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Marching caterpillars (Processionary caterpillars)


January 30, 2024


The Searles Gardening Team

Marching caterpillars, sometimes called processionary caterpillars, are on the march; you will see them in long lines marching from tree to tree. Their scientific name is Ochrogaster lunifer, and they feed on the leaves of the plants at night and can strip a tree quickly as they arrive in their hundreds. The lines can be up to two hundred strong, arriving on mass for a feed.

Another caterpillar that can be confused with the marching caterpillar is Leptocneria reducta (white cedar moth caterpillar). They look similar, and it is mainly the trees they devour that are the difference.

Since they feed at night, they form a mound at the base of the trees at dawn and are covered in a fine thread, often with dropping matted. When they march, they leave the thread produced from their mouth, and during the day, you can see the treads from where they ascend and descend the tree. Once finished with the tree, they will proceed to the next, often following the same route each year. In some regions, they are a regular event, crossing roads and yards as they go to their next plant.

If threatened, they form a mound and the hairy mass that deters most predators. These hairy caterpillars irritate humans and can last a few days, especially if they become infected. It is believed that the hairs from the caterpillar that fall off can cause a pregnant mare to abort, and these caterpillars are a scourge to horse owners. You will note many birds leave these caterpillars alone for this reason, but some, like cockatoos, still dine on them.

They feed on native trees and shrubs and seem particularly partial to wattles. If one caterpillar of the same species comes across a thread of another, they will follow, as is their nature. They march nose to tail to form a line. For the more mischievous of us, if you use the thread to make a circle, they will simply follow it round and round.

They then will separate to develop into a pupate or cocoon when the time is right. They emerge as moths that only live long enough to mate and lay eggs. Interestingly, they do not eat when they are a moth as they have no mouth.

When not in a mound, any pesticide that states it is for caterpillar control will work well. If making up a spray from a concentrate such as pyrethrum, add Spredmax so the spray sticks to the caterpillars, as their spines can cause the spray not to penetrate their bodies. When they have formed the mounds, which can contain hundreds, and the mounds are so layered, the caterpillars on the bottom are protected from the sprays, so reapplying the insecticide will be necessary. They have been known to invade homes as they head to their next tree. Household barrier sprays can help prevent their entry, or if already entered, remember not to touch them with bare hands, and since they shed their spines, do not go barefoot.