Wandering through the Hamarikyu Garden in Tokyo last year I noticed that many pine trees had straw sleeves tied around the branches. A bit more inquiry turned up the information that they are put on in autumn and used to trap pests (which snuggle into the straw for winter) before being taken off and burned in spring.
Komomaki were developed in Japan during the Edo Period (1603-1867) to combat a pine moth (Dendrolimus spectabilis) known in Japanese as matsugareha. The sleeve provides an overwintering site for the pest, which does its damage in spring when the caterpillars are hungry for pine needles. The attack continues through to early summer, leaving the tree unable to feed itself and making it vulnerable to parasites.
This excellent 2012 article from The Japan Times notes that komomaki have been researched and aren’t as effective as once thought – in 5 years of study they trapped more beneficial insects than harmful ones! The sleeves are now most likely used to convey a sense of season, especially in Tokyo where winters aren’t nearly as cold as they once were.
However, the komomaki concept has proved successful at helping to protect Tokyo’s Zelkova serrata street trees from beetle attack (Pyrrhalta maculicollis).