Rice Resistance Buffers against the Induced Enhancement of Brown Planthopper Fitness by Some Insecticides

The brown planthopper, Nilaparvata lugens (Stål)[BPH], is a damaging pest of rice in Asia. Insecticides and rice varietal resistance are widely implemented BPH management practices. However, outbreaks of BPH have been linked to excessive insecticide use—challenging the compatibility of these two management practices. IR62 is a variety with resistance against BPH, the whitebacked planthopper, Sogatella furcifera Horváth [WBPH], and the green leafhopper, Nephotettix virescens (Distant)[GLH]. We compared BPH responses to IR62 and to the susceptible variety IR64 treated with buprofezin, carbofuran, cartap hydrochloride, cypermethrin, deltamethrin, fipronil, or thiamethoxam + chlorantraniliprole. In greenhouse bioassays, cypermethrin, fipronil and thiamethoxam + chlorantraniliprole reduced egg laying on both varieties, and, together with buprofezin, reduced nymph survival to zero. Buprofezin, carbofuran, and cartap hydrochloride stimulated egg laying, and carbofuran increased nymph biomass, but these effects were reduced on IR62. Planthopper populations were ten times higher on deltamethrin-treated rice than untreated rice in a screenhouse experiment. Host resistance failed to buffer against this insecticide-induced resurgence in BPH and WBPH. However, IR62 reduced the effect in GLH. Rice treated with cypermethrin and fipronil had reduced yields compared to untreated controls, suggesting possible phytotoxic effects. We found little evidence of synergies between the two management practices; but host resistance did buffer against the undesirable effects of some insecticides.

Insecticide Resistance Monitoring in Field Populations of the Whitebacked Planthopper Sogatella furcifera (Horvath) in China, 2019–2020

Monitoring is an important component of insecticide resistance management. In this study, resistance monitoring was conducted on 18 field populations in China. The results showed that S. furcifera developed high levels of resistance to chlorpyrifos and buprofezin, and S. furcifera showed low to moderate levels of resistance to imidacloprid, thiamethoxam, dinotefuran, clothianidin, sulfoxaflor, isoprocarb and ethofenprox. Sogatella furcifera remained susceptible or low levels of resistance to nitenpyram. LC50 values of nitenpyram and dinotefuran, imidacloprid, thiamethoxam, clothianidin and chlorpyrifos exhibited significant correlations, as did those between dinotefuran and thiamethoxam, clothianidin, sulfoxaflor, imidacloprid, isoprocarb and buprofezin. Similarly, significant correlations were observed between thiamethoxam and clothianidin, sulfoxaflor and imidacloprid. In addition, the activity of EST in field populations of S. furcifera were significantly correlated with the LC50 values of nitenpyram, thiamethoxam and clothianidin. These results will help inform effective insecticide resistance management strategies to delay the development of insecticide resistance in S. furcifera.

A New Mitochondrial Genome of Sogatella furcifera (Horváth) (Hemiptera: Delphacidae) and Mitogenome-Wide Investigation on Polymorphisms

White-backed planthopper (WBPH), Sogatella furcifera (Horváth), is one of the major sap-sucking rice pests in East Asia. We have determined a new complete mitochondrial genome of WBPH collected in the Korean peninsula using NGS technology. Its length and GC percentages are 16,613 bp and 23.8%, respectively. We observed one polymorphic site, a non-synonymous change, in the COX3 gene with confirmation heteroplasmy phenomenon within individuals of WBPH by PCR amplification and Sanger sequencing, the first report in this species. In addition, this heteroplasmy was not observed in wild WBPH populations, suggesting that it may be uncommon in fields. We analyzed single nucleotide polymorphisms, insertion, and deletions, and simple sequence repeats among the three WBPH mitogenomes from Korea and China and found diverse intraspecific variations, which could be potential candidates for developing markers to distinguish geographical populations. Phylogenetic analysis of 32 mitogenomes of Delphacidae including the three WBPH mitogenomes suggested that Delphacinae seems to be monophyletic and Sogatella species including WBPH are clearly formed as one clade. In the future, it is expected that complete mitogenomes of individuals of geographically dispersed WBPH populations will be used for further population genetic studies to understand the migration pathway of WBPH.

Cloning and Functional Verification of CYP408A3 and CYP6CS3 Related to Chlorpyrifos Resistance in the Sogatella furcifera (Horváth) (Hemiptera: Delphacidae)

The white-back planthopper (WBPH), Sogatella furcifera, mainly harms rice and occurs in most rice regions in China and Asia. With the use of chemical pesticides, S. furcifera has developed varying degrees of resistance to a variety of pesticides. In our study, a chlorpyrifos-resistant population (44.25-fold) was built through six generations of screening with a sublethal dose of chlorpyrifos (LD50) from a field population. The expression levels of ten selected resistance-related P450 genes were analyzed by RT-qPCR and found that CYP408A3 and CYP6CS3 were significantly more expressed in the third instar nymphs of the XY17-G5 and XY17-G6 populations, about 25-fold more than the Sus-Lab strain, respectively (p < 0.01). To elucidate their molecular function in the development of resistance towards chlorpyrifos, we cloned two P450 full lengths and predicted their tertiary protein structures. CYP408A3 and CYP6CS3 were also downregulated after injecting dsCYP408A3, dsCYP6CS3, or their mixture compared to the control group. Moreover, the mortality rates of the dsCYP6CS3 (91.7%) and the mixture injection treatment (93.3%) treated by the LC50 concentration of chlorpyrifos were significantly higher than the blank control group (51.7%) and dsCYP408A3 injection treatment (69.3%) at 72 h (p < 0.01). Meanwhile, the P450 enzyme activities in the dsRNA treatments were lower than that in the control (XY17-G6) (p < 0.01). Therefore, the P450 gene CYP6CS3 may be one of the main genes in the development of chlorpyrifos resistance in S. furcifera.

Characterization of PGRP-LB and immune deficiency in the white-backed planthopper Sogatella furcifera (Hemiptera: Delphacidae)

Peptidoglycan recognition proteins (PGRPs) participate in insect defense against bacterial pathogens by recognizing bacterial cell wall peptidoglycans (PGNs). Here, we identified the PGRP-LB gene in the white-backed planthopper Sogatella furcifera (SfPGRP-LB). SfPGRP-LB is a secreted protein with a typical PGN-binding domain and five conserved amino acid (aa) residues required for amidase activity. Expression analysis showed that the SfPGRP-LB transcript levels were significantly higher in the midgut than in other tissues. Silencing SfPGRP-LB with dsRNA significantly downregulated the expression of Toll pathway genes Toll and Dorsal and Imd pathway genes Imd and Relish after Escherichia coli challenge. However, only Toll and Dorsal expressions were downregulated after Staphylococcus aureus challenge. E. coli and S. aureus challenges rapidly and strongly upregulated SfPGRP-LB expression. Recombinantly expressed SfPGRP-LB (rSfPGRP-LB) had strong affinities for E. coli Dap-type PGN and S. aureus Lys-type PGN and agglutinated the bacteria. However, rSfPGRP-LB inhibited S. aureus but not E. coli growth. Furthermore, rSfPGRP-LB had amidase activity, degraded Lys-type PGN, and destroyed S. aureus cell walls but had no such effects on E. coli Dap-type PGN. Thus, SfPGRP-LB recognizes and binds various bacterial PGNs but only has amidase activity against Lys-type PGN.

A Comparative Analyses of the Complete Mitochondrial Genomes of Fungal Endosymbionts in Sogatella furcifera, White-Backed Planthoppers

Sogatella furcifera Horvath, commonly known as the white-backed planthoppers (WBPH), is an important pest in East Asian rice fields. Fungal endosymbiosis is widespread among planthoppers in the infraorder Fulgoromorpha and suborder Auchenorrhyncha. We successfully obtained complete mitogenome of five WBPH fungal endosymbionts, belonging to the Ophiocordycipitaceae family, from next-generation sequencing (NGS) reads obtained from S. furcifera samples. These five mitogenomes range in length from 55,390 bp to 55,406 bp, which is shorter than the mitogenome of the fungal endosymbiont found in Ricania speculum, black planthoppers. Twenty-eight protein-coding genes (PCGs), 12 tRNAs, and 2 rRNAs were found in the mitogenomes. Two single-nucleotide polymorphisms, two insertions, and three deletions were identified among the five mitogenomes, which were fewer in number than those of four species of Ophiocordycipitaceae, Ophiocordyceps sinensis, Hirsutella thompsonii, Hirsutella rhossiliensis, and Tolypocladium inflatum. Noticeably short lengths (up to 18 bp) of simple sequence repeats were identified in the five WBPH fungal endosymbiont mitogenomes. Phylogenetic analysis based on conserved PCGs across 25 Ophiocordycipitaceae mitogenomes revealed that the five mitogenomes were clustered with that of R. speculum, forming an independent clade. In addition to providing the full mitogenome sequences, obtaining complete mitogenomes of WBPH endosymbionts can provide insights into their phylogenetic positions without needing to isolate the mtDNA from the host. This advantage is of value to future studies involving fungal endosymbiont mitogenomes.

Excessive Nitrogen Fertilization Favors the Colonization, Survival, and Development of Sogatella furcifera via Bottom-Up Effects

Fertilization can trigger bottom-up effects on crop plant–insect pest interactions. The intensive use of nitrogen fertilizer has been a common practice in rice production, while the yield has long been challenged by the white-backed planthopper, Sogatella furcifera (Horváth). High nitrogen fertilization can facilitate S. furcifera infestation, however, how nitrogen fertilizer leads to high S. furcifera infestation and the nutritional interactions between rice and S. furcifera are poorly understood. Here, we evaluated the effects of various levels of nitrogen fertilizer application (0–350 kg/ha) on rice, and subsequently on S. furcifera performance. We found that higher nitrogen fertilizer application: (1) increases the preference of infestation behaviors (feeding and oviposition), (2) extends infestation time (adult lifespan), and (3) shortens generation reproduction time (nymph, pre-oviposition, and egg period), which explain the high S. furcifera infestation ratio on rice paddies under high nitrogen conditions. Moreover, high nitrogen fertilizer application increased all tested rice physical indexes (plant height, leaf area, and leaf width) and physiological indexes (chlorophyll content, water content, dry matter mass, and soluble protein content), except for leaf thickness, which was reduced. Correlation analysis indicated that the specific rice physical and/or physiological indexes were conducive to the increased infestation behavior preference, extended infestation time, and shortened generation reproduction time of S. furcifera. The results suggested that nitrogen fertilizer triggers bottom-up effects on rice and increases S. furcifera populations. The present study provides an insight into how excess nitrogen fertilization shapes rice–planthopper interactions and the consequent positive effect on S. furcifera infestation.

Effect of weather parameters on the population dynamics of White backed planthopper, Sogatella furcifera (Horvath) in rice

A field experiment on the population dynamics of White backed planthopper (WBPH), Sogatella furcifera (Horvath) carried out during kharif and rabi 2016-17 at Regional Agricultural Research Station, Maruteru, West Godavari District of Andhra Pradesh, India revealed that the incidence of WBPH was observed during 36th SMW (September 3-9). Its activity increased during successive weeks up to 44th SMW with two peaks, first peak at 38th SMW (September 17-23) with population of 46/10 hills and second peak at 42nd SMW (October 15-21) with population of 170/ 10 hills during kharif 2016, while WBPH was first noticed during 7th SMW (February 12-18) and population increased gradually during successive weeks and attained peak number during 13th SMW (March 26-April 1) in rabi 2016-17. Correlation studies revealed that WBPH had significant negative relation with morning relative humidity of current week during kharif 2016. None of the abiotic factors has showed significant relationship with the population of WBPH during rabi 2016-17.