Damage to oilseed rape seedlings by the field slug Deroceras reticulatum in relation to glucosinolate concentration

Seedlings of nine commercial cultivars of oilseed rape were exposed to the field slug Deroceras reticulatum immediately after sowing in compost in trays. There was a small reduction in seedling numbers in the presence of slugs which was not related to glucosinolate concentration in seeds or seedlings. However, the number and leaf-area of seedlings with damage symptoms were strongly and inversely related to the total concentration of glucosinolates in seeds and one wk-old seedlings. The presence of barley seedlings as alternative food did not significantly affect this relationship. The glucosinolate concentration of seeds was closely correlated with that of wk-old seedlings. Analysis of individual glucosinolates in four cultivars spanning the range of concentrations found, showed that the concentration of most components declined as total glucosinolate concentration decreased. However, 2-phenyl ethyl-glucosinolate (gluconasturtiin) concentration tended to increase in seeds and 3-indolyl methyl-glucosinolate (glucobrassicin) increased in seedlings as total glucosinolate concentration decreased. Damage by slugs was inversely related to the concentration of those glucosinolates which decreased and was positively correlated with the two compounds which increased as total glucosinolate concentration decreased. The results support the hypothesis that glucosinolates in brassicas protect them from polyphagous herbivores, and, in particular, that an important function of glucosinolates in rape seeds is to protect seedlings from slugs. As glucosinolate concentrations of oilseed rape cultivars continue to decline, so the risk of slug damage to seedlings may well increase.     

Susceptibility of the eggs of the field slug Deroceras reticulatum to contact with pesticides and substances of biological origin on artificial soil

The toxicity of 14 substances, including a number of pesticides, to the eggs of the pest slug Deroceras reticulatum was determined in laboratory experiments. Eggs were kept in contact with a precisely defined artificial soil to which a range of concentrations of the test substances had been applied. Mortality of the eggs was assessed every 24 h and the median lethal doses (LD₅₀) were determined. The herbicides bromoxynil, ioxynil and pyridate + bromoxynil, the insecticides thiocyclam, diflubenzuron and azadirachtin, the molluscicides metaldehyde and methiocarb, and other compounds such as carvone, iron‐EDDHA, saponin, and an extract of Pongamia pinnata, killed the eggs after periods of exposure ranging from 2 to 14 days, depending on the compound and the dose. Only two compounds, the insecticides imidacloprid and teflubenzuron, failed to kill the eggs of D. reticulatum at any of the doses tested. Values of LD₅₀ below 0.01 mg a.i. cm^?2 were obtained for the herbicides bromoxynil, ioxynil and pyridate + bromoxynil, and for the biological pesticide azadirachtin. The feasibility of slug egg control in different contexts is discussed.     

Improving slug baits: the effects of some phagostimulants and molluscicides on ingestion by the slug, Deroceras reticulatum (Miiller) (Pulmonata: Limacidae)

The effects of seven potential phagostimulants and of four molluscicidal compounds on feeding were examined by confining slugs with agar gels containing the chemicals in varying concentrations. Sugars generally increased the amount of gel ingested; sucrose more than glucose, lactose and fructose. Feeding increased with increasing sucrose concentration to a maximum and then fell progressively: the optimum concentration lay between 2.5% and 5%. The sweeteners saccharin and aspartame at concentrations up to 2.5% did not increase feeding. Addition of the molluscicides metaldehyde, methiocarb and ferric acetylacetonate to gels containing 2.5% sucrose progressively reduced feeding at concentrations of 0.001% and above. Metaldehyde reduced ingestion more than methiocarb and ferric acetylacetonate was intermediate. The molluscicidal herbicide Ioxynil deterred feeding completely at concentrations of 0.001%. The implications for slug bait development are discussed.     

Selective grazing of acyanogenic white clover: Variation in behaviour among populations of the slug Deroceras reticulatum

Summary Collections of the slug Deroceras reticulatum were made from grassland sites containing contrasting frequencies of the cyanogenic morph of white clover, Trifolium repens. In choice chamber experiments, slugs obtained from sites with a low frequency of cyanogenic clover showed a significantly greater degree of selective eating of acyanogenic morphs than slugs taken from a site containing a high frequency of cyanogenic clover. Differences in selectivity between populations were caused both by differences in the rate of initiation of feeding on cyanogenic morphs, and by differences in the extent of damage once feeding had been initiated. The implications of these results for the cyanogenic polymorphism of T. repens are discussed.     

Predation on the slug Deroceras reticulatum by the carabid beetles Pterostichus madidus and Nebria brevicollis in the presence of alternative prey

1 Slugs are important pests in many agricultural crops and potential biological control agents are being studied as an alternative to molluscicides. Carabid beetles may be able to reduce slug populations, but their role as control agents may be influenced by the presence of alternative prey. 2 Attacks on the pest slug Deroceras reticulatum (Müller) by the carabid beetles Pterostichus madidus (Fabricius) and Nebria brevicollis (Fabricius) were investigated in the presence of alternative prey (earthworms and Calliphora fly larvae). Consumption of slug eggs and aphids was also investigated. 3 All five prey types were consumed to varying degrees during the experiments. Both beetle species showed a significant preference for Calliphora larvae over slugs. Pterostichus madidus showed a significant preference for earthworms over slugs. No preference was shown between earthworms or Calliphora larvae by P. madidus females or N. brevicollis. However, P. madidus males showed a significant preference for Calliphora larvae over worms. Pterostichus madidus showed no preference between slug eggs and aphids; N. brevicollis showed a significant preference for aphids over slug eggs. 4 The results from this study indicate that generalist beetles will often attack other prey in preferences to adult slugs. Slugs may not be preferred because of their mucus. Other prey items occur frequently in arable soils and generalist carabids may ignore slugs altogether and may only feed on them when slug density is high or other prey are unavailable.     

Feeding behaviour and mortality of the slug, Deroceras reticulatum in relation to control with molluscicidal baits containing various combinations of metaldehyde with methiocarb

In terms of control of slug pests, the differing modes of action of the two molluscicides metaldehyde and methiocarb appear to provide some advantages for the former during warm, dry weather and for the latter during cool, wet weather. With a view to exploiting these different modes of action, the feeding behaviour of Deroceras reticulatum (Müller) presented with molluscicidal baits containing various combinations of metaldehyde with methiocarb was investigated. Voluntary feeding trials conducted in the laboratory with both candidate and standard treatments (containing metaldehyde or methiocarb alone) are described. By combining the two active ingredients in certain ratios the total concentration of active ingredients may be reduced without adversely affecting the efficacy of a bait formulation in terms of slug mortality.     

The effect of stem nematode on establishment and early yields of white clover

The effects of stem nematode (Ditylenchus dipsaci) infestation on the establishment of white clover sown in mixed swards with perennial ryegrass, were investigated in two field trials. Clover cultivars known to have different degrees of resistance were sown on land in which stem nematode was controlled either by aldicarb (Experiment 1) or crop rotation (Experiment 2). The establishment of white clover was greatly improved and subsequent clover yields were inversely related to stem nematode infestation. At the first harvest after sowing, clover yields were 3.6 and 3.3 times greater from aldicarb and rotation treatment plots than from check plots; over the first nine months, total clover yields were increased by 3.5 and 8.9 times by aldicarb and rotation treatments. In both trials, plots of resistant cultivars had less stem nematode infestation and yielded better than the very susceptible cultivar, S184. Stem nematode infestations eventually developed on all plots, after the establishment phase. This is the first report showing that resistant cultivars improve establishment of clover in mixed swards on stem nematode infested soils. In both experiments, plots became dominated by clover and even cv. S184 eventually produced good clover swards. Aldicarb and rotation treatments also controlled clover cyst and root lesion nematodes, and numbers of these were inversely related to first harvest yields. Other soil borne pests and diseases, although not prominent, have not been ruled out as possible confounding factors. There was no correlation between grass yield and aldicarb treatment.     

Effect of nitrogen on interactions between ryegrass mosaic virus (RMV) and Lolium species in mixed swards

The effect of rate of nitrogen applied to three Lolium perenne-based field swards on the development of RMV symptoms and on the impact of the virus on the growth and composition of the swards was examined over two growing seasons. Glasshouse experiments investigated the competition between Lolium spp. in relation to RMV infection and rate of nitrogen. In general, there was a higher incidence of RMV symptoms at the higher rates of nitrogen but the relationship was not consistent in all experiments. RMV infection only significantly reduced the cumulative total yield of the sward that contained the very susceptible Lolium multiflorum cv. Barverdi and was treated with the higher rate of nitrogen. However, yield losses were consistently greater at the higher rates of nitrogen. RMV altered the relative yields of sward components and the effect was greatest after an extended period of severe frost at the end of 1995. There is evidence that the frost severely restricted the yield of RMV-infected L. multiflorum tillers in 1996. Where RMV reduced the yield of susceptible species, there was significant compensatory growth by less susceptible companion species, i.e. L. perenne and Phleum pratense, but the compensation was generally incomplete, resulting in reduced yields for virus-infected swards. RMV had no significant effect on the overall quality of the three field swards but for cv. Barverdi grown in the glasshouse it significantly reduced the percentage organic matter and water soluble carbohydrate, and D value. The reductions were greater at the higher rates of nitrogen.     

Morphological compatibility of white clover and perennial ryegrass cultivars grown under two nitrate levels in flowing solution culture

The effects of nitrate (NO3-) supply on shoot morphology, vertical distribution of shoot and root biomass and total nitrogen (N) acquisition by two perennial ryegrass (Lolium perenne L.) cultivars (AberElan and Preference) and two white clover (Trifolium repens L.) cultivars (Grasslands Huia and AberHerald) were studied in flowing nutrient culture. Cultivars were grown from seed as monocultures and the clovers inoculated with Rhizobium. The 6-week measurement period began on day 34 (grasses) and day 56 (clovers) when the NO3- supply was adjusted to either 2 mmol m-3 (low nitrogen, LN) or 50 mmol m-3 (high nitrogen, HN). These treatments were subsequently maintained automatically. Plants were harvested at intervals to measure their morphology and N content. Cultivars of both species differed significantly in several aspects of their response to NO3- supply. In the grasses, the LN treatment increased the root : shoot ratio of AberElan but did not affect the distribution of root length in the root profile. In contrast, this treatment changed the root distribution of Preference compared with HN, resulting in a larger proportion of root length being distributed further down the root profile. The morphology of white clover Grasslands Huia was for the most part unaffected by the level of NO3- supply. In contrast, AberHerald exhibited different growth strategies, with LN plants increasing their stolon weight per unit length at the expense of leaf production, leaf area and stolon length, whereas HN plants showed reduced stolon thickness, greater leaf area production and stolon length per plant. Cultivars with different morphological/physiological strategies in response to NO3- supply may be of value in the construction of 'compatible mixtures' aimed at reducing oscillations in sward clover content by extending the range of conditions that allow balanced coexistence of species to occur.     

Effects of white clover (Trifolium repens L.) on plant and soil nitrogen and soil organic matter in mixtures with perennial ryegrass (Lolium perenne L.)

To increase our insight into the above- and belowground N flows in grass and grass-clover swards relations between crop and soil parameters were studied in a cutting trial with perennial ryegrass (Lolium perenne) monocultures and ryegrass–white clover (Trifolium repens) mixtures. The effects of clover cultivar on herbage yield, the amount of clover-derived nitrogen, apparent N transfer to companion grass, dynamics of N and organic matter in the soil were estimated.The grass monocultures had very low DM yields (<2.1 t ha^-1) and a low N concentration in the harvested herbage. During 1992–1995 the annual herbage DM yield in the mixtures ranged from 7.0 to 14.3 t ha^-1, the white clover DM yield from 2.4 to 11.2 t ha^-1 and the mean annual clover content in the herbage DM harvested from 34 to 78%. Mixtures with the large-leaved clover cv. Alice yielded significantly more herbage and clover DM and had a higher clover content than mixtures with small/medium-leaved cvs. Gwenda and Retor. Grass cultivar did not consistently affect yield, botanical composition or soil characteristics.The apparent N₂ fixation was very high, ranging from 150 to 545 kg N ha^-1 in the different mixtures. For each tonne of clover DM in the harvested herbage 49 to 63 kg N was harvested, while the apparent N transfer from clover to grass varied between 55 and 113 kg N ha^-1 year^-1.The net N mineralization rate was lower under monocultures than under mixtures. The C mineralization and the amounts of C and N in active soil organic matter fractions were similar for monocultures and mixtures, but the C:N ratio of the active soil organic matter fractions were higher under grass than under mixtures. This explains the lower N mineralization under grass.     

Influence of pH,exchangeable aluminium and 0.02M CaCl2-extractable aluminium on the growth and nitrogen-fixing activity of white clover (Trifolium repens) in some New Zealand soils

The effects of soil acidity on the growth and N₂-fixing activity of white clover in seven acid topsoils and subsoils of New Zealand were investigated using a glasshouse experiment.The application of phosphate (Ca(H₂PO₄)₂) to the soils resulted in very large increases in white clover growth on all soils. The application of phosphate, as well as increasing P supply, also decreased 0.02M CaCl₂-extractable Al levels, but had little effect on exchangeable Al levels.Where adequate phosphate was applied, increasing rates of lime (CaCO₃) resulted in increased plant growth on most soils. N₂[C₂H₂]-fixing activity was increased by the first level of lime for one soil, but generally remained approximately constant or declined slightly at higher rates of lime. Up to the point of maximum yield, white clover top weight was more highly correlated with 0.02M CaCl₂-extractable soil Al than with exchangeable Al or pH. At pH values greater than 5.5, plant yield declined on some soils, apparently because of Zn deficiency. The data suggest that white clover is unlikely to be affected by Al toxicity at 0.02M CaCl₂-extractable Al levels of less than about 3.3 g g^–1. However, there were differences between soils in apparent plant tolerance to 0.02M CaCl₂-extractable Al, which appeared to be caused by differing C levels in the 0.02M CaCl₂ extracts.