Aphids, whiteflies, thrips, and leaf hoppers are among the more common vectors of viruses that cause severe damage in tomatoes. The mode of transmission can be unique to each virus and vector combination, ranging from non-persistent, or passive transport of viral particles on external mouth parts to the more complex internal virus-vector relationships of persistent movement. In non-persistent movement, virus particles are picked up during feeding or probing of infected plants and carried on mouth parts to the next feeding site. The insect acquires the virus within seconds but can only transmit it for hours or days. Long acquisition and transmission times characterize persistent movement where the virus is taken up during feeding, must move through the mid-gut lining to the hemolymph, and ultimately into the salivary glands before it can be transmitted. New infections occur when live viruses are injected along with saliva into the phloem of healthy plants. In general, once they acquire a virus, persistent vectors can transmit it for the duration of their adult lives. 

Aphids: Aphids are small, pear-shaped, insects that feed in groups. They produce live offspring without mating. Aphids can travel from leaf to leaf, and from plant to plant as wingless nymphs, and wingless or winged adults. Adults also can travel for miles when carried by wind. Aphids usually invade fields as winged adults and once established can occur in large numbers on the underside of the newer leaves.

Green peach aphid (Myzus                Green peach aphid (Myzus 
persicae) nymphs.                               persicae) winged adult.

Aphids cause significant damage to tomatoes because they consume plant nutrients and their sucking feeding behavior can cause chlorosis and distortion of the leaves, abscission of blooms, and plant stunting and wilting. Aphids excrete excess plant sap as a sugary honeydew and sooty mold can grow on the honeydew that is left on fruit and foliage, thus reducing fruit quality. Two common tomato aphid pests are: the potato aphid (Macrosiphum euphorbiae), which is large (3mm in length), and pink or green in color, and the green peach aphid (Myzus persicae), which is smaller (1.5mm) and is light to dark green.

The number and diversity of viruses vectored by the numerous genera and species of aphids far exceeds those moved around by other vectors. The ability to probe both surface leaf tissues and the deeper phloem cells without causing significant injury to the host makes aphids efficient virus vectors. Aphids transmit viruses in both a persistent and non-persistent manner. The viruses which are carried by aphids to tomatoes can be acquired and transmitted in seconds and include: cucumber mosaic virus, tobacco etch virus, and alfalfa mosaic virus.

Whiteflies: The greenhouse whitefly (Trialeurodes vaporariorum), sweet potato whitefly (Bemisia tabaci), and silverleaf whitefly (Bemisia argentifolii) are serious insect pests worldwide. Over 500 plant species including weeds and vegetable, ornamental and agronomic crops are attacked. The underside of leaves are colonized by all growth stages. The larval stages are sedentary, while tiny (1 mm) adult flies are mobile, moving short distances from leaf to leaf or plant to plant, or are carried for miles by wind. Once established, populations build up rapidly due to a life cycle of 20 days or less. Whiteflies feed mainly from phloem tissue and cause plant damage similar to that due to aphids. Whiteflies can also cause uneven ripening and a white internal discoloration in fruit. The viruses transmitted by whiteflies are very important in tropical and subtropical regions, but are not confined to these areas. In tomatoes, the adult sweet potato whitefly is an important vector of geminiviruses such as tomato yellow leaf curl, tomato mottle virus, and the numerous Geminiviruses found throughout Mexico, and Central and South America. The adult greenhouse whitefly, Trialeurodes vaporariorum transmits tomato infectious chlorosis virus. Most of these viruses are carried persistently, generally throughout the adult life of the vector. There is no evidence to suggest that viruses are passed to the next generation through the egg.

The greenhouse whitefly                     The silverleaf whitefly 
(Trialeurodes vaporariorum).             (Bemisia argentifolii).

The sweet potato whitefly (Bemisia tabaci).

Thrips: The western flower thrips (Frankliniella occidenfalis) is native to the western USA, but has been introduced into many regions worldwide, while the onion thrips (Thrips tabaci) occurs worldwide. The greenhouse thrips (Heliothrips haemorrhoidalis) is found in greenhouses worldwide where it causes damage to a wide range of ornamental and vegetable plants. Generally, thrips reproduce without mating. The larvae are relatively inactive, but the adults are winged and highly mobile. Adults live up to 20 days, and populations can build up quickly. Thrips feed on the subepidermal cells of the host. The laying of eggs into small developing tomato fruit results in spotting of fruit, and the subsequent feeding by larvae causes scarring. The onion thrips and the western flower thrips are the major vectors of the tomato spotted wilt virus (TSWV). Only the larvae can acquire the spotted wilt virus, however, it is transmitted exclusively by adults. TSWV is not passed to the next generation via the egg, however, there is some evidence this virus may replicate in its vector. Thrips can also spread pollen transmitted viruses by carrying pollen from an infected plant to a healthy plant while feeding.

The western flower thrips                  The western flower thrips
(Frankliniella occidentalis)               (Frankliniella  occidentalis)
 nymphs.                                               adult.

The greenhouse thrips                       The greenhouse thrips 
(Heliothrips haemorrhoidalis)           (Heliothrips haemorrhoidalis)
nymphs.                                                 adult.

Leaf hoppers: Leafhoppers are found in warm, dry regions of North America. They are wedge-shaped, can be up to 3mm long, and are green to greenish-yellow to brown. Nymphs are similar to adults except they lack fully developed wings. Leafhoppers have a very wide host range, including numerous weeds and vegetables. They have sucking mouth parts, and feed on phloem tissues leaving pale, circular spots, or peppery specks in leaves. Adult females make hatch cuts across leaf veins and stems to insert eggs. The life cycle can be completed in 40-45 days if environmental conditions remain favorable. In California, leafhoppers overwinter in weeds, especially those in the foothills of the Sierra Nevada and Coast ranges. In the spring, as weeds die out, leafhoppers move into adjacent tomato fields. The beet leafhopper (Circulifer fenellus), vectors the curly top virus to tomato. This virus is picked up and transmitted 

The beet leafhopper (Circulifer tenellus).

persistently by immature and adult stages of the insect. Once acquired, the virus iscarried to the next instar stage, and can be transmitted throughout the adult life. The curly top virus does not multiply in the vector, nor is it carried to the next generation via the egg. The California Department of Food and Agriculture conducts yearly surveys of beet leafhoppers and conducts a spray program for eradication of this vector to help manage curly top disease in tomatoes. The common brown leafhopper, Orosius argentatus, vectors big bud, a disease caused by a phytoplasma.