Genetic variability in esterases and the insecticide resistance in brazilian strains of Oryzaephilus mercator and Oryzaephilus surinamensis (Coleoptera: Silvanidae)

Oryzaephilus mercator and O. surinamensis are stored grains and processed food pests, the latter being responsible for major economical losses. Polyacrylamide gel electrophoresis was used to analyse esterase patterns during insect development. Seven esterases, three cholinesterases, two carboxylesterases and two acetylesterases, were identified in O. mercator, one of which was proper to adults. Five esterases, of which four were cholinesterases, occurred in O. surinamensis. Strains of O. mercator and O. surinamensis were also exposed to malathion and chlorpyrifos-methyl. According to the LC50 estimates, OmLC-M and OmLA strains of O. mercator and OsLB strain of O. surinamensis were the most resistant to both insecticides. However, higher sensitivity to malathion and chlorpyrifos-methyl has also been verified in some of its esterases. Cholinesterases OmEST-1 and OsEST-5 seem to be involved in this resistance. These results suggest that organophosphate tolerance may be related to genetic variability in esterase isoenzymes.     

Arena size, hole density, and capture of Oryzaephilus surinamensis (Coleoptera: Silvanidae) in grain probe traps

The relationship of size of test arena, number of holes in a grain probe trap body and capture of the sawtoothed grain beetle, Oryzaephilus surinamensis (L.), was determined in simulated field tests conducted in an outdoor screen enclosure exposed to natural temperature fluctuations. Polyvinylchloride (PVC) probe bodies were attached to electronic sensor heads, and insect captures were recorded electronically using an electronic grain probe insect counter (EGPIC) system. In comparisons among PVC probe trap bodies with 60, 132, 252, and 492 holes, tested at 18 insects per kilogram in 4.5, 17, and 40 kg of soft wheat in cylindrical arenas (10.2, 20.3, and 30.5 cm in diameter, respectively), number of holes in the probe trap body had no effect on insect capture, but percentage of insects recovered was indirectly related to size of the test arena. Periodicity of insect capture was determined using the time-stamp data that were recorded by the EGPIC system. Circadian rhythm was observed in the periodicity of the capture that corresponded to foraging activity peaks documented for sawtoothed grain beetles, with activity peaks occurring early in the scotophase. There were shifts in times of peak activity among the different test arena sizes that corresponded to differences in temperature in the grain mass. Increases in both temperature and contact between insects and grain probe in the smallest arenas resulted in higher capture of sawtoothed grain beetles. This research documents additional important factors when evaluating capture of sawtoothed grain beetles in grain probe traps.     

Mechanisms underlying sawtoothed grain beetle (Oryzaephilus surinamensis [L.]) (Coleoptera: Silvanidae) infestation of consumer food packaging materials

The sawtoothed grain beetle, Oryzaephilus surinamensis (L.), is an extremely destructive pest of packaged consumer food products. The beetle is not believed to chew directly through packaging materials, but to use openings or flaws in damaged or improperly sealed packages to gain entry. We investigated the behavioral mechanisms by which the sawtoothed grain beetle infests packages with flaws. Significantly more sawtoothed grain beetles infested consumer food packages that had been punctured with 0.4 mm diameter holes, to simulate packaging flaws that preclude adults, than when packages had no flaws. In a test arena, females laid more eggs into or near the hole in a plastic packaging film, when they were able to contact the food through the hole than when they could not contact the food. First instar larvae placed either 1 mm or 1 cm away entered holes when food was present, indicating that packages could become infested if eggs were laid near holes. In the absence of food, neither adults nor larvae responded to holes. This study has shown the importance of sound packaging in preventing insect infestation.     

Progeny production by Tribolium castaneum (Coleoptera: Tenebrionidae) and Oryzaephilus surinamensis (Coleoptera: Silvanidae) on maize previously infested by Sitotroga cerealella (Lepidoptera: Gelechiidae)

A laboratory experiment was conducted to quantify the effects of infestation of maize by Sitotroga cerealella (Olivier) on progeny production by two common secondary colonizers of grain, Tribolium castaneum (Herbst) and Oryzaephilus surinamensis (L.). Adults of both secondary pest species were allowed to oviposit for 3 wk on intact kernels of 'DeKalb 689', mechanically split kernels, kernels that had been infested for 3 mo by S. cerealella, and kernels that had been infested for 6 mo. Progeny of both species reached highest numbers on 6-mo infested maize. Prior infestation for 6 mo by S. cerealella makes maize a more suitable medium for reproduction by T. castaneum and O. surinamensis, much more than can be accounted for by mere disruption of kernel integrity resulting from larval feeding. The results highlight the importance of limiting establishment by S. cerealella on maize in storage.     

Carboxyesterase degradation of malathion in vitro by susceptible and resistant strains of Tribolium castaneum (Herbst) (Coleoptera, Tenebrionidae)

Malathion monoacid was found to be the major metabolite of malathion produced in vitro by homogenates of susceptible and resistant Tribolium castaneum. Slightly increased production of this metabolite was detected in a malathion specific resistant strain. In all strains, monoacid formation was inhibited by triphenyl phosphate. Partial purification of the enzymes responsible failed to indicate a significant increase in enzyme levels in the resistant strain.     

Malathion resistance in Tribolium strains and their hybrids: Inheritance patterns and possible enzymatic mechanisms (Coleoptera,Tenebrionidae)

(1) The genetics of malathion resistance in two strains of the flour beetle, Tribolium castaneum, was investigated. In CTC-12, resistance is polygenic, while in Kano, it is due to a dominant allele at a single autosomal locus. Reciprocal hybrids with the susceptible control strains bb and pp showed an overdominant response in particular when Kano was the male parent in the original cross. (2) Three possible genetic mechanisms to explain these results are discussed. The model which best explains the genetic results, particularly the difference between the reciprocal crosses, assumes a modifier resistance allele on the Y chromosome. (3) The levels of activity of total esterases, carboxyesterases, mixed-function oxidases, epoxide hydrase, and glutathione transferase in the parent strains and their hybrids were measured quantitatively. Although total esterase activity may not be relevant for the breakdown of malathion, it was inhibited by the pesticide. The activity of the microsomal enzymes was high in CTC-12, low in bb, and intermediate in the hybrids, while carboxyesterases were very active in Kano as well as in the hybrids with bb and low in the latter. These patterns agree with the genetics of resistance in the two strains. A higher level of GSH transferase in the Kano×bb hybrids than in Kano seems to indicate a possible biochemical mechanism for their overdominant resistance.     

Fitness consequences of malathion-specific resistance in red flour beetle (Coleoptera: Tenebrionidae) and selection for resistance in the absence of malathion

Malathion resistance in the red flour beetle, Tribolium castaneum (Herbst), is a worldwide problem and is very stable once it becomes widespread in natural populations. In the absence of insecticide the proportion of resistant phenotypes may rapidly decline but the development of resistance does not always involve reduced fitness. Malathion-specific resistance in T. castaneum seems not to involve any loss of fitness in laboratory or field conditions. Susceptible beetles were in competition with resistant beetles at different initial frequencies and modifications of susceptible gene frequency were estimated in these laboratory populations over 10 generations. A significant decrease in susceptible gene frequency was observed in Tribolium populations over time. The selection coefficient of the susceptible allele was estimated and the fitness of susceptible alleles in all tests was observed to range from 0.89 to 0.93 compared with the fitness of resistant genotypes, which was assumed to be 1. Data provided evidence that the resistant strains exhibited fitness advantages in the absence of malathion. We also compared the biotic potential (fecundity and developmental time) of the susceptible strain, the homozygous malathion-specific resistant strain, and their hybrids. Malathion-specific resistant strains showed an 8 -23% increase in biotic potential relative to the susceptible strain. These findings are consistent with those of malathion-specific resistance in T. castaneum; the fitness of the insects seems independent of the genetic background and the fitness of the resistant insects is not affected by this resistance mechanism.     

Using a Lethality Index to Assess Susceptibility of Tribolium confusum and Oryzaephilus surinamensis to Insecticides

We evaluated knockdown caused by four insecticides: alpha-cypermethrin, chlorfenapyr, pirimiphos-methyl and fipronil against adults of Tribolium confusum Jacquelin Duval, the confused flour beetle and Oryzaephilus surinamensis (L.), the sawtoothed grain beetle. Bioassays were conducted on concrete and metal surfaces. Adults of the tested species were exposed on both surfaces treated with the above insecticides at two doses (low and high). Knockdown assessment was done after 15, 30 and 60 min of adult exposure in the treated surfaces. Also, after 1, 3, 5, 7 and 14 d of exposure, a lethality index was calculated with an equation resulting to values from 0 to 100, where 100 indicated complete mortality and 0 complete survival. We also developed a lethality index by ranking each adult on each surface from 0 to 4, 0: adults moved normally, 1: adults were knocked down, but were able to walk for short intervals, 2: adults were knocked down and unable to walk, but with visible movement of antennae etc., 3: adults were knocked down, with very minimal movement of the tarsi and the antennae and 4: adults were dead (no movement). Knockdown of adults immediately after exposure (15–60 min) was higher for pirimiphos-methyl followed by alpha-cypermethrin, for both dose rates tested and species, but only on the metal surface. The lethality index was nearly 100 for all insecticides after 5d of exposure for O. surinamensis, while for T. confusum the adult lethality index was considerably lower for alpha-cypermethrin, suggesting that that recovery from knockdown occurred. Chlorfenapyr was the only insecticide that was more effective on concrete than on metal, while the reverse was noted for the other three insecticides. These results show that knockdown has different levels, which can be used as indicators of insect mortality or recovery.     

A survey of wild-type strains of Drosophila melanogaster for resistance to malathion and phenylthiocarbamide (PTC)1

A survey of 33 wild-type strains of D. melanogaster for resistance to PTC and malathion has revealed a tendency for strains to be resistant to one, the other, or neither of these compounds. Similar inverse relationships have been reported previously for PTC and DDT, BHC, parathion, and phenylurea. D. simulans may not exhibit similar inverse resistance patterns.     

Control of sawtoothed grain beetles (Coleoptera: Silvanidae) in stored oats by using an entomopathogenic fungus in conjunction with seed resistance

We tested the hypothesis that the entomopathogenic fungus Beauveria bassiana would be more efficacious on oat cultivars that prolonged the immature developmental period of sawtoothed grain beetle, Oryzaephilus surinamensis (L.), a storage pest. However, percentage of reduction in progeny production was similar on whole 'Don' and 'Paul' oats treated with fungus, even though immature developmental time was longer on whole 'Don' than on 'Paul' oats. In our initial test at 10 mg of conidia per kilogram of oats, the number of beetle progeny produced was reduced by 38-67% in whole oats, and there was no effect of the fungus on insects developing on cracked oats. Therefore, we conducted two dose-response studies that showed that adding 150 mg of conidia per kilogram to cracked or whole 'Paul' oats resulted in a 70 and 98% reduction, respectively, in number of progeny produced. No further reduction was obtained by adding 200 mg of conidia per kilogram of cracked or whole 'Paul' oats. Presence of the fungus did not affect developmental time in any of our tests. A previous study showed that cleaned oats should limit insect population growth to allow long-term storage of oats without insect damage. However, the current study shows that if the oats are not cleaned, and not cleaning oats is the normal storage practice, then B. bassiana could be used to help control sawtoothed grain beetles.     

Pyrethroid resistance and mechanisms of resistance in field strains of Helicoverpa armigera (Lepidoptera: Noctuidae)

Pyrethroid resistance was found in 54 field strains of Helicoverpa armigera collected between 1995 and 1999 from 23 districts in seven states of India. LD50 values of the field strains ranged from 0.06 to 72.2 microg/larva with slopes of 0.5-3.1. Resistance was highest in regions where pyrethroid use was frequent (four to eight applications per season). Resistance to deltamethrin was exceptionally high with resistance ratios of 13,570 and 27,160 in two strains collected during February 1998 in central India. Resistance to cypermethrin, fenvalerate and cyhalothrin also was high with resistance ratios of >1,000 in four strains collected from central and southern India. Resistance ratios were below 100 in >50% of the strains tested. Pyrethroid resistance was high in strains collected from the districts in Andhra Pradesh where a majority of the cotton farmer suicide cases in India were reported. Resistance to pyrethroids appeared to have increased over 1995-1998 in most of the areas surveyed. Studies carried out through estimation of detoxification enzyme activity and synergists indicated that enhanced cytochrome p450 and esterase activities were probably important mechanisms for pyrethroid resistance in field strains. Pyrethroid nerve insensitivity also was found to be a major mechanism in some parts of the country where the use of pyrethroids was high. The information presented illustrates the importance of proper insect management programs to avoid the consequences associated with improper insecticide use.     

Boll weevil (Coleoptera: Curculionidae) bait sticks: toxicity and malathion content

Assays of malathion content and toxicity to boll weevil, Anthonomus grandis grandis Bohemian, were conducted on boll weevil bait sticks, now marketed as Boll Weevil Attract and Control Tubes (BWACTs; Plato Industries, Houston, TX). In general, the longer BWACTs were in the field, the lower the mortality of weevils that were exposed to them. Bioassays of weevil mortality correlated with hexane washes of BWACT surfaces showed highly variable mortality when surface malathion fell below approximately 20 ng per 1 microl of hexane, but consistently high mortality (> or = 90%) when surface malathion was above 30 ng per 1 microl of hexane. A linear equation was calculated to predict mortality as a function of malathion on a BWACT surface. Although mortality was related to surface amounts of malathion, it was unrelated to the total amount of malathion present in BWACTs. Similarly, surface malathion was unrelated to the total amount present in BWACTs. As with mortality, amount of surface malathion declined with time, but total malathion did not decline with time. Boll weevils placed on fresh BWACTs tended to accumulate more malathion and died in greater numbers as time spent on fresh tubes increased, but not as time spent on tubes aged in the field (for 5 mo total) increased. Weevils that landed on tubes after a short flight died in approximately the same numbers as those that were placed on tubes using proper methodology. The amount of malathion expected to cause 90% mortality of boll weevils subjected to proper methodology was 47% higher than for a less stringent methodology (34.3 versus 23.4 ng), which demonstrates the importance of strictly adhering to proper methodology; nevertheless, chemical assay of malathion on the BWACT surface proved to be a more consistent measure of BWACT toxicity than bioassay, and it should replace the bioassay.