Across parks and woodlands, those dark sacks signal active pest control and a very real health risk nearby. If you spot one on your weekend walk, give it space, keep children and dogs close, and read on to understand why timing and distance matter.
What those black bags really mean
The black bag is not rubbish. It’s a mechanical trap fitted around a tree trunk to intercept processionary caterpillars as they climb down in groups to pupate. Councils, landowners and foresters install them when monitoring confirms the insects are present. That means the hazard is not theoretical; it’s there, now, around that trunk and in the surrounding air and leaf litter.
Do not touch. Do not pierce. Step back at least 10 metres and keep children and pets on short leads.
The true threat comes from the caterpillars’ barbed hairs. These detach easily, travel on the wind, and stick to skin, clothing and fur. Even when the insects die inside the bag, their hairs remain irritant. You may see printed warnings on the sack, and you might also notice silky nests higher in the canopy. The sack is the trap; the nests are the source.
Why you should keep your distance
What the hairs do to people and pets
Processionary caterpillars carry thousands of tiny hairs that can trigger rashes, eye irritation and breathing problems. The risk rises on warm, dry, breezy days, and after nests or traps have been disturbed. Dogs are especially vulnerable when they sniff or lick contaminated material.
- Skin: itchy, blotchy rash within minutes to hours; sometimes blisters.
- Eyes: gritty pain, redness, watery discharge; urgent care if vision blurs.
- Breathing: coughing, wheezing or tight chest, particularly in people with asthma.
- Pets: drooling, pawing at the mouth, swollen tongue or lips; contact a vet quickly.
Hairs can stay active for months in old nests and inside the trap bag. The risk outlasts the insects.
In Britain, oak processionary moth has established in parts of London and the South East, with caterpillars usually active from late spring into early summer. On the Continent, pine processionary is widespread and dangerous for both pines and pets. Either way, the black-bag trap signals confirmed activity nearby, so treat the area with caution.
How the trunk trap works
The trap uses a simple physical design. A collar fits tightly around the trunk, flared to funnel descending caterpillars into an outlet. A short tube guides them into a black collection sack. Sunlight warms the bag, the insects desiccate, and the system prevents hairs from dispersing widely. There’s no spray, no bait, and no insecticide. Some models add a small weight so the bag stays still in wind, reducing the chance of stray hairs escaping.
When you might see them
| Month | What you may notice |
|---|---|
| April–May | Silky nests on oaks; early caterpillar trails; first traps fitted to trunks. |
| June–July | More caterpillar movement; traps with visible contents; printed warning labels. |
| August–September | Adult moths emerge; traps may remain while hairs in nests still pose a hazard. |
| October–March | Nests persist in trees; hairs remain active; some traps stored until spring. |
What to do if you think you’ve been exposed
Move upwind by 10–20 metres immediately. Keep hands away from your face. Strip outer layers carefully and bag them for washing.
- Rinse skin and hair with lukewarm water and mild soap. Pat dry; do not rub.
- Use sticky tape to lift any remaining hairs from the skin, then discard the tape in a sealed bag.
- Wash clothes, pet bedding and towels at 60°C. Clean leads, boots and pram wheels.
- For eye contact, irrigate with clean water or saline for at least 10 minutes.
- If breathing tightens, wheeze starts or the rash spreads quickly, seek medical advice promptly.
- For pets showing drooling, swelling or distress, contact a veterinary practice without delay.
Reactions can escalate fast. Treat exposure the same day; don’t wait “to see how it goes”.
Your role when you see a trap
Leave the installation alone. Do not lift the collar, untie the bag or check what’s inside. The people who fitted it will remove it safely. Interfering with the system can release hairs, contaminate paths and harm the very tree the trap aims to protect.
If a trap looks torn, note the location and inform the site manager or local authority tree team. Keep others back while you move away. You help more by stepping aside than by attempting a fix without protective gear.
How to tell a trap from litter or a nest
- Trap: black sack connected to a tube and a snug collar around the trunk; often a printed warning.
- Nest: white or greyish silk bundles in the canopy or outer branches; sometimes flecked with brown frass.
- Litter: no collar, no tube, no tidy fit; often snagged on twigs or fencing, not sealed to the trunk.
Bag on the trunk equals trap; web in the crown equals nest. Both mean: keep your distance.
The wider picture across Europe
Milder winters and earlier springs help processionary species spread north. Warmer, drier spells let more caterpillars survive and move between host trees. Holidaymakers driving to pine forests in France, Spain or Italy face higher exposure from January to May, when pine processionary marches on the ground. Keep dogs on short leads, carry saline, and avoid picnic spots under infested pines or oaks.
Tree managers now prefer targeted, low-chemical methods. Trunk traps slot into that toolkit alongside winter nest removal and carefully timed biological treatments. The black bag looks blunt, yet it reduces airborne hairs at head height, and it buys time to protect nearby play areas, school grounds and gardens.
Extra context you can use
Why black? Dark material warms fast, drying the insects and limiting the spread of loose hairs. Why the warning? Even a punctured, empty-looking sack may contain enough hairs to trigger a rash or eye injury. Think of it like broken glass you can’t see. The bag contains the hazard; your job is to avoid disturbing it.
If you manage land with oaks or pines, plan ahead. Map high‑use paths, choose a minimum 10‑metre buffer around infested trees on busy days, add temporary signage during peak months, and schedule pet‑walking routes away from known hotspots. Small, practical steps reduce exposure for hundreds of visitors at a time—and they start with recognising that unremarkable black bag on the trunk.
Thanks for explaining what the black bags are—I’d always thought it was trash. Quick q: is 10 metres enough on windy days, or should we step back farther?