The fall armyworm, Spodoptera frugiperda, which destroys many economic crops such as rice and maize, has recently invaded China.  Insect viruses as biological control agents play important roles in killing pests.  One potential viral insecticide is the environmentally highly infective and virulent densovirus.  We successfully rescued Junonia coenia densovirus (JcDV) using its infectious clone in different insect cell lines and larvae of three insect species.  Results showed that the lysate of cultured insect cells transfected by the JcDV infectious clone killed the 2nd instar S. frugiperda.  The LD₅₀ of homogenate from JcDV-infected Spodoptera litura to the 2nd instar S. frugiperda (1.76×10⁸ viral genome copies per larva during 10 d post infection) was higher than that of the 2nd instar S. litura (7.39×10⁷ JcDV genome copies) or Helicoverpa armigera larvae (9.71×10⁷ JcDV genome copies).  The LT₅₀ of the S. litura homogenate (2.60×10⁹ viral genome copies each larva) to the 2nd instar S. frugiperda was 6.96 d, longer than that of the S. litura (6.18 d) or the 2nd instar H. armigera (5.94 d).  JcDV could infect the fat body of H. armigera, but not S. frugiperda or S. litura.  Although JcDV can infect all three lepidopteran species, their susceptibility to the virus differs.  JcDV has great potential as a biological control agent against pests such as S. frugiperda.

 

ATP-binding cassette transporter C2 (ABCC2) is known to be a receptor for Bacillus thuringiensis (Bt) toxins in several lepidopteran insects.  Mutations in the ABCC2 gene have been genetically linked to field-evolved resistance to the Cry1F toxin from Bt in Spodoptera frugiperda.  Here we generated a SfABCC2 knockout strain of S. frugiperda using the CRISPR/Cas9 system to provide further functional evidence of the role of this gene in susceptibility and resistance to Cry1F.  Results from bioassays showed that the SfABCC2 knockout S. frugiperda strain displayed 118-fold resistance to Cry1F compared with the parental DH19 strain, but no resistance to Vip3A toxin from Bt.  These results provide the first reverse genetic evidence for SfABCC2 as a functional receptor for Cry1F.

The fall armyworm (FAW) Spodoptera frugiperda, which originated in the Americas, is advancing across China and threatening the nation’s maize crops.  Currently, one widely used tool for its control is genetically modified (GM) Bacillus thuringiensis (Bt) maize.  Sufficient content of Bt protein in appropriate plant parts is crucial for enhancing resistance against insect pests.  In this study, we conducted a systematic investigation of Cry1Ab levels in Chinese domestic GM maize DBN9936, which has recently obtained a biosafety certificate, and evaluated its efficacy against FAW.  Quantification of expression levels of Cry1Ab, via ELISA, indicated a spatio-temporal dynamic, with significant variation of mean Cry1Ab, ranging from 0.76 to 8.48 μg g^–1 FW with the Cry1Ab protein level ranked as: V6–V8 leaf>R1 leaf>R4 leaf>R1 silk>VT tassel>R4 kernel.  Among the nine locations, the Cry1Ab levels in DBN9936 of the Xinxiang, Langfang, and Harbin fields were significantly lower than those from Wuhan and Shenyang, and were slightly, but not significantly lower than those from the other four fields.  Furthermore, the artificial diet–Cry1Ab mixture and plant tissue feeding bioassays revealed that DBN9936 has high efficacy against FAW.  The insecticidal efficacy of different tissues against FAW larvae reached 34–100% with a descending order of lethality as follows: VT leaf>R4 leaf>R1 husk>R1 silk>VT tassel>R4 kernel.  Taken together, our results showed that Bt-Cry1Ab maize DBN9936 has potential as a promising strategy to manage FAW.

 

Fall armyworm has invaded China and colonized its populations in tropical and sub-tropical regions of South China since December 2018.  Chemical spray has been widely used to control the pest, which shall lead to resistance evolution.  In this research, we collected five populations of the pest from Yunnan, Hainan, Tibet, and Fujian of China, and tested their susceptibilities to pyrethroid, organophosphorus, oxadiazine, diamide, antibiotics and other types of insecticides (14 insecticides totally) in the laboratory.  Based on the susceptible baseline published from the previous studies, the resistance ratio was 615–1 068-fold to chlorpyrifos, 60–388-fold to spinosad, 26–317-fold to lambda-cyhalothrin, 13–29-fold to malathion, 9–33-fold to fenvalerate, 8–20-fold to deltamethrin, 3–8-fold to emamectin benzoate and 1–2-fold to chlorantraniliprole, respectively.  The median lethal concentration (LC₅₀) of other six insecticides without the susceptible baselines was 148.27–220.96 µg mL^–1 for beta-cypermethrin, 87.03–128.43 µg mL^–1 for chlorfenapyr, 16.35–99.67 µg mL^–1 for indoxacarb, 10.55–51.01 µg mL^–1 for phoxim, 7.08–8.78 µg mL^–1 for M-EBI (the mixed insecticide of emamectin benzoate and indoxcarb) and 1.49–4.64 µg mL^–1 for cyantraniliprole.  This study can be helpful for chemical control as well as for resistance monitoring and management of the pest in China.