Trials were carried out from 1958 to 1961 at Fassouri Plantation in Cyprus to assess the value of dimethoate (0,0-dimethyl S-methylcarbamoylmethyl phosphorodithioate) against California red scale, Aonidiella aurantii (Mask.), on orange, lemon and grapefruit trees. This pest has hitherto been controlled in Citrus plantations by annual programmes of sprays containing parathion, malathion or white mineral oil, singly or in combination.

Sprays were applied to Citrus trees at high pressure by means of hand-lance or motor-driven equipment in such a way that all parts of the tree were thoroughly wetted. Treatments were replicated in Latin square experimental designs. Results were assessed by counting the numbers of scales on samples of leaves, fruits, twigs and bark taken at various intervals after spraying and also, in some trials, before spraying. The earlier trials were concerned with the numbers of scales killed after single applications and the later trials with the level of control resulting from full seasonal spraying programmes.

A preliminary trial in 1958 showed that very high kill of the scale was produced by a spray containing 0·2 per cent, of dimethoate. In 1959 a range of concentrations from 0·01 to 0·15 per cent, was compared with the standard parathion/malathion mixture against the scale on orange and lemon trees. At 0·03 to 0·06 per cent., dimethoate gave a level of control equal to that of the standard spray and at higher concentrations it was superior to the standard. This superiority was confirmed in trials carried out during 1960 using dimethoate concentrations from 0·06 to 0·1 per cent., the observations being extended to include immature stages of the scale. A high kill of all developmental stages was produced by all the concentrations used, and at 0·08 to 0·1 per cent, dimethoate gave higher kills than the standard spraying mixture.

The effect of dimethoate against late-season infestations of fruits was next investigated, using mixtures with white mineral oil as well as dimethoate alone. Standard late-season sprays of mineral oil with or without parathion were included for comparison. Dimethoate alone at 0·08 per cent., or at 0·05 per cent, with 1·0 per cent, mineral oil, gave good control of the scale, on fruit. Both these sprays were superior, to the standard oil or parathion/oil mixtures. The results suggest that dimethoate may be especially valuable against scale on full-sized fruits, the control of which has hitherto been difficult.

During 1961, full seasonal spraying programmes with the standard compounds were compared with others using dimethoate at concentrations suggested by the earlier trials. The treated plots were large and the spraying methods approximated to normal estate practice. The final results were based on the level of infestation of fruits at harvest, and comparisons between standard and dimethoate programmes were made by means of Ebeling's corrected relative infestation index (C.R.I.). Values of the C.R.I, below 100 indicate that the degree of control achieved by the standard has been exceeded by the test spray. In the first trial, where early-season infestation was not heavy, the standard programme consisted of an initial parathion/malathion spray followed later by an application of white oil, and the test programmes comprised sprays of dimethoate at 0·04 or 0·06 per cent., both followed by white oil, or 0·08 or 0·1 per cent, without a second treatment. The C.R.I, values from these four programmes were 24, 22, 58 and 7, respectively, showing that all were better than the standard. The second trial was carried out in a plantation with a heavy early-season infestation of the scale; the same standard programme was compared with dimethoate sprays at 0·04 and 0·08 per cent., both followed by white oil. The C.R.I. values for these two programmes were 144 and 36, respectively. In the third trial single seasonal applications were compared, using a spray containing 1·5 per cent, white oil as the standard. The four test sprays comprised white oil 1·5 per cent, plus malathioii 0·05 per cent., white oil 1.0 per cent, plus dimethoate 0·04 per cent., dimethoate 0·04 per cent., and dimethoate 0·08 per cent. The respective C.R.I, values of the test sprays were 68, 38, 56 and 29.

In trials against other pests of Citrus trees dimethoate gave control of Coccus hesperidum L. at concentrations of 0·04 and 0·08 per cent. It was effective against Aceria sheldoni (Ewing) at concentrations of 0·08, 0·04 and 0·02 per cent., but was found to be ineffective against Phyllocoptruta olcivora (Ashm.).

During the trials a number of different formulations were used, including miscible oils and wettable powders. One of the miscible oils caused severe scorch marks on fruits; this was shown to be due to the formulating materials and not to dimethoate. A certain amount of leaf-drop was noticed in some trials, and this requires further investigation. Dimethoate is a comparatively safe compound from the point of view of hazards to operators, and toxic residues in sampled fruit have been found to lie within the limits that have been suggested as safe.

The principal conclusion from these trials was that dimethoate is a valuable addition to the limited range of insecticides available for use against Aonidiella aurantii. At fairly high concentrations (0·08%) it gives better control than spray mixtures in current use, and this suggests that it may be possible to devise programmes giving satisfactory control with fewer applications. At lower concentrations (0·04%) it is about as effective as present insecticides. Although dimethoate is both a contact and systemic insecticide, observations suggested that the former function was mainly involved in its action against this scale.