Biological and ecological studies of the currant-sowthistle aphid, Hyperomyzus lactucae (L.) (Homoptera: Aphididae) : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Agricultural Science in the University of Canterbury

Abstract

This thesis reports the first comprehensive biological study on the populations of currant-sowthistle aphid, Hyperomyzus lactucae L., alternating between black currants, Ribes nigrum L. and sowthistles, Sonchus app. in the Southern Hemisphere. The study was conducted at the Lincoln College, Horticultural Research Area for a full year during 1976-1977.

Hyperomyzus lactucae is an economic pest of both black-current and lettuce, the latter being an incidental secondary host. The work concentrated on the biology and population dynamics of the aphid on both its normal hosts.

Populations of H. lactucae were sampled weekly to determine the phenology of the aphids. Climatic factors (especially temperature), host plant quality, intraspecific competition and fungal infection, were found to be the most important factors influencing currant-sowthistle aphid populations in the field.

Parasites of the aphid were not found. H. lactucae was found reproducing parthenogentically and viviparously throughout the year on sowthistles, which are the reservoir for lettuce necrotic yellows virus (LNYV) disease, but it was least abundant in autumn and winter.

The peak numbers of apterous H. lactucae on black-currents occurred in early November and on sowthistles during late January. Trapping data from sticky traps near the black-currant plot indicated that there were two marked seasonal peak flights for activity of alate H. lactucae. The first peak in late November to early December was relatively low, where as the second peak in April and May was substantially higher.

The leaf disc technique was used to study the biology and temperature response of H. lactucae at constant temperatures of 15°, 20°, 25°, and 30°c. The developmental threshold for H. lactucae is 10.66°c and the upper limit of development seems to be near 30°c. H. lactucae appears to be a cool-weather aphid as its reproductive rate, longevity and life span were greater under the lower temperatures (15° and 20°c).

The collective effect of different temperatures on developmental rate, reproductive rate and longevity on population growth of H. lactucae was estimated by the intrinsic rate of natural increase (rm). This provides an insight into the life history of the aphid in response to temperature, and is thus an aid in the more accurate timing of sampling.

A sampling plan was developed as an essential aid to the aphid population studies on blackcurrants and in the development of a practical spray programme.