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Green Leaves by June... true for most tree species. The Russian Olive will leaf out very late, but they have silver leaves so that doesn't count here, and even though you might see some good foliage as the temperatures rise in late spring, take a closer look and you could be looking at partially eaten leaves, or just new green twigs. Once the spring is out of the way, late-June in Manitoba, it's important to take a step back and really evaluate how they came through the winter. Are they in good stead as the growing season hits full stride, or are your trees still overcoming last years problems?

The problem at hand

One definite factor that can determine a successful spring is whether your trees have had to cope with any defoliating insects. The 3 we have to deal with mainly in Manitoba all belong to the same order, Lepidoptera, and are Cankerworms, Forest Tent Caterpillar (broad leaf eaters) and Spruce Budworm (coniferous foliage eaters.) Forest Tent Caterpillars are probably the most prominent of the defoliating insects* in Winnipeg and they are whom we will discuss today.

A solitary Forest Tent Caterpillar at mature size. That leaf doesn’t stand a chance.

Our urban forests are a frenzy of activity come mid-May, and Forest Tent Caterpillars are certainly a pest in the flourishing trees. They come to life out of an egg mass laid the previous summer. Laid by an inconspicuous brown moth. If you have a mature tree, these moths will often lay their eggs far away from where you can see them. So when 300 larvae start congregating at the top of your tree, before you've had a chance to catch a glimpse of them, you need to be prepared. If you have any doubt that your trees have been infested with these caterpillars (maybe a tree on your street, or even around the corner, was infested last year, these moths can fly remember!), then this is when you should consider hitting the trees with a biological insecticide**. While these young larvae are vulnerable, the insecticide has maximum affect on their developing bodies. If the insecticide is applied properly most, hopefully all, will perish at this point and there won't be any need for further treatments.

What happens next?

If you don't have your tree treated and you have an infestation, this is the point when hundreds, or thousands, of caterpillars will begin to eat the succulent new leaves that have emerged in the spring. This is when the tree starts to feel it. Younger trees can lose a large amount of their foliage early in the year, mature trees can lose a lesser portion. It's rarely 1 single year that does the damage to mature trees either, but if left untreated for 2,3 or 4 years, these pests can repeatedly defoliate a tree to the point of significant dieback, and when this has to be pruned out, it can disfigure a tree unnecessarily. Younger trees can die as a result of repeated attacks and need to be replaced.

An egg band, late summer on an Amur Cherry.

For the lucky caterpillars that do survive initial treatment, hidden away in the upper canopy or in a perfect crotch, sheltered from the death rain, they will have the whole tree to themselves. Others might migrate from neighbouring plants too as the season goes on. The damage they will do is negligible at this point, but it's the egg laying that becomes the issue.

Regardless, as the larvae grow they stick together for warmth and safety. The Forest Tent Caterpillars operate as a unit, moving around the tree under the command of the most hungry members of the group. As they begin to set off looking for food, they secrete a pheromone and the pack responds by following out toward the canopy. They only finally split off when the twigs become too small, and then it's feeding time! They might be slow eaters, but these insects munch through leaves like Winnie the Pooh with a free pass in a honey store. Together though, their shared body heat keeps them mobile through group thermoregulation, and the sheer volume is enough to discourage predators from attacking. It is also what often alerts tree owners to the problem. A population of up to 300 per egg band, and the potential of multiple bands in a single tree, these caterpillars are extremely obvious and never welcomed at this point. They grow easily to 3” long, but known to reach sizes of 5”, and it can be overwhelming on what to do. You can spray a more potent insecticide, and some will die (if you had sprayed earlier, applying another dose can pick off the survivors, and migrants, to help with prevention of egg laying), but realistically it's time to manually remove these caterpillars before they get a chance to pupate into moths. A domestic pressure washer will get rid of a lot of them successfully, but make sure you squish them when necessary. They do have the potential to make it back to the tree and pretend this whole ordeal didn't happen. If they are within hands reach, just pop a glove on and wipe them off, they don't bite and can just turn to paste in your hands without even feeling it.

The survivors

Unlike the Cankerworm which leaves a leaf skeletonized, the Forest Tent Caterpillar eats all parts of the leaf, minus the main vein.

The ones that reach their destiny of becoming a moth, and reproducing, begin their final journey. About 6 weeks after emergence, battling insecticide, gravity, predation, and jets of water from unruly tree owners, they select their final leaf to use to their benefit. The Caterpillars begin making the cocoon that will be their home for the next couple of weeks or so. Woven out of silk, the caterpillar will wrap the the leaf around its body and seal it. After a vigorous building session, the shelter is safe and strong to stop the weather getting in, but the odd bird can certainly pry out an easy meal. When they come out of their housing, the only priority is to mate. These brown insects have a lifespan of only a few days at this point and they need to reproduce! Male moths hover around a females cocoon to welcome them into the world, often fighting for an available female. The females then lay the egg bands back in the tree, coating it with a protective gel. These will stay on the tree for the next 9-10 months for the next generation to continue their march to insect dominance.

What should I do?

The Moth. Nocturnal workers, but will happily bask in the morning sun.

All that being said, the most important thing is to reduce the stress that the caterpillars will put on the tree. A stressed tree will be more susceptible to future attacks and infections as it concentrates its resources on recovery, rather than growth and defence. A healthy tree is achieved with good insect management, but more importantly good soil management as well. The urban environment is challenging enough for our trees, but if you can look after the things within your control, you can help see your tree reach its full potential.

*At the time of writing. Forest Tent Caterpillars go through peaks and troughs in long term cycles, but are always present in the urban forest.

**With any chemical use, please ensure it is registered for use in your local area. Always follow the labels accurately (any action differing from the labels instructions is against the law), or consult licensed applicators if your trees are too large for you to apply effectively. Applying incorrect products, or in an unsafe manner, can lead to dire consequences ranging from water pollution to wildlife death.

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