The effect of thermal acclimation and relative humidity on the oxygen consumption of three Sitophilus species

The oxygen consumption of adult Sitophilus oryzae (L.). S. granarius (L.) and S. zeamais Motsch, that were acclimated at 27 or 15°C was measured at constant temperatures of from 10 to 30°C and relative humidities of 94, 70 or 50%. Warm-acclimated weevils that were free to move within the respirometers used significantly more oxygen than cold-acclimated weevils in almost all measurement regimens. In general, oxygen uptake was greatest at 70% r.h., intermediate at 94% and lowest at 50%. High oxygen consumption in certain regimens suggested an interaction between the effects of humidity and measurement temperature in S. oryzae and, to a lesser extent, in S. granarius. An interaction between humidity and acclimation temperature was indicated in S. granarius by the finding that the Q₁₀s of warm-acclimated weevils increased as humidity decreased whereas the Q₁₀s of cold-acclimated weevils did not change. The relationships between the logarithm of oxygen consumption and measurement temperature were described in all three species by quadratic functions. The oxygen consumption of S. oryzae closely restrained within the respirometers was not affected by relative humidity. Such weevils consumed more oxygen at 15 and 20°C when warm-acclimated than they did when cold-acclimated; there was no significant difference, however, between the oxygen consumption of warm- and cold-acclimated weevils at either 25 or 30°C.     

Susceptibility of local strains of Sitophilus oryzae (L.) and Tribolium castaneum (Herbst) to insecticides

The toxicity of several insecticides to two local strains of S. oryzae and T. castaneum was determined by exposure to treated papers or wheat. Malathion, lindane, dichlorvos and diazinon were more toxic than DDT, carbaryl and pyrethrins. The S. oryzae strain was very susceptible to lindane (0·43×) and malathion (0·26×), but the T. castaneum strain was found to be tolerant to lindane (1·95×) and malathion (7·6×). The T. castaneum was more tolerant to all tested insecticides than the S. oryzae strain.     

Life history studies of internally feeding pests of stored sorghum: Sitotroga cerealella (Ol.) and Sitophilus oryzae (L.)

A laboratory study of the ecology of single species populations of Sitotroga cerealella (Olivier) and Sitophilus oryzae (L.) was made at 3 temperatures (25, 30 and 35°C) and 3 relative humidities (60, 70 and 80% r.h.). The effects of 2 lower humidities (40 and 50% r.h.) on both species were also studied at 30°C. The optimum conditions for multiplication of S. cerealella were found to be 25–30°C and 60–80% r.h. while for S. oryzae the optimum was more clearly defined (30°C and 80% r.h.). However S. cerealella could tolerate a higher temperature (35°C) and lower humidities (40 and 50% r.h. at 30°C) than S. oryzae. The results are compared with those of other workers on sorghum and other cereals, and discussed in relation to the distribution of the two species in Sudan.     

Diflubenzuron as a grain protectant for control of Sitophilus species

Diflubenzuron, applied to wheat at low doses (0.2–0.6 mg kg⁻¹), prevents development of first generation (F1) progeny of Sitophilus oryzae and S. granarius species except those developing from a short period of oviposition (1–2 weeks) immediately after application. These F1 progeny fail to produce F2 progeny when transferred to wheat dosed with diflubenzuron, and produce very few progeny when transferred to untreated wheat suggesting an effect on fertility in the adult insect. At 30°C, a dose of 0.4 mg kg⁻¹ is adequate to control S. oryzae and S. granarius, although a dose of 0.6 mg kg⁻¹ is required at 20°C. Strategies for use of diflubenzuron are discussed.     

Efficacy of the amorphous silica dust, Dryacide, against Tribolium confusum Duv. and Sitophilus granarius (L.) (Coleoptera: Tenebrionidae and Curculionidae)

Amorphous silica dust (Dryacide) was used to treat wheat at concentrations of 0, 250, 500, 750, and 1000 μg silica dust/g wheat. Adult Sitophilus granarius and Tribolium confusum were placed in the grain which was then incubated at 20 or 30°C and 40 or 60% r.h. Mortality counts were taken after 48 and 168 hr. S. granarius was more susceptible to silica dust than T. confusum under the same conditions. At the same temperature, the toxicity of silica dust to both species was more pronounced at 40 than 60% r.h. T. confusum was more tolerant to silica dust at 30 than at 20°C; whereas, S. granarius was more susceptible to it at 30 than 20°C. Silica dust reduced progeny 100% at 40% r.h. in all the concentrations that had been used. Progeny, however, were produced by S. granarius at 30°C and 60% r.h. but with significantly reduced numbers with increasing dosge. Silica dust had no adverse effect on wheat seed germination, wheat flour, and baking quality.     

A novel use of modified atmospheres: Storage insect population control

The research described here aimed to establish the feasibility of using modified atmospheres (MA) to protect commodities throughout their storage life by using oxygen (O₂) levels that disrupt the life cycles of the target beetle species. Rather than achieving complete mortality of all stages, the aim was to identify more easily obtainable MAs that would kill the most susceptible stage and prevent population growth. Simulated burner gas and nitrogen (N₂) atmospheres with O₂ contents between 3% and 6%, were tested, along with a N₂-based MA with elevated carbon dioxide (CO₂) (10–20%).

Laboratory tests were carried out on five species of stored-product beetles, Cryptolestes ferrugineus, Oryzaephilus surinamensis, Sitophilus granarius, S. oryzae and Tribolium castaneum. After exposure to the MAs for 28 d an assessment was made of the mortality of adults, the number of adults from progeny produced under the MAs and, for the simulated burner gas, the number of adults from progeny produced in a 28-d period after exposure to the MA. The tests were carried out at 20 and 25 °C with 75% and 85% r.h. at each temperature.

The O₂ content preventing population growth varied with species and temperature. For simulated burner gas or N₂ it was about 4% for O. surinamensis, S. granarius and S. oryzae, and about 3% for C. ferrugineus and T. castaneum at 25 °C. At 20 °C it was about 3% for all species tested. When CO₂ was increased to 10% or 20%, reducing O₂ to 5% was sufficient to eliminate emergence of S. granarius at 20°C, but a few individuals emerged at 25 °C. For C. ferrugineus there was a 95% reduction with 5% O₂ plus 20% CO₂ at 20 °C, but not at 25 °C.     

The mortality of Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae) and Sitophilus oryzae (L.) (Coleoptera: Curculionidae) at moderate temperatures

A method using conductive heating is described for quickly obtaining and maintaining moderate temperatures in grain while minimising grain moisture loss. A model is developed to analyse insect mortality at a range of conditions on log-time logit and log-time probit scales. Values of LT 50, LT 99, and LT 99.9 are presented for adult and immature stages of Sitophilus oryzae in wheat from 42 to 48°C at 12% m.c. and for immature stages of Rhyzopertha dominica in wheat from 45 to 53°C at 9, 12 and 14% m.c. Data show that the heat tolerance of all life cycle stages for both species are different, with a degree of progressive change relative to each other as temperature changes. For R. dominica, the thermal tolerance of life cycle stages also changes relatively with changes in grain moisture content. In general, all stages survive longer at a given temperature as grain moisture increases and the effect of moisture on survival increases as temperature decreases. Values for LT 99.9 for the most heat tolerant stage of R. dominica at 9% m.c. range from 78.22 to 2.49 h between 45 and 53°C. At 12% m.c. they range from 96.81 to 3.36 h, and at 14% m.c. from 114 to 4.14 h. In general, at 12% m.c. the heat tolerance of S. oryzae is about 5°C less than R. dominica with LT 99.9 ranging from 37.36 to 3.71 h between 42 and 48°C. At this level of mortality, immature stage three is generally the most heat tolerant stage of R. dominica, whereas it is stage two for S. oryzae. The results are discussed in relation to practical applications of heat disinfestation and in terms of a general model of mortality. The results are also considered in the context of other available information on the heat treatment of both species.     

Susceptibility of stored-product beetles on wheat and maize treated with thiamethoxam: effects of concentration, exposure interval, and temperature

Sitophilus zeamais Motschulsky, the maize weevil, Oryzaephilus surinamensis (L.), the saw-toothed grain beetle, and Tribolium castaneum (Herbst), the red flour beetle, were exposed for 1, 2, 3, and 6 d at 22°C, 27°C, and 32°C on maize treated with 0, 0.5, 1.0, 2.0, or 4.0 ppm thiamethoxam, a new-generation neonicotenoid insecticide. A second series of tests was conducted on hard winter wheat using S. oryzae (L.), the rice weevil, Rhyzopertha dominica (F.), the lesser grain borer, and T. castaneum. Mortality of all species on both commodities generally increased with insecticide concentration, exposure interval, and temperature, and data were described by linear and non-linear regressions with concentration as the independent variable. Mortality of S. zeamais ranged from 58% to 90% on maize treated with 0.5 ppm thiamethoxam, and approached 95–100% as concentration increased to 4 ppm. Oryzaephilus surinamensis appeared to be slightly less susceptible than S. zeamais; mortality ranged from about 18% to 80% at 5 ppm and there was a more gradual increase in mortality as concentration increased. Mortality of T. castaneum generally did not exceed 40% at any concentration unless the beetles were exposed for 6 d. Mortality of R. dominica and S. oryzae was less than 60% when exposed on treated wheat for 1 and 2 d, but increased to nearly 100% when exposed for 6 d at 27°C and 32°C. Mortality of T. castaneum did not exceed 20% at the 1- and 2-d exposures, and approached 100% only when beetles were exposed for 6 d at 32°C. Few F₁ adults of any species were found in treated maize or in treated wheat but the number of F₁ T. castaneum in untreated maize and untreated wheat was very low compared with the other species. Results show that thiamethoxam would be an effective protectant of stored maize seed and stored wheat seed.     

The influence of oxygen on the toxicity of fumigants to Sitophilus granarius (L)

The toxicity of six fumigants—methyl bromide, ethylene dibromide, ethylene oxide, hydrogen cyanide, hydrogen phosphide and carbon tetrachloride to S. granarius was determined in atmospheres containing 0, 1, 3, 20·9 and 100 per cent oxygen. Toxicity was generally increased with decreasing oxygen concentration until the latter was reduced to 1 per cent. In an atmosphere of nitrogen, where no oxygen was present, toxicity was reduced by varying degrees depending on the fumigant; with hydrogen phosphide the insects could not be killed by any concentration of fumigant applied to them in the absence of oxygen. Carbon tetrachloride was most toxic in 20·9 per cent oxygen and below this concentration toxicity was reduced as oxygen was depleted. Tribolium confusum Du V. was found to differ from S. granarius because, in the absence of oxygen, the toxicity of methyl bromide was not reduced.     

Simulation studies on the effects of solar heat on egg-laying, development and survival of Callosobruchus maculatus (F.) and Callosobruchus subinnotatus (Pic) in stored bambara groundnut Vigna subterranea (L.) Verdcourt

The effects of simulated solar heat on oviposition, development and survival of Callosobruchus maculatus (F.) and Callosobruchus subinnotatus (Pic) in stored bambara groundnut, Vigna subterranea (L.) Verdcourt, were evaluated at three high temperatures (40°C, 45°C and 50°C) at a constant, low humidity (30% relative humidity). Exposure to these temperatures for 6 h significantly reduced oviposition in C. maculatus and C. subinnotatus females. Females of both species that were exposed to 50°C laid significantly fewer eggs than those exposed to 40°C; and in the case of C. maculatus, females exposed to 45°C also laid significantly fewer eggs than those exposed to 40°C. The percentage of eggs laid by females of both species that reached adulthood after exposure to 50°C for 2–6 h was significantly lower than the percentage that developed from eggs laid by females that were exposed to 40°C. No adult developed from eggs of C. maculatus exposed for 6 h at 50°C or from eggs of C. subinnotatus exposed for 2 h at this temperature. For both species, no adult progeny subsequently emerged from seeds harbouring first instar larvae when exposed at 50°C for 2, 4 or 6 h. Older larvae of C. maculatus were more tolerant of exposure at 50°C: 26.8, 10.2 and 0.9% of late instar larvae exposed for 2, 4 and 6 h, respectively, developed to the adult stage. In contrast, no adults of C. subinnotatus emerged from seeds harbouring late instar larvae when exposed at 45°C for 6 h nor in seeds exposed to the temperature of 50°C for 4 or 6 h. On average, immature stages of C. subinnotatus were more susceptible to heat treatment than those of C. maculatus.     

Hermetic storage of pearl millet, Pennisetum americanum (L.) Leeke; laboratory studies

The effect of airtight storage of pearl millet, Pennisetum americanum (L.) Leeke, on intergranular atmosphere, insects, mold infection, grain viability and moisture content (m.c.) was investigated in the laboratory. Six hermetically sealed 19-1 cans were used to store millet of three moisture contents (9.9, 12.8 and 15.6%) for 76 days at 27 ± 1°C. One can of grain of each m.c. was infested with 150 adult Sitophilus oryzae L./kg; the others remained uninfested. Carbon dioxide concentrations increased and oxygen concentrations decreased in all cans infested with insects, and in uninfested grain of 15.6% m.c. The changes in atmospheric concentrations were slower in the uninfested grain than in the infested 15.6% m.c. grain. No live insects were found in any treatment, percentages of mold-infested kernels were not significantly different among treatments, and m.c. did not change due to treatment or time. Kernel viability decreased significantly only in the 15.6% m.c. gram. All grain appeared normal; but the 15.6% m.c. grain had a slightly sour odor.     

The influence of relative humidity and thermal acclimation on the survival of adult grain beetles in cooled grain

The influence of humidity and thermal acclimation on the survival of Cryptolestes ferrugineus, Oryzaephilus surinamensis, Rhyzopertha dominica, Sitophilus granarius, S. oryzae and Tribolium castaneum in cooled wheat or flour at 45 and 70% relative humidity was studied in the laboratory. Young adults were held continuously at 30 or 32°C; cooled gradually to 13.5°C and held at that temperature; cooled gradually to 9°C and held at that temperature; or transferred directly to 9°C.

Survival at low temperatures differed, often considerably, between species, being generally shortest in T. castaneum (0.7–16 weeks) and longest in S. granarius (5–40 weeks). There was considerable interaction between the effects of temperature and humidity, with survival being, to varying degrees, shorter in 45% r.h. than in 70% r.h. At 45% r.h., an equilibrium relative humidity common in Australian storages, all species survived when the grain was cooled to 13.5°C but only S. granarius survived 26 weeks in grain cooled to 9°C.

Chill-coma temperature and acclimation temperature were linearly related in all species at each humidity. Neither the slopes nor intercepts of the relationships were influenced by humidity. There was generally an inverse relationship between survival and chill-coma temperature.

At 45% r.h., the increase in survival at 9°C attributable to acclimation ranged from 2-fold in S. oryzae (0.9 weeks) to 3.5-fold in C. ferrugineus (3.8 weeks). Corresponding values for beetles at 70% r.h. ranged from 2.3-fold in R. dominica (2.4 weeks) to 7.9-fold in C. ferrugineus (6.9 weeks).     

The synergism of DDT, deutero-DDT, and methoxychlor in a pyrethrin-resistant strain of Sitophilus granarius (L.) (Coleoptera, Curculionidae)

A pyrethrin-resistant strain of Sitophilus granarius was also cross-resistant to DDT and to less easily dehydrochlorinated analogues such as Prolan. Compounds which were known to inhibit the oxidative detoxication of DDT and pyrethroids, synergized DDT against the resistant S. granarius but not against a susceptible strain. The active compounds were: piperonyl butoxide, sesamex, sulfoxide, Piprotal (‘Tropital’), Synepirin 500, and SKF 525A. Of three compounds known to inhibit the detoxication of DDT to DDE in other insects, two, DMC and piperonyl cyclonene, failed to synergize DDT against either strain of S. granarius; however, WARF-antiresistant was an effective DDT synergist in the resistant strain. Very large factors of synergism were obtained with methoxychlor plus sesamex against both resistant and susceptible weevils.     

Fumigation with carbonyl sulfide: a model for the interaction of concentration, time and temperature

The new fumigant carbonyl sulfide offers an alternative to both methyl bromide and phosphine as a grain fumigant. Separate mathematical models for levels of kill, based on quantitative toxicological studies were developed for adults and eggs of the rice weevil Sitophilus oryzae (L.). These models suggest that fumigation exposure times for carbonyl sulfide will be a compromise between those of methyl bromide (typically 24 h) and phosphine (7–10 d) to achieve a very high kill of all developmental stages. S. oryzae eggs were more difficult to kill with carbonyl sulfide fumigation than the adults. At 30°C, a 25 g m⁻³ fumigation killed 99.9% of adults in less than 1 d, but took 4 d to kill the same percentage of eggs. Models were generated to describe the mortality of adults at 10, 15, 20, 25 and 30°C. From these models it is predicted that fumigation with carbonyl sulfide for 1–2 d at 30 g m⁻³ will kill 99.9% of adults. Furthermore the models illustrate that fumigations with concentrations below 10 g m⁻³ are unlikely to kill all adult S. oryzae. Significant variation was observed in the response of eggs to the fumigant over the temperature range of 10 to 30°C. Models were generated to describe the mortality of eggs at 10, 15, 20, 25 and 30°C. As the temperature was reduced below 25°C, the time taken to achieve an effective fumigation increased. Extrapolating from the models, a 25 g m⁻³ fumigation to control 99.9% of S. oryzae eggs will take 95 h (4 d) at 30°C, 77 h (3.2 d) at 25°C, 120 h (5 d) at 20°C, 174 h (7.5 d) at 15°C and about 290 h (11 d) at 10°C. The role of temperature in the time taken to kill eggs with carbonyl sulfide cannot be ignored. In order to achieve the desired level of kill of all developmental stages, the fumigation rates need to be set according to the most difficult life stage to kill, in this instance, the egg stage.     

A practical trial of pyrethrins-in-oil surface sprays for the protection of bagged grain against infestation by Cadra cautella (Wlk.) (Lepidoptera, Phycitidae) in Kenya

Two formulations of synergized pyrethrins in technical white oil were tested as monthly protective sprays on stacks of fumigated bagged wheat, primarily against Cadra cautella (Wlk.) but also against Sitophilus oryzae (L) and Tribolium castaneum (Hbst.), under warm-temperate storage conditions in up-country Kenya. The formulations were: 0·4% pyrethrins with 2·0% piperonyl butoxide, applied at 50 ml/m², and 0·4% pyrethrins with 0·4% piperonyl butoxide at 20 ml/m².

Results were assessed by recording infestation in samples taken from each stack after 18 weeks storage and five spray applications.

Both treatments gave reasonably good protection against C. cautella but were not satisfactory against S. oryzae or T. castaneum. There was no evidence of any taint in bread made from the treated grain, but the higher application rate caused excessive staining of the bags.

It is concluded that satisfactory control of reinfestation by C. cautella can be expected in practice using 0·4% pyrethrins in oil with only a minimal quantity of added piperonyl butoxide, and that 20 ml/m² is a suitable rate for application to bagged produce.     

Vapours of Acorus calamus oil—A space treatment for stored-product insects

The toxic effects of the vapours of the essential oil of Indian Acorus calamus L. rhizomes on the adults of several stored-product insect pests were studied. The declining order of susceptibility was Callosobruchus chinensis (L.), Sitophilus granarius (L.) and Sitophilus oryzae (L.), with Tribolium confusum J. du V. and Rhyzopertha dominica (F.) being tolerant to all doses and periods of exposure tested in this study. The period of exposure appeared to be the most important factor affecting the efficiency of these vapours rather than the dosage.     

The inheritance of DDT resistance in Sitophilus oryzae (L.) (Coleoptera, Curculionidae) in Queensland

DDT resistance in Sitophilus oryzae (L.) in Queensland was confirmed using a test method of continuous exposure to DDT-impregnated filter paper. When exposed to 1 per cent DDT papers, a laboratory susceptible strain had a median response time of 3 hr, a laboratory resistant strain, 44 hr, and a field resistant strain > 72 hr. The laboratory resistant strain had a resistance factor of × 12·8 for males and × 11·3 for females based on 24 hr responses using a concentration variable. The resistance factor of the field resistant strain could not be measured with the method used.

DDT resistance was controlled by one or more sex-linked, semi-dominant factors and undetermined modifying and segregation distorting factors were present. Segregation aberrations were recorded.     

Repellent effect of pea (Pisum sativum) fractions against stored-product insects

Peas (Pisum sativum) are toxic to some stored-product insects. The repellent effect of fractions of pea seed to stored-product insects was evaluated in multiple-choice tests in which wheat kernels were dusted with fractions rich in either protein, fibre or starch at 0.001 to 10% (wt:wt). There was a negative correlation between pea protein concentration and the number of adults found in grain for Cryptolestes ferrugineus and Sitophilus oryzae, but not for Tribolium castaneum. Pea fibre repelled C. ferrugineus adults but not S. oryzae and T. castaneum. Pea starch did not repel any of the insects. One-week old and 6-week old C. ferrugineus were equally repelled by pea protein. Repellency was detectable 1 h after exposure. Cryptolestes ferrugineus and S. oryzae did not become habituated to the repellent action of pea protein even after 4 weeks of exposure. Habituation was observed, however, when C. ferrugineus was exposed to pea fibre for 4 weeks. In a two-choice bioassay (0 vs. 0.1% and 0 vs. 1% pea protein), the pea-protein-treated grain had significantly fewer insects (C. ferrugineus, S. oryzae, Sitophilus zeamais, T. castaneum, and Tribolium confusum) than untreated grain. The properties of the pea protein fractions seem well suited for developing a natural stored grain protectant.     

Observations on the susceptibility of compressed dried grass and legume to infestation by some storage insects

Under conditions controlled at 25°C and 70% r.h. and under outdoor conditions in southern England, the storage insect pests Sitophilus granarius (L.), Oryzaephilus surinamensis (L.), Cryptolestes ferrugineus (Steph.) and Ephestia elutella (Hueb.) were unable to breed or to survive for long on blocks of compressed, dried forage (grass or legume). The beetles Ptinus tectus Boield. and Trigonogenius globulus Sol. were able to breed on this material under both sets of conditions, but Stegobium paniceum (L.) only at 25°C. At the higher relative humidities associated with outdoor conditions the moths Hofmannophila pseudospretella (Staint.) and Endrosis sarcitrella L. produced thriving cultures. The damage caused by S. paniceum and the moths was severe; in particular, H. pseudospretella completely destroyed the foodstuff during 1 yr of storage at ambient outdoor shade temperatures in Britain.     

Development of four strains of Sitophilus oryzae (L.) (Coleoptera: Curculionidae) on barley, corn (maize), rice, and wheat

Differences in development of four strains of Sitophilus oryzae (L.) (LS-2, Minnesota, Savannah, and Tanzania-90) on diets of hulled barley, yellow corn (maize), brown rice, and soft red winter wheat at 28°C and 55–60 r.h., were analyzed in terms of total progeny production, average time (days) to adult emergence, and rate of emergence with an analysis of variance. Significant main effects of strain and diet were obtained. Across all strains, progeny production was highest on barley and lowest on corn. All measured growth parameters indicated that corn was the least suitable diet of the tested cereals for this species. The Savannah strain of S. oryzae produced significantly more progeny and had shorter development times than the other strains. The strain from Tanzania produced smaller numbers of progeny, had significantly longer development times, and a significantly slower rate of emergence on wheat compared to that of the other strains. The Tanzania strain does not have the major amylase isozyme present in this species and has less than 50% of the total amylase activity found in the other three strains. It is postulated that the strong Tanzania × wheat interaction is caused by combination of naturally-occurring amylase inhibitors in ingested wheat with digestive amylase in this weevil strain and a subsequent decreased efficiency in food utilization.