Millipede Mass Migration

This just in from one of our Master Naturalists:

Has anyone else been experiencing an outright infestation of millipedes at their homes? My folks place and their neighbors the past few days have been swarmed by them. Saturday night the patios must have been covered with thousands upon thousands. They are even finding their way inside, which has become the real concern. I wanted to send out a feeler and see if anyone else is experiencing this and/or knows what can be done to combat them. So far, ant repellent/kill spray works, but the numbers every evening seem to be growing and once the spray dries… they are back on the move. 

Millipedes are detrivores (meaning that they eat dead plant stuff) and are common around accumulated leaf litter, dead wood and therefore yard and garden mulch.  Moisture attracts them, presumable by increasing the breakdown of dead plant materials.  Removing these sites should (eventually) reduce their numbers.

Detrivores are important members of the food chain.  Working in conjunction with fungus, bacteria, millipedes, woodlice, dung flies, and many terrestrial worms, millipedes breakdown organic material to add to soil. Imagine the buildup of tree trunks piled up on the earth's surface up if there was nothing to break them down!

Mass migrations of millipedes are an interesting phenomenon.  They usually occur in periods of drought or excessive rain, either one driving them from their cozy quarters in moist decaying plant material.  Like you and I, they are looking for the most comfortable place to live, and a damp basement or garage may look like Club Med to them in a drought.
Since pesticides have safety concerns and can kill other beneficial insects (bees and butterflies), environmentally friendly measures should be considered first.  From a Clemson University PDF:

"Nonchemical Control: If garden millipedes are occurring in great numbers indoors, it is usually an indication that there is a large population in the area surrounding the home. To control these pests, the most important step is to remove materials that give them shelter in the immediate area around the home. This includes excessive mulch, leaf litter, thick grass, rocks, boards and similar materials.  De-thatching the lawn and mowing closely allows for drier conditions, which reduces the areas where they can live.  Watering in the morning rather than the evening, also gives the lawn a chance to dry before they become active at night.

Try to prevent garden millipedes from entering the house by making sure doors and windows fit tightly, and as many cracks and crevices are caulked as possible. Remember that they may be entering your home from high areas just as easily as low areas.  In most situations, garden millipedes found indoors can be easily removed with a broom or vacuum."

This article from the Missouri Extension has lots of good information, including pesticide recommendations.

"During mass migrations of millipedes, residual pesticide deposits on the soil surface will have very little effect because of the relatively short time of exposure as millipedes move across treated areas. For effective management in these situations, it is desirable to identify the location of the migrating population by inspecting during the night with a flashlight. Look in areas such as outlying grassy and wooded areas. Direct nighttime treatment of millipedes with an appropriately labeled contact pesticide may be needed for several nights in severe cases."

Migrations are time limited events.  Another treatment is waiting them out.  After all, millipedes need love too.
More on Millipedes at Wikipedia  and this past blog.