Beet armyworm, Spodoptera exigua (Hübner), is a serious pest of vegetables in China, and its control is heavily dependent on chemical insecticides. The current resistance status of nine insecticides was investigated in 16 field populations collected from seven provinces of China during 2009-2012. Compared with the susceptible strain WH-S, some field populations evolved various levels of resistance to eight of the nine insecticides tested: emamectin benzoate (4- to 348-fold), indoxacarb (2- to 41-fold), spinosad (5- to 38-fold), chlorantraniliprole (2- to 44-fold), tebufenozide (2- to 87-fold), chlorfluazuron (3- to 31-fold), cypermethrin (79- to 1240-fold), and chlorpyrifos (8- to 3,080-fold), but no significant resistance was detected to chlorfenapyr (0.4- to 7-fold). This indicates that chlorfenapyr has no cross-resistance with these other currently used insecticides. Four consecutive years' resistance screening at two places shows that resistance patterns were different between populations from Luhe (Jiangsu Province) and Fengxian (Shanghai), which are approximately 300 km apart. Resistance levels to chlorpyrifos were much higher in populations from Luhe (877- to 3,080-fold) than from Fengxian (8- to 110-fold). Fengxian populations developed moderate levels of resistance to tebufenozide (13- to 87-fold), but no resistance in Luhe populations (2- to 6-fold). However, Luhe populations developed moderate levels of resistance to chlorfluazuron (21- to 31-fold), but there was no resistance in Fengxian populations (3- to 5-fold). It is suggested that local insecticide selection determined resistance patterns although S. exigua has long-distance migratory potential. Adaptive resistance management tactics (such as rotations) should be designed and implemented based on the resistance patterns of S. exigua for each geographic area.

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BACKGROUNDTrichogramma dendrolimi has been widely used in augmentative biocontrol of lepidopteran pests in China. In mass production of T. dendrolimi using Antheraea pernyi eggs as substitutive hosts, which are large in size, as clutch size is a parameter of importance to produce high quality parasitoids. Here, we aimed to determine the optimal clutch size for the bisexual Wolbachia‐uninfected line (TdB) and Wolbachia‐infected thelytokous line (TdT) of T. dendrolimi reared on A. pernyi eggs.RESULTSA medium clutch size of 42.75 to 62.27 for TdB and 52.93 to 57.14 for TdT was optimal to maximize fitness‐correlated traits of parasitoid individual. The optimal clutch sizes with maximized parameters included adult emergence rate, adult body size, adult longevity, fecundity, and sum of fecundity of all females per brood were 58.31 (86.00%), 42.75 (231.11 μm), 50.92 (2.69 days), 62.27 (150.89 eggs), and 83.25 (7926.33 eggs) for TdB and 57.14 (94.54%), 52.93 (236.97 μm), 53.64 (2.62 days), 56.80 (161.01 eggs), and 70.10 (8579.71 eggs) for TdT. The TdT had a shorter adult longevity, longer development time, and higher adult emergence rate than did its non‐infected bisexual counterpart.CONCLUSIONA medium brood size in a A. pernyi egg host was optimal to produce offspring parasitoids with higher fitness parameters for both bisexual Wolbachia‐uninfected and thelytokous Wolbachia‐infected lines of T. dendrolimi. The determination of optimal clutch size for T. dendrolimi will provide the reference for the quality control of T. dendrolimi production and improvement of the field performance of the wasps. © 2020 Society of Chemical Industry

Thelytokous Wolbachia-infected Trichogramma spp. are widely used egg parasitoids against lepidopteran pests in biological control programs. Wolbachia may manipulate host wasps for superparasitism and is sensitive to temperature. To explore effects of temperature and superparasitism, we compared fitness parameters and Wolbachia-mediated phenotype of thelytokous Wolbachia-infected Trichogramma dendrolimi between those emerging from superparasitised or single-parasitised hosts at 17, 21, 25, or 29 °C. Infected mothers of T. dendrolimi showed reduced superparasitism and parasitism increased with temperature. Wolbachia titre decreased with temperature when females emerged from singly-parasitised hosts, but there was no correlation in superparasitised hosts. Females showed higher Wolbachia titres at 21, 25, or 29 °C when developing from superparasitised hosts. The daily male ratio of offspring increased with temperature, and the day-age threshold for 5%, 50%, or 95% daily male ratio decreased with temperature in both parasitism forms. Females that emerged from superparasitised hosts had a shorter life span and reduced fecundity. These results indicate that Wolbachia may affect host behaviour by increasing superparasitism to enhance its spread, but this has negative effects on thelytokous Wolbachia-infected T. dendrolimi.

BACKGROUND: Bemisia tabaci is one of most notorious pests on various crops worldwide and many populations show high resistance to different types of insecticides. Flupyradifurone is a novel insecticide against sucking pests. B. tabaci resistance to flupyradifurone has been detected in the field, however the mechanism of flupyradifurone resistance has rarely been studied. RESULTS: The flupyradifurone-resistant strain (FLU-SEL) was selected from the susceptible strain of B. tabaci (MED-S) using flupyradifurone for 24 generations. The FLU-SEL strain exhibited 105.56-fold resistance to flupyradifurone, and moderate crossresistance to imidacloprid, but no cross-resistance to other tested neonicotinoids. Synergism tests and metabolic enzyme assays suggested that FLU-SEL resistance can be attributed to enhanced detoxification mediated by glutathione S-transferase (GST) and P450 monooxygenase (P450). Compared with MED-S strain, CYP6CX4 and GSTs2 were significantly overexpressed in FLU-SEL, and silencing CYP6CX4 or GSTs2 increased the mortality of whiteflies to flupyradifurone challenge in FLU-SEL. In addition, silencing CYP6CX4 also increased the mortality of whiteflies exposed to imidacloprid. CONCLUSION: Overexpression of CYP6CX4 and GSTs2 was associated with flupyradifurone resistance, as confirmed by RNA interference. Our findings suggested that metabolic resistance to flupyradifurone might be mediated by P450s and GSTs.

Beet armyworm, Spodoptera exigua (Hübner), is a worldwide pest of many crops. Chemical insecticides are heavily used for its control in China, and serious resistance has been evolved in the field to a variety of insecticides including emamectin benzoate. Through repeated backcrossing to a susceptible strain (WH-S) and selection with emamectin benzoate, the trait conferring resistance to emamectin benzoate in a field-collected population of S. exigua (moderately resistant to emamectin benzoate and strongly resistant to pyrethroids and indoxacarb) was introgressed into WH-S to generate a near-isogenic resistant strain (WH-EB). Compared with WH-S, the WH-EB strain developed a 1,110-fold resistance to emamectin benzoate and a high level of cross-resistance to abamectin (202-fold), with low levels of cross-resistance to cypermethrin (10-fold) and chlorfluazuron (7-fold), but no cross-resistance to representatives of another six different classes of insecticides (chlorantraniliprole, chlorfenapyr, indoxacarb, spinosad, tebufenozide, and chlorpyrifos). Resistance to emamectin benzoate in WH-EB was autosomal, incompletely dominant, and polygenic. Limited cross-resistance in WH-EB indicates that emamectin benzoate can be rotated with other classes of insecticides to which it does not show cross-resistance to delay the evolution of resistance in S. exigua. The incompletely dominant nature of resistance in S. exigua may explain the rapid evolution of resistance to emamectin benzoate in the field, and careful deployment of this chemical within a resistance management program should be considered.