Immature development and adult longevity of the soybean tentiform leafminer (Lepidoptera: Gracillariidae).

The leaf-mining microlepidopteran, Macrosaccus morrisella (Fitch) (Lepidoptera: Gracillariidae), has emerged as a new pest of soybean, Glycine max (L.) Merrill (Fabales: Fabaceae), in Canada and the United States, but little is known about its life history traits. Thus, this study was conducted to assess the immature developmental rate of M. morrisella, from egg to adult emergence, on soybean at different temperatures, and the longevity of adults supplied with water and/or honey at different temperatures. The time to 50% emergence of adults was 71.90, 36.33, 24.62, and 17.83 days at 15, 20, 25, and 30 °C, respectively. The lower developmental threshold of M. morrisella was estimated at 8.96 °C, with 425.04 degree-days required for egg-to-adult development. For adult longevity, time to 50% mortality at 25 °C was 15.00, 4.00, and 2.00 days when adults were provided with 25% (v/v) honey-water solution, water, or nothing, respectively. In a follow-up experiment, time to 50% mortality at 25 °C was 24.00, 6.00, 3.00, and 3.00 days when adults were provided with honey and water (offered separately), honey, water, or nothing, respectively, with a synergistic effect when honey and water were offered simultaneously as opposed to honey or water alone. Finally, when fed 25% (v/v) honey-water solution and maintained at 20, 25, and 30 °C, time to 50% mortality of adults was 26.50, 15.00, and 15.00 days. These results inform the understanding of the basic biology of M. morrisella and will help inform the future development of management programs for this insect in soybean.

Association of voltage-gated sodium channel mutations with field-evolved pyrethroid resistant phenotypes in soybean aphid and genetic markers for their detection.

The frequent use of insecticides to manage soybean aphids, Aphis glycines (Hemiptera: Aphididae), in the United States has contributed to field-evolved resistance. Pyrethroid-resistant aphids have nonsynonymous mutations in the voltage-gated sodium channel (vgsc). We identified a leucine to phenylalanine mutation at position 1014 (L1014F) and a methionine to isoleucine mutation (M918I) of the A. glycines vgsc, both suspected of conferring knockdown resistance (kdr) to lambda-cyhalothrin. We developed molecular markers to identify these mutations in insecticide-resistant aphids. We determined that A. glycines which survived exposure to a diagnostic concentration of lambda-cyhalothrin and bifenthrin via glass-vial bioassays had these mutations, and showed significant changes in the resistance allele frequency between samples collected before and after field application of lambda-cyhalothrin. Thus, a strong association was revealed between aphids with L1014F and M918I vgsc mutations and survival following exposure to pyrethroids. Specifically, the highest survival was observed for aphids with the kdr (L1014F) and heterozygote super-kdr (L1014F + M918I) genotypes following laboratory bioassays and in-field application of lambda-cyhalothrin. These genetic markers could be used as a diagnostic tool for detecting insecticide-resistant A. glycines and monitoring the geographic distribution of pyrethroid resistance. We discuss how generating these types of data could improve our efforts to mitigate the effects of pyrethroid resistance on crop production.

Lack of Evidence for Fitness Costs in Soybean Aphid (Hemiptera: Aphididae) With Resistance to Pyrethroid Insecticides in the Upper Midwest Region of the United States.

Twenty years after the arrival of soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), in the United States, it remains the most economically important arthropod pest of soybean in the Upper Midwest Region. After years of repeated and sustained insecticidal pressures placed on the aphid, resistance to the pyrethroid class of insecticides has been documented in multiple years over a large geographic area. In this study, the fitness of aphid isolates displaying resistant and susceptible phenotypes to λ-cyhalothrin were compared within several experiments over three soybean-growing seasons. Rates of population increase were evaluated on whole plants in the greenhouse, intrinsic rates of increase were calculated from leaf discs in the laboratory, and aphid size and asymmetry were compared through tibial measurements. No evidence of a fitness cost associated with the resistant phenotype was seen in any of our experiments. In contrast, individual resistant isolates occasionally supported significantly higher fitness values than some susceptible isolates. Additionally, a pooled analysis comparing resistant and susceptible phenotypes across years and isolates revealed that, on average, the resistant phenotype had significantly higher fitness values than the susceptible phenotype in most experiments. The lack of reproductive fitness costs associated with the pyrethroid-resistant phenotype raises concerns for longevity of pyrethroid use in soybean aphid management.

Change-point analysis of lambda-cyhalothrin efficacy against soybean aphid (Aphis glycines Matsumura): identifying practical resistance from field efficacy trials.

BACKGROUND: Soybean aphid (Aphis glycines Matsumura) remains the most economically important arthropod pest of soybean in the Upper Midwest Region of the USA. Soybean aphid resistance to the pyrethroid insecticides emerged in 2015; however, the reduction in the efficacy of field applications of pyrethroid insecticides has not been quantified. Based on time-series data from insecticide efficacy trials at two locations, a novel approach of continuous two-phase change point-regression models was used to indicate whether a change in percent control had occurred, and to provide an indication of when and to what degree the percent control had changed. RESULTS: At both locations examined in this study, a significant change point for percent control of λ-cyhalothrin was detected in 2014, thus marking the onset of practical resistance in the soybean aphid. Percent control decreased at a rate of 4.30% and 19.90% per year at these locations. By contrast, percent control for chlorpyrifos remained high over time with no significant change point. CONCLUSION: This research demonstrates that retrospective time-series analysis of insecticide efficacy data can identify the onset and magnitude of practical resistance in the field. This further validates and compliments the other lines of evidence related to pyrethroid resistance in soybean aphid. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Diverse patterns of constitutive and inducible overexpression of detoxifying enzyme genes among resistant Aphis glycines populations.

Understanding the mechanisms of pyrethroid resistance is essential to the effective management of pesticide resistance in Aphis glycines Matsumura (Hemiptera: Aphididae). We mined putative detoxifying enzyme genes in the draft genome sequence of A. glycines for cytochrome oxidase P450 (CYP), glutathione-S-transferase (GST) and esterases (E4 and carboxylesterases-CES). Aphids from clonal populations resistant to pyrethroids from three sites in Minnesota, USA, were screened against a diagnostic LC99 concentration of either λ-cyhalothrin or bifenthrin and detoxifying enzyme genes expression in survivors was analyzed by qPCR. Their expression profiles were compared relative to a susceptible clonal population. We found 61 CYP (40 full-length), seven GST (all full-length), seven E4 (five full-length) and three CES (two full-length) genes, including 24 possible pseudogenes. The detoxifying enzymes had different expression profiles across resistant aphid populations, possibly reflecting differences in the genetic background and pyrethroid selection pressures as the number of constitutively overexpressed detoxifying enzyme genes was correlated with the level of resistance. Our findings will strengthen the understanding of the pyrethroid resistance mechanisms in A. glycines.

Implementation of a Diagnostic-Concentration Bioassay for Detection of Susceptibility to Pyrethroids in Soybean Aphid (Hemiptera: Aphididae).

Soybean aphid, Aphis glycines Matsumura, remains the most economically damaging arthropod pest of soybean in the midwestern United States and southern Canada. Foliar applications of a limited number of insecticide modes of action have been the primary management tactic, and pyrethroid resistance was documented recently with full concentration-response leaf-dip and glass-vial bioassays. Full concentration-response bioassays can be cumbersome, and a more efficient assessment tool was needed. In this study, we implemented a diagnostic-concentration glass-vial bioassay using bifenthrin and λ-cyhalothrin. Bioassays were conducted with field-collected soybean aphid populations to assess the geographic extent and severity of resistance to pyrethroids. In 2017, 10 of 18 and 11 of 21 field populations tested with bifenthrin and λ-cyhalothrin, respectively, had mean proportion mortalities less than the susceptible laboratory population. In 2018, 17 of 23 and 13 of 23 field populations tested with bifenthrin and λ-cyhalothrin, respectively, had mean proportion mortalities less than the susceptible laboratory population. Populations collected after reported field failures of a pyrethroid insecticide generally had mean proportion mortalities less than the susceptible laboratory population. In both years, there was a strong correlation between chemistries, which suggests cross-resistance between these insecticides. The diagnostic-concentration glass-vial bioassays reported here will provide the foundation for an insecticide resistance monitoring program with the ability to determine practical levels and geographic extent of insecticide resistance.

Evaluation of Insect Pests on Edamame Varieties in Minnesota.

Edamame, Glycine max (L.) Merrill, consumption continues to rise in the United States. Improved understanding of production threats, including insect pests, could facilitate increasing production of edamame in the north-central United States. Two years of complimentary field and laboratory experiments were performed to assess insect pest populations on commercially available edamame varieties. Fourteen varieties of edamame and four grain-type soybean varieties were tested in the laboratory with soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), and subsets of these varieties were tested in the field with naturally occurring insect populations. In the laboratory, mean aphid densities on the edamame varieties Agate and Chiba Green did not differ from the aphid-resistant grain-type soybean. Among edamame, aphid densities on Agate, Chiba Green, and Kuroshinju were significantly lower than on Hokkaido Black. In both years of the field experiment, aphid densities were significantly lower on the aphid-resistant grain-type compared to all others varieties. In 2016, aphid populations on edamame varieties did not differ from the aphid-susceptible grain-types. However, in 2017 with greater aphid exposure, differences were seen among edamame varieties, and between edamame and aphid-susceptible grain-types. In both years, potato leafhopper, Empoasca fabae (Harris) (Hemiptera: Cicadellidae), densities tended to be higher on edamame varieties compared to grain-types, and varied significantly among edamame varieties. In a laboratory study, differences were seen among varieties in mean density of trichomes, with grain-types generally having more trichomes than edamame. Results of this research will provide a foundation for development of production recommendations for edamame in the north-central United States.