The role of soil organic matter in the sustainable management of the grass grub Costelytra zealandica (White) in Canterbury pastures

Larvae of Costelytra zealandica (White) have historically been considered as important pests in New Zealand pastures. Control of this species has mainly been based on the green revolution paradigm. In recent years, this paradigm has collapsed leaving behind multiple environmental problems. Interactions among soil organic matter (SOM), the feeding activity of C. zealandica larvae and their amber disease caused by the bacterium Serratia entomophila Grimont et al., were studied in soils from three pastures with the same soil type but different SOM status. A fractionation scheme has been followed to determine the total C and N in labile (soil biomass, cold and hot water extractions) and stable (humic acids, fulvic acids and humins) SOM fractions. In the laboratory, larvae were placed individually in pots containing soil from an old and a young pasture. Pots were inoculated with four doses of S. entomophila and incubated under similar conditions. Larval feeding activity was measured using pieces of carrot root. At the end of the experiment larvae were assessed for infection, mortality, feeding activity and live weight gains. In March 1992, a field experiment was conducted in these pastures. Soil cores were taken, covered with a gauze sleeve, treated, placed back in situ and protected. The following treatments were tested: (a) 5 larvae plus a pathogenic strain of S. entomophila; (b) as for "(a)" but a non-pathogenic strain was added; (c) as for "(a)" but bacteria-free nutrient broth was added; (d) as for "(a)" but H₂O was added and; (e) as for "(d)" Soil was broken down and sieved. Records of insect survival, larval disease and live weight, as well-as plant production were taken for each soil core. Soil from the different treatments was sampled to estimate S. entomophila numbers, total C and N in SOM fractions. Dry matter yields of living root, herbage and plant residues and the amount of total C and N in plant variables were also sampled for each treatment. In March 1993, a laboratory and a field experiment were conducted to evaluate the effect of the application of whey on the growth of S. entomophila, larval health status, herbage, living roots-plant residues yields, and labile SOM fractions. In the laboratory, pots containing soil in which ryegrass seedlings had grown were treated and incubated under similar conditions. Comparisons of these variables were made after testing 4 bacterial doses, additions of whey or H₂O, and the presence or absence of larvae. In the field experiment four treatments were tested: (a) 5 larvae plus a whey application; (b) as for "(a)" but H₂O was applied; (c) as for "(a)" but no larvae were 'introduced and; (d) as for “(c)" but H₂O was applied. After 2 months, variables were evaluated and determinations of total C and N in labile SOM fractions were made. Total C and N present in cold arid hot water extractions combined, representing the labile SOM fraction, showed that in the young pastures there was 45% less C and 37% less N than in the old pasture (P<0.05). Both in young and old pastures approximately 10% of C and N was present in the labile SOM fractions. A 95% higher (P<0.05) population of S. entomophila was present in fresh soil from the older than 'from-the younger pasture. Under laboratory conditions, 63% more (P<0.05) amber disease occurred in the soil with the highest SOM content. After 30 days, larval mortality was closely associated with amber disease and was 50% higher (P<0.05) in the older than in the younger pasture. Carrot consumption of healthy larvae after 15 days was 38% higher (P<0.05) in soil with lower SOM content than in soil with high SOM content. Larval growth was 23% higher (P<0.05) in the soil from the young than in the soil from the old pasture. Under field conditions, the soil with lower SOM content induced a greater larval herbivory; the living roots contained more N and a lower probability of infection by amber disease was observed. Larval mortality attributed to entomopathogens was higher (P<0.05) in the soil with the highest SOM content. The application of S. entomopizila had positive effect (P<0.05) by itself in inducing leaf and root growth regardless of insect herbivory. A reduction (P<0.05) of 29%, 36% and 43% in the amount of total N contained in living plant roots was observed in soils with decreasing levels of SOM attributed to larval herbivory. No correlation was found between the impact of third instars with the reduction of herbage and living roots growth. The impact on growth of herbage and living roots caused by C. zealandica seems to be more severe when physical injuries occur in sensitive parts of the root system at the top five cm of the soil profile. The application of S. entomophila inhibited (P<0.05) soil microbial biomass and reduced the symptoms of other larval diseases. The insect promoted an increase (P<O.05) in the total C and total N present in soil microbial biomass. Negative correlations were observed between larval growth and most of the SOM fractions considered. Reductions in the amount of total N from nonhumic substances were promoted by the presence of larvae. The application of S. entomophila was associated with a reduction (P<0.05) in the amount of total C and N from hot water extractions. A decrease (P<0.05) from 19-34% in the amount of total C and total N present in plant residues may be attributed to the feeding activity of the larvae. This fraction of the SOM is an important source of C for larval growth (P<0.05). The application of whey by itself showed no significant short-term effect on the health status of C. zealandica larvae under microcosm and mesocosm conditions but the whey input interacted positively (P<O.05) with its entomopathogens. Amber disease levels increased (P<0.05) by either combining the whey application with the medium dose of S. entomophila or by enhancing the indigenous background of entomopathogens present in soil from the old pasture. Positive (P<0.05) growth of soil-borne or artificially applied low doses of S. entomophila populations was observed as a response to whey inputs. The medium dose of S. entomophila combined with the pre-application of whey promoted a positive (P<0.05) interaction between the insect and the plant resulting in an increase in the herbage DM production and in its content of total C and N. The interaction whey-insect under low SOM conditions had an effect on reducing (P<0.05) the proportion of clover in the field. The whey application produced a positive effect (P<0.05) on the build up of the total C and N in the labile hot water extraction of SOM under field conditions. Survival of S. entomophila and S. protemaculans was enhanced (P<0.05) by the whey applications where larvae were included, in soil from the old pasture. The SOM effects on plant damage caused by C. zealandica act in a multifactorial way and their global effect may be greater than that due to the addition of the individual components. Efforts should be made to increase the efficiency of the bacterial applications to the soil; to encourage the build up of SOM in order to reduce damage; to increase the presence of indigenous micro-organisms in the soil and to enhance the contribution that these insects make to SOM turnover. These actions are also recommended to prevent pasture damage. Conservation and increased soil fertility may be crucial as to prevent C. zealandica damage within the framework of sustainable agriculture.