Although these arthropods are harmless to people, pets, and plants, millipedes can become quite a nuisance when they occur in the kind of numbers that some unfortunate homeowners are experiencing. The bad news is that we do not have any really good solution for this problem; the good news is that it will go away on its own.
Millipedes feed on decaying organic matter, such as flower bed mulch, leaf litter, grass thatch, etc. In favorable habitats, they can be present by the thousands, but they are rarely seen in dry weather, because they can’t move about when it is dry (unless they are trying to migrate to a moisture source when their original location becomes too dry). Millipedes do most of their wandering at night and seek refuge in moist, dark protected sites during the day.
Despite their hard bodies, millipedes do not conserve moisture very well. This is why they die quickly when they get indoors, whether one sprays with insecticides or not.
Recommended management for millipedes includes:
1} Establish/maintain good exclusion around the house/building. This includes attention to thresholds, door sweeps, door and window seals, structural cracks, etc. Sealing such cracks will help keep out other occasional insect invaders as well and will help conserve energy. Physical exclusion is the most effective thing most home and business owners can do to reduce the number of millipedes--and other insect pests--that invade the building.
2} Use a broom and dust pan or good wet/dry vacuum, or leaf blower to keep dead and dying millipedes cleared off the carport/garage/patio/etc. This takes persistence and patience, but is the best way to avoid having to deal with thousands of dead millipedes crunching under foot.
3} Use cultural practices that help minimize the amount of mulch/leaf litter/grass thatch around the house. Of course, this has to be done before the problem begins. People who have a persistent or recurring problem may want to try to figure out where the millipedes are coming from and take steps to remove/minimize their breeding source. This can be excessive flower bed mulch, accumulations of leaf litter, dethatching the lawn, etc. Houses that have the greatest problems are often located in wooded settings, which results in large numbers of millipedes migrating from leaf litter in the woods. This is one of the most difficult situations to deal with effectively because there is such a large breeding source.
Insecticides are not really very helpful when dealing with millipedes. The problem is that even if you kill all the millipedes that arrive at the house, more just keep coming back--as long as they have the kind of moist humid weather that lets them wander about in the open at night. Excessive use of insecticides can create an even worse problem, one that will not go away on its own with the arrival of dry weather! It is usually better to spend your time and money on exclusion and source reduction than on spraying. Applying an outside perimeter spray in a five to ten foot band around the outside of the building may be somewhat helpful in some situations, but it will not be a total fix. Garden Tech Sevin (Carbaryl) and Ortho® Bug-B-Gon® MAX® Lawn & Garden Insect Killer Ready-to-Spray (bifenthrin) are two treatments that are sold in ready-to-use hose-end applicators, which makes them relatively quick and easy to apply.
For more information see page 13 of Publication 2331, Control Insect Pests in and around the Home Lawn and Bug-Wise Newsletter No. 13 of 2004. Both these resources are easily accessed through www.extension.msstate.edu.