Plant Viruses and Insects

The principal families of insect vectors which cause the most damage to agricultural crops through the spread of plant diseases are in the order Hemiptera, and include the aphids, leafhoppers, delphacid planthoppers and whiteflies. Another important group of insect vectors found worldwide is the order Thysanoptera, the thrips. Other insects also spread plant diseases; however, aphids alone are responsible for spreading the majority of known plant viral diseases, followed closely by the leafhoppers, whiteflies, and thrips. The known number of plant disease vectors within these taxa is large, including Cicadellidae (leafhoppers, containing 49 known vector species), Aphididae (aphids, with the majority of 192 vector species), Aleyrodidae (whiteflies, with 3 vector species) and Thripidae (thrips, with 8 known vector species). Of course, these numbers change every year with the description and discovery of new viral diseases and new insect vectors. Furthermore, a group with only a few insect vectors still may be able to carry and spread a huge number of viral diseases to many different host plants, as occurs in the whiteflies and thrips. In 1997, more than 380 viruses were known to be transmitted by these insect vectors; however, in the last five years the number has increased greatly (about 600 in 2001) and is increasing every year. This dramatic increase has been due in part to our ability to detect and characterize viral diseases more accurately, and also due to increased travel and trade between countries, which often result in the accidental introduction of either diseases or insects from one country into another.