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, for most natural enemies, such detailed knowledge about how slug numbers change with increasing enemy densities is lacking. Furthermore, how the overall suppression of slugs by natural enemies varies with weather, landscape, and management factors in Mid-Atlantic soybean is uncertain.

1) Identify the natural enemies most often found attacking slugs in soybean.
2) Examine environmental factors associated with higher suppression of slugs by natural enemies.

Objective 1: We will take two approaches to identifying natural enemies of slugs in Delaware soybean farms – one that looks for predators of slugs, the other that looks for parasites of slugs. To identify predators, we will deploy pitfall traps on 20 farms to capture arthropods occurring in corn fields in the Spring of 2022, and use molecular gut content analysis to
determine whether captured arthropods had recently consumed grey garden slug (Deroceras reticulatum) or marsh slug (Deroceras laeve). Molecular gut content analysis involves extracting DNA from arthropod guts, sequencing an arthropod gene (usually cytochrome C oxidase I) from that DNA, and comparing generated sequences to sequences curated in a global database to determine what organism that DNA most likely belonged to. To identify parasites, we will
collect slugs from the same 20 farms using bread dough-baited tile traps, and rear them in laboratory colonies with sentinel insects to monitor for the presence of parasitic nematodes. Nematodes will then be identified using a combination of morphological identification and DNA barcoding.

Objective 2: While sampling for predators and parasites in the spring of 2022, we will also take soybean damage ratings so that we can link slug and natural enemy numbers with actual crop damage. We will then revisit the 20 farms sampled in the spring to collect another round of predator and parasite trapping data the fall of 2022. We will combine these data with publicly available data on environmental conditions on and around the farms to model the amount of crop damage, abundance of slugs, and occurrence of predators and parasitic nematodes. Environmental variables that we will consider include temperature, precipitation, soil moisture, amount of plant cover in the field, presence of cover crops, irrigation, pesticide use, and amount of seminatural habitat. We will use a statistical modelling approach that identifies the best set of variables that can then be used to predict the amount of damage and abundance of slugs, predators, and parasitic nematodes in similar landscapes.

Updated July 26, 2022:
Objective 1) Identify the natural enemies most often found attacking slugs in soybean.
We sampled 858 marsh slugs, 160 gray garden slugs, 27 leopard slugs, 24 three-banded slugs, and 1,335 arthropods (beetles, harvestmen, and spiders) from 15 field sites in Delaware and Maryland between March 31 and June 2 in 2022. Efforts are currently underway to identify ground beetles to species prior to DNA extraction for gut content analysis.
Of the 1,018 field-collected marsh and gray garden slugs, 55 died within 3 weeks in captivity. Of these slugs, 10 from Dorchester County and 1 from the University of Delaware farm in Newark tested positive for slug parasitic nematodes. These slug parasitic nematodes are being cultured and efforts are underway to identify them to species. Preliminary morphological assessment suggests that we might have two different nematode species.
2) Examine environmental factors associated with higher suppression of slugs by natural enemies.
Using the field data from Objective 1 in addition to field data collected from 34 field sites in Virginia in 2018-2019, efforts are currently underway to model/identify the environmental factors that explain patterns of slug and natural enemy abundance.

Updated December 19, 2022:
Objective 1) We visited 15 fields from Maryland and Delaware to sample slugs and natural enemies during the spring of 2022. Over this time period, we collected 1,030 slugs and 1,745 natural enemy specimens. 1,472 natural enemy specimens were ground beetles (others were spiders and harvestmen), from which we have so far identified 11 species (we are 38% of the way through identifications). All 11 of these species are potential slug predators according to the literature, and we are beginning to dissect gut and extract DNA to confirm whether these beetles had recently consumed any slugs to determine which species are most critical for slug suppression. Of the 1,030 slugs captured, 30 (2.9%) were found to be infected with parasitic nematodes. These nematodes are currently being maintained in liquid cultures in the laboratory while we work to identify them to species based on morphology and DNA.

2) Examine environmental factors associated with higher suppression of slugs by natural enemies.
We were able to obtain 2 years’ worth of unpublished data from the Shenandoah region of Virginia (provided by Dr. Sally Taylor) and have analyzed the data to identify predictors of slug and natural enemy abundance. These data span 41 corn and soybean fields that vary in cover cropping, tillage, and use of pre-plant insecticides, and that were sampled for slugs and natural enemies from early May to late June in 2018 and 2019. Key findings are that: the positive effect of cover crops was removed by tillage, slugs declined when there were more ground beetles, slugs increased with increasing precipitation, and slugs decreased with increasing temperature. We further found that the only good predictors of ground beetle abundance were weather-related. Ground beetles were less abundant/active when it was hot and dry or when it was cold and wet. We saw a slight decrease in ground beetles in fields with pre-plant insecticides, but this effect was not statistically significant.

Updated June 19, 2023:
1) Identify the natural enemies most often found attacking slugs in soybean.
We collected 1,740 predatory ground-dwelling arthropods from Delaware soybean fields in 2022 and 2,566 in 2023. 80% of these arthropods were ground beetles, representing 15 species: Amara aenea, Amara pallipes, Anisodactylus sanctaecrucis, Chlaenius aestivus, Chlaenius tricolor, Dicaelus elongatus, Harpalus affinis, Harpalus pensylvanicus, Notiobia terminata, Omophron americanum, Poecilus chalcites, Poecilus lucublandus, Pterostichus melanarius, Scarites subterraneus, and Synuchus impunctatus. Notably, these species are all documented to consume slugs to some extent, so future work will examine which of these species are the most voracious slug predators. This level of detail is important, because ground beetles are a diverse group and might respond differently to attempts to promote them on the farm. Of the 1,030 slugs captured in Delaware soybean fields, 30 (2.9%) were found to be infected with parasitic nematodes. While work to identify these nematodes is still underway, we have so far identified one species, Panagrolaimus detritophagus, which is known to opportunistically feed on bacteria colonizing decaying organisms. We plan to continue sampling slugs and isolating nematodes in the search for truly slug parasitic nematodes in Delaware that could be promoted on the farm or used as commercial biopesticides.

2) Examine environmental factors associated with higher suppression of slugs by natural enemies.
We were able to obtain 2 years’ worth of unpublished data from the Shenandoah region of Virginia (provided by Dr. Sally Taylor) and have analyzed the data to identify predictors of slug and natural enemy abundance. These data span 41 corn and soybean fields that vary in cover cropping, tillage, and use of pre-plant insecticides, and that were sampled for slugs and natural enemies from early May to late June in 2018 and 2019. Key findings are that: the positive effect of cover crops was removed by tillage, slugs declined when there were more ground beetles, slugs increased with increasing precipitation, and slugs decreased with increasing temperature. We further found that the only good predictors of ground beetle abundance were weather-related. Ground beetles were less abundant/active when it was hot and dry or when it was cold and wet. We saw a slight decrease in ground beetles in fields with pre-plant insecticides, but this effect was not statistically significant.

We sampled thousands of ground-dwelling predatory arthropods (beetles and spiders) from Delaware soybean fields where they co-occurred with slugs. The majority (80%) of these predatory arthropods were ground beetles, and we identified 15 species that could be potential slug predators. Future work will determine which of these are the most impactful on slugs and how we can promote them on the farm. We found that 3% of slugs captured in the field were infected with nematodes. We have so far identified one species from these nematodes that is mildly pathogenic against slugs. We plan to continue the search for highly pathogenic nematodes that could be used for biocontrol of slugs. Using two years of slug and predator counts from corn and soybean fields representing a diversity of management practices (no-till, reduced-till, preplant insecticides use, cover crops or bare ground), we found evidence to confirm that tillage reduces slugs, even after cover crops provided some benefit to slug populations. We also found that cool and wet conditions were favorable for slugs. Importantly, we found that ground beetles decreased slug numbers in the field. Ground beetles also appeared to be harmed to an extent by the use of preplant insecticides. Altogether, results suggest that practices that promote ground beetles could counterbalance effects of no-tillage and cover crops on slugs.

In 2020, Delaware farms planted 150,000 acres of soybean, averaging 49 bu/acre with a value > $63 million (USDA NASS 2020). Slugs represent a severe, albeit sporadic pest of soybean, for which the costs of treating or not treating can be equally costly. Often, by the time stand loss is evident, the window of opportunity for rescue treatment is long gone. Knowledge about the natural enemy complex that interacts with slugs and of the factors that promote natural suppression of slugs is needed to support an integrated management approach for slugs in lowtill soybean.