Aphids from mangold clamps and their importance as vectors of beet viruses

Mangold clamps are over-wintering sources of the aphid-transmitted beet mosaic, beet yellows and beet mild yellowing viruses, and of several species of aphid, three of the most common in clamps being Myzus persicae, Rhopalosiphoninus staphyleae tulipaellus and R. latysiphon. This study attempted to assess the relative importance of the different species in spreading viruses from clamps. Compared with M. persicae, R. s. tulipaellus and R. latysiphon are seldom trapped in flight, except near large infestations. Alatae of M. persicae and R. s. tulipaellus become common in clamps in April, but few fly below 15d? C., a temperature seldom reached in eastern England in early spring. Flight muscle autolysis, which occurs later in R. s. tulipaellus and R. latysiphon than in some aphid species, also probably prevents many alatae in clamps from flying. We confirmed the importance of clamps as sources of beet viruses, the percentage of infected plants decreasing with increasing distance from infested clamps. M. persicae is shown to be a better vector of beet viruses than the other clamp aphids, and is probably responsible for most virus spread from clamps. R. s. tulipaellus did not transmit beet mosaic virus, but it is a fairly efficient vector of beet yellows and beet mild yellowing viruses, and, although we did not find this species on sugar beet in the field, it probably spreads these viruses from clamps. R. latysiphon did not transmit any of the viruses, and the role of Macrosiphum euphorbiae, Aulacorthum solani and Myxus ascalonicus is probably small.     

Effect of virus yellows on yield of some monogerm cultivars of sugar beet

The yield of plants of monogerm cultivars of sugar beet artificially infected with both beet yellows and beet mild yellowing viruses was, on average, depressed 2–7% for every 100 ‘infected plant weeks’, equivalent to c. £25/ha at 1976 prices. The cv. Vytomo, previously recommended to growers as being tolerant of infection by virus yellows, had a high sugar content and abundant foliage but in field trials its actual yield of sugar was no greater when infected, and lower when virus-free, than that of some other monogerm cultivars.     

Sampling for aphids by traps and by crop inspection

The catches of thirty aphid genera and species during 3 years in a suction trap at 40 ft and on a yellow cylindrical sticky trap at 5 ft at Rothamsted and Broom's Barn are tabulated. Suction-trap catches were the more consistent but both were more effective at recording the first seasonal immigration of Myzuspersicae (Sulz.) and Aphis fabae Scop, than the British Sugar Corporation crop-inspection scheme. The catches show the differential attraction to yellow by different species, the effects of local vegetation, and the seasonal distribution of aphids that are crop pests or potential vectors of viruses.     

THE USE OF SOIL INSECTICIDES TO CONTROL POTATO APHIDS AND VIRUS DISEASES

A replicated trial was done to find whether the insecticides Thimet and Rogor applied in the soil affected the spread of aphid-transmitted viruses from infected to healthy plants with potato crops. The insecticides were applied at planting as activated carbon formulations at rates equal in cost to three sprays with DDT emulsion at 2 lb. DDT per acre. The infected plants were removed (rogued) in late June.
Thimet applied along the furrows with the fertilizer, and Rogor applied in individual doses beneath each tuber, kept the plants free from aphids from a week after the plants emerged until early August. Thimet in individual doses was less effective but greatly decreased the aphid infestation. All treatments prevented or greatly decreased the spread of leaf-roll virus, but they only slightly decreased the spread of virus Y. No treatment damaged the plants or depressed yields significantly.
Tubers harvested from the plots treated with insecticides contained only very small quantities of the insecticides, but shoots from them, when infested with adult Myzus persicae (Sulz.), carried fewer aphids a week after infestation than did shoots from control tubers. Shoots of tubers from treated plots also grew more slowly than those from the controls.
The aphicidal efficiency of Thimet applied as individual doses separated from the tubers by distances of up to 6 in., decreased as the distance increased, but the effect of distance became less as time passed.
Reasons for the differences in the behaviour of the insecticides are discussed, and the possibilities that the method offers to control virus diseases. The application of insecticides to soil promises to be a useful way of controlling the spread of viruses, provided the harvested crop is free from toxic residues.     

THE COMPARISON OF YELLOW CYLINDRICAL, FLAT AND WATER TRAPS, AND OF JOHNSON SUCTION TRAPS, FOR SAMPLING APHIDS

Different traps were compared to find the type most suitable for studying aphid vectors of plant viruses quantitatively.
A Moericke water trap caught more aphids than a flat sticky trap of equal area. A flat sticky trap (930 sq. cm.) caught half as many aphids as a cylindrical trap (945 sq. cm.), which caught about one-third as many as a water trap (1200 sq. cm.) or a Johnson suction trap (9 in. fan) when operated at between 2 and 3 ft. over bare soil.
Yellow traps caught proportionally more Tuberculoides annulatus , and in summer more Capitophorus species than a suction trap, but significantly fewer Anoecia corni, Sitobium spp. and Pemphigus bursarius. Traps with a level surface caught proportionally more Brevicoryne brassicae, Aphis fabae and Myzus persicae , but fewer Anoecia corni and Drepanosiphum plantanoides than vertical cylindical traps. Attraction by colour influences the catch on horizontal traps more than on cylindrical traps because there is less impaction by the wind.
Only suction traps indicate the number of aphids per unit volume of air and are non-selective, but they are expensive and require an electric power supply. Water traps effectively catch those aphids that are attracted to yellow, but they require frequent attention. Sticky traps catch fewer aphids than either suction or water traps, but they can be left unattended for about 2 weeks. Flat sticky traps catch aphids likely to land on a crop, and cylindrical traps show when aphids are in the air, but not if those aphids are able or wanting to land. For routine work cylindrical sticky traps have other advantages; they are cheap and do not require skilled handling, and their catches of alate Myzus persicae have been correlated with the spread of some plant viruses.     

The use of weather data and counts of aphids in the field to predict the incidence of yellowing viruses of sugar-beet crops in England in relation to the use of insecticides

Partial regression equations were calculated that relate the mean percentage of plants infected with yellowing viruses (beet yellows and beet mild yellowing viruses) in sugar-beet crops at the end of August to the number of days during January, February and March when temperatures fell below – 0.3 °C (31-5 °F) and the mean temperatures in April, for the 21 yr, 1951–71, using weather records from Rothamsted Experimental Station. Regression analyses were also made to find the effect of other factors including mean and minimum temperatures for the same months, and also mean counts of ‘green aphids’, mainly of the vector Myzus persicae, on sugar-beet plants during May and June. Significant relationships were established with all factors, but ‘frost-days’ and April mean temperatures accounted for the greatest percentage of the variance in yellows incidence. The calculations were made separately for the years from 1951 to 1958, when no routine advice was given to farmers about aphid control, and 1959–71 when a ‘spray-warning scheme’ was in operation, and many crops were sprayed at critical times to prevent aphid- and virus-spread. Weather factors had the same effects in both periods, but for any particular weather less virus was spread in the second period than in the first, although there were sufficient aphids, i.e. the numbers expected from the prevailing weather conditions. There was no evidence that insecticide treatment used in any one year affected aphid-incidence in subsequent years. Regression analyses on weather variables were also calculated separately for each of seventeen beet-sugar factory collection-areas, using weather records from local weather stations, and also the Rothamsted weather records. Unexpectedly, the fit of the regressions was always better with Rothamsted weather data than with local weather records. Mean yellows-incidence for the different factory areas declined from south to north, and there was a linear relationship with the square root of the latitude above 50 °C. At the same time the correlation coefficients relating yellows-incidence to ‘frost-days’ became smaller and less significant, and those showing dependence