Many populations of the two-spotted spider mite, Tetranychus urticae Koch, have developed high levels of resistance to the pesticide abamectin in China and other countries.  This study developed a near-isogenic line to understand better the inheritance, cross-resistance, and fitness costs associated with abamectin resistance in the field population of T. urticae in China.  We introduced the trait that confers extremely high abamectin resistance in a field-collected population of T. urticae into a susceptible laboratory strain (IPP-SS) to generate an abamectin-resistant near-isogenic line (NIL-Aba).  This process was carried out through multiple backcrossing to IPP-SS and via parthenogenesis and abamectin screening.  Compared with IPP-SS, the NIL-Aba strain had a 25 147-fold resistance to abamectin and a high level of cross-resistance to bifenthrin (288.17-fold), an intermediate level to emamectin benzoate (42.57-fold), and low levels to bifenazate, chlorfenapyr, cyflumetofen, cyenopyrafen, and cyetpyrafen with resistance ranging from 3.18- to 9.31-fold.  But it had no cross-resistance to profenofos.  The resistance to abamectin in NIL-Aba was autosomal, incompletely dominant, and polygenic.  Based on two sex life table parameters, no fitness cost was found in NIL-Aba.  Establishing the NIL-Aba strain provides a reliable basis for an in-depth study of abamectin resistance in T. urticae.  New information on toxicological characteristics and fitness cost should facilitate the management of abamectin resistance in field populations of T. urticae.

Chemosensory proteins (CSPs) perform several functions in insects.  This study performed the gene expression, ligand-binding, and molecular docking assays on the EforCSP3 identified in the parasitoid wasp Encarsia formosa, to determine whether EforCSP3 functions in olfaction, especially in host location and host preference.  The results showed that EforCSP3 was highly expressed in the female head, and its relative expression was much higher in adults than in other developmental stages.  The fluorescence binding assays suggested that the EforCSP3 exhibited high binding affinities to a wide range of host-related volatiles, among which dibutyl phthalate, 1-octene, β-elemene, and tridecane had the strongest binding affinity with EforCSP3, besides α-humulene and β-myrcene, and should be assessed as potential attractants.  Protein structure modeling and molecular docking predicted the amino acid residues of EforCSP3 possibly involved in volatile binding.  α-Humulene and β-myrcene attracted E. formosa in a previous study and exhibited strong binding affinities with EforCSP3 in the current study.  In conclusion, EforCSP3 may be involved in semiochemical reception by E. formosa.

The identification of functional midgut receptors for pesticidal proteins produced by Bacillus thuringiensis (Bt) is critical for deciphering the molecular mechanism of Bt resistance in insects.  Reduced expression of the PxABCB1 gene was previously found to be associated with Cry1Ac resistance in the diamondback moth, Plutella xylostella (L.).  To directly validate the potential receptor role of PxABCB1 and its contribution to Bt Cry1Ac toxicity in P. xylostella, we used CRISPR/Cas9 to generate a homozygous knockout ABCB1KO strain with a 5-bp deletion in exon 3 of its gene.  The ABCB1KO strain exhibited a 63-fold resistance to Cry1Ac toxin compared to the parental DBM1Ac-S strain.  Intriguingly, the ABCB1KO strain also exhibited significant increases in susceptibility to abamectin and emamectin benzoate.  No changes in susceptibility to various other Bt Cry proteins or synthetic insecticides were observed.  The knockout strain exhibited no significant fitness costs.  Overall, our study indicates that PxABCB1 can protect the insect against avermectin insecticides on one hand, while on the other hand it facilitates the toxic effect of the Bt Cry1Ac toxin.  The results of this study will help to inform integrated pest management approaches against this destructive pest.