Protect your bur oaks from gypsy moth caterpillars

At least one of the heritage bur oaks in Champlain Park neighbourhood is being attacked by gypsy moth caterpillars. These little pests seem to love oak leaves.

But combined with a drought that has plagued us since late April, any deciduous tree may be infected by and die from gypsy moth caterpillars.

While some say small trees or saplings are more at risk, a large tree infested with these caterpillars can also die. That’s where your human compassion for the tree world comes into play.

How can you know if your tree is infested, and what can you do? The info provided here (with help from and with thanks to Sharon Boddy and Iola Price!) applies to any species of tree.

A local news article contains good photographs and describes how the moths came to Canada.

Signs of infestation

Look up. If you can easily see the canopy leaves, observe them closely. Do they look like lace? Do they have black caterpillars clinging to them?

Look down. If the tree's canopy is too high to observe, look on the ground under the tree. Do you see pieces of leaf littering the area? The insect chews holes in leaves, and some of the leafy bits fall to the ground.

Use the caterpillars’ daily routine to outsmart them

As it starts to warm up each day, caterpillars descend to the ground. They are looking for a cool hiding place. You may see them hanging in the air on an almost invisible thread similar to a spider’s silk. Sometimes, they just crawl down the tree trunk.

As the day cools in the early evening, the caterpillars start to climb up into the canopy.

NOW is the time to trap and kill caterpillars.

How to kill the critters

1) You need to trap them and drop them into a bucket containing water and dish detergent, which creates a slippery solution. Be sure to mix the soap into the water because water alone will not kill the insect.

2) To trap the caterpillars, wrap the tree trunk in burlap. Wrap a wide strip (17 inch (45 cm) or so) of burlap around the tree trunk at chest height. Tie a string snugly around the centre of the burlap and fold the upper portion down to form a skirt, with the string acting as a belt. The caterpillars will crawl under the burlap to escape the sun and become trapped. Reuse burlap over the course of about 2 weeks. NOW is the time to capture caterpillars.

3) At least once a day, remove the burlap and deal with the caterpillars. Do this at 4 or 5 p.m. if you can only do it once. If you can kill caterpillars twice a day, remove the burlap a second time, between 7 and 9 a.m.

4) WEAR GLOVES AND LONG SLEEVES when dealing with these caterpillars. The caterpillar hairs contain a histamine that can cause an itchy rash.

5) Be careful where you shake out the burlap. Unless you are over concrete or asphalt where you can see and collect them all – they may fall off the burlap, miss the bucket, and end up on the ground where they'll just sneak back up the tree. A tarp laid out on grass is also a good surface on which to shake out the burlap.

6) Because they stick to the burlap, you can pick them off by hand (grab several at a time) and dump them directly into the bucket with soapy water. Leave them in the bucket for at least 24 hours, then dump them down the sewer, or strain them out of the water and put them into your garbage bin, not the green bin!

Alternatives to burlap

Some people are fastening cello tape to tree trunks, sticky side out. You can place more tape at the top and bottom, shiny side out. It seems the caterpillars do not like to touch the shiny material and will wander around the tape leaving them ripe for the picking. If they cross the sticky tape, it traps them when they are crawling up and down the tree.

Some people are using a sticky product (like shortening) to entrap the caterpillars. If you do this, apply it only to the tape - not onto the bark as further tree damage might occur from the sticky goo.

Dealing with drought and this infestation

We are in a Level 1 Drought so WATER YOUR TREES frequently but DO NOT add fertilizer to the water or sprinkle fertilizer around the tree.

Leaf loss is a stress and combined with drought, the impact is doubled. Fertilizer promotes new woody growth but you want leaves only. The tree has to “decide” whether or not to use its starch reserves on new leaves, and water helps promote re-leaf. New leaves will be smaller.

Conifers, especially pines and spruces, may not survive because their reserves are in the needles which have now been eaten and they cannot replace the needles this year.

If it rains, there is a naturally-occurring fungus that multiplies in cool wet weather that will start attacking the caterpillars. And a naturally-occurring virus may also show up. Caterpillars hanging on the tree trunk head down or in a V-shape are being attacked by these diseases. This is normal and to be celebrated.

Not all birds will eat hairy caterpillars. Yellow-billed and black-billed cuckoos are voracious predators of gypsy moth caterpillars and apparently some robins have been seen eating them. Small birds may not take the larger insects and, because there are so many caterpillars, many birds can't keep up.

Life cycle of the insect and dealing with eggs before winter

An instar is a stage in the caterpillar's life. At each instar, it moults its old skin and grows in the new skin until it can't grow any more and then it repeats the process.

The caterpillars go through 5 instars if male and 6 if female (she eats more so there is stored food to make her 300 or so eggs).

Through its five or six instars, a caterpillar can eat 1 square metre of leaf surface.

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The caterpillars will start to pupate in late June or early July; this is a good time to look for pupae and flick them into a pail of detergent and water.

The adults emerge from the pupal cases in August.

The males immediately start to look for females; the females don't fly and once they lay their eggs, they die; a male can mate with several females.

Eggs are laid in August and overwinter and will hatch in the spring of 2022.

Collect the eggs masses by scraping or, as someone suggested, a shop vacuum from August to the spring of 2022. Be sure to get all the eggs - those that fall on the ground will probably survive. Let the egg masses sit in detergent and water for two days to ensure complete kill.

Check any surface: walls, tires, car wheel wells, eaves, undersurface of branches and especially the trunks of trees at what might be below the anticipated snow level.