Background: Seed Myco-priming based on consortium of entomopathogenic fungi is very effective seed treatment against Ostrinia furnacalis herbivory. Maize regulates defense responses against herbivory by the production of defense-related enzymatic and non-enzymatic antioxidants, phytohormones, and their corresponding genes. Jasmonic acid (JA) plays a key role in plant-entomopathogenic fungi-herbivore interaction. Results: To understand how a consortium of the entomopathogenic fungi Beauveria bassiana and Trichoderma asperellum induce changes in the response of maize to herbivory and increase the crop yield, 2-year field experiment, antioxidant enzymes, leaf transcriptome, and phytohormone were performed. Fungal inoculation enhanced the production of antioxidant enzymes and JA signaling pathway more than the normal herbivory. The comparison between single inoculated, consortium inoculated, and non-inoculated plants resulted in distinct transcriptome profiles representing a considerable difference in expression of antioxidant- and JA- responsive genes identified through Weighted gene co-expression network analysis (WGCNA) and expression analysis, respectively. Seed priming with a consortium of B. bassiana and T. asperellum significantly enhanced the expression of genes involved in antioxidants production and JA biosynthesis cascade, with the highest expression recorded at 24-h post O. furnacalis larval infestation. They reduced the larval nutritional indices and survival up to 87% and enhancing crop yield and gross return up to 82-96% over the year 2018 and 2019. Conclusion: From our results we suggest that a consortium of B. bassiana and T. asperellum can be used synergistically against O. furnacalis in maize under field condition and can mediate antioxidants- and JA- associated maize defense response by boosting up the expression of their responsive genes, thereby enhancing crop yield.

Chemical pesticides are largely applied worldwide for the control of agricultural pests, and often cause lethal and sublethal side effects on natural enemies. To determine the compatibility of chemical pesticides, commonly used by maize growers, with Trichogramma ostriniae, the toxicities of four herbicides (pendimethalin, topramezone, glyphosate, and atrazine), three insecticides (imidacloprid, emamectin benzoate, and ethiofencarb), and three fungicides (propiconazole, benzoyl cyclazole, and difenoconazole) were tested under laboratory conditions, measuring residual toxicity to T. ostriniae adults, their lethal effects on the parasitoid's eggs inside the host egg, and on the reproduction of the parasitoid's female offspring. Three tested pesticides including ethiofencarb, glyphosate, and benzoyl cyclazole were slightly harmful to T. ostriniae adults, whereas the other seven pesticides were moderately harmful. The pesticide residues of ethiofencarb, and emamectin benzoate, showed slightly persistence of toxicity to T. ostriniae adults, in contrast to the remaining pesticides. Glyphosate, emamectin benzoate, imidacloprid, ethiofencarb, and propiconazole were classified as harmless, causing < 30% reduction in emergence rate of wasps from host eggs, while atrazine, pendimethalin, topramezone, benzoyl cyclazole, and difenoconazole were classified as slightly harmful, causing 31-52% reduction in parasitoid emergence rates. Furthermore, the number of eggs subsequently laid by T. ostriniae females that had emerged from parasitized host eggs treated with atrazine was significantly reduced compared to which of untreated parasitized host eggs. No negative effects were observed with atrazine treatment on the emergence rates of F₁ wasps, their wing deformity rates and sex ratios. The data presented here indicate that the safety interval between pesticide applications and releases of T. ostriniae wasps is 2-3 weeks, which provide valuable information in designing a sound maize integrated pest management program that combines chemical pesticide applications with augmentative releases of T. ostriniae wasps.

Selecting the candidate agents or species is a fundamental step in developing effective biological control programs. However, no attempts have been made to evaluate the efficacy of Trichogramma Westwood (Hymenoptera: Trichogrammatidae) strains against the Asian corn borer, Ostrinia furnacalis Guenée (Lepidoptera: Crambidae) in Myanmar. In this study, Trichogramma strains were investigated through a series of glass tube bioassays and under a semifield condition. Ostrinia furnacalis or Corcyra cephalonica egg preferences were compared among six strains of Trichogramma ostriniae, three of Trichogramma chilonis, and four of Trichogramma dendrolimi using a choice-test assay design. Significant differences were observed at the inter- and intraspecific levels. Four strains of T. ostriniae and two of T. dendrolimi showed a strong preference for O. furnacalis eggs, while two strains of T. chilonis and one T. dendrolimi strain preferred Corcyra cephalonica Stainton (Lepidoptera: Pyralidae) eggs. The remaining strains showed no preference. Eleven strains were examined on O. furnacalis eggs only, wherein parasitism, sex ratio, and total progeny per female were highest for two strains of T. ostriniae (respectively: 67.6 ± 3.0%, 82.7 ± 2.3%, 49.6 ± 2.8, and 67.6 ± 3.6%, 90.0 ± 2.4%, 42.7 ± 2.6), and one strain of T. dendrolimi (65.2 ± 3.2%, 84.7 ± 2.6%, 46.3 ± 2.8). Parasitism and searching capacity were evaluated for the three, best performing strains in cages containing maize plants. One T. ostriniae from Southern Shan State, Myanmar parasitized ~81% of egg masses, and should be considered a candidate biological control agent against O. furnacalis in Myanmar.

The fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith), is one of the most important insect pests affecting corn crops worldwide. Although planting transgenic corn expressing Bacillus thuringiensis (Bt) toxins has been approved as being effective against FAW, its populations’ resistance to Bt crops has emerged in different locations around the world. Therefore, it is important to understand the interaction between different Bt proteins, thereby delaying the development of resistance. In this study, we performed diet-overlay bioassays to evaluate the toxicity of Cry1Ab, Cry1Ac, Cry1B, Cry1Ca, Cry1F, Cry2Aa, Cry2Ab, Vip3Aa11, Vip3Aa19, and Vip3Aa20, as well as the interaction between Cry1Ab-, Cry1F-, Cry2Ab-, and Vip3Aa-class proteins against FAW. According to our results, the LC₅₀ values of Bt proteins varied from 12.62 ng/cm² to >9000 ng/cm² (protein/diet), among which the Vip3Aa class had the best insecticidal effect. The combination of Cry1Ab and Vip3Aa11 exhibited additive effects at a 5:1 ratio. Cry1F and Vip3Aa11 combinations exhibited additive effects at 1:1, 1:2, and 5:1 ratios. The combination of Cry1F and Vip3Aa19 showed an antagonistic effect when the ratio was 1:1 and an additive effect when the ratio was 1:2, 2:1, 1:5, and 5:1. Additionally, the combinations of Cry1F and Vip3Aa20 showed antagonistic effects at 1:2 and 5:1 ratios and additive effects at 1:1 and 2:1 ratios. In addition to the above combinations, which had additive or antagonistic effects, other combinations exhibited synergistic effects, with variations in synergistic factors (SFs). These results can be applied to the establishment of new pyramided transgenic crops with suitable candidates, providing a basis for FAW control and resistance management strategies.

The “high-dose/refuge” strategy is expected to work most effectively when resistance is inherited as a functionally recessive trait and the fitness costs associated with resistance are present. In the present study, a laboratory selected Mythimna separata strain that have evolved >634.5-fold resistance to Vip3Aa19 was used to determine the mode of inheritance. To determine if fitness costs were associated with the resistance, life history parameters (larva stage, pupa stage, pupal weight, adult longevity and fecundity) of resistant (RR), -susceptible (SS) and heterozygous (R_♂S_♀ and R_♀S_♂) strains on nontoxic diet were assayed. The LC₅₀ values of R_♀S_♂ were significantly higher than that of R_♂S_♀ (254.58 μg/g vs. 14.75 μg/g), suggesting that maternal effects or sex linkage were present. The effective dominance h of F₁ offspring decreased as concentration increased, suggesting the resistance was functionally dominant at low concentration and recessive at high concentration. The analysis of observed and expected mortality of the progeny from a backcross suggested that more than one locus is involved in conferring Vip3Aa19 resistance. The results showed that significant differences in many life history traits were observed among the four insect genotypes. In short, resistance to Vip3Aa19 in M. separata was inherited as maternal and multigene and the resistance in the strain was associated with significant fitness costs. The results described here provide useful information for understanding resistance evolution and for developing resistance management strategies.

Asian corn borer (ACB), Ostrinia furnacalis (Guenée) is a destructive pest of corn and major target of transgenic corn expressing Bt (Bacillus thuringiensis) toxins in China. It is necessary to establish the baseline susceptibility of geographically distinct ACB populations to Cry1Ab protein and estimate the resistance alleles frequency. The median lethal concentration (LC₅₀) and LC₉₅ values of Bt toxin Cry1Ab for 25 geographically distinct populations collected in 2018–2019 ranged from 0.86 to 71.33, 18.58 to 5752.34 ng/cm², respectively. The median effective concentration (EC₅₀) and EC₉₅ values ranged from 0.03 to 10.40 ng/cm² and 3.75 to 172.86 ng/cm², respectively. We used the F₂ screening method for estimating the expected frequency of resistance alleles of the 13 ACB populations, to Bt corn (Bt11 × GA21) expressing the Cry1Ab toxin. The neonates could not survive on the leaves of transgenic maize Bt11 × GA21 with cry1Ab gene, the Cry1Ab resistance allele frequency was still low in each geographic population in the field, ranging from 0.0032–0.0048, indicating that the sensitivity of ACB to Cry1Ab was still at a high level, and there were no viable resistant individuals in the field at present. The susceptibility of 25 populations of ACB collected in China showed regional differences, although the Cry1Ab resistance allele frequency in these ACB populations is still at a low level. This provides essential knowledge for making the decision to commercialize Bt maize, and monitoring resistance development and evaluating resistance management strategies in the future in China.