Some aspects of acclimation to low temperatures in the grain weevils Sitophilus oryzae (L.) and S. granarius (L.)

The ability of Sitophilus oryzae (L.) and S. granarius (L.) to acclimate when transferred from 27°C, a near optimal temperature, to 15° C, a marginal temperature, was examined in terms of chill-coma temperature, dispersal, oxygen consumption and rate of oviposition. The chill-coma temperature of S. oryzae was higher and the increment of acclimation less than that of S. granarius. Acclimation, although well advanced after 2 weeks after transfer, took more than 4 weeks to complete. Re-acclimation after the reciprocal transfer took only 2 weeks. The dispersal of S. oryzae held at 15 and 27° C prior to testing differed when the weevils were released in wheat at 27°C but not when released at 15°C. Cold-acclimated S. granarius dispersed less than warm-acclimated weevils when released in wheat at both 15 and 27°C. The activity oxygen consumption of cold-acclimated weevils was less than that of warm-acclimated weevils. Inverse-shifts of acutely determined R-T relationships appeared more significant than changes in temperature sensitivity. Acclimation to 15° C took 8–10 days and re-acclimation to 27°C about 6–8 days. S. oryzae consumed less oxygen per unit of weight than S. granarius but was more temperature sensitive. Resting oxygen consumption, which was less temperature sensitive than activity consumption, also exhibited inverse acclimation in both species. S. granarius showed evidence of inverse acclimation of oviposition rate after acclimation at 15°C.     

A comparison of the capacity for increase at a low temperature of foreign and Australian populations of Sitophilus oryzae (L.) and S. granarius (L.)

The capacity for increase (r_c) of three populations of S. oryzae and three populations of S. granarius from Great Britain and Canada was determined over 15 and 24 weeks respectively at 15°C in wheat of 14% moisture content. The fertility of these populations at 27°C, their body weight and chill-coma threshold was also measured. With both S. oryzae and S. granarius, values of r_c for the foreign populations fell within the range previously observed when several Australian populations were reared under the same conditions. The cohort generation times (T_c) of the foreign S. oryzae populations were significantly shorter than those of the Australian populations because of the shorter immature development periods (D) of the former. Two of the foreign S. granarius populations had lower values for D than all but one of their Australian counterparts but their values of T_c were within the range previously observed for Australian weevils. When the Australian and foreign populations were considered together, the value of r_c was more strongly correlated with fertility at 27°, an optimal temperature, than with body weight in both S. oryzae and S. granarius. With S. oryzae, r_c was correlated with the chill-coma threshold of cold-acclimated weevils only. There were no correlations between r_c and chill-coma thresholds in S. granarius. It was concluded that the observed differences in r_c were related to differences in the vigour of the populations rather than to physiological differences in cold-tolerance. The likelihood of grain weevils becoming cold-tolerant as a result of grain aeration is discussed and the practical significance of differences in r_c considered.     

Scanning electron microscopy of the spermatheca in Sitophilus granarius (L.)

With scanning electron microscopy the spermathecal organ of Sitophilus granarius (L.) has been examined. The only morphological attachment between this organ and other tissues is to the bursa copulatrix by the sperm duct and to the central nervous system by two nerve fibers. The nerves innervate the muscles connecting the two arms of the U-shaped spermatheca. Glandular cells are grouped together and are connected to the spermathecal lumen by a short collecting duct. Vascular sinuses opening from the body cavity into the body of the gland are observed as pores on the surface of the gland.     

Observations on some effects of temperature and isolation on fecundity of female weevils, Sitophilus granarius (L.) (Coleoptera, Curculionidae)

Egg laying and longevity of individual female grain weevils kept with males throughout life has been compared with those isolated after an initial period of pairing. There was no difference in longevity or egg-laying at 25° but the rate of egg-laying was reduced while longevity was increased in paired females at 20°. Egg-laying by isolated females at 20° fell off rapidly.

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Zusammenfassung Es wurde die Eiproduktion und Lebensdauer von einzelnen Kornkäfer-Weibchen (Sitophilus granarius) verglichen, die entweder während ihres gesamten Imaginallebens mit Männchen zusammen oder die nach einer Woche von den Männchen getrennt gehalten wurden. Bei 25° bestand zwischen diesen beiden Gruppen von Weibchen kein signifikanter Unterschied in der Lebensdauer und der Eiprodukationsrate. Die wöchentliche Eiproduktionsrate gepaarter Weibchen war bei 20° niedriger als bei 25°, dagegen stieg ihre Lebensdauer an. Die Eiproduktion isolierter Weibchen fiel bei 20° sehr stark ab, aber es fand sich kein Hinweis dafür, daß die Spermen nach Übertritt in das Receptaculum bei dieser Temperatur einen Verlust an Beweglichkeit erleiden. Die Ergebnisse werden im Hinblick auf die Ökologie der Art im Getreide diskutiert.

     

The orientation of grain weevils (Sitophilus granarius): Influence of spontaneous turning tendencies and of gravitational stimuli

Summary The paths followed by grain weevils (Sitophilus granarius) in a featureless environment are curved or circular. Animals walking in circles display average angular velocities that may exceed 10°/s. The circling is not due to any asymmetrical damage to the weevil's limbs. Circling weevils walking on a sloping surface follow characteristic paths: on slight slopes the circles become cycloids, which in turn, become relatively straight paths on steep slopes. The mean direction of the cycloidal paths deviates from the direction of the straight paths. Left circlers deviate to the left and right circlers deviate to the right. These results can be explained by a feedback control loop with an additive interaction of internal and feedback signals. The cycloid paths thus result from an unstable state of the feedback loop in which the feedback signal from the gravity receptors is always smaller than the internal signal.     

Inheritance and Characterization of Strong Resistance to Phosphine in Sitophilus oryzae (L.)

Sitophilus oryzae (Linnaeus) is a major pest of stored grain across Southeast Asia and is of increasing concern in other regions due to the advent of strong resistance to phosphine, the fumigant used to protect stored grain from pest insects. We investigated the inheritance of genes controlling resistance to phosphine in a strongly resistant S. oryzae strain (NNSO7525) collected in Australia and find that the trait is autosomally inherited and incompletely recessive with a degree of dominance of -0.66. The strongly resistant strain has an LC50 52 times greater than a susceptible reference strain (LS2) and 9 times greater than a weakly resistant strain (QSO335). Analysis of F2 and backcross progeny indicates that two or more genes are responsible for strong resistance, and that one of these genes, designated So_rph1, not only contributes to strong resistance, but is also responsible for the weak resistance phenotype of strain QSO335. These results demonstrate that the genetic mechanism of phosphine resistance in S. oryzae is similar to that of other stored product insect pests. A unique observation is that a subset of the progeny of an F1 backcross generation are more strongly resistant to phosphine than the parental strongly resistant strain, which may be caused by multiple alleles of one of the resistance genes.     

In vitro studies on the effects of several insecticides on non-specific esterases of Sitophilus oryzae (L.) and Palembus dermestoides (fairm.)

1. Sitophilus oryzae (Curculionidae) and Palembus dermetoides (Tenebrionidae) esterases were studied to determine the correlation between in vitro esterase inhibition and in vivo toxicity of several insecticides.2. K_m values for S. oryzae and P. dermetoides were 0.118 mM and 0.087 mM respectively.3. In vitro esterase inhibition by organophosphates and carbamates showed that for S. oryzae dichlorvos (I₅₀ = 7 × 10⁻⁶ M) and eserine sulphate (I₅₀ = 5 × 10⁻⁵ M) were most potent; for P. dermestoides it was methomyl (I₅₀ = 2.8 × 10⁻⁷ M) and dichlorvos (I₅₀ = 10⁻⁶ M). For both species chlorpyrifos-methyl was least so.4. Only slight correlation between in vitro esterase inhibition and in vivo toxicity of the insecticides could be shown for either species. This is discussed.     

The capacity for increase at a low temperature of some Australian populations of the granary weevil,Sitophilus granarius (L.)

The capacity for increase (r_c) of one laboratory and seven field populations of young adult S. granarius from different sites in Australia was determined over thirty-two weeks at 15°C in wheat of 14% moisture content. The mean value of r_c was 0.0704 ±0.0016 and the populations differed significantly with respect to this parameter. Variation in the net reproductive rate (R₀), which averaged 25.4± 1.29, had a greater effect on the value of I_c than did variation in the cohort generation time (T_c), which averaged 45.7±0.37 weeks. The populations did not differ significantly in terms of adult survivorship and 94% of females were still alive at thirty-two weeks. The maximum rate of oviposition, about ten eggs per female per week, was achieved in the eighteen-twenty week age-interval. About 41% of the immature stages survived to adulthood. Estimates of r_c over a twenty-four week period were only slightly lower than those over thirty-two weeks. The capacity for increase at 15°C of a given population was shown to be correlated with its fertility at 29°C, an optimal temperature, and with its body weight rather than with its cold tolerance, as indicated by its chill-coma temperature, or its previous temperature-history. The temperature experienced by the immature stages had a marked influence on r_c in that weevils reared at 15°C and 27°C had respective values of 0.0654 and 0.0786 when subsequently held at 15°C. The differences in the survivorship and fertility of S. granarius and S. oryzae (L.) when both species were reared at 15°C are considered.     

The capacity for increase at a low temperature of several Australian populations of Sitophilus oryzae (L.)

The capacity for increase (I_c) of one laboratory and seven field populations of young adult S. oryzae from different sites in Australia was determined over a thirty-week period at 15°C, a supposedly marginal temperature, in wheat of 14% moisture content. The average value of I_c was 0.0838 ± 0.0017 and the populations comprised three significantly different groups. Variations in the net rate of increase per generation (R₀) which averaged 34.7±1.71, had a major effect on the value of r_c whereas variation in the cohort generation time (T_c), which averaged 41.6±.23 weeks had only a minor effect. The survivorship of adults of the populations did not differ significantly and 93% of females were alive after thirty weeks. Estimates of r_c based on a fifteen-week period and on an amended value of Tc differed little from those over thirty weeks. The capacity for increase at 15°C of a given population was found to be correlated with its fertility at 27°C and with its body-weight rather than with its cold-tolerance, as evidenced by chill-coma temperature, or with its previous temperature-history. The temperature experienced by the immature stages of weevils had a profound effect on r_c in that weevils reared at 15 and 27°C had respective values of 0.0350 and 0.0707 when subsequently cultured at 15°C. Because all observed values of r_c were higher than expected, possible errors in method were considered. An alternative estimate of R₀ agreed closely with that observed and indicated that only 17% of the immature stages survived at 15°C. Metabolic heating due to the higher than expected population density shortened the duration of the immature stages by 7% but did not affect their survival.     

Threshold temperature and heat unit requirements for the development of the granary weevil,Sitophilus granarius (L.)

The objective of the present investigation was to determine the effects of temperature on the granary weevil, Sitophilus granarius L., in the laboratory so that forecasting models based on heat accumulation units could be developed for the pest. Development of S. granarius reared on wheat grains was studied at three constant temperatures (15, 20 and 25?°C) and relative humidity of 70?±?5%. The developmental time was significantly decreased with the increase in temperature. The developmental threshold temperatures estimated for egg, larva, pupa and from egg to adult were 7.79, 7.31, 14.08 and 9.97?°C, respectively. Based on these thresholds, the developmental stages, respectively, needed 92.75, 393.78, 45.04 and 562.36?day degrees to complete their development. Also, about nine generations were calculated for the pest per year under Assiut prevailing conditions. It seems that temperature of 25?°C is the most favourable developmental temperature.     

Myoneural junctions and neurosecretory endings on spermathecal muscle fibers of two weevils,Sitophilus granarius and Hypera postica

The fine structure of muscle fibers connecting the two arms of the spermatheca and their innervating axons was studied with the electron microscope. The muscle fibers appear to be a sub-set of skeletal and not visceral muscles. Neurosecretory axons with electron dense granules are adjacent to the muscle fibers in young females O-day post-eclosion but not in the ovipositing adult. The typical nerves form synaptic junctions with muscle fibers at all ages but the nerves are divided into two types based on the morphology of the synaptic vesicles they contain, either spherical or flattened.     

Repellent effects of certain plant essential oil,plant extracts and inorganic salts to granary weevil,Sitophilus granarius (L.)

Repellency of six materials, two plant essential oil (clove and flax seed oils), two plant extracts (neem and harmal seed extracts) and two inorganic salts (silica dust and tri-calcium phosphate), to Sitophilus granarius L. were evaluated. Per cent repellency (PR) was determined for each material. Repellent effects of tested materials were proportional to concentration and higher concentration has stronger effect. Neem and flax seed oils had the strongest repellence effect (100%) after 2, 4 and 6?h of treatment at concentrations of 20, 10 and 5?mL/kg grains. Repellency activity of both oils was gradually decreased with the passage of time to reach its lowest level after 12?h of treatment. Their repellency tended to class IV or stronger during the whole exposure period. Neem and harmal seed extracts had maximum repellency (98.7 and 86.5%, respectively) at highest concentration (1000?ppm), while the lowest concentration (250?ppm) had minimum repellency effects (62.9 and 71.6%, respectively). Both extracts showed potent repellent activity against the pest and repellency values ranged between class IV and class V. The repellency action of silica dust and tri-calcium phosphate followed nearly the same trend as shown in case of oil and extracts. High repellent activities of silica dust and tri-calcium phosphate (87.3 and 97.4%, respectively) were observed at concentration of 5?g/kg, whereas the lowest mean of repellency (56.4 and 41.0%) was recorded at concentration of 1.29?g/kg. Thus, both salts exhibited high repellency activity against the weevil and repellency values ranged between class III and class IV.