1997 Annual Report for LNE97-081
Potential of Earthworms as Biocontrol Agents of Scab and Leafminers in New England Apple Orchards
Summary
This project explored the use of the earthworm as a biocontrol agent for apple scab disease and leafminer pupae. It was hypothesized that orchards with a history of ecologically-oriented management practices would encourage earthworm populations, but there was no correlation with the type of management practices. However, levels of total leaf litter removed were >75 percent in 25percent of the orchard evaluations. This could have important implications for decision-making with respect to pest management strategies, because the percentage loss of leaf litter is related directly to the percentage loss of potential inoculum for apple scab and overwintered leafminer pupae.
Objectives
Quantify leaf litter removal by earthworms as a measure of the potential of earthworms to reduce primary inoculum of Venturia inaequalis, the causal agent of scab, and overwintering populations of the apple blotch leafminer and spotted tentiform leafminer in New England apple orchards that use conventional, IPM/low-input sustainable, or certified organic pest and crop management practices or have been abandoned.
Develop case histories of orchard management in selected apple orchards throughout New England that have followed conventional, IPM/low-input sustainable, and certified-organic programs of pest and crop management and compare leaf litter burial with orchard case histories to provide insight into orchard practices favorable or unfavorable to earthworms and leaf burial.
Develop an education and outreach plan that will introduce apple growers, fruit specialists, county extension agents, commercial apple consultants, and others in New England involved in apple production to the project objectives and justification and inform them of the results and potential to increase the sustainability of their orchards, regardless of the crop production system employed.
Approach
This study was intended to be Phase I of an eight-year project that was divided into three phases, with Phase I to be completed in two years. Its main objective was to determine if earthworms have a potential to remove a substantial percentage, i.e., >75 percent, of the leaf litter by bud break under New England conditions. It was expected that Phase I would identify orchards with exceptionally high and poor leaf removal and that scrutiny of a detailed Orchard Information Form completed for each orchard would provide insight into practices and conditions that may be favorable or unfavorable for earthworm removal of leaf litter.
Objective I was evaluated in each orchard by determining the percentage of leaves buried by earthworms in the orchard border, within the orchard, and within the orchard relative to the orchard border. It was expected that the diversity of crop management systems tested would identify orchards with very high and very low leaf burial (earthworm activity). Success occurred in an orchard if leaf burial was within the range (? 75 percent burial) that can be coupled to a sanitation threshold/PAD threshold strategy and tactics that will improve the efficiency of fungicide usage to control scab and above a minimum amount needed to delay the buildup of leafminers on apple leaves to the extent that an action threshold to apply insecticide to control leafminers will be delayed or not reached.
Results
There was no relationship between orchard management practices during the past five years and the amount of leaf litter removed by earthworms and the activities of other organisms that feed on the leaf litter, but leaf litter removal within an orchard was reasonably consistent: only two of 21 sites had a significant change in leaf removal in 1999 compared with 1998. One site had a significant decrease in leaf removal in 1999, i.e., 28 percent compared with 91 percent in 1998, and the other site had a significant increase in leaf removal in 1999, i.e., 69 percent compared with 18 percent in 1998.
A herbicide strip had little impact on leaf removal. Of 24 sites in 1998 and 1999, only two sites had significantly lower leaf removal from a herbicide strop compared to leaf removal from sod.
Leaf removal within an orchard was comparable to leaf removal from plots adjacent to the orchard. Of 21 sites in 1998 and 1999, only two sites had a significant difference in leaf removal between sod plots inside and outside an orchard. One site had a significant decrease in removal outside the orchard, i.e., 97 percent removal inside compared with 27 percent removal outside the orchard. The second site had a significant increase in leaf removal outside the orchard, i.e., 74 percent removal inside compared with 100 percent removal outside the orchard.
There was a significantly high loss of leaf tissue (>75 percent) in five of 22 orchards in 1998 and 9 of 36 orchards in 1999.
Impacts and Potential Contributions
The pest management program in each orchard was classified as either organic, advanced IPM, minimal IPM, or conventional with respect to selecting and scheduling pesticides. It was hypothesized that orchards with a history of ecologically-oriented management practices would have the greatest leaf removal, but there was no correlation with the type of management practices. Because orchards that had been following organic and advanced IPM practices and guidelines were not correlated with the greatest amount of leaf removal, presumably due to the greatest earthworm activity, Phase II was not funded. Thus, this study will not impact in any noticeable way on apple pest management practices. However, it was demonstrated that in approximately 25 percent of the orchard trials, greater than 75 percent of the leaf litter was removed, indicating that if we understood more about the influence of environmental conditions, particularly soil health, on earthworm populations in an orchard, which was to be the main focus of Phase II, it is conceivable that earthworms could play an important role in the management of scab and leafminers.
Reported March 2001