Wednesday, June 12, 2013

Blame it on the rain (Part I): A multitude of millipedes

In the last couple weeks we have gotten a lot of rain. This can have both positive and negative impacts on pests and pathogens. One critter I have been getting a lot of calls and emails about is the greenhouse millipede, Oxidus gracilis (Polydesmida: Paradoxosomatidae).

The greenhouse millipede (Oxidus gracilis) is one of the most common arthropods around. It can be distinguished from other millipedes by the size (~1"), brown color, and especially by the groove going across the top of each body segment
Photo by Matt Bertone
Millipedes are in a group of organisms, closely related to insects and spiders, called myriapods (which translates literally to "many-legs"). At first they may appear to be worms, but they have legs which worms lack. The most familiar are centipedes and millipedes, some of which grow to large sizes (some tropical centipedes are a foot long and can eat bats, lizards and rodents!!). While centipedes are carnivorous and have venom, millipedes are vegetarians that feed on decaying plant matter. Though they cannot bite, many millipedes defend themselves with toxic poisons that either taste bad or can cause sickness - but only when eaten. This is the reason why some millipedes you see are brightly colored and smell like almonds (related to the cyanide compounds they produce):

A larger relative of the greenhouse millipede (Polydesmida: Xystodesmidae) [~1.5" long]
Photo by Matt Bertone
The greenhouse millipede is a non-native species from Asia that has traveled throughout the world with humans. They are abundant under rocks and logs, in basements and in the soil - anywhere where there is some moisture. People tend to notice them sporadically until some event drives them out of hiding. In the case recently, our heavy rains from multiple storms have saturated the soil and make it difficult to survive for the millipedes. Their response is usually to move to higher ground like the foundations and walls of homes and other structures. They may even move inside, though if they get stuck it's bad news for them as they will inevitably die, and bad news for homeowners who have to clean up the carcasses and deal with a distinct odor they produce. In addition, their movement can sometimes be caused by them avoiding chemicals/pesticides that have been sprayed around their habitat, especially the mulch and gardens next to homes.

Greenhouse millipedes invading people's space.
Photo from Stokes Co., NC
Control, especially in the chemical sense, is usually not warranted. Simply vacuuming up the critters is often enough to get rid of them. They pose no harm to people and little to no harm to plants (sometimes becoming important in greenhouse situations - hence the name). So if you are seeing a lot of these worm-like organisms after rains or chemical treatments, be aware that the situation is (A) temporary and (B) the millipedes are harmless (some large ones even make great pets!).

Quick guide to centipedes and millipedes (and other look-alikes):
Does it have legs
> If not then it is likely a worm (though some maggots and other critters are similar):

Typical earthworm.
Photo by Matt Bertone
> If yes, but only up to six true, jointed legs (all on thorax) then it is an insect, likely a larva (caterpillar, grub, etc.):

A click beetle larva (Elateridae) - note only the thorax region has true legs.
Photo by Matt Bertone
> If yes and many legs then you have a myriapod*!
(*unless you have an isopod - pillbugs and sowbugs)

BUT is it a centipede or millipede?
> If it has one pair of legs per segment and is generally fast-moving then it is a centipede (also see house centipedes):

A typical stone centipede (Lithobiomorpha).
Photo by Matt Bertone

If it has two pairs of legs per segment and is generally slow moving then it is a millipede:

A granulate millipede (Polydesmida: Polydesmidae: Scytonotus granulatus)
Photo by Matt Bertone