Begomoviruses are vectored by whiteflies in a circulative persistent manner. The insect swallows the virus through its stylet during phloem feeding.
They belong to: Family: Aleyrodidae ,Order: Hemiptera Genus: Bemisia
Bemisia tabaci disseminates the viruses very powerfully over to vegetables, ornamentals and staple food in worldwide. Begomoviruses have either two (bipartite, DNA-A and DNA-B) or one (monopartite, DNA-A-like), circular ssDNA genomic molecules of about 2800 nucleotides each.
Each DNA particle is separately encapsidated in a 22 nm × 38 nm geminate particle assembled from 110 copies of a single type of protein, the coat protein (CP).
The virion strand of monopartite begomoviruses encodes two genes, V1 and V2; the complementary-virion strand (synthesized during the viral DNA replication) encodes four genes, C1–C4. V1 encodes the CP; the other genes encode proteins involved in virus replication and spread, as well as neutralizing plant defenses.
Bipartite begomoviruses possess a DNA-A (with functions similar to monopartite viruses) and a DNA-B, which contains two genes (one on the genomic strand, the other on the complementary genomic strand) with roles in virus intra- and inter-cellular movement.
Most begomoviruses are phloem-limited and are restricted to the vascular system; a few, like the bipartite Bean dwarf mosaic virus, also invade mesophyll tissue.
- tabaci ingests begomoviruses by inserting its stylets in the vascular system of infected leaves. Virus amounts in an infected plant vary with the age of the tissues;
- Therefore, the amounts of virus ingested and the kinetics of virus accumulation may depend on the virus concentration in the target leaves. B. tabacifeeds on phloem sap by inserting its stylets into the vascular tissue.
The stylet bundle is composed of three joined stylets: the maxillary stylet, which contains the interlocked food canal (through which phloem sap is ingested) and the salivary canal (through which saliva is injected into the plant), as well as two mandibular stylets.
Before plant penetration with their stylets and during stylet movement through the apoplast, whiteflies secrete gelling saliva, forming a saliva sheath around the stylet. After penetrating a sieve tube, whiteflies
secrete watery saliva prior to ingestion.
Both saliva types are produced in the salivary glands. Ingestion is facilitated by hydrostatic pressure in the thieve tubes and stylet. The virus is detectable by PCR in the head of whiteflies 5 min after the beginning of the AAP.
The persistence of Begomovirus in B. tabaci as infective entities for longer than the latent period, sometimes for the entire life of the insect raises the question of replication of the virus in the insect.
Management of Whitefly
Management of heavy whitefly infestations is difficult. The whitefly can be managed by following ways:
- Natural enemies of whiteflies will feed on them. Natural enemies are disrupted by insecticide applications, dusty conditions, or interference by ants
- By avoiding or removing plants that repeatedly host high populations of whiteflies
- Resistant varieties
- Mulches can repel whiteflies from vegetable gardens
- Insect traps can be used to monitor or, at high levels, reduce whitefly numbers
- Insecticidal soaps or oils such as neem oil may reduce but not eliminate populations
- Use of systemic insecticides