| In recent years stored product psocids have received much attention. This group of insects has become a growing problem in grain storages on farms, collection centres, export terminals and warehouses with bagged commodities and grain processing facilities world wide. Increasingly, losses due to psocids have been reported from the United Kingdom, Spain, Australia, Mexico, the United States, Zimbabwe and some Asian countries. Among them the genus Liposcelis are particularly serious stored product pests. Routine fumigations of phosphine in some countries have led to higher levels of resistance in these pests leading to the confirmed failure of almost all the registered grain protectants against these pests. Liposcelis yunnaniensis Li & Li (Psocoptera:Liposcelididae) has been recently discovered in China. The development of a sound pest management program relies heavily on thorough understanding of pest biology. Heat treatment freshly re-emerged as a potential control method for stored product insects and is drawing interest as an alternative to structural fumigations with methyl bromide.The present study examines temperature-dependant developmental rate, temperature thresholds, survivorship, and fecundity of L. yunnanienis. Its population growth with respect to pest preference of different cereals is also verified.Phosphatases are classified according to optimum pH as alkaline phosphatases (ALP) and acid phosphatases (ACP). Their wide distribution in nature, from bacteria to humans, indicates an involvement in fundamental biochemical processes. Despite interest and investigation, the physiological roles of phosphatases are as yet unknown. Owing to the scarcity of L. yunnanienis cultures, biochemical studies were conducted on L. bostrychophila Badonnel. The biochemical characterizations of phosphatases between the susceptible and resistant strains of L. bostrychophila were comparatively analyzed via microplate reader method. This research was funded in part by National Natural Science Foundation of China (30871631).1. Development and reproduction of L. yunnaniensis as a function of temperatureL. yunnaniensis is a newly discovered species originally obtained form the Inner Mongolian Autonomous Region of China. Thus no previous information is available on biology and ecology of the species. Development, survival and reproduction of the L. yunnaniensis psocid were evaluated at ten constant temperatures (20,22.5,25,27.5,30, 32.5,35,37.5,39, and 41±0.5℃). The colonies were maintained on an artificial diet consisting of whole wheat flour, skim milk, and yeast powder (10:1:1) in a temperature controlled room at 27.5±0.5℃, and a scotophase of 24 h. Cultures were set up in glass bottles (250 ml) with a nylon screen cover and kept in desiccators (5000 ml) in which the humidity was controlled with saturated NaCl solution at 75-80%.1.1 Development of L. yunnaniensis at different temperaturesL. yunnaniensis reared at 41±0.5℃resulted in a complete failure of embryonic development. Similarly at 39±0.5℃the insects could complete the egg development but none of them reached the adult stage. Therefore data from 41±0.5℃and 39±0.5℃were excluded from analysis. From 20 to 37.5±0.5℃, the mean development period from egg to adult ranged form 64.3 d at 20℃to 16.1 d at 35℃. Non-linear regression models of various developmental stages, which primarily depend upon the principle of negative growth rate upon reaching the optimum temperature, gave rise to somewhat exaggerated lethal estimates, particularly for egg and preoviposition stage than the actual observed conditions. The upper lethal temperature for egg, nymphs, combined immature stages and preoviposition stage varied from 44.77,38.76,38.17, and 47.84℃, whereas the lower temperature thresholds remained 16.84,20.93,22.52, and 18.84℃respectively. Percent survival from egg to adult stage varied from 56.59 % at 20℃to 31.25% at 27.5℃. The preoviposition period of females ranged from 18.5 d at 22.5℃to 3.1 d at 35℃. L. yunnaniensis produced the most eggs (32.6 eggs/ female) at 35℃and the fewest number of eggs (10.33 eggs/female) was produced at 22.5℃. The weibull frequency distribution gave a good fit to the data set over a full range of temperatures, and populations reared at all the temperatures had type III survivorship curve (c< 1.0).1.2 Life table of L. yunnaniensis at different temperaturesThe intrinsic rate of increase (rm), net reproductive rate (R0), mean generation time (T), and population doubling time (t) of L. yunnaniensis were evaluated at eight constant temperatures. By constructing the age and time specific life tables, the relationship between life table parameters and temperatures were clarified. Within the tested temperature ranges, construction of a life table for every temperature showed that psocids reared at 35℃and 37.5℃had higher intrinsic rates of increase (0.0554 and 0.0461), shorter mean generation times (34.29 and 32.91 d) and the shortest population doubling time (12.52 and 15.03 d), respectively. Based on rm value, these were the best temperatures; however, optimum temperature range for this species could be from 25℃to 37.5℃.2. Comparative population growth of L. yunnaniensis on different dietsPopulation growth of L. yunnaniensis was assessed on eight plain cereals and two artificial diets. The diets comprised of plain cereals were made of wheat, rice, corn, oats, barley and sorghum while two were the artificial diets:(1) Whole wheat:Kibbled wheat:Whole wheat flour and brewer's yeast (10:10:10:0.1, vol:vol) (Nayak and Collins 2001; we refer to this diet as Nayak wheat diet) and (2) Whole wheat flour: Skim milk:Yeast powder (10:1:1) (Leong and Ho 1990, Wang et al.1999), which we refer to as Universal diet that we are currently using in our laboratory to maintain Psocopteran colonies. Among cereals, rice and barley were used with and without hulls to determine hull effect on population growth. We investigated the hypothesis that setae on the outer covering of rice hulls may act like needles that can trigger a physical reaction on the insect epidermis and may lead to the death of insect.2.1 Population growth of L. yunnaniensis on different dietsWe investigated the population growth of the Liposcelis yunnaniensis (Psocoptera: Liposcelididae) feeding on ten different diets. Of those ten diets, eight were made of plain cereals namely wheat (Triticum aestivum L.), corn (Zea mays L.), barley (Hordeum vulgare L.), oats (Avena sativa L.), rice(Oryza sativa L), and sorghum (Sorghum bicolor L.), while two were the artificial diets named Nayak wheat diet and Universal diet. The population growth was recorded as corn> wheat> universal diet> hulled barley> rice (hulls intact)> barley (hulls intact)> sorghum> Nayak wheat diet > oats> hulled rice. After 32 d culture, the initial 5 psocids increased in population size to 41.8±4.26,41.5±4.09 and 39.1±7.64 on corn, wheat and Universal diet, respectively. Meanwhile, psocids feeding on Nayak wheat diet, oats and hulled rice had significantly smaller populations (P< 0.001) with 25.6±2.42,22.5±3.09 and 13.6±2.36, respectively. Rice and barley were included in the diets with and without hulls to determine their effect on population growth. In the case of barley, hulls had no significant effect on population growth, while rice with hulls had significantly higher populations than rice alone. The significant difference is shown by the letters of significance on error bars in the corresponding figure. This study has confirmed the relative level of suitability of different cereals for this species when damaged and we have described a method to get the uniform age adults that can be helpful in research experiments.3. Biochemical characterizations of phosphatases in L. bostrychophilaThe study on the biochemical characterizations of phosphatases showed that significant differences of both ACP and ALP activities existed among different strains of L. bostrychophila. The highest ACP activity was recorded in the susceptible strain, the highest ALP value in PH3-R while lowest value was observed in DDVP-R, suggesting that ACP might play other physiological functions rather than detoxification. In psocids there may be no direct relationship between ALP activity and resistance. Further kinetic analysis revealed that for the resistant strains, both ACP and ALP showed a significantly higher affinity to the substrate than that of the susceptible strain. The catalytic activity of ACP in the susceptible strain was lowest and vice versa for the counterparts of ALP. The higher affinity and higher catalytic activity of ACP in the resistant strains suggested that these enzymes might have developed stronger detoxification and metabolic ability and thus play some role in the resistance of the psocid L. bostrychophila. The inhibition kinetics of insecticides on ACP in vitro showed that dichlorvos and paraoxon possessed some inhibitory effects on ACP in L. bostrychophila. In contrast, cypermethrin and fenpropathrin expressed some facilitated effects on ACP in the different strains of L. bostrychophila. The kinetics of insecticides against ALP in vitro showed that all four compounds possessed some facilitated effects on ALP from the psocid. Compared to the pyrethroids insecticides, the facilitated effects of dichlorvos and paraoxon on ALP were quite poor. It was noticeable that there were some inhibition effects on ALP by paraoxon at higher concentrations.4. Menace of mites in psocoptera cultureMites can deteriorate insect cultures in several ways. These microscopic arthropods occur in the same habitats as the insects and spread with ease throughout laboratory cultures. This proliferation in insect cultures guarantees their vital importance. The history of cultures contamination by mites is long and they can infest cultures like micropropagation for plants, fungal cultures and insectaries. Their evident threat has been recognized in fungal cultures where mite invasion can destroy the culture as a whole. However, these are intolerable in insectaries due to the fact that a standard insect culture should be pure, rigorous and available to utilize in abundance irrespective of the time of use and its size. We identified a mite upto its genus Tyrophagus (Astigmata: Acaridae) in our psocid culture Liposcelis yunnaniensis (Psocoptera:Liposcelididae). Once introduced in a culture it is difficult to irradicate. The probable mechanisms of its continued existence in insect cultures are discussed briefly in this study. |
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