A variety of plant pests are suppressed by the incorporation of cruciferous plant material into soil. Although this effect is attributed to decomposition of glucosinolates into toxic products, little is known concerning glucosinolate degradation in the soil environment. Arenas (30 × 18 × 8 cm) that contained soil amended with 30 g defatted winter rapeseed meal (Brassica napus L.)/kg soil on one half and unamended soil on the other were constructed. Isothiocyanate concentrations in the soil were measured using infrared analysis of CC14 extracts, and ionic thiocyanate (SCN(-)) using ion chromatography on aqueous extracts. Quantities were monitored during a 100-hr time period in conjunction with a wireworm bioassay. Isothiocyanate production reached a maximum of 301 nmol/g soil at 2 hr, but decreased by 90% within 24 hr. Production of SCN(-) reached a maximum of 180 nmol/g soil at 8 hr but persisted longer than isothiocyanate. Separate late instar wire-worms (Limonius infuscatus Mots.) were repelled by the presence of rapeseed meal in less than 24 hr even though the meal was shown in separate experiments not to be toxic. We propose that rapidly produced isothiocyanates are responsible for this repellency, but other products such as SCN(-) may play a role.
Field experiments were conducted to determine growth and yield responses of Pisum sativum L. to defoliation by adult Sitona lineatus (L.). Seedlings grown under conventional (moldboard plowed) and conservation (chisel plowed) tillage treatments were infested for a 1‐week period with 0, 1 and 8 weevils per plant at two times: at 75% field emergence and 1 week later. After the early infestation, defoliation for the control, low and high weevil densities was about 0, 15 and 50%, respectively, while defoliation after the late infestation was about 0, 10 and 35%. An undercompensatory growth response was observed in one experiment after seedlings were subjected to moderate levels of early defoliation. Exact compensation was observed in two experiments after early infestations of low and high Sitona densities. Sitona defoliation reduced the number of pods per plant and pod length in two experiments. However, seed biomass was never significantly reduced. Averaged over all experiments, reduction in seed biomass due to high Sitona densities was 10 and 5% for early and late infestations, respectively. Tillage treatments did not affect Pisum compensatory growth response, although yield components were sometimes greater in conservation tillage than in conventional tillage, possibly due to slightly greater soil moisture in the conservation tillage plots.
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