Since earwigs take shelter in dark, damp locations and are not consuming the mulch itself, we do not think changing the type of mulch to cedar or eucalyptus will have much of a repellent effect. After all, they can also shelter in the folded leaves of plants that they would probably not be able to eat, like toxic milkweed. We cannot find any scientific references to the ability of cedar or eucalyptus mulch to discourage these insects, so it would probably be more cost-effective to just improve the barriers to house entry instead of redoing any mulch. Replacing worn-out weather-stripping around doors or windows or sealing gaps/cracks that allow the earwigs to enter the home would be best, since insecticide barrier sprays are not recommended.

If you want to try replacing mulch, then removing the old mulch might help IF the new material actually does have a repellent quality for earwigs; otherwise, old mulch is always left in place when new is added to replenish the layer, because the organic matter it breaks down into benefits the soil. Bark mulches should not be laid more than two or three inches thick so any plant roots below are not smothered or kept too dry. (This is total depth, whether old + new mulch or only new mulch.) For a 5-foot square area, to create a layer of mulch 2 inches thick would require a little over 4 cubic feet (or less than a quarter of a cubic yard) of mulch.

Earwigs can be drawn to moisture, so if this area is near a leaky faucet, roof gutter overflow or outlet point, or is irrigated regularly, that is likely the main appeal of the location for the earwigs, not the mulch by itself. In that case, addressing the moisture problem so the mulch layer can dry out more thoroughly between any watering the plants need, or fixing any drainage issues that may keep this spot wetter for longer, should help reduce their abundance without needing any other changes.

Miri

I just got this email from you today 8.25.  I think it would be helpful if you put in as your from address header The U of MD extension, as Ask Extension did not register with me.

Further there is much info on the internet about cedar and eucalyptus being helpful ODOR wise in repelling bugs, and even rats. So I  wonder and am confused about your above reply, but thank you for sending it, as some of the info was helpful. For your info. The mulch is around a shrub, there are summer plants in pots near also, but no milkweed. 

If by scientific info available does this rule out the odor repellent property of cedar or eucalyptus. ?

Thanks

The Ask Extension service we participate in is a nation-wide platform used by multiple universities, so aside from the information with our links to UME content at the bottom of our replies (the "fine print" that says "Answered by the Home and Garden Information Center, University of Maryland Extension"), we cannot customize the email header to show University of Maryland Extension as the sender. The auto-reply you should have gotten after the original submission, acknowledging it was received, should have identified itself as Ask Extension, and the link it includes to your question serial number can also let you access the submission for viewing replies if they are not coming through into the email address used on the submission form. (We have found that sometimes clients find a reply in an email spam folder.)

In case we missed a research article, we can ask an entomologist at UMD about whether or not there is scientific evidence that eucalyptus mulch contains enough insect-repelling compounds to be useful as a barrier to home invasion by nuisance insects. HGIC itself does not have an entomologist currently on staff, so our query might take a few days, and we will reply again as soon as we have their feedback.

Miri

We heard back from the entomologist and she agrees that there is no evidence that shows eucalyptus mulch has any reliable insect-repelling properties, particularly for earwigs. Extracts from certain eucalyptus species are sometimes used in other types of insect repellents, but this concentration of active ingredient(s) is not necessarily present in mulch, nor would mulch be regulated to ensure a consistent quality for that particular attribute. She did note that, if you have pets or children in the yard that might ingest mulch, eucalyptus is not safe for dogs, cats, or people.

Miri