The Aphid
Description
The pea aphid (Acyrthosiphon pisum) was introduced into North America along with the legume crops it attacks: pea, lentil, alfalfa, clover. It is now present throughout the continent in most places where these crops are grown. It can also be found infesting some leguminous weeds. The wingless form is large and green, in our region, similar in color to the pea plant, and noticeably long-legged, compared with other aphids. The cornicles, or tail-pipe-like structures on the rear of the abdomen, are long and slender. Two other related and similar species, A. kondoi and A. lactucae also occur in the American West, but are not typically found on pea or lentil. A useful site to help with identification can be found at UCIPM (The University of California Statewide Integrated Pest Management website).
Damage
The pea aphid injures the plant directly by removing sap with its sucking mouthparts. Sufficent numbers of aphids can cause wilting, distortion of leaves, stunting or killing the host plant. When infestations are large, the whitish cast 'skins' or exuviae can be observed covering the plant. Numbers of aphids capable of causing injury to the crop have been estimated as a basis for decisions about when to control them (see Managing The Pea Aphid). Management of the aphid is complicated because it is a vector of several important and injurious viruses affecting pea and lentil.
The Virus
Bean leafroll virus (BLRV)
BLRV is a spherical virus infecting many legumes. BLRV is not transmitted from plant to plant mechanically, i.e. through plant sap. It is transmitted only by aphids, the main vector being pea aphid (Acyrthosiphon pisum). BLRV is transmitted by aphids in a persistent manner, which means the virus requires several hours of a minimum acquisition access period, substantial latent period, and several hours of a minimum access inoculation period. Because of this, BLRV infection can be prevented or limited through a timely application of insecticides. Once acquired by the aphid, BLRV persists for the life of the insect.
In susceptible varieties of pea (Pisum sativum) BLRV causes stunting, chlorosis of the youngest leaves and sometimes downward leaf-rolling. BLRV also causes stunting and yellowing of French bean (Phaseolus vulgaris), chickpea (Cicer arietinum), and lentil (Lens esculenta). Infection in the major perennial hosts, alfalfa (Medicago sativa) and white clover (Trifolium repens), is usually symptomless.
Pea enation mosaic virus (PEMV)
PEMV is a small, spherical virus which infects legumes. PEMV can be transmitted mechanically, through plant sap, however in nature it is transmitted in a persistent manner by at least 10 aphids of which pea aphid (A. pisum) and green peach aphid (M. persicae) are the most important. Minimum acquisition access period in nymphs is only 15 min, and in adults - 2 hours. PEMV has a temperature-dependent latency of 4-70 hours , followed by a minimum access inoculation period of 7-120 seconds. Because of this persistent mode of transmission, PEMV can be controlled through insecticide applications. Once acquired, the virus persists in the aphid body for the life of the insect and is retained through moulting, although it is not transmitted to progeny, and does not multiply in aphids.
Infected pea (Pisum sativum L.) plants develop a slight downward rolling of the trifoliate leaves 4-6 days postinoculation followed by a distinct yellow mosaic on the leaves. The yellow mosaic spots become translucent and clearly delineated. Later, plants develop growth malformation, stunting, and sometimes a top necrosis resulting in the loss of apical dominance. Late in infection, diagnostic blisters or enations (hyperplastic outgrowths perpendicular to the leaf plane and associated with the veins) may develop on the underside of the leaves. Pods are often malformed and warty looking, and contain few if any seeds.
Pea streak virus (PeSV)
PeSV is a long filamentous virus infecting legumes. The virus is easily transmitted mechanically through plant sap. PeSV is transmitted by the pea aphid (Acyrthosiphon pisum) in a non-persistent manner, after a minimum access acquisition period of 15 min. There is no latent period for PeSV transmission, and infectious virus is retained by the aphid for only a short period of time, less than 1 hour.