A susceptible strain of diamondback moth, Plutella xylostella (L.), was used to select for resistance to tebufenozide in the laboratory. After continuous selection with tebufenozide 17 times during 35 generations, a resistant strain was achieved with high resistance to tebufenozide (RR 93.8). Bioassay revealed that this strain showed high cross-resistance to abamectin (RR 35.7), methoxyfenozide (29.1) and JS118 (16.5), and a little to deltamethrin (3.9), but no obvious cross-resistance was found to cypermethrin (1.3), fipronil (1.3), trichlorfon (1.1), chlorfenapyr (1.0), phoxim (0.9) and acephate (0.8). The resistant and susceptible insects had similar development rates, but life table tests indicated that the resistant strain showed reproductive disadvantages, including decreased copulation rate, reproductivity and hatchability. When compared with the susceptible strain, the resistant insects had a relative fitness of only 0.3. This indicated that tebufenozide resistance selected under laboratory conditions had considerable fitness costs in this pest, and therefore rotational use of insecticides without cross-resistance is recommended to delay development of resistance.

We challenged Locusta migratoria (Meyen) grasshoppers with simultaneous doses of both the insecticide chlorantraniliprole and the fungal pathogen, Metarhizium anisopliae. Our results showed synergistic and antagonistic effects on host mortality and enzyme activities. To elucidate the biochemical mechanisms that underlie detoxification and pathogen-immune responses in insects, we monitored the activities of 10 enzymes. After administration of insecticide and fungus, activities of glutathione-S-transferase (GST), general esterases (ESTs) and phenol oxidase (PO) decreased in the insect during the initial time period, whereas those of aryl acylamidase (AA) and chitinase (CHI) increased during the initial period and that of acetylcholinesterase (AChE) increased during a later time period. Activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) decreased at a later time period post treatment. Interestingly, treatment with chlorantraniliprole and M. anisopliae relieved the convulsions that normally accompany M. anisopliae infection. We speculate that locust mortality increased as a result of synergism via a mechanism related to Ca2+ disruption in the host. Our study illuminates the biochemical mechanisms involved in insect immunity to xenobiotics and pathogens as well as the mechanisms by which these factors disrupt host homeostasis and induce death. We expect this knowledge to lead to more effective pest control.

Low temperature induces diapause in locusts. However, the physiological processes and initiation mechanism of diapause are not well understood. To understand the molecular basis of diapause, ‘omics’ analyses were performed to examine the differences between diapause and non-diapause eggs at both transcriptional and translational levels. Results indicated that a total of 62,241 mRNAs and 212 proteins were differentially expressed. Among them, 116 transcripts had concurrent transcription and translation profiles. Up-regulated genes related to diapause included glutathiones-S-transferase et al., and down-regulated genes including juvenile hormone esterase-like protein et al. KEGG analysis mapped 7,243 and 99 differentially expressed genes and proteins, to 83 and 25 pathways, respectively. Correlation enriched pathways indicated that there were nine identical pathways related to diapause. Gene Ontology analysis placed these genes and proteins into three categories, and a higher proportion of genes related to metabolism was up-regulated than down-regulated. Furthermore, three up-regulated pathways were linked to cryoprotection. This study demonstrates the applicability of high-throughput omics tools to identify molecules linked to diapause in the locust. In addition, it reveals cellular metabolism in diapause eggs is more active than in non-diapause eggs, and up-regulated enzymes may play roles in cryoprotection and storing energy for diapause and post-diapause stages.

No abstract

Transgenic Bacillus thuringiensis (Bt) crops play an increasing role in pest control, and resistance management is a major issue in large-scale cultivation of Bt crops. The fitness cost of resistance in targeted pests is considered to be one of the main factors delaying resistance when using the refuge strategy. By comparing 10 resistant Helicoverpa armigera (Hubner) strains, showing various resistance levels to Bt toxin (Cry1Ac), to a susceptible strain, we showed an increasing fitness cost corresponding with increasing levels of resistance. The relationship between overall fitness cost C and the resistance ratio Rr could be described by C = 24.47/(1 + exp([1.57 - Log10Rr]/0.2)). This model predicted that the maximum overall fitness cost would be ~24% (±5.22) in the strains with the highest resistance level. The overall fitness cost was closely linked to egg hatching rate, fecundity, emergence rate, larval survival rate, and developmental duration of adults. Among fitness components measured, fecundity was the most sensitive trait linked to the resistance selection. To integrate the results into simulation models would be valuable in evaluating how variation in fitness cost may influence the development of resistance in pest populations, thus helping to develop enhanced refuge strategies.

Development of resistance to transgenic crops expressing the Cry toxin from Bacterium thuringiensis (Bt) has been the major concern for the long-term success of Bt crops. Alterations in nonbinding site proteinases and Bt toxin receptors are the two types of mechanisms responsible for Bt resistance in resistant insects. However, little is known about the relative contributions of the two types of mechanisms in the early and late phases of the development of Bt resistance. To address the relative contributions of four nonbinding site proteinases including esterase, total protease, chymotrypsin, and glutathione S-transferase in the early and late phases of the development of Cry1Ac resistance, we analyzed the relationships between nonbinding site proteinases and resistance of three groups of Helicoverpa armigera Hübner (Lepidoptera: Noctuidae) strains with different resistance levels because of different geographic origins and selection pressures. Positive correlation (esterase, glutathione-S-tranferases [GST], and chymotrypsin) and negative correlation (total midgut protease) were observed within the low to moderate group II resistant strains. Such correlations were less obvious within the low to moderate group III resistant strains because of only threefold differences in LC50 values. Relative to the unselected susceptible 96S strain, the two highly resistant group I resistant strains BtI and BtR have the same amounts of esterase, GST, and chymotrypsin and disproportionally decreased the amount of total midgut protease. Overall, the low to moderate resistant strains had the lowest amount of the nonbinding site proteinases. The results obtained suggest that alternations in the nonbinding site proteinases probably can only confer low to moderate levels of resistance and thus are enriched in the early phase of the development of Cry1Ac resistance.

No abstract

Many insect pests have evolved resistance to insecticides. Along with this evolution, the sex pheromone communication system of insects also may change, and subsequently reproductive isolation may occur between resistant and susceptible populations. In this study of the diamondback moth, we found that resistant females (especially Abamectin resistant females) produced less sex pheromone and displayed a lower level of calling behavior. Resistant males showed higher EAG responsiveness to the sex pheromone mixture of females, and responded to a broader range of ratios between the two major components compared to the responses of susceptible moths. In addition, wind tunnel experiments indicated that changes associated with insecticide resistance in the Abamectin resistant strain (Aba-R) significantly reduced female attractiveness to susceptible males. Furthermore, mating choice experiments confirmed that non-random mating occurred between the two different strains. Aba-R females with an abnormal pheromone production and blend ratio exhibited significantly lower mating percentages with males from either their own strain or other strains, which corroborates the results obtained by the wind tunnel experiments. The implications of this non-random mating for insect speciation and insecticide resistance management are discussed.