Slugs in the Willamette Valley
Slugs are a key pest in many cropping systems in the agriculture-rich Willamette Valley in western Oregon. While the dominant pest species in field crops is the gray field slug, Deroceras reticulatum Müller, several Arion species can also cause considerable crop damage. Slugs are generalists and many feed on seedlings, foliage, and fruits.
Slug Impact
Crops in western Oregon that are impacted by slugs include small grains and diverse seed crops including various grasses, clovers, and other specialty seed crops, as well as row crops such as beans, Brassicas (e.g., lettuce), fruit (e.g., strawberries) and corn. Slugs also damage Oregon nursery crops (http://oregonstate.edu/dept/nurspest/slugs.htm) and Christmas trees (http://pubs.extension.oregonstate.edu/em9093/slug), and present a significant contamination problem for these commodities when sent out of state (http://westhawaiitoday.com/community-bulletin/hawaii-inspecting-christmas-trees-oregon?qt-popular_quick_tab=2). Slug impact is particularly severe in grass seed crops in the Willamette Valley. In recent years, slug damage has accounted for nearly $100 million in damage to this $500 million industry (http://www.capitalpress.com/Oregon/20150915/expert-offers-options-for-ke...).
Slugs can also have indirect impacts. For instance, slugs can facilitate fertilization of the fungal pathogen that causes choke disease in orchard grass (Hoffman and Rao 2013). The diverse economic impacts of slug damage were highlighted by Oregon farmers during the Slug Summit in May 2015 organized by the Department of Crop and Soil Science at Oregon State University (http://www.capitalpress.com/Oregon/20150326/slugs-remain-a-mystery-experts-say; http://www.vice.com/read/slugs-are-destroying- the-farmland-of-oregons-sustainable-farmers-500).
Factors Affecting Slug Populations
Many factors favor increased slug populations in Oregon’s Willamette Valley. Slugs thrive in the cool, humid, and low-light conditions that prevail in the region from fall through spring. In the early 2000s, the practice of burning straw residue after seed harvest was gradually phased-out in the Willamette Valley, and the straw residue in the fields provided an ideal habitat for slugs. Additionally, farmers in the region adopted no-till production for soil conservation and for improved weed management as part of their overall integrated pest management strategies. This switch in cultivation practice also benefitted slug population growth. For example, soon after farmers adopted no-till practices for grass seed production, a study showed that there were close to 30 times more slugs in no-till ryegrass fields than in conventionally tilled fields. The extra residue provides moisture and a protective habitat for slugs. Improved field drainage has allowed a greater diversity of rotational crops, many of them more advantageous to slug population growth than grass seed crops. The reduction in field flooding itself directly reduces slug mortality.
Slug Management With Baits
Ever since slugs were first observed as pests in Willamette Valley crops, researchers have studied slug biology and behavior, and evaluated slug control options, in collaboration with farmers and with industry support. Slug management has focused on the use of slug baits and poisons added to crops at the time of seedling establishment. Metaldehyde and iron phosphate baits kill slugs but considerable variation in efficacy has been observed depending on the application rate and timing, and prevailing environmental conditions. For example, metaldehyde-based Durham-brand baits works well if conditions are dry for a few days after application. Softer baits such as iron phosphate-based Sluggo brand, tend to be more efficacious when applied early in the fall before fall rains begin in earnest, while the harder, more rain-fast baits such as metaldehyde-based MetaRex brand, provide better control later in fall. Even newer baits that combine iron phosphate with spinosid (Sluggo Plus), and FeEDTA based baits (Slugkill) also provide only limited slug suppression. Basically, when slug populations are high, none of the commercially available baits suppress slugs below damaging levels. At high slug populations, even with three applications in the fall of a medium label rate, grass seed fields still have to be replanted in spring.
Bait Effectiveness
Greenhouse studies have revealed some of the factors affecting impacts of baits. In the Willamette Valley, slugs remain quiescent in subterranean regions in summer, emerge over a 4 to 6 week period in fall once the rains commence, and feed until the onset of near freezing conditions. Thus, slug control has to be effective from crop/seedling emergence in early October until December. Metaldehyde baits are 50% less toxic to slugs at lower temperatures, hence less effective in late fall. In addition, the high moisture levels degrade the baits rapidly. Also, slugs that survive poisoning by baits tend to avoid baits in the future. An additional challenge is the non-target attraction of earthworms to slug baits. While they are not killed by the baits, bait removal by earthworms to their burrows can have a tremendous negative impact on slug control. In fields with good soil tilth and high earthworm populations, 90% of the bait can be depleted within 4-5 days of application by earthworm scavenging.
Slug Management With Biological Control
Models are currently being developed for predicting areas of damaging slug populations within fields, and for insights that can guide future slug control efforts (A.K. Pearson et al. 2005). Besides baits, non-chemical control strategies that are being examined elsewhere have potential for slug management in the Willamette Valley. These include biological control with beetle predators, Sciomzyidae (marsh) fly larvae, and pathogenic nematodes. New approaches, such as the use of RNAi, currently being developed as a pest control strategy for other pests, may also have potential in the future. Slugs are a challenge to manage! New innovative approaches are needed for addressing the slug problem in Willamette Valley crops.
For your information:
Anderson, N.P. 2012. Evaluation of Ferroxx® slug bait for control of gray field slugs in western Oregon. 2011 Seed Production Research, Oregon State University 136:8-9
Anderson, N.P., Hoffman, G.D., and Dreves, A.J. 2011. Evaluation of newly formulated molluscides for control of slugs in western Oregon grass seed fields. 2010 Seed Production Research, Oregon State University 130:15-18
Dreves, A.J. and Fisher, G.C. 2009. Effect of grazing sheep on gray field slug populations in white clover seed fields. 2008 Seed Production Research, Oregon State University 128:62-63
Dreves, A.J., Sullivan, C., and Anderson, N.P. 2015. Slug Control. Pacific Northwest Insect Management Handbook, http://insect.pnwhandbooks.org/ipm/slug-control
Fisher, G.C. and Dreves, A.J. 2009. Fall evaluation of molluscides for control of slugs in third year white clover direct seeded to perennial ryegrass. 2008 Seed Production Research, Oregon State University 128:64-70
Fisher, G.C., DeFrancesco, J.T., and Horton, R.N. 1997. Slug populations in grasses grown for seed. 1996 Seed Production Research, Oregon State University 110:23-25
Fisher, G.C., DeFrancesco, J.T., and Horton, R.N. 1998. Seasonal populations of gray garden slug in four species of grass. 1997 Seed Production Research, Oregon State University 111:28-29
Fisher, G.C., DeFrancesco, J.T., and Horton, R.N. 1999. Laboratory trials to determine efficacy of various baits for slug control. 1998 Seed Production Research, Oregon State University 112:44-45
Fisher, G.C., Dreves, A.J., and Salisbury, S.E. 2008. Summer timing for control of gray field slug in non-irrigated white clover. 2007 Seed Production Research, Oregon State University 127:18-21
Gavin, W.E., Hoffmann, G.D., and Banowetz, G.M. 2010. Cost and benefit in control of the gray field slug in western Oregon. 2009 Seed Production Research, Oregon State University 129:21-24
Gavin, W.E., Fisher, G.C., Dreves, A.J., and Banowetz, G.M. 2009a. Effects of precipitation on molluscicidal efficacy. 2008 Seed Production Research, Oregon State University 128:55-61
Gavin, W.E., Fisher, G.C., Dreves, A.J., and Banowetz, G.M. 2009b. Control of the gray field slug during annual ryegrass establishment. 2008 Seed Production Research, Oregon State University 128:71-76
Gavin, W.E., Mueller-Warrant, G.W., Griffith, S.M., and Banowetz, G.M. 2012. Removal of molluscicidal bait pellets by earthworms and its impact on control of the gray field slug (Derocerus reticulatum Mueller) in western Oregon grass seed fields. Crop Protection 42:94–101
Gavin, W.E., Banowetz, G.M., Griffith, S.M., Mueller-Warrant G.W., and Whittaker, G.W. 2007. Earthworms and their impact on slug control. 2006 Seed Production Research, Oregon State University 126:22-27
Gavin, W.E., Banowetz, G.M., Griffith, S.M., Mueller-Warrant, G.W., and Whittaker, G.W. 2008. Effects of baits and bait alternatives on slug mortality, egg production, and seedling survival. 2007 Seed Production Research, Oregon State University 127:12-17
Gavin, W.E., Banowetz, G.M., Steiner, J.J., Griffith, S.M., and Mueller-Warrant, G.W. 2006. Fast and accurate method for estimating slug densities. 2005 Seed Production Research, Oregon State University 125:32-35
Gavin, W.E., Banowetz, G.M., Griffith, S.M., Mueller-Warrant, G.W., Steiner J.J., et al. 2007. Behavioral and biological effects of weather on the gray field slug in western Oregon. 2006 Seed Production Research, Oregon State University 126:28-33
Hoffman, G.D. and Rao, S. 2013. Association of slugs with the fungal pathogen Epichloë typhina (Ascomycotina: Clavicipitaceae): potential role in stroma fertilization and disease spread. Annals of Applied Biology 162: 324-334. DOI: 10.1111/aab.12024.
Hoffman, G.D. and Rao, S. 2014. Fertilization of Epichloë typhina stromata by mycophagous slugs. Mycologia 106:1-7
LaBonte, J. 2009. Pest risk for the State of Oregon. http://extension.oregonstate.edu/lincoln/sites/default/files/documents/E...
Mueller-Warrant, G.W., Anderson, N.P., Sullivan, C.S., Whittaker, G.W., and Trippe, K.M. 2015. Can knowledge of spatial variability in slug populations help improve stand establishment? 2014 Seed Production Research, Oregon State University 151:4-13
Sullivan, C.S. and Salisbury, S.E. 2015. Evaluating the response of slug populations and activity to tillage practices in annual ryegrass grown for seed. 2014 Seed Production Research, Oregon State University 151:1-3