Field Guide for Integrated Pest Management in Hops

50 SlugsAmy J. DrevesAt-A-GlanceSlugs◆◆Monitor for slugpresence on hillsin early spring.◆◆Cultivatebetween rows todirectly kill slugsor expose themto weather andpredators.◆◆Damagecaused by otherpests such asflea beetles orcucumber beetlescan be mistakenfor slug damage.◆◆Slime trailsindicate thepresence of slugs.◆◆Iron phosphatebait is availablefor slugmanagement(Oregon only).◆◆Bait at plantingtime in an Oregonyard with ahistory of sluginfestation.Pest Descriptionand Crop DamageSlugs are a problem in Pacific Northwesthops primarily in Oregon. While severalspecies can be found in hop yards, the mostcommon is the gray field slug, Deroceras reticulatum(Fig. 94). These soft-bodied mollusksrange in length from ¼ inch to 2 inches andare light gray to dark brown with a networkof mottled colors. The underside of the foot iswhitish with a darker zone. The mantle (i.e.,area on top just behind the head) is roundedat both ends and generally lighter in colorthan the rest of the body. As in all slugs, thereis a respiratory pore behind the mid-pointand on the right side of the mantle. The bodyof the slug behind the keel (i.e., the foot) hasa boat-like shape running down the top to thetail. When disturbed, the watery slime trail ofthis slug turns from clear to milky white.Slugs are most active at night or earlymorning, especially when humidity is highand temperatures are cool. They retreat intocracks, soil crevices, and sheltered areas byday to protect themselves from predators anddehydration. Very little activity takes place inextremely cold or hot weather. Slugs feed onnewly developing shoot tips and leaves of hopplants, resulting in ragged leaves with irregularlysized holes. Damage tends to be heaviestalong the edges of hop yards where weedyor grassy borders serve as a habitat for slugs.When populations are high, slugs can destroythe growing tips of hop shoots.Biology and Life HistoryThe gray field slug completes one totwo generations per year. Young adults or eggsoverwinter under leaf residue, in soil cracks,and in sheltered areas under the soil surface.In the spring, mating and egg-laying usuallyfollow within one to three weeks after slugactivity is noticed. Eggs are laid in clutchesof 10 to 40, totaling 200 to 400 eggs in alifetime. The spherical eggs are laid in a gelatinousmass and are transparent when laid butbecome cloudy just before hatching. The immatureslugs resemble adults but are smaller.The average life span of a slug is nine to 13months. All slugs have both male and femalereproductive organs, so that self-fertilizationand egg-laying can occur in any individual.Monitoring and ThresholdsIn areas where slugs may be present,growers can monitor for slugs by carefullyobserving hop shoots during the pest’s criticalstage of emergence in the early spring. Openbait traps (in Oregon, where bait is registered,see below) or slug blankets/boards can beplaced on the ground near hop hills to monitorfor slugs. After several nights, the trapsshould be turned over and checked for thepresence of slugs. Treatment should be consideredif the field has a history of slug damageor if excessive damage to foliage or growingtips is observed and slugs are determinedto be present.ManagementThe most effective control of slugs canbe achieved in early spring when temperaturesbegin to warm and hop plants start togrow. The hop plant is at its greatest risk ofslug damage when plants are young. Wherebaits are registered, it is best to bait at plantingtime or just before shoots emerge inspring if a yard has a history of slug damage.Managing hop yards so that plants emergequickly in the spring can help to escape theworst period of slug damage.Increased use of irrigation and moistwarm springs favor slugs in hop yards. Soilcultivation in early spring between hopplants can kill slugs and also expose themto predators and desiccation. Birds, frogs,snakes, Sciomyzid flies, harvestmen (daddylong-leg spiders), and carabid ground beetlesprey on slugs. Parasitic nematodes and naturallyoccurring ciliates (protozoans that moveby means of small hairs or cilia) can infectthe bodies of slugs.No chemical treatments/baits forcontrol of slugs are labeled for use on hopsin Washington or Idaho; Oregon has a 24c“Special Local Needs” registration for ironphosphate (Sluggo). Iron phosphate baitsmust be ingested by slugs, and slug deathtakes three to six days. Feeding activity,however, is stopped almost immediately.Iron phosphate baits works at most temperaturesand slugs will not recover afteringesting the bait.See photo opposite page.