Slug Biology and Ecology: A Key to Finding Effective Control Options

1. Slug Biology and Ecology:A Key to FindingEffective Control Options Amber D. Vallotton Virginia Cooperative Extension-Rockingham County Crop and Soil Sciences Extension Agent avallott@vt.edu 540/ 564-3080

2. “What exactly is it you dislike about slugs, Mr. Crabtree?”

3. …EVERYTHING!! • Estimated that in years when favorable slug conditions exist, stand loss can range from 50-90 %

4. Current Control Options Mechanical • TILLAGE • To incorporate surface residue & weeds from previous planting, and any heavily applied manures to disrupt favorable habitat • UNDER NO-TILL • Planting early to avoid peak time of slug activity • Use of row cleaners to remove residues from the furrow at planting, strip tillage, and starter fertilizer Molluscicides • Slug Baits • Metaldehyde  Deadline M-Ps • Iron phosphate  Sluggo

5. Control Options cont’d • Slug distribution often clustered vs. uniform, must scout for “hot spots” • Applied when slugs are at peak activity above soil surface  good coverage is crucial! • Slug may eat bait, but not get a toxic dose, so slug can recover • Control only “buys time” allowing crop to outgrow feeding pressure • Costs might be prohibitive for Deadline M-Ps ($15-20 /acre), but if pressure great enough alternative may be more costly

6. Pest Management Options • Any effective control strategy presupposes a thorough knowledge of the pest • Biology • Morphology • Life Cycle • Activity Levels in relation to the LC • Point where most vulnerable to control • Ecological Relationships • Conditions favoring activity • Feeding Behavior and Preferences in relation to crop and alternate hosts

7. Slug Biology/ Ecology 101: OR A Brief Window into the World of the AMAZING Slug

8. Slug Anatomy: True or False Does a slug have eyes? Yes, simple eyes on Optic Tentacle Does a slug have a tongue? Yes, it is tooth covered, which is used like a rasp for grating food Does a slug have feet? Yes, it has a foot, which causes tell-tale “mucus trail”

9. Slug Life Cycle* FALL & WINTER EGGS SPRING *One Cycle per year Live ~12-15 months JUVENILES JUVENILES ADULTS SUMMER FEEDING JUVENILES

10. Slug Life Cycle & Activity ADULTS EGGS EGGS ADULTS SLUG JUVENILES Corn Planting Soybean Planting Aug Sept May June July Oct April SOURCE: Ron Hammond, OSU, 2006

11. Corn Feeding injury primarily in late-planted corn seeds, embryos, and seedlings (pre- and early-whorl stages) If you can plant earlier, can avoid most damage since most juveniles have not yet hatched

12. Soybeans Planted after juvenile slug hatch, so if slug pressure is high on early vegetative stages, there can be complete stand loss especially since growing point unprotected

13. Conditions Favoring Slug Activity • Cool & Moist between 63-68 °F at 100% RH • Tend to be nocturnal feeders, but during or right after rain are also active • Dependent on available food sources, hunger, and distance from roost to food

14. Generalized Feeding Behavior • Movement by slugs is slow and costly because of the loss of water as mucus trail • Slug feeding strategy minimizes movement • Requires sampling behaviors & sophisticated learning mechanisms • Used to assess a huge number of potential food items • Smell crucial • Food availability & accessibility also big factors • Sequence of steps involved in feeding

15. Slug Feeding Behavior ATTRACTIVE SMELL ENCOUNTERED MOVE TOWARDS FOOD LOCOMOTION REJECT B/C DETERRED Inhibition or Food Texture Hard AVERSION LEARNING FOOD TOUCHED W/ TENTACLE & LIP IF ATTRACTED, SLUG FEEDS TOXIC? FEED TILL FULL DIGESTION PREFERENCE LEARNING NUTRITIOUS Adapted from B. Speiser, “Food and Feeding Behaviour”, in The Biology of Terrestrial Molluscs (2001)

16. Slug Feeding Preferences • Slug diet varied  plants, fungi, animals, and soil particles • Good evidence that slugs prefer specific types of plants • Preferences • Learned behavior = test & either like or don’t like • Based partly on presence or absence of toxic plant compounds (deterrent)  Crops highly palatable & low SPC • Softness or hardness of leaf surfaces • Eat all plant parts at all stages of plant growth  Fresh green, mature, dying, and decaying plant residues

17. Approaching Slug Management from a Biological/ Ecological Angle • In No-Till situations, do slugs prefer certain cover crops over others? • If so, is slug activity higher in these cover crops and damage to subsequent crops greater? • Other important questions about our cropping systems • Attractant vs deterrent to slugs? • Food composition  Higher palatability or toxicity issues? • C:N in plant  Greater satisfaction of nitrogen demand? • Provide more desirable habitat or shelter? • Offer more roosting sites? • Provide more protected egg-laying sites?

18. 2008 Slug Research Plots • Located in Mauzy • Two Main Rotations: • Vetch-Corn (VC) and Rye-Soybean (RSB) • For each rotation, three roll down dates for cover crop • Four replications within each rotation

19. In each subplot, beer traps used as attractants • Measured slug counts, slug length, and species

20. Roll-down Planting Planting

21. ADULTS EGGS EGGS ADULTS SLUG JUVENILES Corn Planting Soybean Planting Aug Sept May June July Oct April

23. As temperatures reached around 90°F, slug pop’s declined quickly in plots rolled down for several weeks (T1, T2), whereas # slugs in newly rolled plots (T3) remained more stable • T3 cover provided more stable environment for slugs, whereas other plots had degraded • T2 spike likely due to huge variability within T2 plots Total number slugs trapped between May 28 and July 1 for each roll-down date.

24. We see similar trends like in the RSB where slug populations declined quickly in plots rolled down for several weeks (T1, T2), whereas slugs in newly rolled plots (T3) remained more stable • T2 more similar to T1 roll-down with only slight spike Total number slugs trapped between May 28 and July 1 for each roll-down date.

25. Overall, total number of slugs similar between the two crop rotations • Variability between plot reps in each crop rotation accounted for differences • This variability demonstrates that slug distribution may tend to be clustered and spotty in the field rather than uniformly distributed this has important implicationsfor control application practices and the need to scout well before putting out bait • Predominant type of slug was the gray garden slug (>97 %) with slight occurrences of the marsh and dusky slug • Slugs consistently measured 1 to 3 cm in length, followed by slugs < 1 cm; very few were > 3 cm  majority slugs trapped were juveniles

26. Future Directions • Lot of work needs to be done to understand slug feeding preferences and its usefulness in developing control options • Run second year experiment similar to last year • Sample slugs earlier to get better idea of early-season pressure • See if any trends emerge when combining both year’s data • Conduct another experiment • Plots comparing several different cover crop treatments • With/without N • Determine associated slug presence to better understand feeding preferences

27. THANK YOU!