Agricultural Importance

While aphids are among the most serious agricultural problem insects, only about 250 species are considered to be agricultural pests. Pest aphids may affect only a very specific host (e.g., Brachycorynella asparagi, asparagus aphid), or group of related hosts, such as crucifers (Brassicaceae) (e.g., Brevicoryne brassicae, cabbage aphid). Some, however, are quite polyphagous (e.g., Aphisgossypii, cotton aphid; Myzuspersicae, green peach aphid), with an extremely wide host range. Some common polyphagous pest aphids represent sibling species complexes that are morphologically identical but differ in karyotype. They comprise anholocyclic clones, or biotypes, that differ in host preferences, ability to transmit diseases, or resistance to pesticides.

Aphids cause damage in several ways. They can build to high population densities and damage plants directly, by removing enough sap to cause withering and eventual plant death. If not washed off, aphid excrement, or honeydew, can build up enough on plants to serve as a medium for the growth of sooty molds, impairing photosynthesis and plant development, and eventually promoting other fungal diseases. Salivary secretions of some aphids are phytotoxic, causing stunting, leaf deformation, and gall formation. Even if the feeding effects of aphids are not apparent, they may affect plant hormone balances, changing host metabolism to their advantage, thus essentially hijacking the plant's physiological functions.

The aphid vectoring of stylet-borne and circulative plant viruses is the most serious problem to agriculture posed by aphids. Stylet-borne viruses occur on the aphid's epidermis and are not aphid specific. These viruses are acquired quickly and transmitted during the aphid's probing of the plant's epidermis. They are nonpersistent, however, and the aphid's infectiousness is lost upon molting. Circulative viruses, in contrast, live internally in the aphid's gut. The aphid must feed for a while to acquire these viruses, which require an incubation period before they can be successfully transmitted. They are persistent, however, and once infected, the aphid remains a vector throughout its life. The virus—aphid—plant linkage is fairly specific for circulative viruses, and a given virus is transmitted by only one or few aphid species. Virus-infected plants often show an aphid-attractive yellowing and have increased free amino acids, so aphids benefit by virus transmission.

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