Processionary caterpillars get their common name because they move in a line head-to-tail. The scientific name for the Bag-shelter moth and it’s Processionary caterpillars is Ochrogaster lunifer.
Moths hatch in October-November.
Female on the left is a ground nester and the right is a tree-hugger. Females are approximately 5 cm across and have a tuft of white or yellow scales at the tip of the abdomen. These scales are irritating, and the female deposits them around her eggs to protect the egg mass.
Moths possess non-feeding mouth parts and only live a few days. After mating, the female deposits a single cluster of 150-550 eggs in an egg mass, covering the eggs with a thick white or golden mat of deciduous scales and white or golden hairs from her anal tuft.
The eggs hatch as first instar larvae which do not feed. Second instar larvae generally appear in December and begin feeding, moving up into the canopy and feeding on the outer leaves for a few hours each day, returning to the base of the tree at night.
Later instars feed at night. Watch a video of the caterpillars foraging at night here.
The larvae grow and moult synchronously in a silk nest at the base of the tree trunk, on a branch, or in the canopy. The larvae remain together through the 8 instar stages (I to VIII) and will remain in the same nest unless the tree is defoliated (photo below). If the tree is defoliated, larvae leave the tree in a single-file procession in search of a secondary host. Larvae from different clutches on the same tree will often aggregate to form one large cohort.
Larvae of the 3rd to 6th instar are densely covered with microscopic urticating hairs that are continually shed and may become airborne. These hairs or setae are found on the dorsal surface of the eight abdominal segments. Setae are 150-250 um in length and 5-10 um in diameter and are tubular. They have pointed tips and break at the slightest touch, releasing a mixture of different proteins, the most important of which is a histamine liberating toxin (thaumetopoein or a closely related compound). Each caterpillar may possess hundreds of thousands of urticating hairs. Hairs that contact human skin are often broken open by mechanical action and release a histamine liberating protein that causes dermatitis.
Egg masses and nests
The moth appears to lay eggs on preferred host trees. The initial egg mass is a spherical white or golden, fluffy mass about the size of a golf ball. The white mass is made up of scales from the posterior end of the moth abdomen, deposited over the eggs to provide protection. As the caterpillars grow inside the egg mass, the egg mass changes appearance and increases in size and becomes a caterpillar nest or bivouac.
There are different locations for egg masses and nests and it seems possible that there may be about five forms. These five forms may be associated with different sub-types or species of processionary caterpillar.
1. Egg masses are white and laid at the base of trees, commonly Acacia sp. (wattles) from very young saplings to older trees. Over time as the caterpillars develop, the egg mass develops into a nest. The ground nests develop into aggregations of caterpillar frass (poo), exoskeletons (cast skins), and other organic debris, held together with silk (spider web-like material) from the caterpillars. The nests may be subtle and difficult to distinguish from dead leaf matter around the base of the tree and are sometimes seen up to a half a metre or more from the tree trunk. Caterpillars tend to sleep inside the nest during the day as a big ball. There is often a silken trail going from the nest up the trunk of the tree and caterpillars use this to navigate to and from the food in the canopy. It helps to use a stick and disturb the material at the base of trees when searching for nests. When a nest is opened it will contain caterpillars and lots of frass and shed exoskeletons.
Egg masses may also be deposited up higher in trees where trunks fork or on branches. They may be harder to spot when up higher. There are three broad forms of elevated nests, forming the other nest forms.
2. Elevated nest in Acacias and possibly other trees. Recent observations have identified elevated nests appearing as spherical bags with a leathery outer skin and adherent to branches up off the ground. These nests have been observed in wattles and in eucalypts in QLD and NSW and may be less than a metre off the ground or up to several metres depending on how high the trees are.
3. Elevated nests in taller Eucalypts. In some areas (Southern QLD for example) elevated nests may appear as hanging guourd or pear shapes with the same leathery outer skin. Nests may be quite high and quite prominent.
4. Tree hugger nest types. These nests appear as a flattened form made up of a mixture of silken material and leathery skin like material. The nests are tightly adherent to the trunk or branches of trees (mostly Eucalypts) and may be difficult to identify because they are more subtle in appearance. These nests appear less commonly than the other types.
It is not known whether these nest types are all simply variants of the nests from the same species of caterpillar, or whether they might be associated with different types or even species of processionary caterpillar. The caterpillars appear morphologically similar. More research is needed to determine whether these forms may represent different sub-types of processionary caterpillar.
Observations of nest types appear to suggest that the same nest forms appear repeatedly in the same host species. There are some wattles where the nests appear to be almost exclusively at the base of trees. There are other wattles where nests appear to be repeatedly of the elevated form. We don’t know enough yet about host species and nest types to be confident of which trees might be major host species.
Our experience in observing egg masses and nest development is that many egg masses fail to develop (at least 50%). This is likely to be a normal part of biology. This is based on observations of egg masses in December and then repeat observations of nests in the same area in March. A number of early developing nests will also coalesce to form single larger nests.
Caterpillar movement between trees
Caterpillars tend to nest on the tree on which they are feeding. If there is sufficient suitable feed (likely to be younger growth at the tops of trees), the caterpillars will usually develop through most or all stages on the same tree. Caterpillars may choose to move to another tree if they defoliate the host tree, or if they decide that the foliage is not right in some way. In some cases it is possible to see trees that havebeen defoliated and processionary caterpillars may be one potential cause (there are many other things that may cause this, including other insects, diseases or nutritional/water stress so don’t leap to the conclusion that you have processionary caterpillars every time you see a denuded tree). When the caterpillars decide they want to move to a new host, they generally come down a move in a single file, maintaining head to tail contact. It is unclear how they determine a direction to travel but they can be found some distance from trees. When the caterpillars find another host tree, they may bivuoac at the base of the tree and then form a new nest either at the base of the tree or up off the ground.
Moths may preferentially lay eggs on selected host trees. However, when caterpillars move from one tree to another they appear to be capable of surviving on a much wider variety of host. This has led to the concept that there may be primary host trees where egg masses are more likely to appear, and then secondary host trees where caterpillars may subsequently be found.
While the triggers that may initiate movement from one tree to another are not well defined, it seems likely that this movement is unlikely to occur until several weeks or a few months after egg masses are laid.
Final instar larvae disperse about May (range may be from April to June depending on location and climate), travelling up to 200 m from the host tree and undergo pupal diapause underground during the dry season, with pupation then resuming and completing a few weeks before the next season’s summer rains in October-November.
While caterpillars seem most likely to dig into the soil to pupate it is possible that they may be found in some unusual places. There are reports of caterpillars being seen in the wire holes in fence posts though it is not clear if these were processionary caterpillars.
Boom and bust
Like many insect populations, processionary caterpillars seem to exist in a long term boom and bust cycle. Populations will increase over a number of years until they may be so common as to be considered in plague proportions. This is often followed by a bust of rapid decline in populations and they may be virtually undetectable in an area for some years. A number of factors may influence these boom and bust periods including climate, tree growth, flowering events and natural predators. Booms may occur at intervals of several (5 to 10 years) apart.