Andrew Wilcox

Influence of beetle banks on cereal aphid predation in winter wheat

An experiment was conducted to investigate whether polyphagous predators dispersing from a within field overwintering refuge (‘beetle bank’) could reduce cereal aphid numbers in an adjacent crop of winter wheat. To assess the impact of predators emigrating from the beetle bank on populations of Sitobion avenae in the crop, polyphagous predator densities were manipulated using exclusion barriers placed at set distances away from the beetle bank. Results from the pitfall trap data indicated that the exclusion barriers were effective in significantly reducing the number of ‘boundary’ Carabidae, ‘open-field’ Carabidae, Staphylinidae, Linyphiidae and Lycosidae in the enclosed areas compared to the control areas by 56, 31, 46, 24 and 57%, respectively. Aphid numbers were significantly greater in the enclosures, where they were 34% higher compared to the control areas during the aphid peak. The impact of polyphagous predation decreased with increasing distance away from the beetle bank and the distribution of polyphagous predators in relation to the beetle bank varied over time. The results are discussed in terms of the peak activity periods of the different predatory groups and the consequences for integrated crop management (ICM).

Aromatic plant production on metal contaminated soils.

Field and container experiments were conducted to assess the feasibility of growing aromatic crops in metal contaminated areas and the effect of metals on herbage and oil productivity. The field experiments were conducted in the vicinities of the Non-Ferrous Metals Combine (Zn-Cu smelter) near Plovdiv, Bulgaria using coriander, sage, dill, basil, hyssop, lemon balm, and chamomile grown at various distances from the smelter. Herbage essential oil yields of basil, chamomile, dill, and sage were reduced when they were grown closer to the smelter. Metal removal from the site with the harvestable plant parts was as high as 180 g ha(-1) for Cd, 660 g ha(-1) for Pb, 180 g ha(-1) for Cu, 350 g ha(-1) for Mn, and 205 g ha(-1) for Zn. Sequential extraction of soil demonstrated that metal fractionation was affected by the distance to the smelter. With decreasing distance to the smelter, the transfer factor (TF) for Cu and Zn decreased but increased for Cd, while the bioavailability factor (BF) for Cd, Pb, Cu, Mn, and Zn decreased. Scanning electron microscopy and X-ray microanalyses of contaminated soil verified that most of the Pb, Cd, Mn, Cu, and Zn were in the form of small (<1 microm) particles, although there were larger particles (1-5 microm) with high concentrations of individual metals. This study demonstrated that high concentrations of heavy metals in soil or growth medium did not result in metal transfer into the essential oil. Of the tested metals, only Cu at high concentrations may reduce oil content. Our results demonstrated that aromatic crops may not have significant phytoremediation potential, but growth of these crops in metal contaminated agricultural soils is a feasible alternative. Aromatic crops can provide economic return and metal-free final product, the essential oil.

Effects of different grass treatments used to create overwintering habitat for predatory arthropods on arable farmland

An experiment was conducted to investigate the suitability for overwintering of polyphagous arthropods of five different grass species (Arrhenatherum elatius, Dactylis glomerata, Phleum pratense, Festuca rubra and Cynosurus cristatus) and a natural regeneration treatment on mid-field overwintering refuges (beetle banks). Polyphagous predator density and composition differed significantly between the six treatments with A. elatius and D. glomerata supporting the highest predator densities and C. cristatus and the treatments left to regenerate naturally having the lowest. The results are discussed in terms of species composition within the different treatments, cost of creating a beetle bank, and the implications for the biological control of cereal aphids.

Potential enhancement of degradation of the nematicides aldicarb, oxamyl and fosthiazate in UK agricultural soils through repeated applications.

BACKGROUND The potential for enhanced degradation of the carbamoyloxime nematicides aldicarb and oxamyl and the organophosphate fosthiazate was investigated in 35 UK agricultural soils. Under laboratory conditions, soil samples received three successive applications of nematicide at 25 day intervals. RESULTS The second and third applications of aldicarb were degraded at a faster rate than the first application in six of the 15 aldicarb-treated soils, and a further three soils demonstrated rapid degradation of all three applications. High organic matter content and low pH had an inhibitory effect on the rate of aldicarb degradation. Rapid degradation was observed in nine out of the ten soils treated with oxamyl. In contrast, none of the fosthiazate-treated soils demonstrated enhanced degradation. CONCLUSION The potential for enhanced degradation of aldicarb and oxamyl was demonstrated in nine out of 15 and nine out of ten soils respectively that had previously been treated with these active substances. Degradation of fosthiazate occurred at a much slower rate, with no evidence of enhanced degradation. Fosthiazate may provide a useful alternative in cases where the efficacy of aldicarb and oxamyl has been reduced as a result of enhanced degradation.

Factors affecting the yield of winter cereals in crop margins.

Yields of arable crops are commonly lower on the crop margins or headlands, but the nature of the relationship between yield and distance from the crop edge has not been clearly defined, nor have the reasons for lower marginal yields. Surveys of 40 winter wheat headlands were carried out in 2 years to determine how yield changed with distance, and what factors might influence this relationship. Two field experiments were also conducted over 3 years in winter cereal headlands, in which the effect of distance was measured under conservation headland and conventional (fully sprayed) management. Yields in the headland surveys varied from 0.8 to 10.2 t/ha. An inverse polynomial regression model was fitted to yield and weed data. Best fits were obtained by using separate parameters for each site. Adjusting yields to take account of weed dry matter improved the non-linear fit between yield and distance from crop edge. Field experiments provided similar results but the non-linear relationship was not as apparent. There was a negative relationship between soil compaction, as measured by a cone penetrometer, and yield in one field experiment, where soil density values were relatively constant. No relationship was found between pattern of nitrogen fertilizer application and yield. Conservation headland management resulted in lower yield at one experimental site, especially in the third year, but not at the other site. Where yields were affected, weed dry matter was higher in conservation headland plots than in fully sprayed plots. Although greater weed competition appears to account for at least part of the observed yield reductions on headlands, the role of other factors, particularly soil compaction, needs further study. Increased weed infestation may be an indirect result of reduced crop competition caused by other adverse conditions.

Effect of non-inversion tillage on field usage by UK farmland birds in winter

Capsule Several guilds of wintering farmland birds showed preferences for cereal fields established by non-inversion tillage, rather than ploughing. Aims To compare the effects of cereal crop establishment methods using non-inversion tillage and ploughing on field use by wintering farmland birds. Methods Cereal fields on commercial farms, established by non-inversion tillage or conventional ploughing, were censused for birds over the winter months of 2000 to 2003, using standard whole-field count methodologies. Multivariate logistic regression methods were used to assess the difference in bird use between fields with the two crop establishment methods, whilst controlling for the effects of a variety of other variables. Results In late winter, Skylarks Alauda arvensis, granivorous passerines and gamebirds occupied a greater proportion of fields established by non-inversion tillage than conventional tillage. Conclusions As well as documented benefits for resource protection, such as soil and water conservation, non-inversion tillage methods appear to enhance suitability of winter cereal fields for foraging birds. Future studies could usefully identify the mechanisms, in terms of food abundance and sward structure, which drive these preferences.

Degradation of the nematicide oxamyl under field and laboratory conditions

The persistence of nematicides such as oxamyl can vary greatly in field conditions. The objectives of the present studies were: i) to compare oxamyl degradation in soils with different properties; ii) to quantify and examine the influence of various abiotic factors on oxamyl degradation; iii) to establish the validity of using simulated models to predict the degradation in the field; and iv) to examine if a second application of oxamyl to the same soil 13 or 26 weeks after the first application enhances degradation. The first two studies included field measurements of oxamyl concentration and parallel laboratory incubations. For the field measurements, soils were collected from each of ten potato (Solanum tuberosum) fields in Shropshire, UK, immediately after application of oxamyl on the day of planting and then at weekly intervals for the duration of the two experiments. After each collection, oxamyl was extracted and its concentration determined. For the laboratory incubations, soils were collected from the same ten sites immediately prior to field application and received one application of oxamyl in the laboratory at the same day (day 0). The PERSIST model was then used to predict oxamyl degradation in the field (modelled degradation). Modelled degradation was then compared with the measured degradation up to 91 days (study 1) or 56 days (study 2) after application. In study 3, an extra application of oxamyl to that in the field at day 0 was made in the laboratory at 13 or 26 weeks after application. There were wide variations in the persistence of oxamyl between the ten sites, with the field half-life ranging from 10 to 24 days. Degradation in the field was significantly greater at site 4, where it could not be detected 28 days after application. At other sites, the chemical persisted for 42-63 days and was still detectable at two sites 91 days following application. Soil temperatures had a greater impact on oxamyl degradation than rainfall accounting for up to a maximum of 79% of the variation. The short persistence at site 4 was attributed to the combination of warm and moist conditions in a higher pH soil. The PERSIST model predicted the same rate of decline of oxamyl as actually occurred in the field at only four (sites 5, 6, 7 and 8) sites. At the other sites, degradation in the field occurred at more rapid rates than predicted. This could be as a result of the model not allowing for the movement of nematicide by leaching, or because enhanced degradation of nematicides occurred at these latter sites, or due to a combination of these factors. The wide variation in half-lives and the behaviour of soils after subsequent additions of oxamyl in study 3 were suggestive of complex microbial dynamics even under controlled conditions. Further studies would be required to establish the influence of soil microflora together with that of abiotic parameters on oxamyl degradation.