Genetic and phenotypic comparisons of viral genotypes from two nucleopolyhedroviruses interacting with a common host species, Spodoptera litura (Lepidoptera: Noctuidae)

Baseline Susceptibility, Cross-Resistance, and Sublethal Effects of Broflanilide, a Novel Meta-Diamide Pesticide, in Spodoptera litura

Spodoptera litura is a damaging and notorious insect pest of agricultural crops that has developed resistance to various insecticides. Broflanilide is a novel pesticide with a unique mode of action that displays high efficiency against lepidopterous larvae. We here determined the baseline susceptibility of a laboratory strain of S. litura to broflanilide and 10 other popular insecticides. Furthermore, we measured susceptibility and cross-resistance using three common insecticides in 11 field-collected S. litura populations. Broflanilide caused the highest toxicity among all tested insecticides, with the laboratory strain and all field-collected populations showing high susceptibility. Moreover, no cross-resistance was detected between broflanilide and the other tested insecticides. We subsequently evaluated the sublethal effects of broflanilide and found that treatment with the 25% lethal concentration (LC25) prolonged the development duration in the larvae, reduced the pupation rate and pupae weight, and decreased egg hatchability. Finally, the activities of three detoxifying enzymes were measured in S. litura after treatment with the LC25 dose. The results suggested that enhanced cytochrome P450 monooxygenase (P450) activity could be involved in broflanilide detoxification. Overall, these findings demonstrate the strong toxicity and significant sublethal effects of broflanilide in S. litura and suggest that increased P450 activity may be associated with broflanilide detoxification.

Herbivore-induced tomato plant volatiles lead to the reduction of insecticides susceptibility in Spodoptera litura

Herbivore-induced plant volatiles (HIPVs) have been associated with plant-plant-herbivorous-natural enemies communication and an enhanced response to the subsequent attack. Spodoptera litura is a serious cosmopolitan pest that has developed a high level of resistance to many insecticides. However, the underlying molecular and biochemical mechanism by which HIPV priming reduces S. litura larval sensitivity to insecticides remains largely unknown. This study was conducted to explore the potential of volatile from undamaged, or artificially damaged, or S. litura-damaged tomato plants on the susceptibility of S. litura to the insecticides beta-cypermethrin indoxacarb and chlorpyrifos. We found that larvae exposed to volatile from S. litura-damaged or artificially damaged tomato plants were significantly less susceptible to the three insecticides than those exposed to volatile from undamaged tomato plants. Elevated activities of detoxifying enzymes [cytochrome P450 monooxygenases (P450s), glutathione S-transferases (GSTs), and esterases (ESTs)], were expressed in S. litura larvae exposed to volatile from S. litura-damaged tomato plants than those exposed to volatile from undamaged tomato plants. Similarly, seven detoxification-related genes [GSTs (SlGSTe1, SlGSTo1, and SlGSTe3) and P450s (CYP6B48, CYP9A40, CYP321A7, and CYP321B1)] in the midgut and fat body of larvae were up-regulated under exposure to volatile from S. litura-damaged tomato plants. Increased volatile organic compounds emissions were detected in the headspace of tomato plants damaged by S. litura compared to the undamaged plants. Collectively, these findings suggest that HIPVs can considerably reduce caterpillar susceptibility to insecticides, possibly through induction-enhanced detoxification mechanisms, and provide valuable information for implementing an effective integrated pest management strategy.

Complete Genome Sequence of an Alphabaculovirus from Choristoneura diversana

The genome sequence of a baculovirus isolated from Choristoneura diversana is 122,827 bp long and contains 150 putative open reading frames (ORFs). The virus is closely related to alphabaculoviruses isolated from insect species of the genus Choristoneura.

Genotype assembly, biological activity and adaptation of spatially separated isolates of Spodoptera litura nucleopolyhedrovirus

The cotton leafworm Spodoptera litura is a polyphagous insect. It has recently made a comeback as a primary insect pest of cotton in Pakistan due to reductions in pesticide use on the advent of genetically modified cotton, resistant to Helicoverpa armigera. Spodoptera litura nucleopolyhedrovirus (SpltNPV) infects S. litura and is recognized as a potential candidate to control this insect. Twenty-two NPV isolates were collected from S. litura from different agro-ecological zones (with collection sites up to 600 km apart) and cropping systems in Pakistan to see whether there is spatial dispersal and adaptation of the virus and/or adaptation to crops. Therefore, the genetic make-up and biological activity of these isolates was measured. Among the SpltNPV isolates tested for speed of kill in 3rd instar larvae of S. litura, TAX1, SFD1, SFD2 and GRW1 were significantly faster killing isolates than other Pakistani isolates. Restriction fragment length analysis of the DNA showed that the Pakistan SpltNPV isolates are all variants of a single SpltNPV biotype. The isolates could be grouped into three genogroups (A-C). The speed of kill of genogroup A viruses was higher than in group C according to a Cox' proportional hazards analysis. Sequence analysis showed that the Pakistan SpltNPV isolates are more closely related to each other than to the SpltNPV type species G2 (Pang et al., 2001). This suggests a single introduction of SpltNPV into Pakistan. The SpltNPV-PAK isolates are distinct from Spodoptera littoralis nucleopolyhedrovirus. There was a strong correlation between geographic spread and the genetic variation of SpltNPV, and a marginally significant correlation between the latter and the cropping system. The faster killing isolates may be good candidates for biological control of S. litura in Pakistan.