, but how small these differences can be is unknown. Response of Cryptolestes ferrugineus (Stephens) adults to 0-6 °C temperature differences in 1 m wheat (12.5% moisture content, w.b.) columns was determined in 24 h. Similarly, the moisture response of the adults was determined in 1 m grain columns with a 1 percentage point difference in the wheat moisture content (12.5 and 13.5%) at 25 °C in 24 h. Adults were highly temperature and moisture sensitive and were able to respond to the lowest temperature difference of 1 °C and moisture difference of 1 percentage point within the wheat column. The temperature preference of the adults was confirmed with the recovery of about 78% of insects from the middle warmer sections of wheat at 25 or 30 °C when there was no temperature difference at other sections of the wheat columns. Irrespective of the temperature differences, on average of about 29% of adults moved towards the warmer end with the highest recovery of 47.3% observed at the temperature difference of 6 °C. The adult recovery from high-moisture locations decreased with an increase in distance away from the point of insect introduction (0.05-0.45 m). About 14% of adults moved to the furthest location of high-moisture ends (13.5% moisture content wheat) at 0.45 m. This study provided valuable insights for the development of mathematical models to predict 3D insect movement and distribution in storage grain bins.

Biology, Ecology, and Behavior of Rusty Grain Beetle (Cryptolestes ferrugineus (Stephens)).

Cryptolestes ferrugineus, the rusty grain beetle, is a cosmopolitan pest that has adapted to cool and warm climates due to its unique biology, ecology, and behavior. The rusty grain beetle is a pest of high economic importance; hence, understanding their biology, ecology, and behavior could be useful in designing effective management strategies. An extensive literature survey was conducted using the databases Web of Science and Scopus. Information on country-wise publications from 1949 to 2023 on C. ferrugineus was provided, and a table illustrating the distribution of C. ferrugineus was also presented to demonstrate the global significance of C. ferrugineus. We overviewed their life stages, morphology, and factors influencing their biology, ecology, and behavior, such as refuge-seeking behavior, flight activity, mating behavior, interspecific interaction with other species, movement, and distribution. Mathematical models focusing on C. ferrugineus population dynamics and movement were also presented. In order to advance our knowledge on C. ferrugineus, the following possible avenues for future research were outlined: application of molecular markers and population genetic approaches to understand their evolutionary history; mechanisms responsible for adaptation and resistance to insecticide; interspecific interaction in storage facilities and wider landscapes; and identification of microbial roles in the ecology, behavior, and control of C. ferrugineus.

Survival of Plodia interpunctella (Hübner) larvae treated with 98% N2 and the life history of their next generation.

Understanding the development and reproduction of insects surviving controlled atmosphere treatment may help in developing sound pest management strategies. The developmental duration, survival percentage, and oviposition of Plodia interpunctella and its F1 generation were determined after the fifth instar larvae (the last-stage larvae) were exposed to 98% N2 for different exposure times. The survival percentage of the last-stage larvae treated with 98% N2 for 6, 4, 1.5, and 0 day was 70, 80, 91, and 100%, respectively when measured 24 h after treatment. The survival percentage of the last-stage larvae that developed to pupae was 37, 55, 73, and 96%, corresponding to the different exposure times. The developmental time needed to pass from pupa to adult emergence of specimens treated as the last-stage larvae were 8, 7, 6, and 6 days corresponding respectively to high N2 treatment after 6, 4, 1.5, and 0 day of exposure. The mean number of eggs laid by the subsequent females developed from the treated last-stage larvae was 35, 66, 81, and 123, respectively. The oviposition inhibition ratio of the F1 generation decreased by more than 33% compared with that of the parental generation. When the last-stage larvae were exposed to 98% N2 for longer than 4 days, the immature developmental time of surviving individuals in the F1 generation was delayed more than 6 days due to slower egg hatching and longer development of the first and second instar larvae stages. The population trend index of the F1 generation was lower when raised from the treated last-stage larvae than those from untreated controls.

Amplification refractory mutation system based real-time PCR (ARMS-qPCR) for rapid resistance characterization of Tribolium castaneum to phosphine.

Resistance of Tribolium castaneum to phosphine is related to point mutations in DNA code corresponding to amino acid changes associated with a core metabolic enzyme dihydrolipoamide dehydrogenase (DLD), but the mutation patterns vary among different resistant populations. Thus, there is a great need to develop a cost-effective method to detect core mutations in T. castaneum, which would be the key factor to understand the molecular basis of phosphine resistance. Amplification refractory mutation system-based quantitative Real-Time PCR (ARMS-qPCR) is an ideal method that can rapidly detect point mutations. Here, the P45S and G131D mutations existed in the DLD of T. castaneum selected from strong Chinese resistance phenotypes, and the DLD P45S mutation, which represents a strong phosphine resistance allele, was confirmed as the most abundant mutation to determine strong resistance genotypes. Our study found that 85 out of 120 beetles carried the P45S resistance allele, including 51 homozygous and 34 heterozygous individuals. Moreover, there was a strong linear relationship (R2 = 0.917) between the resistance ratio and the resistance allele frequency among the strongly resistant populations. Our data showed that the ARMS-qPCR method that we developed could rapidly determine strong resistance phenotypes of T. castaneum to phosphine by detecting the DLD P45S mutation. These results not only provide a detailed example for developing an ARMS-qPCR-based method to characterize pesticide resistance, but also support further elucidation of the molecular basis of phosphine resistance.

Movement and Distribution of Cryptolestes ferrugineus (Coleoptera: Laemophloeidae) Adults Under Different Temperature Differences in Different Lengths of Horizontal Grain Columns.

Understanding insect movement and distribution is critical for developing an effective insect pest management protocol. Movement and distribution of adult Cryptolestes ferrugineus (Stephens) in response to nominal temperature differences of 5, 10, 15, and 20°C in 1- and 2-m horizontal columns filled with 12.5% moisture content wheat were studied in 24, 48, and 72 h of movement times. In the wheat columns without temperature differences, adults had a diffusion distribution pattern caused by their random movement. Adults showed bias movement to the warmer grain in wheat columns with temperature differences and preferred the warmer grain regardless of the magnitude of temperature differences in less than 24 h. Their distribution did not have significant differences among different movement times in 1- or 2-m columns. About 40% of adults moved to the warmer wheat located at 0.4 to 0.5 m in the 1 m column but did not move to the warmer wheat located at 0.9 to 1 m in the 2-m column. Therefore, length of grain column influenced detection of warmer grain by insects. Adults under different temperatures had a similar response as that under linear temperature gradients.

Effects of Insect Density, Movement Period, and Temperature on Three-Dimensional Movement and Distribution of Adult Cryptolestes ferrugineus (Coleoptera: Laemophloeidae).

Knowledge on three-dimensional (3D) movement and distribution of Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae) in grain bulks assists in the prediction of their distribution inside a bin. The following experiments were conducted to determine the 3D dispersal patterns of adult C. ferrugineus in wheat with 14.5% moisture content: 1) at various insect densities (0.35, 1.77 and 3.53 A/kg (adults/kg) at 20°C and in 24 h movement period; 2) in different movement periods (6, 24, and 72 h) at 20°C and 0.35 A/kg insect density; and 3) at different temperatures (20, 30 and 35°C) at 0.35 A/kg density in 24 h movement period. To create the densities of 0.35, 1.77, and 3.53 A/kg, 100, 500, and 1,000 adults were introduced in about 285 kg wheat, respectively. The 285 kg of wheat was kept in 343 mesh cubes, which in turn were packed in a wooden box. The introduced adults were counted at the end of the movement periods. Adult C. ferrugineus tended to move downward from the point of introduction, and then diffused throughout the grain bulk. The effects of insect densities, movement periods, and temperatures on the dispersion pattern of insects were similar in 1D columns, 2D chambers, and 3D grain bulk.

Mathematical Modeling of Population Dynamics of Trogoderma granarium (Coleoptera: Dermistidae).

Khapra beetle, Trogoderma granarium Everts, is one of the economically important quarantine pests that mainly feeds on food grain and proteinaceous materials. Its total development time lasts approximately 40-45 d under favorable environmental conditions. Extreme temperatures, high relative humidity (RH), high larval densities, or low food quality can induce a larval diapause, where the insect can survive for up to a few years, occasionally feeding and molting. Ecological modeling is a helpful tool to study the population dynamics of biological systems. Physi-Biological age method is based on temperature-driven development rate, and factors such as RH and food quality were considered as multipliers. The objective of this study was to develop mathematical models to calculate the survival and development of adults, eggs, larvae, pupae, and oviposition and diapause under different environmental conditions such as temperature, RH, and food quality. Algorithms were developed to simulate the population dynamics for each day and coded in C++. The developed models were validated against the literature data and evaluated using linear regression, R2, and MSE. Population dynamics were simulated under Canadian grain storage conditions, and the developed models predicted that the diapausing larvae survived the extremely cold conditions found in Canadian grain. In contrast, other stages did not survive. The surviving larvae developed to pupae and adults, and females began laying eggs once the temperature became warmer in the grain bins.

Three-dimensional Movement and Distribution of Cryptolestes ferrugineus (Coleoptera: Laemophloeidae) Adults in Stored Wheat Under Constant Temperatures and Moisture Contents.

Understanding the movement and distribution of insects inside a grain bin is crucial to develop an effective stored grain management protocol. The three-dimensional movement and distribution of adult Cryptolestes ferrugineus (Stephens) at 20 and 30°C were determined in a 0.7 × 0.7 × 0.7 m3 (internal dimensions) wooden box filled with wheat of uniform moisture contents (12.5 ± 0.1%, 14.5 ± 0.1%, and 16.5 ± 0.1% wet basis). The wheat at a constant moisture content was filled into 343 mesh cubes (0.1 × 0.1 × 0.1 m3) and placed inside the wooden box. The center mesh cube in the box had one hundred adult insects introduced at the beginning of the movement. After 24 h, the 343 mesh cubes were removed from the wooden box in less than 45 min. Finally, the contents of each mesh cube were sieved, and the insects counted. Each experiment was replicated three times. A maximum of 17% of insects stayed at the introduced cube (center of the wooden box). About 50-88% of the introduced adults moved downward from the introduction location at the studied temperatures and moisture contents. This 24 h study showed that C. ferrugineus movements in three dimensions follow a diffusion pattern in the horizontal direction and move downward due to the 'drift' effect and geotaxis in the vertical direction.

Attractiveness of Male and Female Adults of Cryptolestes ferrugineus (Coleoptera: Laemophloeidae) to Conspecifics With and Without Grain.

Whether stored-grain insects can communicate with each other inside stored-grain bulks is an important question for the development of pest management programs. Movements of the individual adults of Cryptolestes ferrugineus towards caged adult(s), in the presence or absence of wheat, were studied inside an apparatus (10 cm length), using an infrared camera. The numbers of the caged adults were 1, 20, or 50 of females or males, and 100 or 200 mixed-sex adults. Without grain, both males and females moved towards the caged single male, but not the caged single female. With grain, neither males nor females moved towards the caged single male or female. When 50 males were added to the cage, females did move significantly towards the caged males. There were trends for introduced males and females to move towards caged males at higher densities.

Mortality of Different Stages of Plodia interpunctella (Lepidoptera: Pyralidae) at Three Temperatures in Controlled Atmosphere of High Nitrogen.

The lethal exposure time to controlled atmospheres of high nitrogen at stored grain temperatures is an important information for control of stored-product insects. The mortality of 1-d-old egg, 1-wk-old (first or second instar) larva, 3-wk-old (fourth or fifth instar) larva, and 1-d-old pupa of Plodia interpunctella (Hübner) was determined at 18 ± 1, 23 ± 1, and 28 ± 1°C in 98% N2 mixed with air. At 18°C, the lethal exposure times to achieve 100% mortality were 12.7 ± 0.7, 16.3 ± 0.3, 19.7 ± 0.7, and 14.7 ± 0.7 d for 1-d-old egg, 1-wk-old larva, 3-wk-old larva, and 1-d-old pupa, respectively. Temperature had significant effect on the lethal exposure time, and increase of the temperature significantly decreased the lethal exposure time. The order of the insect stages from the highest to lowest for LT50 values was follows: 3-wk-old larva > 1-wk-old larva > 1-d-old pupa ≥ 1-d-old egg. The minimum lethal exposure times required to kill all stages of P. interpunctella were about 20, 16, and 12 d at 18, 23, and 28°C, respectively.

Control of Three Species of Stored-Product Insects in Wheat Treated With Steam and Hot Air.

The effect of 105°C steam or hot air on adult mortality of three species of stored-product insect pests outside wheat kernels of 12.5, 14.5 and 16.5% moisture content was investigated. The species were Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae), Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae), and Sitophilus oryzae (L.) (Coleoptera: Curculionidae). In the case of S. oryzae, young adults and immature stages inside wheat kernels were also tested. The mortality of insects inside kernels was higher at lower moisture contents of wheat treated with hot air, whereas moisture content did not significantly affect mortality of insects treated with steam. In the hot air treatment, all adults of the three species outside kernels had 100% mortality when the treatment time was 75 s for wheat with 16.5% moisture content, and 60 s for 12.5 and 14.5% wheat. In the steam treatment, the time to reach 100% mortality of adults outside kernels was 1 s at any moisture content and without significantly affecting germination. The young adults and immature stages of S. oryzae inside kernels required 90 s to reach 100% mortality in hot air, whereas 3 s was needed in steam. The treatment to reach 100% mortality of insects inside kernels caused a 20% drop in germination in steam and 81% drop in hot air.

Influences of Stored Product Insect Movements on Integrated Pest Management Decisions.

Insect movement inside and outside grain bulks and processed products influences pest management decisions. Movement allows insects to find essential food resources, shelters (refuges), warmer and/or humid locations, mating and egg-laying sites, even when they are rare in fields, buildings, mills, warehouses, and inside grain masses. This review discussed the advantages and disadvantages of stored product insect movements, and the influence of insect mobility on some integrated pest management practices. Insect movement (1) results in clumped insect spatial distributions and thus makes large sample sizes necessary for monitoring; (2) makes trapping more efficient, but is influenced by many factors; (3) allows control methods to be effective, but requires pest management programs to be area-wide; (4) makes eradication of quarantine pests difficult and commodities are quickly re-infested; and (5) results in a diverse genetic pool and speeds the development of resistance to pesticides. Any element of an IPM approach should use the knowledge of insect movement. Reasons for the difficult interpretation of cryptic movement behaviours of insects were provided and future research areas were suggested.

Static and dynamic methods to determine adsorption isotherms of hemp seed (Cannabis sativa L.) with different percentages of dockage.

Adsorption and desorption isotherms of hemp seeds with 0%, 5%, 10%, 15%, and 20% of dockage were determined using the salt solution static (SSS) method. The wet hemp seeds with 0% dockage were also dried at 30℃ with 50% RH, 35℃ with 30% and 50% RH, and 40℃ with 30% and 50% RH inside a thin-layer dryer (thin-layer dynamic method). The hemp seeds with different percentages of dockage showed hysteresis, and this hysteresis became more obvious with the decrease of temperature. At the same condition, the equilibrium moisture content of hemp seeds with 0% dockage was approximately 0.5 percent points lower than that of the hemp seeds with dockage. The best equation to fit the equilibrium moisture content data under constant temperature and RH was the modified GAB equation for both adsorption and desorption isotherms. The constant rate period of drying was observed for <0.75 hr when drying air RH was 30% or when drying air temperature was 40℃. The Henderson and Pabis model was the best model to fit the thin-layer drying data. The equilibrium moisture contents measured by the SSS method were lower than those measured by the thin-layer dynamic method when temperature was ≤35℃.

Demography of Rusty Grain Beetle in Stored Bulk Wheat: Part I, Population Dynamics at Different Temperatures and Grain Bulk Sizes.

Population dynamics of rusty grain beetle, Cryptolestes ferrugineus (Stephens; Coleoptera: Cucujidae), was studied using different sizes of grain bulks (patches) at various temperatures. The temperatures were 21, 25, 30, 35°C, T-decrease (30°C in the first 4 wk and then decreased 1°C /wk), and T-increase (21°C in the first 2 wk and then increased 1°C /wk). Number of adults and offspring and infested wheat kernels were counted every 4 wk up to 24 wk (31 wk for the T-decrease). The grain bulk patches used were: small (50 ml inner volume, 0.03 kg wheat), medium (2.6 liters inner volume, 2 kg wheat), and large (18 liters inner volume, 14 kg wheat). All of the correlation coefficients between the insect numbers and kernel infestation percentage were ≥0.63. Two types of the population dynamic curves were observed: insect number or density continually increased with time during the entire experiment, or there was a rise then a fall in insect number or density over time, giving a peak number or density. The peak insect density was approximately 400 to 500 adults/kg of wheat for all patches at 30°C or lower. At 35°C, the peak densities of live adults were 3,956 ± 630, 2,094 ± 34, and 1,003 ± 70 adults/kg of wheat in small, medium, and large patches, respectively. Patch size influenced insect population dynamics at 35°C. Insect number inside large patch was more dependent on the previous insect number than that inside small patches.

Demography of Rusty Grain Beetle in Stored Bulk Wheat: Part II. Mathematical Modeling to Characterize and Predict Population Dynamics.

Data collected in Part I of this study were further analyzed by using mathematical modeling methods. Out of the nine unstructured population models tested, no model could fit the insect numbers under all of the tested conditions. This analysis showed that Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae) inside small patches (50 ml volume) had different characterization of population dynamics from that inside large patches (18 liter volume) and had different population demography when the insect number at the previous time was different. The key factor analysis showed that the first two main factors influencing the population dynamics were the temperature and the previous insect numbers. The total numbers of insects increased with the increase of sum of degree days. However, the degree day model developed based on the constant temperatures could not predict insect numbers under fluctuating temperatures. A newly developed model, which used the result of the unstructured population models, key factor analysis, and the degree day model, could explain about 66% of the insect numbers under fluctuating temperature conditions.

Chill-coma and Minimum Movement Temperatures of Stored-Product Beetles in Stored Wheat.

The minimum temperature at which stored-product insects can move inside stored grain bulks with different grain moisture contents is not known. The movement of adults of four stored-product insects, Cryptolestes ferrugineus (Stephens), Tribolium castaneum (Herbst), Sitophilus oryzae (L.), and Sitophilus zeamais (Motschulsky), at low temperatures inside jars or bulks of wheat with 11.1 or 16.0% moisture content was measured. Spontaneous walking stops (SMh), CCT (no movement after shaken), and minimum movement (TMmin, caught in probe pitfall traps) temperatures were determined at stepped-decrease of temperature. The ranges of SMh, CCT, and TMmin temperatures of the four stored-products beetles were 4.0 to 9.0°C, 2.0 to 8.0°C, and 6.0 to 11.5°C, respectively. The TMmin was ∼3°C higher than the SMh for all the tested beetles. C. ferrugineus had the lowest SMh, CCT, and TMmin temperatures, whereas S. zeamais had the highest values of these determined temperatures. C. ferrugineus under a faster stepped-decrease of temperature had higher SMh, CCT, and TMmin temperatures than that under a slower stepped-decrease of temperature, while adults of T. castaneum were not influenced by the temperature decrease rate. The two species of Sitophilus did not survive to the end of the experiment at the faster stepped-decrease of temperature. Two grain moisture contents did not affect these determined temperatures for all tested species.