Isolation and characterization of a baculovirus associated with the insect parasitoid wasp, Cotesia marginiventris, or its host, Trichoplusia ni

Transmission Electron Microscopy Observation of Morphological Changes to Cryptophlebia Leucotreta Granulovirus following Ultraviolet Irradiation

Cryptophlebia leucotreta granulovirus (CrleGV), a double-stranded DNA virus (genus Betabaculovirus, family Baculoviridae), is highly infective to the citrus insect pest Thaumatotibia leucotreta. The South African isolate CrleGV-SA is formulated into a commercial biopesticide and registered for use in several countries. In South Africa, it is used as a biopesticide in a multi-faceted integrated pest management approach for citrus crops involving chemical and biological control methods. The virus nucleocapsid is surrounded and protected by an occlusion body (OB) composed of granulin protein in a crystalline matrix. Like all other baculoviruses, CrleGV is susceptible to ultraviolet (UV) radiation from sunlight. This reduces its efficacy as a biopesticide in the field and necessitates frequent respraying. UV damage to baculovirus biopesticides is detected by means of functional bioassays. However, bioassays do not give an indication of whether any structural damage has occurred that may contribute to functional loss. In this study, transmission electron microscopy (TEM) was used to observe damage to the OB and nucleocapsid (NC) of CrleGV-SA, following controlled UV irradiation in the laboratory to mimic field conditions. The resultant images were compared with images of non-irradiated CrleGV-SA virus. TEM images of irradiated CrleGV-SA samples revealed changes to the OB crystalline faceting, a reduction in the size of the OBs, and damage to the NC following UV exposure for 72 h.

Few indirect effects of baculovirus on parasitoids demonstrate high compatibility of biocontrol methods against Tuta absoluta

BACKGROUND

Combining different biocontrol agents, particularly micro- and macroorganisms, can contribute to new and sustainable pest control approaches. Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) is one of the most destructive pests of solanaceous crops. An emerging management strategy consists of biological control using microbial insecticides such as baculoviruses, but with limited efficacy. Thanks to their high target specificity, baculoviruses can be used simultaneously with natural enemies such as parasitoids for improved control of T. absoluta. However, potential indirect nontarget effects of baculoviruses on parasitoids can result from overlapping resource requirements. We assessed whether ovipositing parasitoid females discriminated against virus-treated hosts and examined the outcome of within-host competition between the hymenopteran parasitoids Necremnus tutae (Reuter) (Eulophidae) and Dolichogenidea gelechiidivoris Marsch (Braconidae), and the Phthorimaea operculella granulovirus (PhopGV, Baculoviridae) that infects T. absoluta larvae.

RESULTS

Female D. gelechiidivoris discriminated against virus-treated hosts, whereas N. tutae did not. We found few indirect virus-related effects depending on the species, the sex, and the time of virus treatment. Effects were ambivalent for D. gelechiidivoris offspring and ranged from increased male longevity when infection occurred before parasitization to reduced emergence and male longevity when infection occurred after parasitization. N. tutae offspring showed a longer development time and shorter male longevity when they developed in virus-treated hosts.

CONCLUSION

The virus had a low impact on parasitoid offspring. In rare cases, adverse effects were detected; however, the low magnitude of these effects is unlikely to reduce the fitness of parasitoid offspring, therefore both parasitoids seem compatible with the baculovirus for control of T. absoluta. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Insect prophenoloxidase: the view beyond immunity

Insect prophenoloxidase (PPO) is an important innate immunity protein due to its involvement in cellular and humoral defense. It belongs to a group of type-3 copper-containing proteins that occurs in almost all organisms. Insect PPO has been studied for over a century, and the PPO activation cascade is becoming clearer. The insect PPO activation pathway incorporates several important proteins, including pattern-recognition receptors (PGRP, β GRP, and C-type lectins), serine proteases, and serine protease inhibitors (serpins). Due to their complexity, PPO activation mechanisms vary among insect species. Activated phenoloxidase (PO) oxidizes phenolic molecules to produce melanin around invading pathogens and wounds. The crystal structure of Manduca sexta PPO shows that a conserved amino acid, phenylalanine (F), can block the active site pocket. During activation, this blocker must be dislodged or even cleaved at the N-terminal sequence to expose the active site pockets and allow substrates to enter. Thanks to the crystal structure of M. sexta PPO, some domains and specific amino acids that affect PPO activities have been identified. Further studies of the relationship between PPO structure and enzyme activities will provide an opportunity to examine other type-3 copper proteins, and trace when and why their various physiological functions evolved. Recent researches show that insect PPO has a relationship with neuron activity, longevity, feces melanization (phytophagous insects) and development, which suggests that it is time for us to look back on insect PPO beyond the view of immunity in this review.

Isolation and characterization of baculoviruses from three major lepidopteran pests in the semi-arid tropics of India

Baculoviruses were isolated from three major lepidopteran pests, Helicoverpa armigera, Spodoptera litura and Amsacta albistriga in the semi-arid tropics during natural epizootic conditions at ICRISAT fields, Patancheru, Andhra Pradesh, India. Biological, morphological and biochemical analysis identified these isolates as Nucleopolyhedroviruses (NPVs). Scanning electron microscopy of the occlusion bodies (OBs) purified from diseased larvae revealed polyhedral particles of size approximately 0.5-2.5 μm [Helicoverpa armigera Nucleopolyhedrovirus (HearNPV)], 0.9-2.92 μm [Spodoptera litura Nucleopolyhedrovirus (SpltNPV)] and 1.0-2.0 μm [Amsacta albistriga Nucleopolyhedrovirus (AmalNPV)] in diameter. Transmission electron microscopy of thin sections of OBs of the three isolates revealed up to 5-8 multiple bacilliform shaped particles packaged within a single viral envelope. The dimensions of these particles were 277.7 × 41.6 nm for HearNPV, 285.7 × 34.2 nm for SpltNPV and 228.5 × 22.8 nm for AmalNPV. Each of HearNPV and AmalNPV contained up to 6 nucleocapsids and SpltNPV contained up to 7 nucleocapsids per envelope. The estimated molecular weights of the purified OB (polyhedrin) protein of the three NPVs were 31.29-31.67 kDa. Virus yield (OBs/larva) was 5.18 ± 0.45 × 10(9) for HearNPV, 5.73 ± 0.17 × 10(9) for SpltNPV and 7.90 ± 0.54 × 10(9) for AmalNPV. The LC50 values of various NPVs against 2nd and 3rd instar larvae indicated 2.30 × 10(4) and 1.5 × 10(5) OBs/ml for HearNPV, 3.5 × 10(4) and 2.4 × 10(5) OBs/ml for SpltNPV and 5.6 × 10(4) and 3.96 × 10(5) OBs/ml for AmalNPV. The lethal time required to cause 50% mortality (LT50) for these three species were also defined. This study has shown that the NPVs infecting three major lepidopteran pests in India are multiple NPVs, and they have good potential to use as biocontrol agents against these important pests.