Codling Moth Caterpillar (Worm from an Apple)

2008 January 5

What’s worse that biting into an apple and finding a worm?
Biting into an apple and finding half a worm!

We never spray our apple trees, and so it is pretty common to have little fellas like this in the apples. This is one of the reasons I like to cut an apple into quarters to eat it, instead of just eating it whole. That way, I have a chance to see if there is a worm inside, and to separate it and its “frass”[2] from the part of the apple I want to eat.

Codling moth dorsal view

Codling moth caterpillar ventral view

Anyway, I found this codling moth caterpillar in an apple that I cut up to eat in October. It would have grown up to be like one of these Cydia pomonella moths. This is one of those cases where Bug Guide is really letting the team down. They don’t have any picture of the caterpillars yet[3], their “info” page has hardly any actual info, and all in all we’re going to have to get our information from somewhere else.

“Somewhere Else” being, in this case, Common Fruit Tree Pests, by Angus H. Howitt [4]. The codling moth came out of Asia with the ancestral apple trees, and is probably the single biggest cause of economic damage to apples (in general, infested apples are not considered acceptable as “eating apples”, and are only used for making juice). The adults emerge in the spring, just about the time the apple blossoms drop their petals, and lay eggs either directly on the young fruits or on nearby leaves. The eggs hatch, and then the new caterpillars promptly bore into the apples where they eat, and eat, and eat for about 3 weeks.

In Michigan, they then pupate, emerge as moths in the middle of the summer, and lay a second batch of eggs. From what I’ve seen on our trees, the apples that get infested in the early stage often don’t develop properly and fall off of the tree. The apples that get infested by the second brood are the ones that look normal aside from the fact that they have holes drilled in them (and a worm inside). The caterpillars like to enter the apple through the stem end or the blossom end, although they will punch right through the side of the apple when they come out to pupate. Usually, when you cut open an infested apple the worm is kind of cozied up in the middle with the seeds, and most of the apple is actually in pretty good shape for eating. It’s just the psychological issues with eating something with “a worm” in it that gives people trouble [5].

In the autumn the caterpillars crawl out of the apples, spin a cocoon in a protected spot (under loose bark, in woodpiles, in brush, etc.) and wait out the winter, pupating in the spring. They then emerge as adult moths in the spring, and it all starts over.

It appears that a big source of codling moth infestation is apples that are left on the ground in the fall. If the apples are all picked and either eaten or juiced before the caterpillars can get out, it is possible to keep them from maturing, so that there will be fewer of them in the spring[8].

What I found really disturbing in Howitt’s book was his introductory comments about how people used to keep codling moths out of the apples. It seems that the first real pesticide for them, in the early 1900s, was — you ready for this? — Lead Arsenate! Yes, you read that right. They were spraying a compound of lead and arsenic onto apples to kill the worms, right up until about 1946![6] Not that things like DDT, Sevin, and Guthion are all that great for you either, but I kind of draw the line at toxic heavy metals that will accumulate in the soil and probably be incorporated in the fruit. On top of all that, the reason that they stopped using it was that the caterpillars started becoming resistant to it in just about 25 years! They then proceeded to become resistant to DDT over a span of about 10 years, and are currently showing resistance to the organophosphate insecticides that are being used now. Part of the issue is that they are only exposed to the insecticides for a brief period, when they are flying as moths or immediately after the eggs hatch. Once they burrow into the fruit, the caterpillars are well protected and impossible to kill without drastically contaminating the apples. Anybody using sprays on apples therefore has to time it just right, and part of the “resistance” strategy for the moths is simply to spend as little time outside of the apples as possible.

This is a good opportunity to have a bit of a look at caterpillar anatomy. If you pick up a butterfly or moth caterpillar, you’ll notice that they have two different kinds of legs[7]:


The first set of six legs, just behind the head, are on the three body segments that are going to become the thorax in the adult. These “true legs” tend to be pointed, grasping appendages for hanging onto whatever the caterpillar is eating at the time. The legs on the rear of the body, the “prolegs”, aren’t so much actual arthropod-style legs as they are fleshy protruberances with suction cups on the end. They are very good for holding on to smooth surfaces, and they are lost when the caterpillar metamorphoses into an adult.

The head is basically a hole-boring machine, with mouthparts that are adapted to chew into apple flesh.


The head is practically the only hard part of the body, with the rest being just a tube of digestive organs being dragged along for the ride. The caterpillar has pretty negligible eyes (in fact, I’m not even 100% sure that the indicated spot is an eye), and it breathes through holes along its side (breathing spiracles). These will probably be easier to see in some other caterpillars that I will post in later entries.

[1] Bahahahaha! Oh, you’ve heard it? Never mind, then.

[2] “Frass” is the technical term for insect waste droppings, in case you didn’t guess that yourself.

[3] I just submitted these to BugGuide, so the next person looking for a codling moth caterpillar will be able to find one there.

[4] Copies of this book are available from the MSU Cooperative Extension Service here, for $10/copy. It’s a good book to have if you are trying to grow fruit trees anywhere in the Midwest.

[5] Agriculture does wonders for curing people of squeamishness about their food. Once you see the wheat come out of a combine harvester, squirming with grasshoppers, stink bugs, lady beetles, and similar things (sometimes making up around 10-20% of the volume of the wheat), you get kind of accustomed to the whole idea that you’ve been eating bugs all your life without realizing it. Or, at least, food that has been fondled by bugs. I won’t even go into details about milk and egg production, I think you can guess most of the gory details there if you give it some thought.

[6] Personally, I’d rather eat the occasional caterpillar.

[7] I just recently downloaded the latest version of ImageJ, the free software from the National Institutes of Health that I use for cropping and cleaning up my pictures. While poking around with it, I realized that it had the ability to conveniently add text labels and arrows to the pictures. So, now I can directly point out features of interest instead of just hoping that people can figure out what I am talking about from the text. The next thing is figuring out how to choose the text and arrow colors so that I get good contrast. It has a weird setup where, to choose a text and arrow color, you need to pick it off of an image, and if your text and arrow have to go across several parts of the image it can be kind of hard to find a color that will give uniformly good contrast. I think I’ll need to make a “color and shading pallette” image that can be used to pick off colors that don’t actually occur in a given picture.

[8] An extreme example of this happened here unintentionally a few years ago: we had this massive infestation of forest tent caterpillars in the early summer that defoliated pretty much every tree in the county. While it (mostly) didn’t kill the trees, it did make the fruit trees (like the apples) drop their entire crop. And then, the next year the apple trees were so busy building up their reserves again that they didn’t even bloom, so no apples that year either. By the time the apples actually came back, the year *after* that, the codling moths had been practically wiped out by lack of food for two full years. The fruit that year, and for a couple years afterwards, was practically perfect. Even now, almost six years later, I’m not seeing all that much worm damage.

9 Responses
  1. January 17, 2008


  2. January 17, 2008

    And the blogosphere strikes again as the Backyard Arthropod Project contributes to Mankind’s Store of Knowledge!

    Well done, sir!

  3. abkaiser permalink*
    January 18, 2008

    Actually, this was one of my favorite posts so far. Cool pictures of the spiracle, true legs and prolegs. I’m doing some writing where I had to research a bit of bug anatomy. It’s cool to see the real-life bug parts!

  4. Ryan permalink
    October 12, 2008

    Thank you. We found one of these guys burrowed in a bath towel. We also have apple trees, and it’s October. Thanks for the help in Identifying what this is.

  5. Tanya permalink
    October 11, 2012

    Thank you for this information. My son complained something was on him, shook out his pants and found this guy. Are these worms dangerous? We picked apples a few days ago, also live near several apple trees & it is October.

  6. October 12, 2012

    Tanya: Don’t worry, they are completely harmless. It was probably just trying to find a spot to make a pupa. In fact, if you accidentally eat one while eating an apple, they are even nutritious!

  7. Tanya permalink
    October 13, 2012

    Thank you Tim for the quick response. And I ‘ll remember the nutritious part!

  8. aviella permalink
    June 10, 2015

    Those suckers bite too! Somehow they keep getting in my house and my 3 year old son was playing with one and he suddenly started screaming in pain and said it poke a hole in him. Big tears and screams.

  9. June 11, 2015


    I don’t know what it is that is getting into your house, but it isn’t these caterpillars. They don’t bite, and they don’t get far from the fruit tree they are infesting. There are other things that look vaguely caterpillar-like and might possibly bite (mostly predatory beetle larvae, or possibly lacewing larvae), but they are quite different from codling moth caterpillars.

Comments are closed.