Slugs are a persistent threat to Maryland soybean, typically infesting < 20% of soybean acreage but causing significant yield loss when populations reach high densities (Musser et al. 2018, 2019). The sporadic-but-severe nature of slug damage makes management frustrating. Ironically, insecticides make slug problems worse by killing predators but leaving slugs unharmed (Douglas et al. 2015). Molluscicides (e.g., metaldehyde or iron phosphate, applied as a bait) can be effective, but are too costly and prone to washing away with rain to be relied upon as a preventative treatment (Bailey 2002). By the time slug damage is evident, though, it may already be too late to achieve control with a molluscicide. This is a classic “damned if you do, damned if you don’t” conundrum. Moving forward, we need to understand what factors put a soybean field at greater risk of economic damage by slugs so that we can manage our farms to avoid situations
where stand loss becomes unacceptably high.
Natural predators and parasites (enemies) of slugs are a perhaps underappreciated ally in our battle against slugs. A variety of ground beetles, spiders, marsh flies, and nematodes are known to consume slugs at different parts of the slug life cycle (Barker 2004). These natural enemies are themselves influenced by a number of factors, including weather, tillage, pesticide use, and cover crops (Everts et al. 1989; Vernavá et al. 2004; Le Gall & Tooker 2017; Rivers et al. 2018). Sometimes, the link between natural enemy numbers and slug numbers appears clear. For example, a common ground beetle (Pterostichus melinarius) readily consumed a common slug (Deroceras reticulatum) in a small grains farm in the UK, and slug numbers dropped with increasing beetle numbers (Symondson et al. 2002). However, it is unclear how farms can leverage these natural enemies.
The Maryland Grain Producers Utilization Board and the Delaware Soybean Board provided support for Dr. Crossley shortly after his arrival at University of Delaware to begin examining the natural enemies that could make a dent in slug populations and the factors that promote these natural enemies. Dr. Crossley, along with help from Dr. David Owens and the effort of a dedicated PhD student (Thabu Mugala), were able to sample >1,000 slugs (~3% of which harbored slug parasitic nematodes) and >1,000 ground beetles that are currently being identified to species before using molecular techniques to confirm which species are important slug predators. At least two types of slug parasitic nematodes were collected (with obvious morphological differences), one type was from marsh slugs, the other from leopard slugs. We propose to continue this line of research, and will specifically focus in 2023 on finding more slug parasitic nematodes and developing a method for maintaining nematodes in liquid culture (as opposed to continually feeding them living slugs), which will greatly enhance our ability to identify and manipulate them for slug pathogenicity experiments.
For this project, I propose to: 1) Collect and identify slug parasitic nematodes from Maryland soybean fields, and 2) Develop a liquid culturing technique to maintain slug parasitic nematode colonies in the lab.