Viruses are highly variable, submicroscopic particles consisting of either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) and usually, but not always, with a protein cover.
Viruses multiply or replicate only in living cells, primarily those of bacteria, plants, insects, and vertebrates.
The best-known viruses associated with insects are those that are pathogenic to insects, higher plants, or vertebrates. David (1975) and Vaughn (1974) have reviewed the viruses affecting insects, and Falcon (1976) discusses their use in controlling pests.
From the standpoint of numbers of species of insects, the most important pathogens are the DNA-containing nuclear polyhidrosis viruses (NPV), DNA granulosis viruses (GV), and RNA cytoplasmic polyhedrosis viruses (CPV). Lepidoptera catterpillars and larval sawflies are most susceptible to these viruses. The polyhedrosis viruses are characterized by polyhedral crystal
ADVERTISEMENTS:
Carter (1973) lists 261 viruses that are transmitted to plants by insect vectors. Aphids transmit the largest number of viruses, mostly those producing the mottled “mosaic” diseases.
Aphids also rank first in numbers of species of vectors. Some species may vector numerous viruses, for example, Mazus persicae is the vector of more than 100 viruses.
Leafhoppers rank next in importance and are noted as vectors of persistent propagative viruses. Recently, one group of diseases called the “yellows diseases” has been discovered to be caused not by viruses, but by organisms called mycoplasmas in the phloem that resemble minute bacteria (Whitcomb, 1973; Whitcomb and Davis, 1970).
Like bacteria, mycoplasmas are prokaryotes, i.e., the nucleus lacks a membrane. The cells are highly variable in shape and uniquely lack a cell wall. Mycoplasmas were previously known as pathogens of mammals and birds, but not associated with arthropods. It has been estimated that as many as 350 species of plants may be involved in the natural cycle of aster yellows disease and its vector, the aster leafhopper, Macrosteles fuscifrons (Cicadellidae).”
ADVERTISEMENTS:
Of historical interest is the tulip “breaking” virus that causes the petals of red or purple flowers to be beautifully striped.
The first record was published in 1576, making this the oldest known plant disease (McKay and Warner, 1933). Broken tulips became greatly admired is Europe.
Paintings of the sixteenth and seventeenth centuries often depict the attractive blossoms. Demand was heightened because the bulb of a strikingly patterned flower might degenerate or rapidly die. Various methods of cultivation were alleged to increase breaking, but the capricious cause was unknown.
A colossal gambling craze developed in Holland in 1634-1637 called tulipomania.” Fortunes were traded for single prized bulbs. The episode was finally stopped by the government (Frylink, 1954). We now know that the virus is transmitted mechanically by several aphids, including Myzus persicae.
ADVERTISEMENTS:
Viruses pathogenic to vertebrates that are transmitted by arthropods are collectively called “arboviruses,” an abbreviation of “arthropodbome viruses.” These contain RNA as the nucleic acid. Out of more than 200 arboviruses, 50 infect humans (Horsefall and Tamm, 1965).
Yellow fever, dengue and various encephalitis virsus are the most important. Mosquitoes are the vectors for the great majority of arboviruses, followed by ixodid ticks, Culicoides flies (Ceratopogonidae), and Phlebotomus flies (Psychodidae) and possibly laclapid mites.
Yellow fever is an arbovirus transmitted by the mosquito Aedes aegypti. This disease may be fatal in up to 10 per cent of human cases. Hundreds of thousands have died, especially in tropical Africa and South America. The yellow fever mosquito was originally from Africa, but now occurs worldwide as a vector in urban environments.
Although controlled in cities and towns, the virus persists in tropical forests. Here it is transmitted among monkeys by forest mosquitoes such as Haemagogus in the New World and various Aedes in Africa. Jungle yellow fever is transmitted to humans living in forest clearings and working in forests.