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 garden slug, Deroceras reticulatum, several Arion species can also cause considerable crop damage. Slugs are generalists and feed on seedlings, foliage, and fruits. Crops in western Oregon that are impacted by slugs include small grains and diverse seed crops including various grasses, clovers, cover crops like radish, and vegetables. Slugs also damage nursery crops and Christmas trees, and present a significant contamination problem for these commodities when sent out of state. 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 orchardgrass.  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.   

Many factors favor slug development in the Willamette Valley, and facilitate build-up of their populations. 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 this switch in cultivation practice also benefitted slug population growth.  Soon after farmers adopted no-till 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.  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.

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 row 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, Durham baits works well if conditions are dry for a few days after application.  Softer baits such as Sluggo tend to be more efficacious when applied early in the fall while the harder (more rain-fast baits) baits such as MetaRex provide better control later in fall. 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 medium rate bait and poison applications in fall ($50 /acre), grass seed fields have to be replanted in spring.

Greenhouse studies have revealed some of the factors affecting impacts of baits and poisons.  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 and are 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 be deterred by baits similar to their aversion to eating plants that have previously poisoned them.  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 by earthworm scavenging within 4-5 days of application. 

Models are currently being developed for predicting areas of damaging slug populations within fields for insights that can guide future slug control efforts. Besides baits and poisons, 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 and entomopathogenic 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. This will be the focus for the slug researcher to be hired by Oregon State University with support from the Oregon legislature.

Bibliography

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