Insecticides against Aphid Vectors of Potato Viruses.

Abstract

Experiments were carried out on potato in Holland in 1951 to investigate the effects of Systox (diethyl 2-(ethylmercapto)ethyl thiophosphate [demeton]) and DDT on the Aphid vectors of leaf-roll and virus Y, their natural enemies and the yield of tubers, and the effect of demeton on the spread of the two viruses [cf. R.A.E., A 43 61], of which the former is of the persistent type and the latter non-persistent. The experimental plots were surrounded and divided from each other by double rows of densely-sown oats, which have been shown to prevent migration between plots. In those treated with demeton, the middle row was planted with tubers from infected plants, and ten rows of virus-free tubers were planted on either side. The plants were sprayed with 0.1 per cent. demeton at about 117 gals, per acre on 25th May, when they first came up, and at about 180 gals. on 7th, 16th and 22nd June, 3rd, 14th and 25th July, and 3rd August. No infected tubers were planted in the DDT plots, and a spray of 0.4 per cent. wettable DDT was applied on llth and 20th June, and 3rd, 13th and 25th July. Control plots were sprayed with water only.

Aphid counts, the results of which are shown in tables, were made some 8-9 days after spray applications, by examining 10-50 plants and counting, on one stem, the Aphids on the largest leaf at the bottom, the largest leaf halfway up, and the whole of the apex, including incompletely opened leaves. Of the Aphids observed, only Myzus persicae (Sulz.) and Aphis nasturtii Kalt. were sufficiently numerous to be considered. The demeton-treated plots remained free from Aphids until 29th June, and though large numbers of alates of M. persicae were present from that date until 24th July and reproduced, no second-instar nymphs were found. In a subsidiary test, alates of M. persicae confined with leaves of a plant treated 8-9 days previously all died within 24 hours. Colonisation between treatments, and nymphal development, had therefore been rendered impossible. Spraying the upper surfaces of the leaves with DDT initially reduced the populations of M. persicae and spraying both surfaces did so still further, but populations were ultimately higher on the treated plots than on the controls. The effects of the insecticides on A. nasturtii were similar. One of the control plots was sprayed, with demeton in error on 16th June, and subsequent observations showed that no Aphids survived on the plants until about 12 days later, the effect of this single treatment on the population as a whole being comparable with that of regular intensive spraying with DDT.

Counts of natural enemies of the Aphids (Syrphids and Coccinellids) appeared to indicate that they were killed by the demeton treatments, but the authors point out that they could have been affected by them only during the actual spraying period, and furthermore that the density of predators is apparently dependent on the Aphid population, so that a reduction in the latter automatically results in a reduction in oviposition by the former. There was some evidence, however, that the increase in Aphids following treatment with DDT was more directly due to destruction of natural enemies. Counts on 16th July showed that the number of leaves shed per stem was not affected by demeton but was 1.5 when both leaf surfaces were treated with DDT, 2.5 when only the upper surface was treated and 3.5 for no treatment. Both insecticides increased the yield of tubers slightly but significantly.

Two tubers from each of the plants (except those used as sources of virus infection) were harvested and planted in 1952 in such a way that two fields resulted in each of which each plant had the same position as the parent plant in the previous year. Each field was repeatedly examined for virus disease, and the results are shown on figures and discussed. If one or both of the resulting plants showed infection it was considered that the parent plant had become infected in 1951. It thus appeared that about 20 per cent. of the plants exposed to leaf-roll and sprayed with demeton had become infected, as compared with 34.1 per cent. in the controls. The distribution of the infected plants was regular throughout the sprayed plots, however, indicating that the treatment had suppressed the influence on adjacent plants of the sources of infection with a persistent virus, whereas in the controls the percentage of plants infected was considerably higher in the first 3-4 rows parallel to the sources of infection than in the other rows (except those near the oat screens, where alates attempting to leave the plots had been prevented from doing so). The percentage of plants infected with leaf-roll in the 4th-8th rows in both demeton-treated and control plots was 18-28, and it is assumed that this represents infection due to Aphids that were already infective when they entered the plots.

In the plots that contained the sources of infection with virus Y and were treated with demeton, infection in the other rows averaged 60.4 per cent., ranging from 87 in the row adjacent to the source of infection to 42 in the 8th and 9th rows. When no source was present and the plots were treated with demeton, 16.9 per cent. of the plants became infected, and when a similar plot was sprayed with water (and once, in error, with demeton) the percentage was 13.8. In the last two cases, infection is ascribed to transmission by Aphids carrying the virus when they entered the plots. The validity of the method employed for assessing virus infection is discussed, and it is concluded that the experiments explain the frequently observed absence of correlation between Aphid counts and the incidence of infection. Since the whole of the standing Aphid population was destroyed in the plots sprayed with dciiifton, the infection that occurred must have resulted from immigrant alates. These are highly mobile and usually escape detection by ordinary methods of population assessment. The difference in the spread of leaf-roll and virus Y agrees very well with the theory of persistent and non-persistent viruses [cf. 29 26]. Virus-free Aphids that acquired the leaf-roll virus from plants treated with demeton died too soon to become infective, but those acquiring virus Y became infective long before they died [cf. 43 61]. No known aphicide could much reduce the spread of virus Y by Aphids entering the field. Treatment with demeton will have little effect on virus transmission if there are no sources of infection within the field when the Aphids arrive. In practice, however, these are usually present, since it is seldom possible to remove all infected plants in time, especially those infected with leaf-roll, the symptoms of which do not appear sufficiently early. Spraying the whole field would make it possible to postpone the removal of plants infected with leaf-roll until symptoms become distinct and prevent the dissemination of infected Aphids at the time of removal. As demeton is dangerous to the worker, however, it is suggested that the spray be applied only to infected plants and those suspected of infection, roguing being carried out at a later date. If a dye were added to the spray, the roguing could be done by unskilled persons. (Abstract retrieved from CAB Abstracts by CABI’s permission)