Despite being around for the whole winter-cropping season, earwigs, slaters and millipedes are establishment pests (particularly in canola), and they generally aren’t a problem once plants grow and become vigourous enough.
In fact, one of the few ‘defences’ that we have against them, are good growing conditions to help crops emerge and achieve early growth and vigour.
But why are we so reliant on plants outrunning feeding by earwigs, slaters and millpedes?
In this article, we outline the four main challenges surrounding these invertebrates in broadacre crops in south-eastern Australia.
Note that in this article, the term ‘earwigs, slaters and millipedes’ refers to the following specific species that are known to reach pest status:
Two slater species, the pill bug (Armadillidium vulgare) and common rough woodlouse (Porcellio scaber).
1. The role of natural enemies appears limited
When the Russian wheat aphid spread across south-eastern Australia in 2016, some reassurance and hope was found in the observation that several parasitoids and generalist predators preyed upon the unwelcome aphid.
By 2020, we had become confident that beneficials play a pivotal role in Russian wheat aphid management on farms here in Australia.
Unfortunately, this scenario is not what has happened with all introduced invertebrates.
Often when an exotic species enters a new area, there are no or very few natural predators or parasites present in the environment to fight back and supress populations (which is why biosecurity is so important!).
Pest species of earwigs, millipedes and slaters in Australia are mainly introduced species from Europe.
Sure, they have some natural enemies in Australia.
Vertebrates such as birds and lizards, and generalist invertebrate predators like spiders and carabid beetles, would likely snack on earwigs, millipedes and slaters.
There is even a native parasitic nematode, Rhabditis necromena, that kills the black Portuguese millipede and can be purchased commercially for release.
However, earwigs, slaters and millipedes can build up to very high numbers in paddocks, and to date there have not been any natural enemies (that we know of) that we can rely on to effectively slow the tide of feeding damage on establishing crops.
2. They are somewhat unpredictable
Slaters and millipedes are actually detritivores and breakdown dead organic matter in soils and plant litter (detritus), playing a beneficial role in agricultural ecosystems.
They don’t need living plants to persist and survive.
But slaters and millipedes do sometimesfeed on some establishing crops. We have seen it in laboratory experiments and have received reports of damage from growers and agronomists over the years.
The issue is that we do not understand what triggers these species to turn their attention to crop plants in some circumstances. Nor can we predict under what conditions damage is likely to occur.
3. Stubble management is not always an option
Crop stubble provides a cool, moist habitat that helps invertebrate survival, and also provides a source of organic matter to feed detritivores like slaters and millipedes.
And with stubble retention practices common in broadacre cropping, this is thought to be a factor contributing to the build-up of slaters, millipedes, and earwigs on farms.
Soil tillage is likely to help if these species reach pest status, as it would disrupt their habitat and expose them to drier, more unfavourable conditions.
For example, as the European earwig typically constructs it nests under soil clods and seeks shelter during the day, tillage and farming practices which disturb soil and reduce stubble loads, may help to reduce breeding sites and suppress populations.
However, with the move towards retaining stubble, tillage is not compatible with all farming systems and it is not a realistic option for many growers.
4. Chemical control options are extremely limited
Once plants have emerged, there are very limited options for managing earwigs, slaters and millipedes in crops.
There are no foliar/bare earth insecticides registered for control of these species in broadacre crops.
Synthetic pyrethroids and organophosphates insecticides may not affect earwigs, slaters and millipedes. These species are nocturnal and hide during the day. Furthermore, they are ground dwelling invertebrates and are often protected by heavy stubble loads.
Using broadspectrum insecticides will also have negative consequences on beneficial invertebrates that play a crucial role in controlling other crop pests that will arise throughout the season.
In previous years, some protection against these species was provided by methiocarb-based slug and snail bait, however this has now been discontinued. Note that metaldehyde-based slug and snail baits will not kill earwigs, slaters and millipedes.
A new bait (Transcend®) has been registered for control of earwigs, slaters and millipedes, however at the time of writing this article, it can only be applied in “Agricultural non-crop areas, industrial & commercial situations where invertebrate pests congregate”.
Research by our team has shown that some seed treatments can kill the European earwig, black Portuguese millipede and pill bug (see this study or contact us for more information), but seed treatments alone may not be enough to protect seedling under a high pest pressure.
A tip on monitoring earwigs, slaters and millipedes
Feeding by earwigs, slaters and millipedes creates typical ‘chewing’ damage (e.g. chewed foliage, severed stems, missing cotyledons) and can be mistaken for feeding by beetles, moth larvae and slugs/snails.
If you are unsure what could be responsible, shelter traps are good for identifying slaters and millipedes. These could be a bathroom tile, hessian matt, pot plant or drum placed on the soil surface and checked underneath after a day or two.
With earwigs, an effective way to trap them is with rolled up pieces of corrugated cardboard placed in the field.
Not all earwigs, slaters and millipedes in paddocks are pests, with many species providing beneficial services to soils and farms.
For example, the native, common brown earwig (Labidura truncata) is a generalist predator and can feeds on some soft-bodied insects like moth larvae (see video below).
Furthermore, researchers have been looking into crop susceptibility to feeding by key species, and not all broadacre crops are equally susceptible to damage by earwigs, slaters and millipedes.
For help understanding the risk to a particular crop, contact our PestFacts south-eastern team (firstname.lastname@example.org).