Millipedes attacking sweet potatoes
and carrots in Ontario - the latest info
Part 1: Tracking millipede populations in the field
Millipedes are considered beneficial in vegetable production due to their
habit of feeding on decaying plant material, which helps to incorporate
organic matter into the soil. However, under certain conditions millipede
populations can build to high levels and may damage the roots and seedlings
of a variety of crops. In recent years, millipedes have become an increasing
problem in a number of Ontario crops, including no/low-till field corn
and ginseng.
In a recent Hort Matters article, we noted that over the last few years
there have been numerous incidents where large numbers of millipedes were
found in root vegetables where crop damage had been attributed to wireworms.
At the time, we did not have enough information to know whether millipedes
were emerging as a new pest of root vegetables in Ontario, or whether
they were simply secondary feeders entering these crops after being damaged
by other pests. In the summer of 2007 a research project was conducted
by University of Guelph researchers and OMAFRA staff to address the role
of millipedes as potential pests of Ontario root crops.
Field Sampling
Sampling was conducted throughout the 2007 growing season in three sweet
potato and three carrot fields in southwestern Ontario. The purpose of
this research was to observe changes in millipede numbers throughout the
season, determine where in the field they were located, and compare different
monitoring methods. Three trap types were buried in the soil at 30 sites
per field: (1) potato bait (a piece of white potato placed in a fine mesh
bag), (2) corn bait (Fig. 1, a quarter cup of untreated corn placed in
a fine mesh bag, then soaked in water for 12 hrs) and (3) a pitfall trap
(Fig. 2, a plastic cup partially filled with antifreeze). The traps were
left in the ground for one week, then the number and species of millipedes
was recorded. Fields were visited every other week, from shortly after
planting (May for carrots, June for sweet potatoes) until early September.
At the end of the season, sweet potatoes and carrots were harvested from
each sample site, and assessed for pest damage.
Figure 1 - Example of a corn bait
trap.
Figure 2 - Example of a pitfall
trap, covered with a canopy (plastic dairy container lid and galvanized
wire) to protect it from rain.
What we saw:
- Millipede populations varied considerably among fields for both carrots
and sweet potatoes (Figs. 3 and 4). This was likely due to differences
in soil moisture, soil organic matter and texture and crop production
practices, but more study is needed to determine exactly what conditions
contribute to high populations.
- In carrot fields, millipede populations were highest early in the
season, dropping down to very low numbers during the hot dry periods
of June to August, 2007 (Fig 3). In contrast, millipede populations
were higher in sweet potato fields, and tended to remain high throughout
the season (Fig 4). In one sweet potato field, we found more than 2000
millipedes even after a prolonged period of high heat and little rain!
What accounts for this difference? It may be due to the fact that, once
sweet potato vines close over, the field is essentially covered by a
dense crop canopy that helps to keep soil moist. Additionally, sweet
potato weeds are commonly left on the soil after hand hoeing, which
may create localized areas of cool, moist conditions that favour millipedes.
Carrots, on the other hand, do not form such a dense canopy, and the
carrot fields in this study were more severely impacted by the limited
rainfall of 2007.
- Millipede populations also varied within the field. We tended to find
more millipedes in traps placed closer to the centre of the field than
in traps located adjacent to the field edge.
- In both carrot and sweet potato fields, we found that pitfall traps
were better at collecting millipedes earlier in the season. Later in
the season, after crop canopies filled in, corn bait traps became more
efficient at trapping millipedes (Figs. 5 and 6).
- End of season crop damage varied from 0 to 57% of sweet potatoes sampled.
Damage to carrots ranged from 0 to 30%, but was generally much lower
than that seen in sweet potato fields. However, we cannot directly relate
end-of-season damage with millipede populations because feeding occurs
strictly underground, and may have been partly due to other soil pests.
To address this problem, we conducted laboratory studies to observe
the type of damage caused by millipedes over time. These results will
be reported in a subsequent article.
Figure 3. Total number of millipedes
captured per sweet potato field (all trap types)
from June to September, 2007
Figure 4. Total number of millipedes
captured per carrot field (all trap types)
from May to September, 2007.
Figure 5. Total number of millipedes
captured per trap type in sweet potato fields
from June to September, 2007.
Figure 6. Total number of millipedes
captured per trap type in carrot fields
from May to September, 2007.
So in a nutshell, millipedes definitely can be present in large numbers
in root vegetable fields, with populations tending to be higher in the
middle of the field. In contrast to what we previously believed, the environment
does not need to be extremely wet to favour development of high millipede
populations. In fact, a closed crop canopy and developing crops may actually
favour millipedes in dry conditions because they act as sources of moisture.
We were also able to identify sampling techniques that were effective
at detecting millipedes in the field. In part 2 of this article, we will
report results of lab studies designed to identify millipedes and study
the damage they do to root crops.
Acknowledgements
The authors wish to thank the Canada-Ontario Research and Development
Program (CORD), an initiative of the federal-provincial territorial Agricultural
Policy Framework and the Fresh Vegetable Growers of Ontario for funding.
Thanks also go to Drs. Sears and Hallett, L. O'Keefe and T. Marrowa of
the University of Guelph. A special thank you to all cooperating sweet
potato and carrot growers who collaborated on this project.
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