Relation of the Feeding Process of the Pea Aphid to the Inoculation of Pea Enation Mosaic Virus1

Abstract

The probing behavior of 100 nonstarved pea aphids was compared with that of 504 aphids starved 2–3 hr. Forty-eight nonstarved aphids did not probe during a 5-min observation period; the 52 which did probe did not tend to initiate short probes. The probing behavior of non-starved aphids is generally erratic and difficult to characterize. Starved pea aphids, on the other hand, tend to initiate 1 or more probes of 5–25 sec before settling down to probes of longer duration. Additional observations revealed that pea aphids tap and/or slide the rostrum over the plant surface before probing. The rate of penetration of pea aphid stylets in pea leaves was studied by measuring exposed stylets of aphids anesthetized with CO₂. A maximum penetration of 5μ was attained by 60 pea aphids in a 10-sec probe. Of 100 observations each at 30, 60, and 300 sec, the means were 8μ, 18μ, and 79μ, respectively. Observations of sheath saliva left behind in pea leaves subsequent to probes by 112 pea aphids revealed the following information about stylet penetration: (1) all probes by the pea aphid are accompanied by secretion of sheath saliva; (2) penetration of the epidermis is primarily intercellular; (3) during short probes (< 27 sec) penetration beyond the epidermis does not occur; and (4) during longer probes the parenchyma is penetrated primarily intercellularly, with a more rapid rate of penetration in interveinal than veinal areas. Viruliferous pea aphids are capable of inoculating pea enation mosaic virus (PEMV) to pea leaves during single probes as short as 7 sec. An increase in the rate of transmission occurs with an increase in the duration of inoculation probes from 6–60 sec. Further increases in the rate of transmission of this circulative virus occur during probes of from 1–5 min. Inoculation of PEMV to veinal areas is 2 or more times as effective as inoculation to interveinal areas. These studies show that the pea aphid can inoculate PEMV intercellularly to the epidermis, and to the interveinal and veinal parenchyma of the pea leaf.