Field experiments on the control of wireworms

SUMMARY 1. This paper describes an attempt to find out whether it is possible to test chemical control measures against wireworms by a field techniqpe similar to that used in fertilizer and varietal experimentation. A full account is given of three field experiments using old grassland on a heavy ?clay-with-flints? soil. The arrangements and sizes of the plots were as follows: (1) 5 ? 5 Latin square, 1/60 acre; (2) 3?10 randomized blocks, and (3) 6?8 randomized blocks, 1/200 acre. 2. Wireworm populations were ascertained in every case by sampling the soil before and after treatment. The Ladell flotation machine was found rather too small and a modification of the technique using oil drums was adopted. Later, a larger and improved form of the machine was used. In the first experiment six samples (9?9?5 in.) were taken in each plot, making a total of 150 soil sampleswn each occasion. In the second, four samples (9?9?5 in.) per plot were taken. making a total of 120 before treatment, but after treatment the size of the samples was reduced to 6?6 ? 5 in., the number of samples remaining as before. In the third experiment two samples (6?6?6 in.) per plot were taken, to talling 96 on each occasion. Local control was introduced in the first two experiments. In addition preliminary experiments were carried out using potatoes and cabbage-leaf as baits as a means of estimating wireworm populations. No relationship was found between the number of wireworms obtained in the baits and the actual number in the soil. 3. The wireworms were grouped according to size in order to ascertain if there was any differential action of the fumigants on the various hetars. No conclusive evidence was obtained. The mean density of the original wireworm population was 65, 335 and 277 per sq. yd. respectively in the three experiments. The uneven distribution of the wireworms in the soil resulted in high sampling errors accounting for most of the experimental error. This aspect of the work is discussed fully by W. G. Cochran in the Appendix. No relation was found between the pH of the soil and the density of the wireworm population. 4. The untreated controls showed changes of -23, +1 and -9% between the first and second sampling respectively. The biggest drop was obtained when the second sampling was done in July and the smallest difference when the second sampling was done in May. It is suggested that the big drop in July might be due to a downward movement of the wireworms to escape the heat, but the evidence is insufficient to prove this point. 5. Fumigants ?K? and ?S? (mixture of o- and p-dichlorbenzene) reduced the population by 69 and 62 %, but had an adverse effect on the crop (sugar-beet) in both cases, due to insuEcient interval between its application and the drilling of the seed. There was also a residual effect the following year, a reduction in the number of the wireworms and an increase in the plant stand (barley), although there was no sigdicant increase in the final crop yield. 6. Chlorpicrin redilced the population by 46 and 33 %, but there was no harmful effect on the crops (sugar-beet and barley). No residual effect was detected. 7. Fumigant ?M? (sodium cyanide) reduced the population by 45 % and increased the yield of the crop (sugar-beet). The residual effect was negligible. 8. Crude naphthalene (?creosote salts?) reduced the population by 60 %, tar + lime by 29 % and ammonium carbonate by 20 %, on summer fallow. 9. Other treatments which all proved ineffective were chlordinitro-benzene, lime alone, and superphosphate.