The cunning little bag moth, never seems to move and looks just like the bark of its host plant. As kids we would bring them inside and leave them overnight and then try to see where they had attached themselves the following morning. Believe me they do move. The bag moth loves ornamental conifers such as cupressus and larix they will also chew on your Manuka, elm or acacia bonsai given the chance.
When the bag moth eggs hatch the caterpillars weave themselves a tight little silk sleeping bag with small pieces of bark and leaf incorporated for camouflage. The sleeping bag even has a draw string in the top so when the caterpillar is disturbed it can pull the cocoon shut and wait out the danger.
The caterpillar will stay encased for many months by which time the cocoon will be several times larger than the one in the photo, sometimes up to 80mm long. The metamorphosis from caterpillar to pupa to moth all takes place in the relative safety of their carefully maintained bag and when the male moth hatches he crawls out the bottom of the case and shoots off in search of a female. The male is a small black furry moth with four quite small wings. The female moth has no wings and a grossly swollen abdomen supported by six stubby legs. She will lower her abdomen out through a hole in the bottom of her bag and wait for a passing make. Once fertilized she will crawl back into the bag and lay her hundred or so eggs and die. When the eggs hatch small caterpillars fall from the case and move off to spin the own sleeping bag.
On large specimen trees the bag moth will hardly be noticed with some browning on conifers and the odd hole on other trees. On a bonsai where aesthetics are important they need to be removed. It should be easy enough to keep an eye out and use digital control, just pick them off. I think they are quite cool and worth moving to another plant away from your trees, but that’s up to you. The only time they can disperse from a host plant is when the baby caterpillars drop out the bottom of the bag. Some will spin small lengths of silk which act as a parachute and catch the wind, otherwise they will stay put.
The other reason not to squish them is they are quite heavily preyed upon by a small parasitic fly. This fly lays its eggs on the edge of the previous nights feeding site, so when the caterpillar resumes feeding the following night it swallows them. The eggs hatch inside the caterpillar and eat it from the inside out, starting with the non essential parts first. So if you do catch one of these case moths after it has eaten a hole in your prize elm remember there are worse things than a quick squish under the heal of your boot.