Interactions Among Organic Fertility, Mustard Green Manures, and Insect Biocontrol by Entomopathogenic Nematodes

Interactions Among Organic Fertility, Mustard Green Manures, and Insect Biocontrol by Entomopathogenic Nematodes

Summary

Growers are using various management practices for crop health. We are evaluating the influence of these practices on entomopathogenic nematodes (EPNs) as biocontrol agents. A field experiment augmenting EPNs into organic and conventional soil amendments (manure and chemical fertilizer respectively) showed that organic plots reduced EPN infection rates compared to conventional plots. Predation/interference seems to be an important factor limiting augmentation of EPNs in these plot types. Survey data show that organic and conventional potato fields have endemic EPNs infecting hosts at similar rates. However, more potato beetle attacking EPNs seem to be associated with organic fields. Surveys also showed that fields incorporating mustard green manures reduced EPN infection rates. Subsequent laboratory studies showed similar effects of mustard green manure on EPN infection.

Objectives/Performance Targets

Our project has 5 objectives: 1) to survey the Columbia Basin for soil competitors and entomopathogenic nematodes (EPNs) for potential control agents, 2) to investigate inoculative releases of entomopathogenic nematodes in conventional and organic soil fertility, 3) to determine whether a combination of two entomopathogenic nematodes with different host finding behaviors increases Colorado potato beetle (CPB) control, 4) to determine whether mustard affects efficacy of entomopathogenic nematodes, and finally, 5) to determine whether mustard green manures affect densities of alternative prey and pupation of Colorado potato beetle.

Accomplishments/Milestones

We have made progress on several of the objectives to date. In order to investigate objective 1, a survey of EPNs was conducted in the Columbia Basin of Washington in the summer of 2004 and 2005. In 2004 three organic potato fields, five conventional fields, and three conventional fields where potatoes followed plantings of mustard green manure were surveyed near Moses Lake, Othello, and Prossor, WA. In 2005 three conventional potato fields, and three conventional fields and three organic fields where potatoes followed plantings of mustard green manure were surveyed. The greater wax moth, Galleria mellonella, is highly susceptible to entomopathogens and was used as sentinel prey in field bioassays. Groups of five larvae were placed in mesh sacks (10 sacks/field) and buried at a depth of 10-15 cm, similar to CPB pupation depth, for 48 hrs. The sacks were then collected and monitored for nematode infection over 2 weeks. The analysis of the survey suggests that EPNs are common in potato fields in the Columbia Basin (35% infection rate). Thus, conservation of resident EPNs could form one component of a potato beetle IPM program. Mustard green manures, used for the control of plant feeding nematodes, may also be antagonistic to EPNs as shown by a decrease of infection. Finally, organic and conventional fields housed similar EPN densities, although species diversity may be higher under organic management. So far, the nematodes have been identified to genera, however, need to be identified to species to get a better idea of diversity. In addition, more survey work must be done to have enough replication in production potato fields.

Objective 2 was heavily investigated in the summer of 2005. To investigate the augmentative biological control of the CPB using EPNs we conducted an extensive field study at the Washington State Othello Research Station. The study was setup as a factorial design with treatments comparing organic (chicken-cow manure mixture) soil fertility to conventional (chemical fertilizer) soil fertility. Two levels of EPN rate were applied to each fertilizer type. A mixture of two species known to infect CPB were used and applied using a backpack sprayer at a low rate (100 million nematodes/acre) and a high rate (1 billion nematodes/acre). In addition there were control plots where no EPNs were applied to fertility plots to get a baseline of natural EPN infection in the field. Each treatment was replicated 8 times. To examine the influence of soil fertility on EPN effectiveness two hosts were used. Similar to the survey G. mellonella was used throughout the field season because they were available in large quantities and are commonly used as indicators. The CPB last larval stage, most susceptible life stage to EPN attack, was also used when the density of this life stage was at its highest. Groups of five larvae were placed in mesh sacks (5 sacks/plot) and buried at a depth of 10-15 cm for 48 hrs. The sacks were then collected and monitored for EPN infection over 2 weeks. For both hosts there was a significant rate effect showing that low EPN rates provided lower infection than high EPN rates that resulted in high infection. More importantly there was also a significant soil fertility effect showing that organic soil fertility, regardless of rate, rendered lower infection than conventional soil fertility. We hypothesized that organic soil fertility, because of improved soil attributes from organic soil amendments, would be more conducive to EPNs than conventional fertility. Although these data contradict what was hypothesized, several factors may limit the effectiveness of EPN augmentation in organic systems. Currently, we suspect that predation and/or interference may be a factor suppressing infection in organic plots. Several soil samples were collected and a dehydrogenase enzyme activity measurement was conducted to examine soil microbial activity. These initial data show that more soil microbial activity is occurring in organic plots compared to conventional plots. This suggests that perhaps there is a greater suite of soil microbes that include other entomopathogens and nematode predators. Several observations have been made finding Beauvaria bassiana, another entomopathogen, and Arthrobotys, the nematode trapping fungus, a nematode predator. The factors limiting augmentation using EPNs is in its infancy and needs to be investigated further.

We conducted a series of laboratory experiments in petri dishes to determine whether mustard affects the efficacy of entomopathogenic nematodes (objective 4). As seen in field surveys, there was a decrease in parasitism when mustard green manure was present. A series of 2×2 completely randomized factorial designs were used (mustard ± X nematode ±) giving a total of 4 treatments. Five G. mellonella larvae were placed in Petri dishes with filter paper, 1ml of treatment solution (either a water control, or mustard extract diluted in water), and 50 infective EPN juveniles (Heterorhabditis bacteriophora) per larva. The response variables were the percent of larvae dead and infected. These laboratory assays support our field survey work, with decreasing EPN infection in the presence of mustard extract. The next step in this effort is to screen several commercially available EPN species to determine levels of susceptibility between and within genera.

Impacts and Contributions/Outcomes

Overall, this project is investigating the influence of different management regimes on the effectiveness of EPNs as biological control agents. Several growers are directed toward using integrated pest management strategies. With the addition of using EPNs as a management strategy this project will and has educated growers (2004 and 2005 potato field day) that not all nematodes are pests. Growers have been able to learn the way EPNs look, work, and can be applied. In addition, the experiments show the compatibility of EPNs with different field strategies (i.e. organic, conventional, and other soil amendments that may influence EPNs) and give growers a sense of what is practical with this control option. EPNs can be applied in the same fashion as a pesticide (e.g. backpack sprayer) and may be compatible with other pest control options. EPNs are also naturally occurring in fields without prior application and infecting at high rates. Thus, surveys from this project suggest that conservation of EPNs could play a role in an integrated pest management program. To conclude, these studies may help us be better able to suggest strategies that growers can adopt to make EPNs an effective biological control option in conjunction with pest control options that are currently in place.

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