The effect of toxin-producing Fusarium fungi on behavior of the rice weevil Sitophilus oryzae (Coleoptera,Dryophthoridae)

The paper describes laboratory tests in which the behavior response of adult rice weevils Sitophilus oryzae L. to the presence of seven species of micromycetes of the genus Fusarium (F. graminearum, F. culmorum, F. cerealis, F. poae, F. sporotrichioides, F. langsethiae, and F. sibiricum; 3 strains for each species) infecting cereals was characterized. The chemicals of unkown structure, released by the fungi, can have both attractive and repellent effects on the weevils; in some cases a neutral response was observed. The strains of Fusarium species characterized as weak pathogens (F. langsethiae, F. poae, and F. sibiricum) in most cases stimulated attractive and neutral responses. Relatively strong pathogens (F. cerealis, F. culmorum, F. graminearum, and F. sporotrichioides) generally had a repellent effect. The results obtained are discussed in the context of possible relationships between Fusarium fungi and the rice weevil during their utilization of cereals as a shared food substrate.     

Glutathione S-transferase detoxification as a potential pyrethroid resistance mechanism in the maize weevil, Sitophilus zeamais

Insecticide resistance patterns among 16 Brazilian populations of the maize weevil, Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae), were recognized by surveying resistance to three organophosphates (chlorpyrifos‐methyl, malathion, and pirimiphos‐methyl) and three pyrethroids (cypermethrin, deltamethrin, and permethrin). Two population clusters were obtained: one with three populations (Bragança Paulista, Cristalina, and Nova Andradina) showing low frequency of cypermethrin resistance (13–36%) and negligible frequency of deltamethrin resistance (2–9%); and another with six populations (Campos dos Goytacazes, Ivinhema, Patos de Minas, Penápolis, Uberlândia, and Venda Nova) showing low to negligible levels of pyrethroid resistance (0–23%). The remaining seven populations, including a susceptible, and a DDT‐ and pyrethroid‐resistant reference populations (Sete Lagoas and Jacarezinho, respectively), were significantly different from each other and from the two recognized clusters. In contrast with pyrethroid resistance, organophosphate resistance was negligible except for chlorpyrifos‐methyl in two populations (Fátima do Sul and Penápolis). There was no correlation between geographic distance and the Mahalanobis distance estimated from the resistance pattern ordination of the populations by canonical variate analysis, suggesting local selection and/or broad dispersal of resistant populations by grain trade. The results of biochemical in vitro studies measuring the activity of detoxification enzymes (esterases and glutathion S‐transferases) in conjunction with canonical correlation analysis suggest a major involvement of enhanced conjugation by glutathione S‐transferases (> 2‐fold increase) in pyrethroid resistance and, in the case of cypermethrin resistance, enhanced phosphotriesterase activity.     

Rostrum structure and development in the rice weevil Sitophilus oryzae (Coleoptera: Curculionoidea: Dryophthoridae)

A documentation and review of weevil rostrum growth is made through examination of the developmental life stages in the rice weevil Sitophilus oryzae (Linnaeus). Histological and morphological examinations are made utilizing light, fluorescent, and electron microscopy. In S. oryzae, rostral tissue begins proliferating in the late 4th instar larva and continues through to the pupal stage, with the majority of rostrum growth taking place in the prepupa. Adult cranial and rostral morphology is also reviewed, focusing on structures that may be pertinent to phylogeny reconstruction. The weevil rostrum is essentially an extension of various head sclerites that are basal to the mouthparts. Therefore, while the mouthparts are fairly similar to other Coleoptera in basic form, the head is markedly different due to its anterior extension. By understanding the more noticeable details of rostrum growth and structure, this study may serve as a foundation for comparative studies of a similar nature and as a basis for beginning research on the genetic nature of rostrum formation and evolution throughout the weevil clade.     

Pectin methylesterase from the rice weevil,Sitophilus oryzae (L.) (Coleoptera: Curculionidae): Purification and characterization

A pectin methylesterase was purified to apparent homogeneity from the adult rice weevil, Sitophilus oryzae (L.), by Q-Sepharose and S-Sepharose chromatographies followed by high-performance anion-exchange chromatography. The resulting preparation is the first pectin methylesterase which has been purified from any animal species, although at this point we cannot rule out the possibility that the enzyme is produced by a symbiotic microorganism. The molecular mass of the enzyme was estimated as 38 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This mass is similar to those of pectin methylesterases previously isolated from bacteria, fungi, and plants. The purified enzyme had a broad pH optimum between 6 and 7, which appears consistent with the enzyme's probable site of action, the gut.     

Influence of maternal age on the fitness of progeny in the rice weevil, Sitophilus oryzae (Coleoptera: Curculionidae)

We studied the effects of maternal age on fitness of progeny in the rice weevil, Sitophilus oryzae L. (Coleoptera: Curculionidae). Five-, 20-, and 50-d-old female rice weevils were used to study the effects of maternal age on the lifetime fecundity and longevity of their daughters. In addition, we determined the effects of maternal age on the weight and survivorship of daughters' progeny. Daughters of 5- and 20-d-old weevils lived longer, and the numbers and weights of the progeny of these daughters were higher than for daughters of 50-d-old weevils. Survivorship of immature grand-offspring of 5-, 20-, and 50-d-old female weevils was similar. None of the fitness characteristics of the daughters and grand-offspring of 5- and 20-d-old weevils that were measured differed significantly. We believe maternal age effects on rice weevil progeny fitness may at least partly be acting through maternal age effect on egg size. Individuals that developed from younger 5- and 20-d-old weevils had a greater fitness than those produced by older 50-d-old females. Our study shows maternal age is impacting life history parameters that influence population dynamics across generations. Therefore, maternal age could significantly affect population development and have far reaching implications for pest management and simulation modeling of rice weevil populations.     

Influence of olfactory stimulants on resistance/susceptibility of pearl millet, Pennisetum americanum to the rice weevil, Sitophilus oryzae

The orientation responses of the rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae) as influenced by the olfactory stimulants emanating from either resistant or susceptible pearl millet, Pennisetum americanum, kernels were studied using a Y-tube olfactometer. The rice weevils were more attracted towards odours from millet grains than to blank controls and to susceptible entries more than to resistant entries, in general. However, there were exceptions. Odours from some resistant entries caused statistically the same, or even more positive orientations than those from some susceptible entries, and this suggests that the relative resistance of an entry and its olfactory attractiveness may not be strongly related, or that the resistance to infestation might be influenced by factors other than attractiveness. The partial damage of the millet kernel pericarp by abrasion slightly increased the orientational response of the weevils to odours from some susceptible and some resistant entries alike.     

Low temperature shock and chill-coma consequences for the red flour beetle (Tribolium castaneum) and the rice weevil (Sitophilus oryzae)

Insects face several (environmental) abiotic stressors, including low temperature, which cause the failure of neuromuscular function. Such exposure leads insects toa reversible comatose state termed chill-coma, but the consequences of this state for the organism biology were little explored. Here, the consequences of the chill-coma phase were investigated in two of the main stored product pest species – the red flour beetle Tribolium castaneum (larvae and adults) and the rice weevil Sitophilus oryzae (adults). For this purpose, a series of low-temperature shocks were used to estimate the chill-coma recovery time (CCRT), survival, nutrition and weight gain/growth of T. castaneum (larvae and adults) and S. oryzae, as well as the development of T. castaneum life stages. The relatively long CCRT was characteristic of beetle larvae, at different low-temperature shocks, and CCRT increased with decreasing temperatures and increasing exposure intervals for both pest species. The survival was little affected by the low-temperature shocks applied, but such shocks affected insect feeding and growth. Tribolium castaneum larvae was more sensitive than adults of both insect species. Moreover, the relative consumption and weight gain of S. oryzae adults were lower than those of T. castaneum adults and mainly larvae, while feeding deterrence was not affected by low temperature shocks, unlike food conversion efficiency. Low-temperature shocks, even under short duration at some temperatures, significantly delayed development. The lower the temperature and the higher the exposure period, the more delayed the development. Thus, the physiological costs of chill-coma are translated into life-history consequences, with potential implications for the management of this insect pest species in stored products and even more so on red flour beetles and rice weevils.     

Deltamethrin‐induced feeding plasticity in pyrethroid‐susceptible and ‐resistant strains of the maize weevil,Sitophilus zeamais

Phenotypic plasticity contributes to the adaptative evolution of populations exposed to new or altered environments. Feeding plasticity is a component of phenotypic plasticity not usually considered in insect strains adapted to insecticide‐altered environments, but which may either accentuate or mitigate insecticide resistance. This is a concern in the pyrethroid‐resistant strains of the maize weevil Sitophilus zeamais Motsch. (Col., Curculionidae), and the reason for this study. A pyrethroid‐susceptible and two pyrethroid‐resistant strains of maize weevil were subjected to free‐choice and no‐choice tests with maize grains sprayed with increasing doses of the pyrethroid, deltamethrin. The insects from the pyrethroid‐resistant strains exhibited higher feeding avoidance with increased deltamethrin doses than insects from the susceptible strain when subjected to free‐choice tests. The strains of maize weevil physiologically resistant to pyrethroids were also behaviourally resistant to deltamethrin – an additional management concern. The resistant strains avoid deltamethrin‐sprayed grains and are less nutritionally affected by this compound, with divergent responses from the susceptible strain with increased doses of deltamethrin. Furthermore, the higher relative growth rate and consequently higher efficiency of food conversion observed in the insecticide‐resistant strains were significant even without insecticide exposure, indicating that these traits are stimulus‐independent and may persist even without further insecticide selection, potentially limiting the options available for their management.     

Mating behaviour and reproductive output in insecticide‐resistant and ‐susceptible strains of the maize weevil (Sitophilus zeamais)

Insecticide resistance is a broadly recognised and well‐studied management problem resulting from intensive insecticide use, which also provides useful evolutionary models of newly adapted phenotypes to changing environments. Two common assumptions in such models are the existence of fitness costs associated with insecticide resistance, which will place the resistant individuals at a disadvantage in insecticide‐free environments, and the prevalence of random mating among insecticide‐resistant and ‐susceptible individuals. However, cases of insecticide resistance lacking apparent fitness disadvantages do exist impacting the evolution and management of insecticide resistance. Assortative mating, although rarely considered, may also favour the evolution and spread of insecticide resistance. Thus, the possible existence of both conditions in the maize weevil (Sitophilus zeamais), a key pest of stored cereals, led to the assessment of the mating behaviour and reproductive fitness of insecticide‐resistant and ‐susceptible weevil strains and their reciprocal crosses. The patterns of female and male mating choice also were assessed. Although mating behaviour within and between weevil strains was similar without mate choice, mating within the resistant strain led to higher reproductive output than within the susceptible strain; inter‐strain matings led to even higher fertility. Thus, no apparent fitness cost associated with resistance seems to exist in these weevils, favouring the evolution of this phenotype that is further aided by the higher fertility of inter‐strain matings. Mate choice reduced latency to mate and no inter‐strain preference was detected, but female weevils were consistent in their mate selection between 1st and 2nd matings indicating existence of female mating preference among maize weevils. Therefore, if female mate selection comes to favour trait(s) associated with insecticide resistance, higher reproductive fitness will be the outcome of such matings favouring the evolution and spread of insecticide resistance among maize weevil populations reverting into a management concern.     

Massive presence of insertion sequences in the genome of SOPE, the primary endosymbiont of the rice weevil Sitophilus oryzae

Bacteria that establish an obligate intracellular relationship with eukaryotic hosts undergo an evolutionary genomic reductive process. Recent studies have shown an increase in the number of mobile elements in the first stage of the adaptive process towards intracellular life, although these elements are absent in ancient endosymbionts. Here, the genome of SOPE, the obligate mutualistic endosymbiont of rice weevils, was used as a model to analyze the initial events that occur after symbiotic integration. During the first phases of the SOPE genome project, four different types of insertion sequence (IS) elements, belonging to well-characterized IS families from gamma-proteobacteria, were identified. In the present study, these elements, which may represent more than 20% of the complete genome, were completely characterized; their relevance as a source of gene inactivation, chromosomal rearrangements, and as participants in the genome reductive process are discussed herein.     

Toxic and deterrent effects of two volatile oils against cowpea weevil,Callosobruchus chinensis (Coleoptera: Bruchidae)

The fumigant effect of Mentha piperita and Ocimum basilicum oils and their mixture against adults and eggs of Callosobruchus chinensis (L.) was evaluated. Bioassay experiments showed that Ocimum basilicum oil was significantly effective against adult and egg stage which was the least effective. However, in the fumigant toxicity experiments against adult stage, Ocimum basilicum oil at a dose of 1.0 μl/38.5 ml air caused 100% mortality (LC₅₀ = 1.88), the mixture of both oils at a dose of 6.0 μl/38.5 ml air caused 100% mortality (LC₅₀ = 10.3) and Mentha piperita oil at a dose of 80.0 μl/38.5 ml air caused 80.0% mortality (LC₅₀ = 41.224) during a one day exposure period. Regarding the oviposition deterrent activity, Ocimum basilicum oil achieved 100% oviposition deterrent (at a dose of 0.5 μl/38.5 ml air) followed by the mixture of Mentha piperita and Ocimum basilicum oils, which achieved 71.22% oviposition deterrent (at a dose of 1.0 μl/38.5 ml air) and Mentha piperita oil, which achieved 39.6% oviposition deterrent (at a dose of 5.0 μl/38.5 ml air). The essential oils and their mixtures studied here determined a significant decrease in the number of eggs hatched and in the emergence of adults. The eggs failed to hatch on using Ocimum basilicum oil at a dose of 0.6 μl/38.5 ml air. However, the number of eggs hatched decreased to 7.4 on using a mixture of oils at a dose of 2.0 μl/38.5 ml air and 14.0 with Mentha piperita oil at a dose of 80.0 μl/38.5 ml air compared with 25.0 eggs hatched in the control experiments. On the other hand, the percentage reduction in emerging adults were 100% for Ocimum basilicum oil, 90.9% for a mixture of the two oils and 72.7% for Mentha piperita oil. Results showed that Ocimum basilicum oil and Ocimum basilicum oils plus Mentha piperita oils in blend are potential alternatives to synthetic fumigants in the treatment of durable agricultural products. Successful adoption of plant oils in the protection of food commodities promises an eco-friendly option compatible with international biosafety regulations.     

Characterization of chitinase from Shewanella inventionis HE3 with bio-insecticidal effect against granary weevil,Sitophilus granarius Linnaeus (Coleoptera: Curculionidae)

A 40 kDa chitinase from Shewanella inventionis HE3 was purified (ChiA-Si40) and characterized. Using fermentor with an optimized medium for 48 h at 37 °C, enzyme activity was enhanced by 10-times compared to those using shaking-flask-culture. Purified chitinase is a homogenous monomer with molecular mass of 40 kDa. Its N-terminal residues revealed significant identity with glycoside hydrolase family 18 (GH18) chitinases from Gammaproteobacteria. Using colloidal chitin as a substrate, its finest activity was accomplished at pH 4 and a temperature of 70 °C. Its catalytic efficiency (k_cat/K_m) was superior to that of some bacterial GH18 chitinases and commercial enzyme, Chitodextrinase®. For scale-up and with regards to the improvement of ChiA-Si40 with PEG 6000 storage stability (6 months), the atomizing process was more pronounced than that of lyophilizing. Bio-assay of ChiA-Si40 against grain weevil Sitophilus granarius, indicates that it had an efficient insecticidal effect. About 10–100 % mortality rates were obtained 1-h after insect came in contact with ChiA-Si40. Histological study clearly demonstrated that luxury larval mid-gut, peritrophic-membrane, and epithelial-cells have been affected considerably after ChiA-Si40 treatment. These properties make ChiA-Si40 a potential bio-insecticidal agent for the biological control of S. granarius that is popular among insect pests of stored grains in Algeria.     

Cytochrome c oxidase inhibition in the rice weevil Sitophilus oryzae (L.) by formate, the toxic metabolite of volatile alkyl formates

Volatile alkyl formates are potential replacements for the ozone-depleting fumigant, methyl bromide, as postharvest insecticides and here we have investigated their mode of insecticidal action. Firstly, a range of alkyl esters, ethanol and formic acid were tested in mortality bioassays with adults of the rice weevil, Sitophilus oryzae (L.) and the grain borer, Rhyzopertha dominica (F.) to determine whether the intact ester or one of its components was the toxic moiety. Volatile alkyl formates and formic acid caused similar levels of mortality (LC(50) 131-165 micromol l(-1)) to S. oryzae and were more potent than non-formate containing alkyl esters and ethanol (LC(50)>275 micromol l(-1)). The order of potency was the same in R. dominica. Ethyl formate was rapidly metabolised in vitro to formic acid when incubated with insect homogenates, presumably through the action of esterases. S. oryzae and R. dominica fumigated with a lethal dose of ethyl formate had eight and 17-fold higher concentrations of formic acid, respectively, in their bodies than untreated controls. When tested against isolated mitochondria from S. oryzae, alkyl esters, alcohols, acetate and propionate salts were not inhibitory towards cytochrome c oxidase (EC 1.9.3.1), but sodium cyanide and sodium formate were inhibitory with IC(50) values of 0.0015 mM and 59 mM, respectively. Volatile formate esters were more toxic than other alkyl esters, and this was found to be due, at least in part, to their hydrolysis to formic acid and its inhibition of cytochrome c oxidase.     

Microstructure of mandibulate mouthparts in the greater rice weevil,Sitophilus zeamais (Coleoptera: Curculionidae)

The greater rice weevil, Sitophilus zeamais, is a serious pest of stored grain. It chews a hole in the grain and deposits an egg inside. For the purpose of excavating a tunnel, these weevils are equipped with effective mandibulate mouthparts for penetrating and boring holes. The mouthparts of the weevil are a long slender snout, which consist of a labrum, a pair of mandibles, a pair of maxillae and a labium. Mothparts exhibit typical morphology of phytophagous coleopteran beetles and have characteristics of chewing mouthparts. Mandibles are connected to the head capsule through dicondyous articulation, which allows movement along one single rotating axis. Both labrum and labium are fused to the snout and form the upper and lower lips, respectively. Along the depressed surface of the snout, a number of short sensory hairs are sparsely arranged. The distal apexes of the maxillary and labial palpi are deeply depressed into a sensillar field, and only one type of the basiconic sensilla, which function as chemo‐ or gustatory receptors, occurs in both sexes.     

Progeny of the maize weevil,Sitophilus zeamais,is affected by parental exposure to clove and cinnamon essential oils

The interest in and utilization of botanical insecticides, particularly essential oils, has become increasingly relevant to the control of insect pests. However, the potential ecotoxicological risks or flaws (including sublethal effects on the targeted pest generation and its subsequent progeny) of this pest control tool have been neglected frequently. Here, we evaluated the effects of sublethal exposure to clove, Syzygium aromaticum (L.) Merrill & Perry (Myrtaceae), and cinnamon, Cinnamomum verum J. Presl (Lauraceae), essential oils on adult (F0) maize weevils, Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae), and the physiology (e.g., body mass, respirometry, and grain consumption) and population dynamics (e.g., daily emergence and sex ratio) of their progeny. Longevities of the parents were negatively affected by the essential oils in a concentration‐dependent manner. Parental sublethal exposure to clove oil (0.17 μl cm^?2) accelerated offspring emergence but delayed the emergence of females compared to males. Parents that were sublethally exposed to clove (0.17 μl cm^?2) or cinnamon (0.35 μl cm^?2) essential oils produced heavier offspring. Parental sublethal exposure to cinnamon essential oil accelerated offspring emergence (at 0.70 μl cm^?2), delayed female emergence (at 0.17 μl cm^?2), and enhanced grain consumption (at 0.35 and 0.70 μl cm^?2) of the progeny. Thus, our findings indicate that sublethal exposure to clove and cinnamon essential oils is capable of promoting transgenerational effects in S. zeamais that can negatively impact the control efficacy of such products.     

Monitoring of rice weevil, Sitophilus oryzae, feeding behavior in maize seeds and the occurence of supernumerary molts in low humidity conditions

Rice weevils, Sitophilus oryzae (L.), complete their development from egg to early adulthood inside grains of wheat, rice or maize, but little is known about their feeding behavior within the seeds. An ultrasonic insect feeding monitor was used to characterize the feeding patterns of individual rice weevils as they developed in maize grains. Weevils reared in grains held at ca. 40% r.h. took longer to develop from egg to pupation than those reared in grains kept at ca. 70% r.h. Feeding patterns revealed that larvae developing in grains held at ca. 70% r.h. had only four instars, whereas supernumerary molts (five instars) occurred in certain individuals reared in grain held at ca. 40% r.h. The supernumerary molts may represent a response of the insect to stresses associated with low moisture levels in the seeds.     

Efficacy of plant essential oils against two major insect pests of coffee (Coffee berry borer,Hypothenemus hampei,and antestia bug,Antestiopsis intricata) and maize weevil,Sitophilus zeamais

Coffee berry borer, antestia bug and maize weevil are serious pest of coffee and maize, respectively. Bioassays of plant essential oils were conducted with coffee berry borer, antestia bug and the maize weevil. Essential oils of Thymus vulgaris, Aloysia sp., Ruta chalepensis, Chenopodium ambrosioides and Cymbopogon nardus resulted in 80%–90% mortality of coffee berry borer, whereas essential oils of C. ambrosioides, T. vulgaris and R. chalepensis achieved 87.5%–92.5% mortality of antestia bug. Essential oils of C. ambrosioides caused significantly the highest percentage mortality of the maize weevils (95%) with “Lethal Dose” (LD₅₀) values of 2.202 ml 100 ml^?1 within 24 h after treatment, whereas essential oils of Mentha spicata, T. vulgaris and R. chalepensis resulted in 82.5%, 77.5% and 73.5% mortality, respectively. The potential of plant essential oils for the management of coffee berry borer, antestia bug and maize weevil for the small-scale farmers is discussed.     

Toxic effects of some insecticides,herbicides, and plant essential oils against Tribolium confusum Jacquelin du val (Insecta: Coleoptera: Tenebrionidae)

Cereals are staple food for many countries and are grown on millions of hectares of land, but much of the harvest is wasted due to losses by pests. To minimize these losses, many pesticides are used which are damaging to the environment and human health. There are debates to get rid of these chemicals but they are still in use at large scale. An alternative control strategy for insect pests in storage houses is the use of botanicals. In this study, four plant essential oils, two plant extracts, two herbicides, and two insecticides were used against Tribolium confusum and the comparison of toxicity was made by calculating LC₅₀ and LT₅₀ values. LC₅₀ values were higher for abamectin (2.09–10.23 mg/L) and cypermethrin (3.41–11.78 mg/L) insecticides followed by neem essential oil (7.39–19.24 mg/L) and citrus extract (10.14–24.50 mg/L). However, LC₅₀ values were maximum in case of jaman plant extract (22.38–176.42 mg/L) followed by two herbicides, Logran (19.66–39.72 mg/L) and Topik (29.09–47.67 mg/L) However, LC₅₀ values were higher for topic herbicide (24.098 ppm) and jaman essential oil (16.383 ppm) after four days of treatment. Abamectin and cypermethrin insecticides, neem essential oil and citrus plant extract also killed adults of T. confusum quicker as compared other essential oils, extracts and herbicides. Results revealed that botanical formulations being environmentally safe could be used instead of highly hazardous pesticides for stored products’ pests. This study also elaborates the non-host toxicity of herbicides commonly applied in our agroecosystem.     

The influence of rice plant age on susceptibility to the rice water weevil,Lissorhoptrus oryzophilus

Plant age‐ and plant stage‐related changes in the resistance of rice, Oryza sativa, to its most important insect pest in the US, the rice water weevil (Lissorhoptrus oryzophilus), were investigated in a series of field and greenhouse choice and no‐choice studies. Rice plants were susceptible to infestation by rice water weevils over a broad range of plant ontogenetic stages, from at least the early vegetative stage to well into the reproductive stage. There was, however, a clear preference expressed by rice water weevils in both choice and no‐choice experiments for plants in (or nearly in) the tillering stage of development, with pre‐tillering and reproductive stage plants less preferred. The relationship between rice plant age and susceptibility to weevils is thus nonlinear. This study constitutes one of the most thorough studies to date of the relationship in a grass species between plant age and susceptibility to herbivores. The results provide a biological explanation for observed patterns of weevil infestations and a rationale for the cultural practice of delayed flooding.