Epidemiology of lettuce necrotic yellows virus in South Australia. III.* Virus transmission parameters, and vector feeding behaviour on host and non-host plants

Abstract

When Hyperomyzus lactucae are given acquisition feeds of 24 hr on Sonchus oleraceus infected with lettuce necrotic yellows virus (LNYV) a temperature-dependent latent period must be completed before the virus can be transmitted. The mean duration of latent period is 18.0, 9.2 and 5.4 days at 15, 20 and 28°C respectively. On completion of the latent period, H. lactucae transmit LNYV consistently. Within 24 hr of reaching the fourth instar, nymphs which have developed on virus-infected S. oleraceus are capable of transmitting the virus, adult apterae and alatae transmit more efficiently than nymphs, and the efficiency of transmission is similar for both morphological forms of the aphid. The longer the inoculation feed by viruliferous H. lactucae, the greater are the chances for successful transmission of the virus to either S. oleraceus or lettuce seedlings. The inoculation thresholds are between 5 and 30 min on S. oleraceus and 1–5 min on lettuce. During this time stylets penetrate the epidermal layer only, which indicates that aphids can inoculate without penetrating the vascular tissue. Transovarial transmission has been demonstrated in viruliferous viviparae apterae, but the rate of passage of virus to progeny is low; out of 73 sets of progeny 5 sets acquired LNYV maternally. It appears that nymphs which acquire the virus maternally complete the latent period at or shortly after birth. At 20 and 28°, the longevity of aphids transmitting LNYV is less than that of those not transmitting.

When apterous H. lactucae freshly removed from S. oleraceus are allowed to probe on lettuce they generally walk off the plant; however, the tendency to settle in a feeding position is considerably increased when they are 'pretreated' by starving them in continuous light at a lowered relative humidity of 65–70%. Salivary secretions are deposited on the leaves of both S. oleraceus and lettuce when either pretreated or untreated aphids are caged on them. Depth of probing increases with time up to 30 min on both hosts, but after this aphids feeding on lettuce tend to withdraw their stylets. Pretreated H. lactucae given access to ³²P-labelled plants took up 17 times more sap from S. oleraceus than from lettuce.

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*Part II, Aust. J. Agric. Res., 22: 231 (1974).