The Dark Side of Millipedes

Looking back into past issues of Hort Matters and other OMAFRA newsletters, it would seem that we've published a number of articles that fall into the "When Good Bugs Go Bad" category. It would be nice if there were just good guys and bad guys and the good guys were always good. Nature has not provided us with such a luxury. Under certain environmental conditions, even beneficial insects (and their relatives) can seemingly turn against you. In the last couple of weeks, we've seen damage to several crops in Ontario caused by … millipedes!

Before we go any further, it is important to distinguish between millipedes and other inhabitants of the soil frequently encountered during scouting activities. Millipedes are sometimes confused with wireworms (larvae of click beetles - truly "bad guys" from an agricultural standpoint), as well as centipedes and symphylans. Millipedes are typically dark in colour (brown or grey-black) and have hard, cylindrical many-segmented bodies. The number of body segments increases with age and varies by species. Each body segment has 2 pairs of legs (Figures 1 and 2), except for the first three segments behind the head, which have a single pair each. "Millipedes" translates roughly to "thousands of feet" or "thousand leggers"; while this isn't entirely accurate, they do indeed have many legs. When disturbed, millipedes will curl up tightly (note that dead ones are particularly foul-smelling). In contrast, wireworms have three well-defined body sections (head thorax abdomen) and three pairs of true legs on the thorax. Both wireworms and millipedes may require several years to complete their development. If you need some assistance in identifying these, visit the the OMAFRA website.

Figure 1: Millipedes are typically dark in colour (brown or grey-black) and have hard, cylindrical many-segmented bodies

Figure 2: Each body segment has 2 pairs of legs, except for the first three segments behind the head, which have a single pair each

Millipedes are important scavengers, feeding on decaying organic matter and mixing it through the soil. In other words, millipedes are normally considered beneficial. Unfortunately, under some circumstances they can cause crop damage. Millipedes have high moisture requirements and thrive in damp, dark places, particularly those with protected areas and crop residues or heavy mulch. Last fall's wet soils and this spring's prolonged cool wet soil have led in some cases to a population explosion of millipedes. Under high population pressure, millipedes will eat emerging and young plants. In this article we use ginseng as an example, but other crops may also be at risk. When wet conditions persist and millipede populations are high enough, they can eat new shoots and roots as the seed germinates (Figure 3). In the case of ginseng, where the hard-coated seed cracks just before germination, the seed pulp may also be eaten away (Figure 4). Plants that make it to the soil surface and begin to extend above the straw may be chewed off where they break through the soil (Figure 5). Clearly, this is another case of when good bugs go bad!

Figure 3: Millipedes can eat new shoots and roots as the seed germinates

Figure 4: In the case of ginseng, where the hard-coated seed cracks just before germination, the seed pulp may also be eaten away

Figure 5: Plants that make it to the soil surface and begin to extend above the straw may be chewed off where they break through the soil

So what can you do at this point? Not much. Soil insecticides do not provide good control of millipedes. The good news is that millipedes are extremely sensitive to soil moisture levels. As soils warm up and dry out, millipede populations may return to normal. Don't let this transgression sour you on millipedes as beneficials. They play an important role in breaking down organic material and improving soil structure.

Related Links

• Be a Better Farmer: Understand the Living Soil: Good Bug or Bad? At first glance it can be hard to tell

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