A seedling that was fine at dusk and chewed off at the base by morning is almost always slugs or snails, not an insect. The giveaway is the silvery slime trail left behind — mucus laid down to help them move, which dries to a trail that can look like dust of gold in morning light. No trail, and the damage is more likely a caterpillar (see White Butterfly and Cabbage Moth on NZ Brassicas if the holes are on kale or broccoli).

Identification

Sign Likely cause What it means
Silvery, slightly slimy trail on soil or leaves, sometimes dried to a dusty sheen Slug or snail movement Confirms mollusc activity even if you don't see the pest itself
Seedling stem chewed through at soil level, top of the plant lying nearby Slug or snail Young seedlings are the most vulnerable stage
Ragged holes in leaves, often starting at the edges Slug or snail feeding Rasping mouthparts (a "radula") tear rather than cut cleanly
Visible shelled pest on foliage or soil Snail Shell distinguishes it from a slug
Soft-bodied pest with no shell, often under mulch or pots by day Slug Same feeding damage as snails, just without the shell
Damage worst after rain or evening watering Cool, wet, humid conditions Their most active window

Why Seedlings Take the Worst Hit

Slugs and snails feed on decaying plant matter, algae and living leaves and stems, and they're most active on cool, wet nights with high humidity — exactly the conditions common through a NZ autumn and spring. During the day they shelter somewhere moist and dark: under rocks, pots, pipes, or dense mulch close to the bed, then come out to feed once it's dark and damp.

A mature plant has enough leaf and stem to shrug off some grazing. A seedling doesn't — a single night's feeding on a stem only a few millimetres thick can take the whole plant down, which is why slug and snail damage shows up disproportionately in the weeks right after transplanting or direct sowing, on lettuce, brassicas and other soft-leaved crops in particular. Starting seedlings indoors or under cover and transplanting them out larger (see When to Start Seeds Indoors in NZ Cool/Mountain Zone) shortens the window a plant is at its most vulnerable size while still outdoors.

What Actually Works

Not every commonly-suggested control pulls its weight. Ranked roughly by effectiveness for the effort involved:

  • Remove shelter first. Rocks, pots, pipes and anything else sitting damp against the bed give slugs and snails somewhere to hide during the day — clearing it away or checking and baiting under it directly reduces the population sheltering near your plants, rather than just treating symptoms.
  • Iron-based baits (iron EDTA or iron chelate pellets). These are the standard low-toxicity option sold at NZ garden centres for vegetable beds — pets and wildlife can be around them, unlike older metaldehyde-based pellets, and they still work by getting the pest to stop feeding and retreat to shelter, where it dies. Scatter thinly rather than in heaps, and reapply after heavy rain washes bait away.
  • Handpick at night with a torch, especially in a small garden. Slugs and snails are most active after dark, so a five-minute check an hour or two after sundown catches far more than a daytime look ever will.
  • Beer traps have a real but limited effect. Slugs are genuinely drawn to the yeasty smell, but most that visit a trap drink and climb back out rather than drowning, and the attraction only reaches a metre or so — you'd need a trap every metre of bed edge for meaningful coverage, not one trap for the whole garden. Useful as one layer, not a standalone fix.
  • Copper tape and diatomaceous earth are the least reliable options for NZ conditions. Both rely on a dry barrier effect that copper and DE lose once wet, and slugs are most active precisely when everything is wet. Worth trying around a pot or raised bed edge, but don't count on either alone to protect open ground through a rainy stretch.
  • Metaldehyde-based pellets work but carry real risk. They're effective, but toxic enough to dogs, cats and birds that they're a poor fit for a food garden pets or children have access to — iron-based baits are the safer default for vegetable beds specifically.

Protecting Seedlings and Transplants

The highest-risk window is the two or three weeks right after seedlings go into the ground, so put the effort there rather than spreading it evenly across the whole season:

  • Scatter iron bait around a bed the evening before transplanting, not after damage shows up.
  • A cut-bottom plastic bottle or cloche over an individual seedling is a cheap, complete physical barrier for the most vulnerable stage.
  • Keep mulch pulled a few centimetres back from seedling stems — mulch right up against a young plant is both moisture and shelter in one place.

Prevention for Next Season

  • Clear away rocks, pots, boards and other daytime shelter from around vegetable beds rather than just at planting time — it's a standing population reduction, not a one-off.
  • Water in the morning where practical rather than the evening, so the soil surface has a chance to dry out before the pests' most active hours after dark.
  • Check under mulch and empty containers near the garden periodically through wet, mild weather — that's when numbers build fastest, not just after visible damage appears.

The Bottom Line

Slime trails and stems chewed off at ground level point to slugs and snails, not an insect. Habitat removal and iron-based bait do the most real work; handpicking adds a fast, chemical-free layer for small gardens. Beer traps help a little over a very short range, and copper tape or diatomaceous earth are the weakest options once conditions turn wet — which is exactly when these pests are at their worst. Protect seedlings specifically in their first few weeks, since that's the stage a night of feeding can actually kill the plant outright.

Growing seedlings that need protecting this season? Check timing for your area: NZ Temperate Planting Calendar or NZ Cool/Mountain Planting Calendar.