Could biorational insecticides be used in the management of aflatoxigenic Aspergillus parasiticus and its insect vectors in stored wheat?

Microbial vectoring capacity by internal- and external-infesting stored product insects after varying dispersal periods between novel food patches: An underestimated risk

Understanding the ability of internal- and external-infesting stored product insects to vector microbes is important for estimating the relative risk that insects pose to postharvest commodities as they move between habitat patches and in the landscape. Thus, the aim of the current study was to evaluate and compare the microbial growth in novel food patches at different dispersal periods by different populations of Sitophilus oryzae (e.g., internal-infesting) and Lasioderma serricorne (e.g., external-infesting). Adults of both species collected from laboratory colonies or field-captured populations were either placed immediately in a novel food patch, or given a dispersal period of 24 or 72 h in a sterilized environment before entering a surrogate food patch. Vectored microbes in new food patches were imaged after 3 or 5 days of foraging, and microbial growth was processed using ImageJ while fungal species were identified through sequencing the ITS4/5 ribosomal subunit. We found that increasing dispersal time resulted in multiple-fold reductions in microbial growth surrogate food patches by L. serricorne but not S. oryzae. This was likely attributable to higher mobility by S. oryzae than L. serricorne. A total of 20 morphospecies were identified from 13 genera among the 59 sequences, with a total of 23% and 16% classified as Aspergillus and Penicillium spp. Our data suggest that there is a persistent risk of microbial contamination by both species, which has important food safety implications at food facilities.

Selection for resistance to pirimiphos-methyl, permethrin and spinosad in a field strain of Sitophilus oryzae: resistance risk assessment, cross-resistance potential and synergism of insecticides

Indiscriminate use of insecticides in food storage facilities for controlling insect pests has deleterious effects on the environment and non-targeted organisms in the premises. Continuous use of insecticides may result in resistance development in insect pests, which compel the stakeholders to increase the dosage of insecticides to manage resistant insect pests. The increased dosage of insecticides ultimately may result in contamination of stored food stuff that affects human health. The present study was planned to generate data that will be helpful to delay resistance development and to reduce environmental pollution. A field strain of Sitophilus oryzae, one of the most common insect pests of stored foodstuff, was selected separately with pirimiphos-methyl, permethrin, or spinosad for five consecutive generations. The selected strains were studied for resistance risk assessment, time taken to develop resistance to insecticides after continuous exposure in the selection process, preliminary mechanism of resistance, and whether the development of resistance due to the selection with a particular insecticide could develop cross-resistance to other insecticide or not. In comparison to a laboratory susceptible reference strain, the insecticide-selected strains revealed rapid development of resistance against insecticides as a result of selection process: 31.05-fold resistance to pirimiphos-methyl, 156.49-fold resistance to permethrin, and 65.6-fold resistance to spinosad. The selected strains did not show cross-resistance to insecticides to with these strains were not exposed during selection experiments, i.e., strain selected with pirimiphos-methyl did not show cross-resistance to spinosad and permethrin. In the synergism bioassays, the synergists (S,S,S-tributyl phosphorotrithioate and piperonyl butoxide) significantly reduced resistance of the selected strain against insecticides to with these were selected, revealing the probability of metabolic mechanism of resistance. The present study revealed high risks of resistance development to pirimiphos-methyl, spinosad, and permethrin under consistent selection pressure. Lack of cross-resistance among insecticides provides an opportunity to use insecticides in rotation instead of increasing dosages to manage resistant insects that will ultimately pollute the environment.

Resistance to grain protectants and synergism in Pakistani strains of Sitophilus oryzae (Coleoptera: Curculionidae)

The widespread use of insecticides for the management of insect pests in storage facilities and food industries have caused insecticide resistance a frequent issue worldwide. Nonetheless, this issue has been little explored in Pakistan that resulted in control failures and increased dosage of insecticides. In the present study, insecticide resistance to chlorpyrifos-methyl, pirimiphos-methyl, permethrin and spinosad was surveyed in five field strains of Sitophilus oryzae: FSD-SO, GJR-SO, DGK-SO, MTN-SO and BWP-SO, collected from five different localities of Punjab, Pakistan, and contrasted with an insecticide susceptible reference strain (Lab-SO). Dose-mortality bioassays were performed in glass vials containing insecticide-treated rice grains, and lethal doses (LD₅₀ and LD₉₅) were calculated and compared using the ratio tests. In comparison to the Lab-SO strain at LD₅₀ and LD₉₅ levels, field strains exhibited: 24.51 to 52.80 and 36.55 to 69.31 resistance ratios (RRs), respectively, for chlorpyrifos-methyl; 15.89 to 45.97 and 55.12 to 194.93 RRs, respectively, for pirimiphos-methyl; 39.76 to 108.61 and 61.33 to 130.12 RRs, respectively, for permethrin; 4.23 to 27.50 and 6.28 to 41.00 RRs, respectively, for spinosad. In the synergism experiments using the Lab-SO and the most resistant strains against each insecticide, the enzyme inhibitors (PBO and DEF) failed to synergize toxicity of insecticides in the Lab-SO strain; however, toxicity of chlorpyrifos-methyl, pirimiphos-methyl and permethrin significantly enhanced in the resistant strains of S. oryzae, suggesting possibility of metabolic mechanism of resistance. In addition, activities of detoxification enzymes (CarE, MFO and GST) were significantly higher in resistant strains compared to the Lab-SO strain. The results revealed presence of insecticide resistance in field strains of S. oryzae that necessitate the need to develop a resistance management strategy.

A Systematic Review of the Behavioral Responses by Stored-Product Arthropods to Individual or Blends of Microbially Produced Volatile Cues

Microbes are ubiquitous and play important ecological roles in a variety of habitats. While research has been largely focused on arthropods and microbes separately in the post-harvest supply chain, less attention has been paid to their interactions with each other. Up to this point, there has been no attempt to systematically describe the patterns of behavioral responses by stored-product insects to microbially produced volatile organic compounds (MVOCs). Thus, our aims were to evaluate whether stored-product arthropods were primarily and significantly attracted, repelled, or had a net neutral effect (e.g., unaffected or mixed) by MVOCs presented as (1) complex headspace blends or (2) single constituents and known mixtures. In total, we found 43 articles that contained 384 sets of tests with different combinations of methodology and/or qualitative findings, describing the behavioral responses of 24 stored-product arthropod species from two classes, four orders, and 14 families to 58 individual microbial compounds and the complex headspace blends from at least 78 microbial taxa. A total of five and four stored-product arthropod species were significantly attracted and repelled by MVOCs across odor sources, respectively, while 13 were unaffected or exhibited mixed effects. We summarize the biases in the literature, including that the majority of tests have occurred in the laboratory with a limited subset of methodology and has largely only assessed the preference of adult arthropods. Finally, we identify foundational hypotheses for the roles that MVOCs play for stored-product arthropods as well as gaps in research and future directions, while highlighting that the behavioral responses to MVOCs are complex, context-, and taxon-dependent, which warrants further investigation.

Susceptibility to indoxacarb and synergism by enzyme inhibitors in laboratory and field strains of five major stored product insects in Pakistan

Resistance to commonly used grain protectants and fumigants in stored product insect pests necessitates the need to explore alternative substances. Indoxacarb is a reduced-risk oxadiazine insecticide that is generally used in field crops, but there are limited reports of its susceptibility in stored insect pests. The objective of this study was to determine susceptibility to indoxacarb in laboratory and field strains of five major stored product insects: Rhyzopertha dominica (Fabricius), Sitophilus zeamais (Motschulsky), S. oryzae (Linnaeus), Tribolium castaneum (Herbst), and Oryzaephilus surinamensis (L.), using dose-mortality bioassays on wheat grains. In most of the cases, the susceptibility of laboratory strains of all the studied pests were significantly higher than the corresponding field strains. The LD₅₀ and LD₉₉ values (mg a.i./kg of grains) of field strains ranged from 0.13 to 0.38, and 3.44 to 24.76, respectively (for R. dominica), 0.26 to 0.55, and 5.26 to 19.37, respectively (for S. oryzae), 0.41 to 1.01, and 13.11 to 22.46, respectively (for S. zeamais), 0.67 to 1.37, and 15.43 to 43.44, respectively (for T. castaneum), and 0.52 to 0.92, and 18.06 to 61.63, respectively (for O. surinamensis). Synergism bioassays implementing piperonyl butoxide or S,S,S-tributyl phosphorotrithioate along with indoxacarb on selected field strains revealed enhanced susceptibility to indoxacarb. The study demonstrates relative susceptibility to indoxacarb in major stored product insects. Synergism results support the probability of metabolic-based mechanisms responsible for mitigating indoxacarb toxicity. The results might be helpful for monitoring future variation in susceptibility to indoxacarb in the selected insect species and for setting field rates.

Spinosad resistance affects biological parameters of Musca domestica Linnaeus

Musca domestica is one of the major cosmopolitan insect pests of public health importance. Spinosad is considered an eco-friendly insecticide used for the management of M. domestica and other pests of significant concern. Cases of resistance against spinosad in M. domestica have been reported from some parts of the world; however, there are no reports of any negative effects of spinosad resistance on the fitness/biological parameters of M. domestica. To investigate fitness costs, a near isogenic M. domestica resistant strain (Spin-R) was constructed using Spin-UNSEL-susceptible and Spin-SEL-resistant strains sharing a close genetic background. We found significantly reduced rates of adult eclosion, fecundity, egg hatching, survival, and lengthened developmental time in the Spin-R strain. Moreover, the values of different fitness parameters like biotic potential, mean relative growth rate, intrinsic rate of natural increase, and net reproductive rate, were also significantly reduced in the Spin-R strain, which reflect fitness costs most probably linked with spinosad resistance. The presence of fitness costs suggests likely instability of resistance to spinosad in M. domestica, which can be reverted by relaxing spinosad selection pressure and rotation with alternate insecticides. The wise use of insecticides will ultimately help to manage resistance in this pest and minimize environmental pollution.