Moth Balls, Cedar and What Deterrents Really Do

Moth balls

A Fumigant, and the Old Ones Are Not Legal

Moth balls work by subliming - turning from solid straight to vapour - and it is the vapour that does the work. That has one enormous practical consequence: they only do anything in a sealed container. In an open wardrobe or a drawer that gets opened daily, the vapour concentration never gets near what would affect a larva. It smells strongly and achieves very little.

The older naphthalene sort was withdrawn from sale in the UK, and the Office for Product Safety and Standards still issues recalls when non-compliant naphthalene moth balls turn up for sale here. If you have inherited a box of them, they are not something to start using.

They are also genuinely hazardous - to children in particular, and the vapour is unpleasant to live with. Given they need a sealed container to work at all, and a sealed container on its own excludes moths anyway, the sealing is doing most of the work. Which rather makes the point.

Cedar and lavender

Pleasant, and Not Insecticides

Cedar blocks, cedar rings, lavender bags and the various scented sachets are repellents at best. There is a reasonable case that fresh cedar oil deters some adults from settling. There is no case at all that any of them kill larvae that are already feeding.

Two further problems. The oil in a cedar block is largely gone within a year or so unless it is sanded back, so most of the cedar in most wardrobes is decorative wood. And a repellent does nothing about the eggs and larvae already in the garment when you put the sachet in.

Use them if you like the smell. Do not use them instead of cleaning, sealing and checking, which are the three things that actually work.

Pheromone traps

A Measuring Instrument, Not a Treatment

These are sticky traps baited with a synthetic version of the female’s sex pheromone, so they catch male moths only. They will never catch a larva, a pupa or an egg, and they will not catch a female.

As monitors they are genuinely useful and we use them. They tell you whether there is still activity, roughly where in the property it is concentrated, and whether a treatment is working - put several out, compare the catch between rooms, and the room with the source is usually obvious.

As a control they do essentially nothing. A trap filling up is information, not progress, and "the trap is catching loads so it must be working" is precisely backwards.

What does work

Three Dull Things

Cleaning. Wash or dry-clean wool before storing it. It removes the soiling that attracts egg-laying and it kills what is already on the garment.

Sealing. A rigid lidded box or a vacuum bag is a physical barrier. Nothing gets in, and nothing that is already in gets out to the rest of the house.

Disturbance. Vacuum the edges. Move the furniture twice a year. Turn the rugs over. Empty the wardrobe out once a year and look at the base. It is unglamorous and it is more effective than everything in the rest of this article put together.

Add heat or freezing for anything suspect, and that is the whole of domestic moth prevention.

Short answers

Carpet Moth Questions

Going further

Carpet Moth Guides