Slugs are a persistent threat to Delaware 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 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. In 2023, Dr. Crossley, along with help from Dr. David Owens and the effort of a dedicated PhD student (Thabu Mugala), regularly sampled a total of 17 fields, yielding a total of 1,531 slugs (1,043 marsh slugs, 488 gray garden slugs). Only 20 marsh slugs (~2%) melted and produced nematodes, a low but typical proportion (~3% of slugs melted in 2022). These slugs originated in just two sites. No gray garden slugs were infected with nematodes. Identification is ongoing, but so far we have identified three of the nematode species. One of them, Panagrolaimus detritophagus (these critters don’t have common names, sorry), is not considered a strict parasite, but instead uses slug hosts to disperse and feeds on bacteria in the host and environment. The other, Pristionchus pacificus, is an obscure species that has only been recorded parasitizing scarab beetles. Finding it in a slug is exciting, but warrants further investigation to determine pathogenicity. The third species was identified to the genus Oscheius, which includes species that are known slug parasites. We are most excited about finding this nematode. We plan to continue identifying these nematodes, and to eventually conduct pathogenicity tests to verify the potential of these nematodes to serve as effective slug parasites.