Dear Alice,

My question is about those straw belts that get wrapped around the trunks of pine trees a meter or so above the ground. I know they have something to do with fooling insects into laying their eggs in the straw instead of the bark, but could you give us more details? For example, what the heck are the belts called? When are they applied? What beetle or disease is involved? Most importantly, is this technology exportable and would it help my sister in Colorado save her pine forest?

Margaret O., Kitakyushu, Fukuoka Prefecture

Dear Margaret,

Those belts are called komomaki and they are indeed a form of pest control, but the way they work is a little different from what you described. The komomaki method was developed in Japan several hundred years ago to combat a pine pest called matsugareha. It's a moth found throughout Japan as well as in Siberia and on the Korean peninsula and Sakhalin Island. In English it's usually referred to as the "pine moth," and its scientific name is Dendrolimus spectabilis.

You are correct that the belts are put on trees to trick the pests into doing something they wouldn't normally do, but the straw is provided not for egg-laying but rather as a nice place to spend the winter. This particular pest does its damage in the spring, as a caterpillar, when it emerges from hibernation with an unstoppable appetite for pine needles. Left to its own devices, the matsugareha caterpillar eats voraciously from March to June, depriving the tree of its means of feeding itself and making it vulnerable to attack by parasites.

To stop this, some clever gardener during the Edo Period (1603-1867) figured out that the best way to combat this pest is not in spring, when the caterpillars are already out in force, but months earlier when they're getting ready to hibernate. Normally the caterpillars make their way to the ground and bury themselves in fallen leaves, but if offered cozy straw matting on the way down they will happily settle in there for the winter instead. This makes it easy to eliminate them; in the spring, gardeners simply remove the matting along with the pests and burn the whole affair.

Traditionally, komomaki were applied in late October or early November to coincide with one of two of the nijūshi sekki, the 24 divisions of the old calendar: either sōkō, the beginning of the frost season, or rittō, the official first day of winter. The komomaki were then left on the tree until a day in March called keichitsu, which is by tradition the day when hibernating insects emerge. The komomaki are removed and burned, pests and all, and the ashes returned to the soil as fertilizer.

In recent years, however, the efficacy of komomaki has come under question as new research shows that the traps catch natural enemies of the matsugareha in higher numbers than the pests themselves. Chikako Niiho and colleagues at the Himeji Institute of Technology in Hyogo Prefecture, for example, conducted a five-year study of the straw wraps applied to pines on the grounds of Himeji Castle, removing each mat in the spring and determining the exact count for each species caught. In 2005, the year with the highest yield (1,577 insects), spiders accounted for the largest percentage of the catch (37 percent), followed by assassin bugs (Velinus nodipes) (31 percent) and cockroaches (12 percent). In fact, the researchers found only six pine moth caterpillars, accounting for just 0.3 percent of the total. In all five years of the study, komomaki trapped far more ekichū (beneficial insects) than gaichū (harmful insects).

In part because of such findings, komomaki are no longer used on the pine trees around the Imperial Palace in Tokyo or in the Kyoto Gyoen National Garden in Kyoto. In the nine traditional gardens maintained by the Tokyo Metropolitan Park Association, however, komomaki are still applied to pine trees in the winter but largely to convey a sense of the season. "We know they are not as effective in pest control as was once thought," a spokesperson told me. "But komomaki are an important fūbutsushi (seasonal item) we can use to create a winter feel in a garden. Such devices are particularly needed in Tokyo because it so rarely snows here anymore."

I'm intrigued by your idea about exporting komomaki technology. You told me in a follow-up e-mail that the pests in your sister's trees are the mountain pine beetle (Dendroctonus ponderosae) and the engraver beetle (Ips pini), both of which damage trees by boring into the bark. I don't know enough about the life cycle or habits of those insects to say whether trapping them on the trunk would work, or when the komomaki would have to be applied, but I can tell you that the method has been successfully adapted to combat other pests.

In recent years, the Tokyo Metropolitan Government has used a komomaki-like band trap to protect keyaki (zelkova) trees from a beetle called nirehamushi (Pyrrhalta maculicollis). The idea was developed in 2003 to combat a severe infestation in the trees lining the wide streets in Nishi-Shinjuku near the Keio Plaza Hotel and the metropolitan government headquarters, and worked so well that the traps have been used in every subsequent year to keep the nirehamushi population under control. In June and July, workers tie a cloth band around the trunk of trees that show signs of infestation in order to trap the caterpillars on their way down the trunk. A week or so later, workers remove the bands and collect the trapped caterpillars. It's a laborious process, but it's actually more effective and better for the environment than using pesticides.

"In any case, we would not want to use chemicals in an area with such high pedestrian traffic," a spokesperson said. "But our goal is not to completely eliminate the pests, which would upset the natural ecosystem, but to reduce their numbers to levels that the trees can survive."