Spongy Moth Eggs

2023 October 29

This past summer, we had a fairly serious outbreak of Spongy Moths[1], Lymantria_dispar. They defoliated a lot of apple and poplar trees. The two small apple trees we recently planted in the front yard were hit so bad that we were out there picking them off in handfuls, and chucking them into paper grocery bags[2]. They pupated in July[3], and on August 4 Sandy found this one on the side of a maple tree laying eggs. The egg mass is that brown patch under her wings, and was a little bit bigger than a nickel[4].

After doing away with her, the egg mass looked like this spongy bit of fluff stuck to the tree (hence the name “spongy moth”).

It popped right off of the tree, so I took it into the house for more pictures. It basically looks like nothing quite so much as a mass of loose felt or a piece of dryer lint. The little hairs in it are stiff and bristly, and kind of irritating to the skin if you touch it. A bit like fiberglass.

Flipping it over, we can see the eggs, which are perfectly round and about the size of pinheads. They are also kind of hard, they feel a bit like tiny ball bearings if you roll them around with your fingers.

The egg masses overwinter, and hatch out into caterpillars in the spring. They will then either crawl up the tree trunk that they were laid on and start chowing down, or if they don’t happen to like that tree they will spin out a silk thread for the wind to catch, and waft themselves to another tree to try again. The adult females can’t fly and lay their eggs a few inches away from where they pupated, and so it is the parachuting caterpillars that are mainly responsible for spreading them.

So, if you see these tan bits of spongy fluff stuck to trees in your yard, you should probably smash them if you don’t want them to defoliate your tree next spring.

[1] These were formerly called “gypsy moths”, which was still the common name in the US the first time I posted about them back in 2011, and the second time I posted about them in 2015. Why the name change? Well, several reasons. First, “gypsy” is something of an ethnic slur, and even if it wasn’t, ethnic groups generally aren’t keen on having destructive pests named after them. Second, calling them “spongy moths” lets people know what they should be looking out for to prevent infestations – the spongy egg masses on tree trunks. And third, it was never really clear why they were called “gypsy moths” in the first place. They were never called anything like that back in Europe, where they are native, and the French common name for them is “spongieuse”, which means “spongy”, like you would expect.

[2] It’s really too bad there is no market value for sacks of hairy caterpillars. We couldn’t feed them to the chickens, because they are too bristly for chickens to eat. There are only a few types of birds that will eat hairy caterpillars, like cuckoos and catbirds. And they first beat the caterpillars around to knock off as many hairs as possible. Even then, there are still hairs remaining that embed themselves in their stomachs, and I have read that they periodically shed their stomach linings to get rid of the embedded hairs.

And if you are wondering why I don’t have any pictures of the sacks of caterpillars, it is because we killed them by folding up the bag and then stomping on it. You don’t want to see that.

[3] We noticed a couple of things this year that were different from the spongy moth caterpillars we had seen in previous years. They were generally smaller, and we found a lot of them that looked like they had either died horribly of various diseases, or been sucked dry by predators (probably the predatory stink bugs that we have seen in the past raiding tent caterpillar nests). And only a tiny fraction of the caterpillars seem to have successfully pupated, most likely because of parasitoid flies. We had steadily escalating spongy moth numbers every year for the last few years, but it looks like this year might have been the peak before the crash.

[4] Chickens don’t like the egg masses, either. Too bristly. In fact, it sounds like the only life stage that is preyed on to any great extent are the pupae, which are regularly eaten by mice.

One Response
  1. Steve Plumb permalink
    October 31, 2023

    Ah mice, very useful but somewhat hard to direct.
    We have spongy moths in Maine but low numbers for many years. The last major outbreak was quite awhile ago. The last 7 years the problem has been the browntail moth similar but the hairs are much more irritating. I’ve been considering bringing in cuckoos but Amazon doesn’t list them for sale. Fortunately this summer was very damp and the numbers have dropped. Like you with the spongies, lots of shriveled up caterpillars. The state says the ones that have a j shape are the product of a fungus (Entomophaga aulicae). Our damp year has enabled it to spread widely. None to soon as some of the oaks have been defoliated multiple times and I thought they might not recover.
    Link to state forest pest pdf with picture on fungus killed caterpillar on page 8.

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